Background of the InventionThis application claims priority from United States Patent Application SerialNo. 10/014,415, filed December 14, 2001.
This invention relates generally to a combustion system for gas turbine gasgenerators, gas turbine engines, or other heat devices, which can produce significantadvantages including low levels of pollutants, namely oxides of nitrogen, carbonmonoxide, and unbumed hydrocarbons. Specifically, the present invention relates tosingle stage, controllable fuel/air ratio combustors for gas turbine engines and gasgenerators using fuel/air premixer assemblies with controlled variable premixer exitgeometry.
Although gas turbine devices such as engines and gas generators do notproduce the majority of the nitrogen oxide emissions released into the earth'satmosphere, reducing those emissions will reduce the total and, in that regard, manycountries have enacted laws limiting the amounts that may be released. The reactionof nitrogen and oxygen in the air to form nitrogen oxides, like almost all chemicalreactions, proceeds faster at higher temperatures. One way to limit the amount of NOxformed is to limit the temperature of the reaction. The NOx produced in gas turbinedevices is produced in the combustion process where the highest temperature in thecycle normally exists. Therefore, one way to limit the amount of NOx produced is tolimit the combustion temperature.
Various attempts have been made to limit the combustion temperature andthereby NOx production in both "single stage" combustors (i.e., those having only asingle combustion zone where fuel and air are introduced) and "multistage"combustors, including pilot burners where several, serial connected combustion zones having separate fuel and air introduction means are used. U.S. Patent No. 4,994,149,U.S. Patent No. 4,297,842, and U.S. Patent No. 4,255,927 disclose single stage gasturbine combustors where the flow of compressed air to the combustion zone and thedilution zone of an annular combustor are controlled to decrease the concentration ofNOx in the turbine exhaust gases. In the above combustors, essentially unmixed fueland air are separately admitted to the combustor, with mixing and combustionconsequently occurring within the same chamber.See also Japanese Laid-Open No.55-45739. U.S. Patent No. 5,069,029, U.S. Patent No. 4,898,001, U.S. Patent No.4,829,764, and U.S. Patent No. 4,766,721 disclose two stage combustors.See alsoGerman Gebrauchsmuster, 99215856.0. Again, however, fuel and air are provided toeach stage at least partially unmixed with complete mixing occurring within therespective combustion zones.
Attempts also have been made to utilize separate premixer chambers toprovide a premixed fuel-air flow to a combustor. Japan Laid-Open Application No.57-41524 discloses a combustor system which appears to premix only a portion of thetotal fuel flow to a multistage can-type combustor in a separate mixing chamber priorto introduction to the staged combustion chambers. In U.S. Patent No. 5,016,443, alarge number of separate fuel nozzles are used to inject fuel into an annular premixerchamber. However, the complexity of the above constructions employing multiplefuel nozzles and fuel splitting devices can lead to control difficulties, as well as a highinitial cost.
Single stage combustor systems using external premixers are known based onthe previous work of the present inventor, such as are disclosed,e.g., in U.S.5,377,483; U.S. 5,477,671; U.S. 5,481,866, U.S. 5,572,862; U.S. 5,613,357; and U.S.5,638,674. There systems provide close control of the fuel/air ratio by premixing allof the fuel for combustion with essentially all the combustion air using a venturi-typemixing tube, and introducing the mixture to the combustion zone of the combustor.Significant reductions in gaseous and particulate emissions have been achieved by gas turbine engines and modules over a broad range of operating conditions, employingthe inventions disclosed in the above-listed patents.
It is, however, desired to provide an improved premixer system for a singlestage combustor that can reduce "flash backs" from the combustor into the premixer,which can occur when the flame speed is greater than the velocity of the fuel/airmixture in the premixer. Flash backs can adversely affect the mechanical integrityand performance of the premixer system and related structure. Specifically, it isdesired to provide a premixer system that can reduce flow separation in the premixercaused by the geometrical configuration of the premixer components. Flow separationcan cause flash backs into the premixer.
It is further desired to provide a premixer system that can reduce pulsations inthe delivery of fuel/air mixture from the premixer into the combustion chamber.These can occur from lack of flame stability in the combustor due to excessivevelocities of, as well as variations in, the mixture velocity exiting the premixer.Pulsations can adversely affect the combustor liner and engine structure.
It is further desired to provide a premixer system that can deliver fuel/airmixture into the combustion chamber in a manner that reduces the impingement offlow onto the combustor liner while maintaining a comparatively simple geometricconfiguration of the overall design. Impingement of the flow onto the liner wall canlead to carbon buildup and decrease heat transfer performance and increase thermalfatigue.
It is further desired to provide an apparatus that is relatively less complex thanother state of the art annular combustor apparatus and systems thereby facilitating easeof operation, lower initial cost and maintenance of the apparatus, and substantiallyimproved fuel/air control by the avoidance of matching a large number of separatepremixers.
Test experience from the development of low emission gas turbine combustorsof the type described in,e.g., U.S. 5,377,483, indicate that undesirable combustion pulsations can occur which are dependent on both the velocity of the fuel/air mixturebeing ejected from the premixer mixing tube, as well as the composition of the fuel/airmixture itself. The geometry of the combustor as a whole will also influence theemission of nitrous oxides. Where the exit area for the mixing tube is fixed, thevelocity of the discharged fuel/air mixture can vary during idle and full power by afactor of three. In order to avoid undesired combustion "flash back" into the premixerand reduce emissions, a certain minimum velocity of the charge―well above theflame speed of the utilized fuel―should be provided.
A desired minimum velocity in the case of a typically used fuel, such asdieselfuel #2, is approximately 20-30 m/sec. At this velocity, the thickness of the boundarylayer found at nozzle wall surfaces during operation is not great, which allows for awell performing combustor with essentially no "flash back" at low power levelsincluding idle conditions. At full power, however, and depending on the type ofturbine engine, the nozzle discharge velocity can increase to 100 m/sec for fixed exitflow areas. It has been found that at this higher level of velocity, flame stabilization isdifficult to maintain and the flow of the fuel/air mixture will impinge onto adjacentcombustor liner walls.
In accordance with the present invention, as embodied and broadly describedherein, the premixer apparatus for mixing fuel and compressed air from respectivesources to provide a fuel/air mixture to a combustor comprises a mixing tube havingan entrance configured for receiving fuel and compressed air, an axis, and an exitconfigured for discharging a fuel/air mixture into the combustor; and a mixture valveassociated with the mixing tube exit for varying a fuel/air mixture discharge velocityinto the combustor.
Preferably, the premixer includes a compressed air flow path between thecompressed air source and the mixing tube entrance, a fuel flow path between the fuelsource and the mixing tube entrance, and an air valve and a fuel valve disposed in therespective flow paths for controlling the fuel/air ratio of the fuel/air mixture, wherein the mixture valve varies the exit velocity of the controlled fuel/air ratio dischargedmixture.
It is also preferred that the mixture valve comprises a mixing tube exit memberand a valve member positioned adjacent a mixing tube exit member and defining withthe exit member an exit flow area, one of the valve member and the exit memberbeing movable along the mixing tube axis relative to the other to vary the exit flowarea and the mixture exit velocity.
It is further preferred that the exit member is a nozzle assembly fixed to themixing tube and the valve member is a skirt co-axially surrounding the nozzleassembly with respect to the mixing tube axis and having a skirt end, wherein the skirtend and the nozzle assembly define a cylindrical-annular exit flow area, the flow areaand the mixture velocity varying with axial positions of the skirt end and the nozzleassembly during the relative movement therebetween.
Altemately, the exit member is a mixing tube axial end, the valve member is avalve plate; wherein the mixing tube axial end and the valve plate define a cylindrical-annularexit flow area; and wherein the flow area and the mixture velocity vary withrelative axial positions of the valve plate and the mixing tube axial end during therelative movement therebetween.
And it is still further preferred that the apparatus include a sensor for sensingmixture pressure upstream of the mixing tube exit, an actuator operatively connectedto one or the other of the valve member and the mixing tube exit member, and acontroller operatively connected to the pressure sensor, the actuator varying themixing tube flow exit area and the mixture exit velocity in response to the sensedpressure.
Also in accordance with the present invention, the method for controlling thevelocity of a fuel/air mixture discharged from premixer apparatus, the apparatushaving a fuel/air mixing tube with an entrance flow-connected to respective sources offuel and compressed air, an axis, and an exit for discharging the fuel/air mixture,comprises the steps of providing a mixture valve including a mixing tube exit member and a valve member together defining an exit flow area; and moving one of the valvemember and the mixing tube exit member relative to the other to increase or decreasethe exit flow area whereby the fuel/air mixture velocity is respectively decreased orincreased.
Still further in accordance with the present invention, the gas turbine gasgenerator operable with a fuel source and having an air compressor, a turbine, a shaftassembly interconnecting the air compressor and the turbine, and a combustoroperatively connected to provide combustion gases to the turbine, comprises one ormore premixers each having a mixing tube having an entrance for receiving fuel fromthe fuel source and compressed air from the air compressor to be mixed to provide afuel/air mixture, an axis, and an exit configured for discharging the fuel/air mixture tothe combustor; and a mixture valve associated with said mixing tube exit for varying avelocity of the fuel/air mixture discharged into the combustor.
And still further in accordance with the present invention, the method forcontrolling the flow of a premixed fuel/air mixture to a combustor of a gas turbine gasgenerator, which has a compressor for providing a source of compressed air along apath, a fuel conduit connectable to a source of fuel, and premixer apparatus forproviding a fuel/air mixture including a mixing tube having an entrance in flowcommunication with the compressed air path and the fuel conduit, and an exit in flowcommunication with the combustor. The method comprises the steps of controllingthe flow rates of compressed air and fuel to the mixing tube entrance using respectivevalves located in the compressed air path and the fuel conduit upstream of the mixingtube entrance to provide a mixture with a desired fuel/air ratio, and controlling thevelocity of the fuel/air mixture with the desired fuel/air ratio flowing through themixing tube exit using a mixture valve positioned within the combustor.
Other advantages of the invention will be set forth in part in the descriptionwhich follows, and in part will be apparent from the description, or may be learned bythe practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out inthe appended claims.
The accompanying drawings, which are incorporated in and constitute part ofthe specification, illustrate a preferred embodiment of the invention and, together witha description, serve to explain the principles of the invention.
- Fig. 1A is a schematic cross-section of a predecessor gas turbine enginemodule utilizing a single stage combustor system having controlled fuel/air ratio;
- Fig. 1B is a schematic end view of the apparatus shown in Fig. 1A taken in thedirection AA in Fig. 1A;
- Fig. 2 is a schematic cross-section of a predecessor gas turbine engine modulewith an alternative version of the combustor system shown in Fig. 1A;
- Figs. 3A-3C are detailed cross-sectional views of a test version of the preferredfuel/air premixer component of the apparatus shown in Fig. 1A;
- Fig. 4 is a detailed cross-sectional view of an engine version variation of thefuel/air premixer shown in Figs. 3A-3C;
- Fig. 5 is a schematic cross-section of another predecessor gas turbine enginemodule utilizing a single stage combustor system having a controlled fuel/air ratio;
- Fig. 6 is a schematic cross-section of an alternative premixer constructionwithout an integrated compressed air flow valve, for use in the gas turbine enginemodule shown in Fig. 5;
- Fig. 7 is a schematic cross-section of yet another predecessor gas turbineengine module having a single stage combustor with controlled fuel/air ratio;
- Fig. 8 is a schematic cross-section of yet another predecessor gas turbineengine module having a single stage combustor with controlled fuel/air ratio;
- Figs. 8A is a schematic cross-section of the premixer assembly taken alongline 8A-8A of Fig. 8;
- Fig, 9 is a schematic cross-section of the premixer assembly taken along line9-9 of Fig. 8;
- Fig. 9A is a schematic cross-section of a variation of the premixer assemblyshown in Fig. 9 using a cylindrical air valve, and Fig. 9B is a schematic cross-sectionof a further modification of the premixer assembly in Fig. 9A;
- Fig. 10 is a perspective view of a preferred nozzle assembly for use in theengine modules depicted in Figs. 8 and 9;
- Fig. 11 is a perspective cross-sectional view of the nozzle assembly of Fig. 10;
- Fig. 12 is a schematic cross-section of an alternate premixer assembly exitnozzle configuration;
- Fig. 13 is a schematic cross-section of yet another predecessor gas turbineengine module and having a can-type combustor;
- Fig. 13A is an enlargement of the air valve component depicted in Fig. 13;
- Fig. 13B is a schematic cross-section of the nozzle of Fig. 13 assembly takenalongline 13B-13B;
- Fig. 14A is a schematic cross-section of still another predecessor gas turbineengine module having a single stage combustor and controlled fuel/air ratio;
- Fig. 14B is a schematic perspective end view of a part of the engine module ofFig. 14A;
- Fig. 14C is a schematic cross-section through the engine module part depictedin Fig. 14B taken along theline 14C-14C;
- Fig. 14D is an enlargement of the portion of Fig. 14A showing the premixerassembly;
- Fig. 15A is a longitudinal, schematic cross-section of yet still anotherpredecessor engine having a single stage combustor with controlled fuel/air ratio;
- Fig. 15B is a partial end view of the embodiment in Fig. 15A;
- Fig. 16 is a schematic cross-section of a gas turbine engine module having amixture valve to control premixer exit velocity made in accordance with the presentinvention;
- Fig. 17A is a schematic detail cross-section of an alternate mixture exit valvemember configuration, and Fig. 17B is a schematic detail cross-section of an alternatemounting configuration for the embodiment of Fig. 16;
- Fig. 18A is a schematic cross-section of a further embodiment of the gasturbine engine module of the present invention, Fig. 18B is a schematic end view of amultiple premixer variation of the embodiment of Fig. 18A, and Fig. 18C is aschematic cross-section of the configuration in Fig. 18B taken along the line AA;
- Figs. 19A-19C are schematics of yet another gas turbine engine embodimentof the present invention which uses variable premixer exit geometry to provide amixture for controlling mixture exit velocity for use especially with annularcombustors, Fig. 19 being an outside plan view, and Figs. 19B and 19C being a cross-sectionview and a detail view, respectively, taken along the line 19B-19B;
- Figs. 20A and 20B are schematic illustrations of a variation of the variable exitgeometry premixer of the Figs. 19A-19C embodiment but adapted for can-typecombustors;
- Fig. 21A is a schematic cross-section of portion of a gas turbine enginecombustor with yet another premixer embodiment having variable exit geometry forcontrolling exit velocity and angular distribution of the discharged fuel/air mixture,and Fig. 21B is a detail of exit nozzle components of the premixer depicted inFig. 21A; and
- Figs. 21C and 21D are schematic cross-sections of a portion of a variation ofthe combustor and premixer embodiment shown in Figs. 21A and 21B.
Reference will now be made to the present preferred embodiments of theinvention, which are illustrated in the accompanying drawings. Specifically, theembodiments of the parent invention are shown in Figs. 16-20 which show gas turbineengines with Premixers having variable geometries for control of the mixture exitvelocity. However, a review of related predecessor gas turbine engine and premixer combustor systems will facilitate a better understanding and appreciation for thepresent invention.
With initial reference to Fig. 1A, there is shown a predecessor combustorsystem of the present inventor, which system includes aspects of the present inventionand is designated generally by the numeral 10.System 10 is depicted as being used inconjunction with radial gasturbine engine module 12. Gasturbine engine module 12included apressure housing 14 within which was mountedshaft 16 rotatable aboutaxis 18. Mounted on one end of ashaft 16 wasradial turbine 20 for drivingcentrifugal compressor 22 mounted at the opposed end ofshaft 16. In theconfiguration depicted in Fig. 1A, gasturbine engine module 12 power is taken outthrough a mechanical coupling arrangement shown generally at 24 adjacentcentrifugal compressor 22. However, the combustor system of the present inventionlike the configuration in Fig. 1A can be utilized in a gas generator in association, e.g.,with a free power turbine" (see Fig. 5A), a "free-jet" propulsion unit (not shown), orany other turbine engine system version as one skilled in the art would immediatelyrealize. Also, the present invention is not limited to use in a radial gas turbine engineor gas generator module but, at least in its broadest extent, could be used with axial ormixed axial-radial turbine engine and gas generator modules as well.
With continued reference to Fig. 1A, gasturbine engine module 12 operatesgenerally as follows. Air enterscentrifugal compressor 22 in a direction designated bythearrows 26, is centrifugally accelerated to increase its velocity, whereupon it entersdiffuser 28 to increase static pressure. The compressedair exiting diffuser 28 iscollected in aplenum chamber 30. Thereafter, compressed air fromplenum 30 ismixed with fuel from afuel source 32 by means ofpremixer 60 ofcombustor system10, to be described in more detail hereinafter, to produce hot exhaust gases whichflow pastinlet guide vanes 34 toradial turbine 20, where power is extracted. Theexhaust gases fromturbine 20 are ducted to the atmosphere or to a subsequent enginemodule. In the case of free power turbine arrangement, thegases exiting turbine 20would be ducted to the free power turbine for extraction of further power.
The combustor system in Fig. 1A included a cylindrical housing defining acombustion chamber, the housing having an axis and having at least one inlet portadjacent one axial chamber end. Importantly, the portion of the chamber adjacent theone axial chamber end comprised a single stage combustion zone. An exhaust waspositioned at the opposite axial chamber end, with the portion of the combustionchamber adjacent the opposite axial chamber end comprising a dilution zone. Thehousing further has aperture means in the form of dilution ports in flowcommunication with the dilution zone.
With continued reference to Fig. 1A,combustor system 10 included annularcombustor liner housing 40 ("housing" or alternatively referred to as a "liner"), whichis generally toroidal in shape. Although Fig. 1A is illustrated with an annular housing,a "can-type" cylindrical housing could also be used.Housing 40 is contained withinpressure vessel 14 and defines anaxis 42 essentially coincident with gas turbineengine module axis 18.Housing 40 is closed ataxial end 44 except forinlet port 43,but is open ataxial end 46 to form an annular exhaust port (or combustor exit) 48.Exhaust port 48 is in flow communication withradial turbine 20 throughchannel 50past inlet guide vanes 34.
With continued reference to Fig. 1A,toroidal chamber 52 defined byhousing40 comprised two generally axial sections with different functions.Section 54adjacentaxial end 44 comprised a single stage combustion zone andsection 56adjacent housing end 46, comprises a dilution zone. A plurality ofapertures 58a, 58bwas provided inhousing 40 opening intodilution zone 56. Dilution ports 58a areaseries of apertures formed in the outer peripheral surface ofhousing 40, whiledilutionports 58b are a series of apertures formed in an inner peripheral surface ofhousing 40,relative tohousing axis 42. The aperture means generally comprisingdilution ports58a, 58b provided for the introduction of compressed air into thedilution zone 56 ofcombustion chamber 52 from compressed air conduit means which will be describedin more detail hereinafter. However, dilution apertures need not be placed in bothinner and outer walls of the combustion liner. For example, aperture 58a may be eliminated ifapertures 58b are used and sized to accommodate the entire dilution flowrate.
At least one fuel/air premixer disposed outside the cylindrical housing wasprovided for mixing a portion of the compressed air flow with fuel to provide afuel/air mixture and delivering the mixture to the combustion zone through the inletport. The fuel/air premixer included means for receiving the compressed air, meansfor receiving the fuel and also chamber means for flow-smoothing the receivedcompressed air and for mixing the received compressed air and fuel. With continuedreference to Fig. 1A,combustion system 10 further included a single fuel/air premixerdesignated generally by the numeral 60.Premixer 60 includeshousing assembly 62for receiving the compressed air from conduit means which will be described in moredetail hereinafter, and asingle fuel nozzle 64 for receiving fuel fromfuel source 32viafuel line 66.Fuel nozzle 64 depicted in Fig. 1A is an "air-blast" type fuel nozzleespecially advantageous for use with liquid fuel to provide atomization and thusenhance vaporization. However, use of an "air blast" nozzle with gaseous fuel canprovide benefits in terms of providing an initial mixing of the fuel with air prior toadmission to the venturi element which will be described hereinafter. Therefore, thecombustion system of Fig. 1A was, like the present invention, not restricted to the useof liquid fuel or an "air-blast" fuel nozzle, but gaseous fuel and other types of fuelnozzles, such as swirling-type nozzles, can be used as well.
Fuel/air premixer 60 further included mixing chamber means in the form ofventuri 68 havingventuri inlet 70 disposed within fuel/airpremixer housing assembly62 andventuri exit 72 connected toinlet port 43.Venturi 68 defines aflow axis 74,andfuel nozzle 64 is positioned to deliver a fuel spray intoventuri inlet 70substantially alongaxis 74. The cross sectional flow area and dimensions ofventuri68 are chosen to provide vigorous and complete mixing of the fuel and compressed airwithin the venturi chamber and a directed flow of the resulting mixture alongventuriaxis 74 tocombustion zone 54, such as indicated schematically byarrow 76. Theflow area ofventuri exit 72 should be chosen such that minimum velocities of the mixture (i.e., during idle) are greater than the flame propagation speed of the fuel/airmixture. Flame holder means such as depicted schematically as 78 may be providedproximate venturi exit 72 to enhance the stability of combustion incombustion zone54.
As best seen in Fig. 1B, mixingventuri 68 is disposed such thatventuri axis 74is oriented substantially tangentially with respect tohousing axis 42 such that theincoming fuel/air mixture is caused to swirl aboutaxis 42 within thecombustion zone54. It has been found using the premixer construction to be described in more detailhenceforth thatcombustion chamber 52 can be adequately fed by using only a singlefuel/air premixer fed by a single fuel nozzle. As in Figs. 1A and 1B, the presentinvention contemplates the possible use of multiple fuel/air premixers, particularly forsituations wherein the radial "thickness" ofcombustion chamber 52 is small relativeto the outer radius thereof, as measured fromaxis 42.
The combustor system included an ignitor disposed on the cylindrical linerhousing at a location adjacent the intersection of the flow axis of the venturi. Withcontinued reference to Fig. 1B,ignitor 79 is positioned near the intersection offlowaxis 74 andhousing 40, and extends at most only a short distance intocombustionzone 54.Ignitor 79 is thus ideally positioned to intercept the fuel/air mixtureemanating frompremixer 60 to initiate combustion. Once started, the swirling hotcombustion gases inzone 54 provided auto ignition of the fuel/air mixture andignitor79, which was electrical, is normally shut off.
In the predecessor combustion systems, compressed air conduit means wereprovided interconnecting the compressor exit and the fuel/air premixer for delivering aportion of the compressed air flow to the premixer compressed air receiving meansand for delivering essentially the remaining portion of the compressed air flow to theaperture means for providing dilution air to the dilution zone. With continuedreference to Fig. 1A, compressed air conduit means designated generally by thenumeral 80 includes generallyannular passageway 82 disposed betweenpressurehousing 14 andhousing 40.Passageway 82 extends between compressedair receiving plenum 30 and a ring-shapedplenum 84 and is formed as part ofpressurevessel 14 adjacent the turbine exhaust section. Fuel/airpremixer housing assembly 62is connected to receive compressed air fromplenum 84 for eventual communication totheventuri inlet 70 as explained previously.Plenum 84 is shown having a circularcross section but other shapes, configurations and locations are possible and areconsidered within the scope of the present invention.
As can be appreciated from the schematic in Fig. 1A,passageway 82 isconfigured such that the compressed air flowing therein provides cooling forhousing40, particularlyhousing portion 86 immediately surrounding thecombustion zone 54where the highest combustion temperatures are expected.Portion 86 ofhousing 40 isconstructed for convection cooling only, with no film-cooling necessary. That is, inportion 86 ofhousing 40, the housing acts to seal off the compressed air flowing inpassageway 82 from the fuel/air mixture being combusted incombustion zone 54.This construction provides for control of the fuel/air ratio of the mixture incombustion zone 54 and permits operation as a "single stage combustor" with adesired lean fuel/air ratio. Such an operation can yield low levels of NOx andunburned fuel and fuel by-product levels. As will be discussed henceforth, theparticular construction of the combustor system permits extraordinarily low levels ofNOx in comparison with other state of the art combustion systems.
Passageway 82 essentiallyenvelopes combustion chamber 52 to provideconvection cooling and also to supply compressed air todilution ports 58a and 58b.Passageway 82 also may include achannel 82a for channeling compressed air flow forcooling the portion of thepressure vessel 14adjacent turbine 20, as is shown inFig. 1A. Turbineinlet guide vanes 34 may be film cooled inlet guide vanes and maybe fed frompassageway 82 or 82a. Also, compressed air conduit means 80 caninclude aseparate passageway 88 interconnecting the compressedair receivingplenum 30 and air-blast fuel nozzle 64 when such a nozzle is used, particularly withliquid fuel operation.
As would be understood from the foregoing discussion in conjunction withFig. 1A, compressed air conduit means 80 acts to channel a portion of the compressedair flow to the fuel/air premixer 60 and to channel essentially the remaining portion ofthe compressed air flow to thedilution ports 58a and 58b. The compressed air flownot channeled to either the fuel/air premixer or the dilution ports, namely the air usedto cool theinlet guide vanes 34, is very small and in any event does not disturb thefuel/air ratio in the combustion zone but merely results in a small further dilution ofthe exhaust gases prior to entry intoturbine 20.
Further, valve means are disposed in the compressed air flow path fordetermining the compressed air flow rate to the premixer. The compressed air valvemeans is especially important where the speed of the compressor, and thus thevolumetric flow rate of compressed air, is essentially independent of the fuel flowrate, such as the application depicted in Fig. 1A. As embodied herein and withcontinued reference to Fig. 1A,valve 90 is positioned in fuel/airpremixer housingassembly 62 for determining the rate of compressed air flow fromplenum 84 toventuri inlet 70.Valve 90 is continuously adjustable, and a suitable construction ofvalve 90 will be discussed in more detail hereinafter in relation to the description ofone preferred construction of the fuel/air premixer of the present invention. When thevalve opening changes, the pressure drop over the premixer changes, resulting in anincrease or decrease of air mass flow to the dilution zone. Thus, this variation anddividing of the air flow happen outside the combustor proper.
Fig. 2 disclosescombustor system 110 having an alternate configuration forthe compressed air conduit means. Components having the same or similar functionrelative to the embodiment in Figs. 1A, 1B are given the same numeral but with a"100" base. In the compressed air conduit means designated generally as 180 inFig. 2, adistribution conduit 181 is provided between compressedair collectionplenum 130 andannular passageway 182 surrounding housing 140, and fuel/airpremixer housing assembly 162 is directly connected todistribution conduit 181upstream ofpassageway 182.Valve 190 is disposed at the connection between fuel/airpremixer housing assembly 162 anddistribution conduit 181 to positivelydivide the air flow into a first portion flowing to fuel/air premixer 160 and theremainder topassageway 182 via distribution conduit portion 181a. As comparedwith the construction in Fig. 1A, where substantially all of the compressed air portionflowing to the premixer was first used to cool at least a part ofliner housing portion86 definingcombustion chamber 52, none of the compressed air portion flowing tofuel/air premixer 160 is used to coolportions 186 of housing 140 definingcombustionzone 152. However, the Fig. 2 embodiment does allow for the direct control of thecompressed air fractions flowing to the fuel/air premixer versus the compressed airflow fraction flowing to thedilution ports 158a and 158b. The configuration shown inFig. 1A may be preferred nonetheless because of an ease of construction in assemblyof the various components, principally the fuel/air premixer wherein the valve can beintegrated directly with the fuel/air premixer housing, as will be discussed in moredetail henceforth.
Further in accordance with the predecessor combustor system, fuel conduitmeans was provided interconnecting the fuel supply and the premixer fuel receivingmeans, the fuel conduit means together with the premixer fuel receiving meansestablishing a flow path for all the fuel to the premixer. Fuel valve means is disposedin the fuel flow path for determining the fuel flow rate therein. With reference againto Fig. 1A,fuel line 66interconnects fuel source 32 withfuel nozzle 64.Fuel valve92 is disposed infuel line 66 immediately upstream offuel nozzle 64, which isdepicted as being an "air-blast" type fuel nozzle particularly suitable for use withliquid fuels, as stated previously.
Still further, the combustor system of Figs. 1A and 1B includes controllermeans operatively connected both to the compressed air valve means and the fuelvalve means for essentially controlling the respective flow rates of the compressed airportion and the fuel delivered to the premixer to provide a preselected lean fuel/airratio mixture through the inlet port to the combustion zone. As depictedschematically in Fig. 1A,controller 94 which can be either mechanical or electric (e.g., a microprocessor) is interconnected withcompressed air valve 90 to essentiallycontrol the flow rate of the compressed air flowing directly toventuri inlet 70. Whilea small portion (typically 5% or less), of the total compressed air flowing to fuel/airpremixer 60 can travel throughconduit 88 when an "air-blast" nozzle is utilized, thecontrol provided byvalve 90 of the remaining 95+% of the compressed air flow isexpected to achieve adequate overall fuel/air ratio control. Moreover. for situationsutilizing gaseous fuel, such as natural gas as provided in the Example to be discussedhereinafter,conduit 88 could be eliminated such that all of the compressed air flow tothe fuel/air premixer will be under the control of the compressed air flow valve.
Also as depicted in Fig. 1A,controller 94 is operatively connected to fuelvalve 92 to meter the fuel flow tofuel nozzle 64. As one skilled in the art wouldappreciate,controller 94 can act to control both the fuel flow and the compressed airflow to fuel/air premixer 60 to achieve a single preselected fuel/air ratio mixture overthe entire operating range of the gas turbine engine module so that the mass flow ofthe combustible mixture would change as a function of the load. Or, alternatively,controller 94 can be configured to provide a sequence of preselected fuel/air ratiomixtures as a function of load. One skilled in the art would be able to select and adapta suitable controller for a particular, application based on the present disclosure andthe general knowledge in the art.
In operation, and with reference to Figs. 1A and 1B, compressed air fromcompressed air receiving means 30 is channeled via passageway/envelope 82 over theoutside surface ofhousing 40 for coolinghousing 40, and particularlyportions 86which surroundcombustion zone 54. A portion of the compressed air flowing inpassageway 82 is admitted toplenum 84 and then flows to fuel/air premixer 60 via theinterconnection between fuel/airpremixer housing assembly 62 and 84 as controlledbycompressed air valve 90 viacontroller 94. Inventuri 68, the compressed airportion is mixed with the fuel fromfuel nozzle 64, possibly with a small additionalportion of compressed air ifnozzle 64 is a "air-blast" type nozzle, and is injected along theventuri axis 74 throughinlet port 43 and intocombustion zone 54 ofcombustion chamber 52.
As shown in Fig. 1B, swirling flow and combustion is provided incombustionzone 54 by locatingventuri axis 74 tangentially with respect toaxis 42 of the housing.The direction of orientation of theventuri axis 74 is chosen to give 2 specific angulardirection (clockwise or counterclockwise) with respect to the direction of rotation ofthe turbine in order to provide some aerodynamic unloading of the inlet guide vanes.For the configuration depicted in Fig. 1A and 1B where the fuel/air mixture isadmitted to achieve a clockwise swirling combustion incombustion zone 54 asviewed in the direction AA, the direction of rotation ofturbine 20 also would be in theclockwise direction. After combustion of the fuel/air mixture inzone 54, the hotexhaust gases pass todilution zone 56 where dilution air fromdilution ports 58a, 58breduce the average temperature of the exhaust before it is ducted viachannel 50 pastinlet guide vanes 34 toturbine 20 for work-producing expansion.
The control of combustion afforded bycombustion system 10 as well as inaccordance with the present invention through the complete mixing of the fuel and airoutside the combustion chamber in the fuel/air premixer, including completevaporization of the fuel if liquid fuel is used, together with the control of the fuel/airratio of the mixture delivered to the combustion chamber allows for significantreductions in NOx levels and the levels of unburned fuel and fuel by-products, asmentioned earlier. Furthermore, the utilization of essentially the total amount ofcompressed air flow to either combust the fuel or to dilute the exhaust gases upstreamof the turbine provides considerable reduction of peak combustor temperaturesresulting in longer life for combustor liners compared to conventional combustordesigns.
As previously mentioned, the fuel/air premixer of the Figs. 1A and 1Bconstructions, as well as the preferred premixer of the present invention, includes acompressed air receiving means, a venturi having an inlet operatively connected to thecompressed air receiving means with air flow smoothing means, a fuel receiving means including a nozzle with an exit positioned to deliver a spray of fuel into theventuri inlet substantially along the venturi axis, and valve means associated with thecompressed air receiving means for determining the compressed air flow rate to theventuri inlet. With reference to Fig. 3A, fuel/air premixer 260 includes air receivingmeans in the form ofhousing assembly 262. Components having a like or similarfunction to those disclosed in the embodiments of Figs. 1A and 1B will be designatedby the same numeral but with a "200" base.Housing assembly 262, in turn, includeshousing 300 andhousing support 302 for mountinghousing 300 onpressure vessel214 of gasturbine engine module 212.Housing support 302 is hollow and, inaddition to supportinghousing 300 and the components contained therein, acts tochannel compressed air fromplenum 284 tohousing 300. In the construction shownin Fig. 3A, coolingshroud member 303 is positioned between combustionchamberliner housing 240 andpressure vessel 214 for establishing theflow path 282, at leastin the vicinity ofportions 286 ofhousing 240 that define the boundary of thecombustion zone 254.Shroud member 303 also defines withpressure vessel 214,plenum 284 for collecting the compressed air portion for eventual transmission tohousing 300 viahousing support 302.
With continued reference to Fig. 3A, fuel/air premixer housing 300 is dividedinto upstream anddownstream compartments 304, 306 respectively bydivider plate308.Aperture 310 is provided individer plate 308, and a butterfly-type valve plate290 is mounted for rotation inaperture 310. In the Fig. 3A embodiment, heorientation ofvalve plate 290 inaperture 310 is controlled through control arm 312(see Fig. 3B) to provide a selective degree of obstruction and, hence, pressure drop. Inthe orientation ofvalve plate 290 shown in Figs. 3B and 3C, a minimum amount ofobstruction occurs withvalve plate 290 being oriented perpendicular to thedividerplate 308, corresponding to a "zero" setting of theangular calibration plate 314 shownin Fig. 3C. A position ofcontrol rod 312 corresponding to either "9" position onindicator 314 would result in the greatest amount of obstruction and pressure drop inthe compressed air portion flowing throughaperture 310. As one skilled in the art would realize, the degree of obstruction and thus control of the compressed air flowbetweenupstream compartment 304 anddownstream compartment 306 could bevaried by changing the angular orientation ofcontrol rod 312 between the "zero" and"9" positions, thereby controlling the compressed air flow rate to the balance of thefuel/air premixer 260 which will now be described in more detail.
Divider plate 308 includes anadditional aperture 316 in which is mountedinlet 270 ofventuri 268.Venturi inlet 270 is configured and mounted todivider plate308 such that a smooth transition exists between the upper planar surface ofdividerplate 308 and the inner surface ofventuri inlet 270.Venturi 268 extends throughupstream housing compartment 304,housing support 302,past pressure vessel 214,combustion chamber liner 303, and connects to housing 240 at the location ofinletport 243. As described previously in relation to the embodiment depicted in Fig. 1A,theventuri axis 274 which corresponds generally to the flow direction of the fuel/airmixture inventuri 268 is oriented to provide a substantially tangential admissiondirection with respect to the axis (not shown) of annularcombustion chamber housing240.
With continued reference to Fig. 3A,fuel nozzle 264 is mounted indownstream compartment 306 with thefuel nozzle exit 318 positioned to deliver aspray of fuel intoventuri inlet 270 alongventuri axis 274.Fuel nozzle 264 is of the"swirling" spray type which utilizesports 320 and swirlvanes 322 to channel some ofthe compressed air swirl the fuel entering throughfuel port 324 before releasing thefuel spray throughexit 318. Also shown in Fig. 3A is perforated flow-smoothingelement 326 positioned in thedownstream compartment 306 and surroundingfuelnozzle exit 318 andventuri inlet 270, to avoid uneven velocities and separation in theventuri which otherwise could result in "flame holding" in the venturi. While a smallpressure drop is introduced by its incorporation, theperforated element 326 has beenfound to provide increased stability for the compressed air flow fromdownstreamcompartment 306 past thefuel nozzle 264 and intoventuri inlet 270, without anyseparation at the lip ofventuri inlet 270.
Fig. 4 shows a variation of the preferred fuel/air premixer depicted in Figs.3A-3C, which variation is designated generally by the numeral 360. Componentshaving the same or similar function to those described in relation to the Figure 1A, 1Bembodiment are given the same numerals but with "300" base. Fuel/air premixer 360includes aventuri 368 which hasinlet 370 which extends slightly above the surface ofdivider plate 408. Also,fuel nozzle exit 418 extends a distance intoventuri inlet 370.One skilled in the art would realize that the optimum performance of thefuel nozzle364 in conjunction with the venturi 368 (as well asnozzle 264 andventuri 268 in thevariation shown in Figs. 3A-3C) may vary from application to application and that thepositioning offuel nozzle exit 418 along theventuri axis 374 in the vicinity ofventuriinlet 370 may be adjusted to determine the optimum position. However, it isanticipated thatperforated screen element 426 would provide flow stability for theFig. 4 embodiment as well. Finally, the Fig. 4 embodiment incorporates contemplatedrefinements in the construction of the fuel/air premixer compared to the constructionshown in Fig. 3A, such as the use of integral, bell-shapedhousing 400.
As mentioned previously, the present invention advantageously can be adoptedfor applications such as gas turbine gas generator modules used in conjunction withfree power turbines or free jet propulsion units, which gas generator modules may notrequire the use of a compressed air flow valve and associated controller functions.Fig. 5A depicts schematically such an engine system constructed in accordance with apredecessor combustion system which includes aspects of the present invention anddesignated generally by the numeral 500.Engine 500 comprises gas turbinegasgenerator module 512, includingcombustor system 510 to be discussed in more detailhereinafter and freepower turbine module 513.Free turbine module 513 includes freeturbine 513a which is depicted as an axial turbine, but could be pure radial or mixedaxial-radial as the application may require. In comparison with the Fig. 1A enginesystem where power was extracted from gearing 24 connected toshaft 16, power istaken from theengine system 500 in the Fig. 5A embodiment via gearing associatedwithfree turbine shaft 513b. Although shown coaxial withaxis 518 of the gas generator module,rotational axis 513c offree power turbine 513 could be angularlydisplaced to meet the requirements of theoverall system 500.
In the subsequent discussion, like components relative to the embodiment inFig. 1A will be given the identical numeral but with a "500" prefix, for example.
Specifically, gas turbinegas generator module 512 includes a mechanicallyindependent spool, namelycentrifugal compressor 522 andradial turbine 520mounted for dependent rotation onshaft 516, insidepressure housing 514. Thus,shaft516 can rotate independently offree turbine shaft 513b althoughgas generator 512andfree turbine module 513 are interconnected in the gas flow cycle.Module 512also includescombustor system 510 withcombustor liner housing 540 which iscontained withinpressure housing 514 and which receives premixed air/fuel fromexternal premixer 560 throughinlet port 543 alongventuri axis 574.Venturi axis 574is oriented tangentially with respect toaxis 542 of annularcombustor liner housing540 to provide efficient, swirling combustion and also to partially unload inlet guidevanes 534, as discussed previously in relation to the Fig. 1A embodiment. SeeFig. 5B.
Fig. 5B also depicts a position ofignitor 579 onliner housing 540 adjacent theintersection ofventuri axis 574. While it may eventually be possible to locate theignitor in a relatively cooler environment, such as in the premixer, and therebyprolong ignitor life and further decrease the number of penetrations inliner housing540, the location depicted in Fig. 5B is useful where it is necessary to ensure light-offbecause of the low velocities of the fuel/air mixture in the annular chamber.
In the construction depicted in Figs. 5A and 5B,housing liner 540 andpressure housing 514 cooperate to form passages for the compressed air flow fromcompressor plenum 530. Also included in this engine isannular cooling shroud 583disposed between, and radially spaced from both,housing liner 540 and thecircumferentially adjacent portion ofpressure housing 514. As can be appreciatedfrom the figures, coolingshroud 583 andhousing liner 540 cooperate to form part ofthepassageway 582 for convectively cooling the combustor chamber defined byliner 540 while coolingshroud 583 andpressure housing 514 cooperate to formannularplenum 584 to collect the portion of the compressed air flow to be channeled topremixer 560 for mixing with the fuel. In the Fig. 5A embodiment, as in theembodiment shown in Fig. 1A, a portion of the compressed air is taken from thepassageway leading from the compressor exit after providing convective cooling andis then channeled to the premixer for mixing with fuel, but the Fig. 5A arrangementcan be made more structurally compact than the ring-shapedplenum 84 in Fig. 1A.Furthermore, coolingshroud 583 provides radiation shielding of the adjacent parts ofpressure housing 514 from the relativelyhot liner housing 540, allowing the use ofless expensive materials and increasing the service life of the pressure housing.
The balance of the compressed air flow inpassageway 582 is channeledthroughdilution apertures 558b. There are no dilution ports corresponding to theports 58a in the Fig. 1A embodiment, butdilution ports 558b include two separatecircumferential port sets 558b1 and 558b2.Divider 559 and the sizing ofports 558b1and 558b2 causes dilution air flowing throughports 558b2 to first flow throughpassageway 582apast turbine shroud 557. One skilled in the art would be able toperform the required sizing analysis to provide adequate distribution of the dilution airto achieve desired turbine shroud cooling. The elimination of film cooling providesfor control over the fuel/air ratio in thecombustion zone 554 and is one of the highlysignificant benefits and advantages of the present invention, as explained previously.
Fig. 5A also shows (in dotted line)conduit 588 leading fromcompressor exitplenum 530 topremixer 560 in the event "air-blast" type liquid fuel nozzle is utilized,for reasons explained previously. Although shown penetrating compressor plenum-exit530 axially inclined in Fig. 5A for clarity, the inlet toconduit 588 would betangential and in the axial plane of the compressor exit to capture the total dynamichead. One skilled in the art would be able to design an appropriate inlet configurationgiven the present description.
Aside from the small amount of compressed air that may be required tooperate an air blast-type liquid fuel nozzle and, possibly, for inlet guide vane cooling, all of the compressed air is used to convectively cool at least part ofliner housing 540before being used for mixing with the fuel or for dilution. This construction optimizesthe convective cooling capacity of the compressed air inventory. Although not shown,the present invention is also intended to include a gas generator variationcorresponding to the Fig. 2 embodiment where the compressed air flow portion usedfor mixing with the fuel is not first used for convective cooling. The simplifiedconstruction of such a system might outweigh the reduction in cooling capacity andtherefore be desired for certain applications.
As depicted in Fig. 5A, air is channeled frompassageway 582 throughannularplenum 584 for mixing directly with the fuel inpremixer 560. Fig. 5A depictscompressedair valve 590 by broken lines to indicate that the valve is optional. It maybe used for "fine tuning" the fuel/air ratio during operation, it may be preset to a fixedopening for operation, or it may be eliminated entirely, for the following reason. Inengine system 510, the speed ofcompressor 522 and thus the compressed air flow rateis essentially proportional to the fuel flow over the operating range. Hence, grosscontrol of the fuel/air ratio to a preselected lean value can be achieved automatically.The function ofcontroller 594 acting to control fuel flow tofuel nozzle 564 fromsource 532 throughfuel valve 592 thus becomes similar to that of a conventionalthrottle responsive to power demands.
Whilepremixer 560 channels all the fuel/air mixture tocombustion zone 554required over the intended operating range ofengine system 510, an auxiliary fuelsupply system such assystem 596 depicted in Fig. 5B may be used to provide a richermixture for start-up and idle conditions.System 596 includes a conventionalfuelspray nozzle 597 fed from fuel source 532 (see Fig. 5A), and the auxiliary fuel flowrate can be controlled bycontroller 594 throughvalve 598. In the disclosedconstruction,spray nozzle 597 is positioned to penetrateliner housing 540adjacentventuri outlet 572 and disposed radially. However,nozzle 597 can be positioned in anopposed tangential orientation relative to venturi 570 (not shown) to enhance mixingwith the fuel/air mixture entering throughventuri 570. Other positions, constructions and orientations ofspray nozzle 597 are, of course, possible and are considered to fallwithin the general teachings herein.
Fig. 6 is a schematic of an alternative "valve-less" premixer design whichcould be used inengine system 510, and which is designated generally by the numeral860.Premixer 660 includeshousing 662, fuel nozzle 663 which is of the type havingperipheral swirl vanes 665, andventuri 668 oriented withventuri axis 674 tangentialto the combustor axis (not shown). Also, perforated flow-smoothingmember 667surrounds nozzle 664 and the entrance to venturi 668, for reasons explainedpreviously in relation to the corresponding components in the "valved" embodiment inFig. 3A.Premixer 660 additionally includes heating means such as electricresistanceheater jacket 669 surrounding the throat area ofventuri 668 and operatively connectedto a power source (not shown) via electrical leads 671. During start up and usingliquid fuels, a film of fuel tends to collect on the venturi inner surface.Heater jacket669 augments vaporization of this fuel film and thus promotes the overall mixing ofthe fuel and air in the premixer. During operation, the temperature of the compressedair portion flowing past the outer surface ofventuri 668 fromplenum 684 mayprovide sufficient heat for vaporizing a liquid film, or prevent the formation of aliquid fuel film altogether, thereby dispensing with the need for continued activationofheating jacket 669.
Fig. 7 schematically depicts yet another engine construction that mayadvantageously utilize the combustor of the present invention, namely, a gas turbineengine system such as described in my previous patent U.S. Patent No. 5,081,832, thedisclosure of which is hereby incorporated by reference. In Fig. 7,engine system 700includeshigh pressure spool 711 and mechanically independent low pressure spool709. Low pressure spool 709 includeslow pressure compressor 701 which is driventhroughshaft 702 bylow pressure turbine 703. The compressed air exitinglowpressure compressor 701 flows through diffuses 704 and entershigh pressurecompressor 722 for further compression. As components ofhigh pressure spool 711high pressure compressor 722 is driven byhigh pressure turbine 720 viashaft 716. Gases exhausted fromhigh pressure turbine 720 are diffused indiffuser 705 and thenexpanded inlow pressure turbine 703. For reasons explained more fully in U.S.Patent No. 5,081,832, net power is taken fromengine system 700 via gearing 724connected toshaft 716 ofhigh pressure spool 711. Low pressure spool 709 is usedprincipally to supply pre-compressed air tohigh pressure spool 711 and possibly todrive engine support systems (e.g., lubrication).
As seen in Fig. 7,engine system 700 includescombustor system 710 toprovide hot combustion gases tohigh pressure turbine 720 by combusting fuel with aportion of the compressed air fromhigh pressure compressor 722. Importantly,combustor system 710 usesexternal premixer 760 which includes fuel nozzle 764(which may be an "air-blast" type receiving compressed air directly fromcompressor722 viaconduit 788 with a tangential inlet-shown dotted) andventuri 768 to supplyfully premixed fuel/air tangentially toannular combustion zone 754 defined bylinerhousing 740. Coolingshroud 783 andliner housing 740 cooperate to define part ofconvective cooling passageway 782, while coolingshroud 783 and thecircumferentially adjacent portion ofpressure housing 714 cooperate to formannularplenum 784 to channel a portion of the compressed air topremixer 760. The balanceof the compressed air flow is used for additional convective cooling and finallydilution, using a configuration and construction similar to that shown in Fig. 5A.
However, the engine system configuration shown in Fig. 7 is intended forproducing power at essentially constant high pressure spool shaft speed. Like theFig. 1A embodiment, the total compressed air flow rate will not automatically adjustto a changed fuel flow in the manner ofgas generator module 512 in the Fig. 5Aembodiment. As a result,combustor system 710 specifically includes compressedairvalve 790 integrated withpremixer 760 and under the control orcontroller 794, whichalso controlsfuel valve 792, to achieve a preselected lean fuel/air ratio. It isunderstood that, although not shown, the Fig. 7 embodiment could include featuresdescribed in relation to the other embodiments, including a liner-mounted ignitor,auxiliary fuel spray system, staged dilution ports, etc.
Fig. 8 schematically depicts yet another engine configuration thatadvantageously utilizes certain aspects of the present invention. With initial referenceto Fig. 8, a combustor system is shown and designated generally by the numeral 810.(Note, the upper portion ofcombustor system 810, like shown in several other figures,is a cut-away view, illustrating the upper cross-sectional half of the system.)System810 is depicted as being used in conjunction with radial gasturbine engine module812. Gasturbine engine module 812 includes apressure housing 814 within which ismountedshaft assembly 816 rotatable aboutaxis 818. Mounted on one end ofshaftassembly 816 isradial turbine 820 for drivingcentrifugal compressor 822 mounted atthe opposed end ofshaft assembly 816. In the configuration depicted in Fig. 8, powerfrom gasturbine engine module 812 is taken out through a mechanical couplingarrangement shown generally at 824 adjacentcentrifugal compressor 822. However,the combustor system of the present invention can be utilized in a gas generator inassociation e.g., with a "free power turbine," a "free-jet" propulsion unit, or any otherturbine engine system version as one skilled in the art would immediately realize.Also, the present invention is not limited to use in a radial gas turbine engine or gasgenerator module but, at least in its broadest extent, could advantageously be usedwith axial or mixed axial-radial gas turbine engines and gas generator modules aswell.
With continued reference to Fig. 8, gasturbine engine module 812 operatesgenerally as follows. Air enterscentrifugal compressor 822 in a direction designatedby thearrows 826, is centrifugally accelerated to increase its velocity, whereupon itenters diffuser 828 to increase static pressure. The compressed air exiting diffuser828 is collected in aplenum 830. Thereafter, a portion of the compressed air fromplenum 830 is mixed with fuel from afuel source 832 by means ofpremixer assembly860 ofcombustor system 810, to be described in more detail hereinafter, to producehot exhaust gases which flow pastinlet guide vanes 834 toradial turbine 820, wherepower is extracted. The exhaust gases fromturbine 820 are ducted to the atmosphereor to a subsequent engine module. For example, in the case of free power turbine arrangement, thegases exiting turbine 820 would be ducted to the free power turbinefor extraction of further power.
The combustor system includes a cylindrical combustor liner defining acombustion chamber, the liner having an axis and having one or more inlets adjacentone axial chamber end. The portion of the chamber adjacent the one axial chamberend comprises a single stage combustion zone. With continued reference to Fig. 8,combustor system 810 includesannular combustor liner 840 which is generallytoroidal in shape.Housing 840 is contained withinpressure vessel 814 and defines anaxis 842 essentially coincident with gas turbineengine module axis 818.Liner 840 isclosed ataxial end 844 except forinlet 843, but is open ataxial end 846 to form anannular combustor exit 848. (If multiple premixers are utilized, it should beunderstood that additional inlets may be provided in the liner to accommodate theadded premixers.)Combustor exit 848 is in flow communication withradial turbine820 throughchannel 850 past inlet guide vanes 834.
With continued reference to Fig. 8,toroidal chamber 852 defined byliner 840comprises two generally axial sections or portions with different functions.Region854 adjacentaxial end 844 comprises a single stage combustion zone (e.g., acombustion volume) andregion 856adjacent liner end 846, comprises a dilution zone.A plurality ofports 858 are formed in the outer peripheral surface ofliner 840 andopen intodilution zone 856.Dilution ports 858 provide for the introduction ofcompressed air into thedilution zone 856 ofcombustion chamber 852 from acompressed air conduit, which will be described in more detail hereinafter.Alternatively, compressed air may be delivered into the dilution zone through asecond set of dilution ports (not shown) provided as a series of apertures formed in aninner peripheral surface ofliner 840 by redirecting compressed air from the premixerinto the dilution zone.
Further, one or more fuel/air premixer assemblies are each disposed relativethe cylindrical liner and is provided for mixing a portion of the compressed air flowwith fuel to provide a fuel/air mixture and for delivering the mixture to the combustion zone through the respective liner inlet. The fuel/air premixer assemblyincludes an air inlet for receiving the compressed air, a fuel inlet for receiving the fueland also a mixing tube for flow-smoothing the received compressed air and formixing the received compressed air and fuel. Essentially all of the air used duringcombustion is delivered to the combustion zone through one or more fuel/air premixerassemblies. The combustion zone is otherwise sealed off from receiving compressedair except through the premixer assembly.
With reference to Figs. 8 and 8A,combustion system 810 further includes asingle fuel/air premixer assembly designated generally by the numeral 860.Premixerassembly 860 includeshousing assembly 862 for receiving the compressed air throughanair inlet 861 from an air conduit (described later), and afuel nozzle 864 forreceiving fuel through afuel inlet 865 fromfuel source 832 viafuel line 866.Fuelnozzle 864 depicted in Fig. 8 is an "air-blast" type fuel nozzle that mixes the fuel withswirling compressed air that is especially advantageous for use with liquid fuel toprovide atomization and thus enhance vaporization. However, use of an "air blast"nozzle with gaseous fuel can provide benefits in terms of providing an initial mixingof the fuel with air prior to admission to the venturi element. Thus, the combustionsystem of the present invention is not restricted to the use of liquid fuel or an "air-blast"fuel nozzle, but gaseous fuel and other types of fuel nozzles, such as otherswirling-type nozzles, can be used as well. As shown in Fig. 8A, anauxiliary fuelnozzle 867 may be provided for use during the start-up sequence ofcombustor system810.
The mixing tube, such as a venturi, has a flow axis substantially radiallydisposed with respect to the combustion liner axis, an inlet adjacent one mixing tubeaxial end, and a nozzle assembly at the opposite mixing tube axial end. The mixingtube inlet is flow connected to the premixer air inlet and the premixer fuel inlet. Themixing tube is connected to the liner inlet, and the nozzle assembly extends into thecombustion chamber along the flow axis to deliver the fuel/air mixture within thecombustion zone.
With continued reference to Fig. 8,premixer assembly 860 further includes amixing chamber in the form of a venturi-type mixing tube 868 having mixingtubeinlet 870 disposed within fuel/airpremixer housing assembly 862 and connected toliner 840 atinlet 843. Further, mixingtube 868 has anozzle assembly 872 fordelivering fuel/air mixture into the combustion chamber that is connected to a portionof the mixing tube that extends intocombustion zone 854. Mixingtube 868 defines aflow axis 874, andfuel nozzle 864 is positioned to deliver a fuel spray into mixingtube inlet 870 substantially alongaxis 874. The cross-sectional flow area anddimensions of mixingtube 868 are chosen to provide sufficient residence time toobtain vaporization and mixing of the fuel and compressed air within the mixing tubeand to direct the flow of the resulting mixture along mixingtube axis 874 tonozzleassembly 872. Preferably, the minimum residence time of particulate matter in themixing tube should be on the order of 5-10 milliseconds for the high mass flow rateconditions associated with power operation. Some engine configurations such asrecuperated designs where the combustion air is at an elevated temperature, maydictate these low residence times to avoid pre-ignition of the fuel/air mixture in themixing tube. Although the preferred mixing tube depicted in Fig. 8 is a venturi-typemixing tube 868, one skilled in the art would appreciate that other geometricalconfigurations are possible, including conically or cylindrically shaped mixing tubes,for example.
As further shown in Fig. 8, compressed air conduit includes generallyannularcooling passageway 882 disposed betweenliner 840 and a second, outerannular liner841.Passageway 882 extends betweencompressed air plenum 830 anddilution ports858. Fuel/airpremixer housing assembly 862 is connected to receive compressed airfromorifices 885 inliner 841 for eventual communication to the mixingtube inlet870 by delivering the air throughplenum 884 and valve 890 (discussed later).
As can be appreciated from the schematic in Fig. 8,passageway 882 isconfigured such that the compressed air flowing therein provides cooling forliner840, particularlyliner portion 886 immediately surrounding thecombustion zone 854.Portion 886 ofliner 840 is constructed for convection cooling only, with no film-cooling.That is, inportion 886 ofliner 840, the liner acts to seal off the compressedair flowing inpassageway 882 from the fuel/air mixture being combusted incombustion zone 854.Passageway 882envelopes combustion chamber 852 toprovide convection cooling and also to supply compressed air todilution ports 858.This construction provides for control of the fuel/air ratio of the mixture incombustion zone 854 and permits operation as a "single stage combustor" with adesired lean fuel/air ratio. Such an operation can yield low levels of NOx andunburned fuel and fuel by-product levels.
Further shown in Fig. 8A, avalve 890 is positioned in fuel/airpremixerhousing assembly 862 for determining the rate of compressed air flow fromplenum884 to mixingtube inlet 870.Valve 890 is continuously adjustable, and a suitableconstruction ofvalve 890 can vary, but is depicted as a butterfly-type. When the valveopening changes, the pressure drop over the premixer changes, resulting in an increaseor decrease of air mass flow. A controller 894 (depicted schematically), which, forexample, can include a microprocessor, is interconnected withvalve 890 to essentiallycontrol the flow rate of the compressed air flowing directly to mixingtube inlet 870.Controller 894 is also operatively connected to a fuel valve to meter the fuel flow tofuel nozzle 864. As one skilled in the art would appreciate,controller 894 can act tocontrol both the fuel flow and the compressed air flow topremixer assembly 860 toachieve preselected fuel/air ratios―e.g., preselected in accordance with atmosphericconditions, operating conditions, and fuel-type―over the entire operating range of thegas turbine engine module.Controller 894 could provide infinitely variable fuel/airratios or step-type ratios. One skilled in the art would be able to select and adapt asuitable controller for a particular application based on the present disclosure and thegeneral knowledge in the art.
With reference to Figs. 9-11,nozzle assembly 872 extends along the mixingtube flow axis into the combustion chamber and has one or more ports for distributingthe fuel/air mixture within the combustion zone. The nozzle assembly further may have at least one channel for each nozzle assembly port, wherein each channel isangled away from the mixing tube flow axis and terminates at a nozzle assembly portfor distributing the fuel/air mixture within the combustion zone.
Specifically,nozzle assembly 872 is positioned withincombustion chamber852, and haschannels 901 defined by the geometrical configuration ofend cap 903andinterior side walls 905 ofnozzle assembly 872.Side walls 905 can be configuredas an extension member for mixingtube 868 or can have different geometrical shape.Nozzle assembly 872 further includesports 907 defined byend cap 903 andside walls905.Ports 907 are in flow communication withchannels 901 and distribute fuel/airmixture withincombustion zone 854. Fins orribs 909 are additionally provided toconnectend cap 903 toside walls 905.
Due to the beveled or sloped surfaces of the nozzle assembly (and in particularchannels 901), the flow of the fuel/air mixture is directed away fromflow axis 874, ascan be seen by the arrows in Fig. 11. That is, the flow of the fuel/air mixture can bediverted in a desired direction by utilizing surfaces of varying geometricalorientations. Although several channels and nozzle assembly ports are depicted, it isunderstood that the present invention can be achieved by utilizing only a singlechannel and associated port. However, at least two ports for delivering the fuel/airmixture in opposed angular directions relative to the liner axis is particularlybeneficial in utilizing the overall combustion volume.
Further, the structural components of the nozzle assembly (and in particularchannels 901) can be configured to direct the fuel/air mixture into the combustionzone in a variety directions, with the flow preferably not impinging the walls of thecombustion liner. For example,channels 901 of thenozzle assembly 872 could beconfigured so that the fuel/air mixture flows into the combustion zone in substantiallyradial or mixed radial-axial directions away from the mixing tube flow axis. Further,the flow could be directed in multiple directions relative to the liner axis, e.g., along atleast two generally opposed, substantially tangential angular directions relative to thecombustion chamber liner axis as is shown by the arrows in Fig. 9. Moreover, thechannels 901 could also be configured to direct flow in more than two directionsrelative to the mixing tube axis, such as is depicted in Figs. 10 and 11.
It should be further understood that the aforementioned geometry ofnozzleassembly 872 advantageously provides a flame holding effect by causing the suddenexpansion and recirculation of the exiting fuel/air mixture in the vicinity ofend cap903.
That is, the configuration ofend cap 903, for example, providesareas 911 forthe circulating fuel/air mixture to bum outsidenozzle assembly 872adjacent ports907. Flame holding is beneficial in providing a stable flame nearports 907 in order tomaintain a steady flame front to stabilize combustion during the varying operatingconditions.
Preferably, the total cross-sectional area ofports 907 are collectively about70-90% of the cross-sectional area of mixing tube 868 (generally indicated atreference point 913) in order to accelerate the fuel/air mixture and thereby increase themixture velocity delivered intocombustion chamber 852 relative to the velocity in themixingtube 868. The significance of this feature can be appreciated fromunderstanding that flames fromchamber 852 could otherwise ignite fuel withinmixingtube 868 when the flow of fuel/air mixture is at a low speed relative to theflame speed incombustion zone 854. By utilizingports 907, sized to increase thevelocity of the flow of fuel/air mixture, the likelihood that flame fromcombustionchamber 852 will "flash back" into the mixing tube is reduced. Further, by increasingthe velocity of the flow, it is believed that the boundary layer alongchannels 901 andatports 907 is reduced, thereby eliminating low velocity regions where the flame fromcombustion chamber 852 can creep along the surfaces ofnozzle assembly 872 andflash back into mixingtube 868. It is also believed that the aforementioned geometryis particularly useful when compressed air variations occur in mixingtube 868, whichotherwise could cause variable flame fronts or pulsations withincombustion chamber852. The increased pressure atports 907 also can dampen the minor variation incompressed air velocity in the premixer and reduce such pulsations. These advantages are useful in maintaining the structural integrity of the combustor system and itsindividual components, and thus provide a benefit to the integrity and performance ofthe overall gas turbine engine itself.
FIG. 9A depicts a variation of the construction shown in Figs. 8 and 9 with theprincipal differences being that the premixer 860' includes a cylindrical-type air valve890' in place of the butterfly-type air valve 890 and an asymmetric nozzle assembly872' arrangement. Air valve 890' has a rotatableinner cylinder section 890a', whichprogressively increases or diminishes the amount thatvalve outlet opening 890c' isoccluded to permit more or less air flow through valve 890' upon rotation of thecylinder/sleeve 890a' about axis 890b'. One skilled in the art would understand thatother cylindrical valve constructions could be used.
Fig. 9A also depicts a nozzle assembly 872' havingasymmetric nozzle ports907a' and 907b' configured to minimize the amount of fuel/air mixture impinging onthe axially rear wall ofliner 840. That is, the configuration of the flow directingsurfaces 901a' and 901b' ofnozzle end cap 872a' are configured to admit the fuel/airmixture intocombustion zone 854 predominantly in the tangential direction withrespect toaxis 842 of the combustion chamber while still admitting some of thefuel/air mixture into other regions (i.e., to the right and left of theventuri axis 874 inFig. 9A). This asymmetric nozzle port arrangement permits more effective utilizationof the combustion volume while minimizing fuel/air mixture impingement on theliner wall, which can lead to carbon build up, uneven heat transfer, and increasedthermal stress-caused distortions.
Fig. 9B is a modification of the construction shown in Fig. 9A with thecylindrical-type air valve 890" spaced a greater distance from the portion ofpremixerhousing 862 supporting theventuri mixing tube 868. It is expected that spacingairvalve 890" a greater distance from the premixer housing will help reduce theunavoidable asymmetries in the compressed air flow field exitingair valve 890" andallow the compressed air flow to be distributed more evenly in the premixer housingleading to the inlet ofventuri mixing tube 868. This will minimize the pressure drop along the air flow path from the air valve to the venturi inlet and allow a highermaximum power level for the engine while maintaining low emission levels.
It should be appreciated that an exit nozzle assembly can be connected to amixing tube by installation methods known to those skilled in the art. For example, asdepicted in Figs. 10 and 11,nozzle assembly 872 may have aflanged connection 915andattachment locations 917 for connecting the nozzle assembly to a mixing tubehaving a mating flanged structure. Alternatively, a mixing tube can incorporate thenozzle assembly into its overall structure.
With continued reference to Figs. 8 and 9, the mixing tube is connected to theliner so the flow axis of the mixing tube is aligned to generally intersect the liner axis.However, at least some of the channels of the exit nozzle are formed to direct fuel/airmixture in the combustion zone in a substantially tangential direction with respect tothe liner axis. This radial orientation of the mixing tubes can provide a more precisesliding fit between the mixing tube and the combustor liner because the combustorinlet opening is less elongated. This results in less leakage, and less lateral movementand thermal distortion during operation.
Specifically, controlled swirling flow and combustion is provided incombustion zone 854 by orientingnozzle assembly 872 so the fuel/air mixture willflow in a direction generally betweenliner wall 840a andliner wall 840b. Mixingtube 868 is radially mounted toliner 840 so that mixingtube flow axis 874 generallyintersectsliner axis 842. It should be appreciated that alignment need not be precise,so long as divided flows of the fuel/air mixture can be directed bynozzle assembly872 into the combustion chamber without appreciably impingingliner walls 840a and840b. Although some impingement of liner wall can be expected, it is preferred tominimize the amount of fuel/air mixture impacted on a given surface in order toreduce the amount of carbon deposited on such a surface during the combustionprocess. Carbon deposits can eventually insulate areas of the liner, causing problemsof thermal fatigue and localized overheating of the combustion chamber.
In operation, and with reference to Figs. 8-11, compressed air fromplenum830 is channeled viapassageway 882 over the outside surface ofliner 840 for coolingliner 840, and particularly portions which surroundcombustion zone 854. A portionof the compressed air flowing inpassageway 882 is admitted toplenum 884 throughorifices 885 and then flows to fuel/air premixer assembly 860 via the interconnectionbetween fuel/airpremixer housing assembly 862 andplenum 884 as controlled bycompressed air valve 890 viacontroller 894. This portion of the compressed air isessentially all the compressed air used for combustion (except for inadvertent leakageand compressed air that may be used to power an air-blast type fuel nozzle). Inmixingtube 868, the compressed air portion is mixed with the fuel fromfuel nozzle864, again possibly with a small additional portion of compressed air ifnozzle 864 isa "air-blast" type nozzle, and is directed along the mixingtube axis 874 tonozzleassembly 872, where the fuel/air mixture is divided into paths alongchannels 901 andaccelerated out ofports 907 intocombustion zone 854 ofcombustion chamber 852.By the orientation and sizes of thenozzle assembly ports 907, the designer can controlthe distribution and direction of the fuel/air mixture within the combustion volume.
After combustion of the fuel/air mixture inzone 854, the hot exhaust gasespass todilution zone 856 where dilution air fromdilution ports 858 reduces theaverage temperature of the exhaust before it is ducted viachannel 850past vanes 834toturbine 820 for work-producing expansion.
The control of combustion afforded bycombustion system 810, whichincludes aspects of the present invention, through the complete mixing of the fuel andair outside the combustion chamber in the fuel/air premixer, including completevaporization of the fuel if liquid fuel is used, together with the control of the fuel/airratio of the mixture delivered to the combustion chamber allows for significantreductions in NOx levels and the levels of unburned fuel and fuel by-productsemanating fromengine module 812, as mentioned earlier. Furthermore, the efficientutilization of essentially the total amount of compressed air flow to either combust thefuel or to dilute the exhaust gases upstream of the turbine provides increased efficiency, considerable reduction of peak combustor temperatures resulting in longerlife for combustor liners compared to conventional designs.
The system described is expected to provide low emissions at all power ratingsfor high inlet temperature gas turbine applications while keeping variable geometryflow apparatus away from and outside the hot combustor area.
Alternatively, as seen in Fig. 12, another predecessor construction of thenamed inventor having aspects of the present invention is illustrated. In particular,nozzle assembly 972 has asingle channel 1001 for directing the flow of fuel/airmixture in a direction that is generally tangential to the combustion chamber axis dueto the downwardly sloped surfaces ofchannel 1001.Nozzle assembly 972 furtherincludes asingle port 1007 in flow communication withchannel 1001 for distributingfuel/air mixture withincombustion chamber 952. Preferably, the total cross-sectionalarea ofport 1007 is about 70-90% of the cross-sectional area of mixing tube 968(generally indicated at reference point 913) in order to increase the acceleration of thefuel/air mixture delivered intocombustion chamber 952.
Although the above descriptions relate to radially mounted mixing tubes whichhave a nozzle assembly that extends into the combustion chamber, the presentinvention and its advantages can employ other mixing tube positions andconfigurations. For example, it should be appreciated that a mixing tube may beconnected to the liner so the flow axis of the mixing tube is slightly tangentiallyaligned to the liner axis. As such, the mixing tube's exit nozzle or other like structurecan be oriented to direct the flow of the fuel/air mixture tangentially into thecombustion zone and preferably minimize impingement of flow onto the liner whilemaintaining a simple geometric configuration at the liner inlet compared toconstructions such as depicted, e.g., in Fig. 1B, whereventuri axis 74 is substantiallytangentially oriented with regard toliner axis 42.
Further, the present invention may be utilized by a can-type combustorconfiguration such as shown in Fig. 13. In Fig. 13,combustor system 1100 includes acombustion chamber 1112 includingcombustion zone 1113 defined bycombustion chamber liner 1114. Aroundliner 1114 is disposed, in spaced relation,pressure vessel1116, which partly functions as a cooling shroud. Apremixer assembly 1126 includesanair valve 1128 and a venturi-type mixing tube 1130, a portion of which is disposedoutsideliner 1114, and anozzle assembly portion 1132 disposed to deliver a fuel/airmixture withincombustion zone 1113 ofchamber 1112.Fuel nozzle assembly 1138mounted inpremixer housing 1139 delivers a spray of fuel into a mixingtube inletregion 1131, where it is mixed in mixing tube 1130 with compressed air in an amountpartially controlled byvalve 1128 that is fed bycompressor 1102. As shown inFig. 13,valve 1128 is a cylindrical-type three-way valve with rotatable sleeve 1128a(although other types of valves are possible) and can direct air to venturi mixing tube1130 or tosecondary dilution ports 1140 inliner 1114 viabypass conduit 1142 andmanifold 1144 (as taught earlier in this specification).
Fig. 13A is an enlargement of the portion of Fig. 13 showingair valve 1128including rotatable sleeve 1128a, which is a circular segment that can act as a sealagainst about 1/3 of the inner circumference of the valve. Sleeve 1128a can be rotatedby an actuator (not shown) about axis from a position totally obscuring the entrance1142a to bypass conduit 1142 (as shown in solid in Fig. 13A) to a position blockingair flow to venturi mixing tube 1130 via premixer housing 1139 (shown in dottedFig. 13A), and allowing full bypass flow to secondary dilution ports (not shown).
For engine applications requiring multiple premixers, an air valve can beprovided for each can combustor (as shown in Fig. 13A) or for each pair ofcombustors, such as depicted in the Fig. 14A-14D embodiment (to be discussedinfra),and then connected to a common actuator which would move all the valvessimultaneously, in the same way as variable stator blades are moved on axialcompressors. One skilled in the art thus would be able to easily adapt the presentinvention, to be discussed subsequently, to such engine applications.
With continued reference to Fig. 13A,primary dilution ports 1160 receive aportion of the compressed air fromcompressor 1102 at a point upstream of manifold1128b ofvalve 1128. The dilution portion is dependent upon the pressure drops through the respective flow paths as well as the number and sizing ofdilution ports1160, as one skilled in the art would readily understand. The portion ofliner 1114definingcombustion zone 1113 is purposefully sealed off from receiving air exceptthrough mixing tube 1130 disposed in chamber inlet 1113a in order to maintaincontrol of the fuel/air ratio and provide low emissions, and a gap 1130a. Gap 1130a isprovided between mixing tube 1130 andpressure vessel 1116 to pass combustion airsufficient for idle operation. This arrangement simplifies the construction of the airvalve which no longer has to pass the (low) flow necessary for idle operation.
Nozzle assembly 1132 is depicted as part of mixing tube 1130 and extendinginto thecombustion chamber 1112 at the center of the can-type combustor liner 1114.As further shown in Fig. 13B,nozzle assembly 1132 has anend plate 1135 withsurface convolutions 1135a forming four channels that direct the fuel/air mixturewithinchamber 1112 throughports 1133, thereby optimizing the availablecombustion volume. A total of fourports 1133 are depicted as symmetricallyarranged about mixing tube axis 1130a but an asymmetric arrangement with fewer ormore ports can be used. Preferably still, the collective area atports 1133 fornozzleassembly 1132 should be between about 70 and 90% of the largest cross-sectionalarea of the mixing tube 1130 in order to increase the velocity of fuel/air mixtureadmitted intochamber 1112 throughports 1133. It is believed that theaforementioned configuration will likewise achieve the benefits described fornozzleassembly 872 of the Fig. 8 embodiment.
Although shown with a three-way valve 1128 that is highly useful inapplications requiring high bypass air flow (i.e., past the cooling channels formed byliner 1114 and pressure vessel 1116) during low power applications, can-typecombustor system 1100 can be used with a two-way air valve as described elsewherein this specification. Also,combustor system 1100 is depicted in use with an axial-typeengine havingaxial compressor section 1102 andaxial turbine section 1104, theengine axis being shown schematically as 1106 in Fig. 13.Combustor system 1100 using a can-type combustion chamber can be used in engine configurations employingradial and mixed axial-radial type compressors and turbines, as well.
It is also understood that one or more of the combustor systems can bepositioned circumferentially aboutaxis 1106 with the hot gas output of each collectedand distributed inturbine inlet plenum 1108 providing low emission operation for theengine.
Figs. 14A-14D show a configuration of a gas turbine engine havingcombustion apparatus which could advantageously utilize the present invention.Specifically, Fig. 14A shows a sectional view throughgas turbine engine 1210 havingcompressor section 1214 andturbine section 1216 operatively connected for rotationaboutengine axis 1218.Engine 1210 includesannular combustor chamber 1220,defined byliner 1222, withcombustion zone 1224 anddilution zone 1226.Coolingshroud 1228 surroundsliner 1222 to provide flow passageways for convection coolingofliner 1222 particularly in the vicinity ofcombustion zone 1224. As with the otherembodiments discussed previously,combustion zone 1224 is sealed off from thecooling air flowing throughpassageways 1262 and 1268 (see Fig. 14D) betweenshroud 1228 andliner 1220. Thus thecombustion zone 1224 receives air forcombustion essentially only as part of the fuel/air mixture delivered tocombustionzone 1224 through premixer assembly 1230 (to be discussed in more detailhenceforth) and thus constitutes a "single stage" combustion zone.
With continued reference to Fig. 14A,premixer assembly 1230 includes a pairof premixers 1232 (only one being shown in Fig. 14A) each having a venturi-typemixing tube 1234 positioned to receive fuel fromfuel nozzle 1236 and air frompremixer housing 1238 throughventuri inlet 1240. Eachventuri mixing tube 1234 isconfigured to deliver fuel/air mixture alongventuri axis 1242 and throughnozzleassembly 1244 intocombustion zone 1224.Nozzle assembly 1244 is constructed ofextension member 1244a andend cap 1244b having its surface contoured to providechannels andports 1246a, b for distributing the fuel/air mixture withincombustionzone 1224, generally at an angle with respect toventuri axis 1242. See Figs. 10 and 11 for examples. Although not seen in Fig. 14A, the ports 1246 also provide a flowdirection for the fuel/air mixture that is in opposed angular directions with respect toaxis 1242. Also as seen in Fig. 14A,premixer housing 1238, which surroundsventurimixing tube 1234 and mountsfuel nozzle 1236, is itself mounted toseparable endportion 1250a of engine pressure vessel 1250.
Fig. 14B is a perspective schematic view of an end portion ofengine 1210,which provides an understanding and appreciation for the highly advantageousconfiguration ofengine 1210. As seen in Fig. 14B, the pair ofpremixers 1230 aremounted to the separable pressurevessel end portion 1250a at essentiallydiametrically opposed positions with respect toaxis 1218.Premixer assembly 1230also includes a single, cylindrical-type air valve 1252 also mounted on pressurevesselend portion 1250a.Air valve 1252 is activated byactuator 1253 to control the flow ofcompressed air for combustion to bothpremixers 1232 along air paths throughmanifold 1254 and a pair ofdistribution conduits 1256.Distribution conduits 1256can be of a variety of shapes depending on the space limitations afforded by thebalance of the components of the combustion apparatus and the engine. However,they should be configured to provide a minimum pressure drop and present essentiallyidentical flow restriction characteristics.Distribution conduits 1256 are shown withbellows connectors 1258 leading tocompressed air inlets 1260 in each ofpremixers1232. Also,air valve 1252 is angularly disposed with respect toaxis 1218 to beessentially equidistant from each ofpremixers 1232 to provide a compact arrangementforpremixer assembly 1230 and to help ensure equal pressure drops betweenair valve1252 and theindividual premixers 1232. Although not shown in Fig. 14B, one orboth of thedistribution conduits 1256 can be purposefully made with a slightly higheror lower flow resistance than the other to allow flow balancing at the time ofconstruction. Alternatively, preset flow restrictors could be used indistributionconduits 1256 to ensure proper flow balancing between the premixers, but such aconstruction would entail increases in the overall restriction in the compressed airflow path and thus is not presently preferred.
As a consequence of the configuration ofpremixer assembly 1230 includingthe mounting of not only premixers 1232 but alsoair valve 1252 on separable pressurevessel end portion 1250a, theentire premixer assembly 1230 is removable along withpressurevessel end portion 1250a. As best seen in Fig. 14A, upon removal of theturbine exhaust pipe 1262,premixer assembly 1230 can be removed along withpressurevessel end portion 1250a. This ease of assembly/disassembly is a significantadvantage for the configuration of the combustion apparatus shown in Figs. 14A-14D.
Importantly, theindividual premixers 1232 are oriented and constructed suchthat the flow axes 1242 ofventuri mixing tubes 1240 are both radially disposed andaxially inclined with respect toaxis 1218. That is, the extensions ofventuri axes 1242intersect or pass in close proximity to engine/combustion chamber axis 1218 while atthe same time exhibit angles of significantly less than 90° with respect toaxis 1218 asis depicted schematically in Fig. 14B. This orientation effectively utilizes thenormally wasted annular space surrounding the turbine exhaust pipe andadvantageously provides a smaller overall "envelope" diameter forengine 1210, ofimportance in applications requiring a minimized axial profile, that is, a minimizedoverall engine O.D., such as in certain aircraft applications. Moreover, the moreeffective utilization of the combustion space incombustion zone 1224 may allow theaxial length ofcombustion chamber 1220 to be reduced, while maintaining sufficientresidence time in the combustor to reduce CO and NOx levels to acceptable values.The axial shortening ofcombustion chamber 1220 has the advantage of reducing thetotal heat transfer area that must be cooled bypassageways 1262 and 1268 (see Fig.14D). The reduction in the required cooling air flow leads to a more effective use ofthe available supply of compressed air, particularly in recuperated engine applicationswhen the recuperated return air would be hot.
With reference now to Fig. 14A and to Fig. 14C, which is a cross-sectionthrough theair valve 1252 anddistribution manifold 1254, the principal combustionair flow path to the premixer assembly can be seen. In particular, air flows from theradial compressor unit 1214 first along thecooling passages 1262 formed between thecombustion chamber liner 1222 and thecooling shroud 1228. In the vicinity of theend of thecombustion chamber 1220 proximate the singlestage combustion zone1224, a portion of the compressed air flows outward throughapertures 1264 incooling shroud 1228 and is collected inplenum 1266 formed by coolingshroud 1228andpressure vessel portion 1250a.Apertures 1264 may have any form and number aslong as the remaining cooling air has guidance and maintains the correct velocity.
Fromplenum 1266, the compressed air flowspast air valve 1252 and intodistribution manifold 1254 where it splits with essentially half going to each of therespective premixers (not shown in FIG. 14C). The remainder portion of thecompressed air, that is, the portion not flowing through theapertures 1264, flows todilution ports 1269 (Fig. 14A) alongpassageway 1268 along the inner portion of theannular combustion chamber 1220. Because combustion is essentially completed inthe vicinity of thedilution zone 1226 where the dilution air is added, the air travelingalongpassageway 1268 does not undergo combustion but only mixes with the hotcombustion products prior to enteringnozzle guide vanes 1215 and thenturbine unit1216 to provide efficient air flow and heat value management.
As shown in Fig. 14C,air valve 1252 is a cylindrical-type valve having arotatableinner cylinder section 1252a that can progressively close off or open flowpaths through the air valve under the control of a fuel/air controller (now shown) viaactuator 1253 as in previous embodiments. While other types of air valves can beused, such as butterfly valves, etc., cylindrical valves have been found to exhibit morepredictable flow characteristics and be less subject to aerodynamic oscillations at alow flow rates and thus are presently preferred. While thecylindrical air valve 1252shown in Fig. 14C is a "two-way air valve" the configuration could be modified toinclude a three-way valve used in conjunction with a second set of dilution ports.Such a construction is depicted in dotted lines in Figs. 14A, 14B, and 14C whichshowsbypass conduit 1270 interconnected with secondary dilution ports 1272 (Fig.14A) and is similar to the system shown in Fig. 13 at 1144. The benefits andadvantages of such a bypass configuration are set forth in my copending application Serial No. 08/892,397 filed July 15, 1997 and my provisional application Serial No.60/038,943 filed March 7, 1997, the contents of both of which are hereby incorporatedby reference.
Fig. 14D is an enlargement of the premixer cross-section shown in Fig. 14Aand shows in more detail certain additional features of the preferred design.Specifically, Fig. 14D showsventuri mixing tube 1234 havingcylindrical flange 1280which defines an annular opening withpremixer housing 1238. This annular openingis configured and sized to pass an amount of compressed air sufficient for operation ofengine 1210 at idle conditions. That is, the air flowing throughopening 1282 is takenfrom thesame plenum 1266 that supplies air to the premixers throughair valve 1252but bypassesair valve 1252 and thus is not directly controlled by it. This arrangementallows for simplification in the design ofair valve 1252 inasmuch as it is not requiredto pass a minimum amount of air to sustain combustion at idle operation.Opening1282 can be configured to have predictable and thus easily controlled air flow rates.
Also shown in Fig. 14D is a flow-evening grid 1284 mounted inpremixerhousing 1238 to surroundventuri mixing tube 1234 in the vicinity ofinlet 1240. Thefunction ofgrid 1284 is to redistribute the flow enteringpremixer housing 1238 viainlet 1260 and to even out other flow asymmetries arising from the structural featuresof thepremixer housing 1238 in order to obtain a more even circumferential inflowintoventuri inlet 1240.Grid 1284 can have an array of evenly spaced anddimensioned orifices or the array can be asymmetric in either orifice positioning ororifice dimensions in order to achieve the desired redistribution of the flow about theventuri entrance 1240.
Also depicted in Fig. 14D is a circumferential indent 1222a incombustionliner 1222 which is intended both to retard the axial flow of combustion products incombustor 1220 to gain more residence time and thus lower CO levels, and tostrengthen the structure against buckling.Nozzle assembly 1244 can clearly be seento be asymmetric in terms of theoutlet ports 1246a and 1246b formed by thecooperation ofnozzle end cap 1244b and extension member 1244a. As discussed previously, the asymmetries in the nozzle exit ports are intended to allow betterdistribution of the fuel/air mixture within the volume of the combustion zone whileprecluding excessive direct impingement of the fuel/air mixture on proximate portionsof the combustor liner. That is,exit ports 1246a and 1246b provide fuel/air mixtureflows at different angles with respect toventuri axis 1242 and are related to theorientation of the nozzle in the combustion chamber. And, as in the embodimentdisclosed in Figs. 8, 9, 9A and 9B, the total exit area of thenozzle exit ports 1246aand 1246b can be made less than the maximum cross-sectional flow area in venturi-typemixing tube 1234 to provide acceleration through nozzle ports in order to reducethe possibility of "flash backs" and burning within the venturi mixing tube itself.Generally, the area of the maximum flow area is at the end of the diverging portion ofthe venturi region for venturi-type mixing tubes.
While a single pair ofpremixers 1232 is shown in the Fig. 14A-14Dembodiment, two or more pairs could be used, each pair feeding an angular sector ofthe combustion chamber and having a single air valve and respective distributionmanifold and distribution conduits located between the associated premixers. Ingeneral, particularly for larger engine sizes, it is highly useful to have multiplepremixers to provide a substantially even gas velocity distribution in all portions ofthe combustion zone, to minimize variations in heat transfer to the liner. The shape,location and number of the nozzle ports, such asports 1246a,b in the Fig. 14A-14Dembodiment, also can impact on the gas velocity distribution and should be taken intoaccount.
Alternatively, multiple premixers can be used each with an associated air valveand actuator, but with the actuators interconnected, e.g., by a rotating ring to provideuniform control. A still further alternative uses a single air valve interconnected withmultiple premixers via a doughnut-shaped plenum. Such a configuration is depictedschematically in Figs. 15A and 15B which show a longitudinal cross-section and endview, respectively, ofengine 1310 havingmultiple premixers 1312 each with aseparate fuel nozzle 1314. Asingle air valve 1316 controls the flow of combustion air todistribution plenum 1318 which feeds eachpremixer 1312. The cross-sectionalflow areas ofplenum 1318 are made large enough so that the pressure drop along theflow paths fromvalve 1316 to the individual premixers is substantially the same, toensure balanced flow.Air valve 1316 can be mounted on the circumference ofpressure vessel 1320 and preferably is of the "cylindrical" type discussed in previousembodiments. As seen in Fig. 15A, compressed air flow entersair valve 1316 directlyfrom the compressor (not shown) throughpassage 1322 betweenpressure vessel 1320andcooling shroud 1324 and also from coolingpassage 1326 betweenshroud 1324andliner 1328 throughaperture 1334. Circumferential seal 1330 blocks compressedair flow frompassages 1322 and 1326 directly intoplenum 1318.Air valve 1316 is a"three-way valve" shunting excess compressed air directly to secondary dilution ports(not shown) viaconduit 1332.
The present invention, as broadly described and claimed hereinafter representsa further and significant improvement of the foregoing single stage combustionapparatus and methods in that it provides a variable mixing tube exit geometry forcontrolling the velocity of the fuel/air mixture discharged to the combustor.
Previous pre-mixer systems of the single stage, constant fuel/air ratio type, asdiscussed above, have as their main elements an air valve, fuel nozzle, venturi-typemixing tube and a fixed, constant area venturi exit nozzle. At varying loads, the airvalve admits varying air mass flows to match the varying amounts of fuel added.With the constant area venturi exit nozzle used in the above constructions, the exitvelocity of the premixed charged could vary appreciably, for example from less than20 m/sec to more than 60 m/sec in a typical single shaft turbine engine. At the lowerend, the achievement of stable combustion could cause problems and at the higherend, pressure losses and charge impingement on the combustor walls could beharmful. The variable geometry venturi exit of the present invention would enablepre-mixer operation with a constant, selected exit velocity of, for example 30 m/sec,independent of power rating, or within a range above and below predetermined minimum and maximum exit velocity limits, respectively. This would provide thefollowing advantages:
- 1. Enhance predictable combustion performance over the entire loadrange.
- 2. Avoid flash-back at low loads and impingement at high loads.
- 3. Reduce pressure losses at higher loads that can cause venturi air"starvation".
- 4. Better utilization of combustor volume at high fuel/air mass flow rates.
Fig. 16 shows a first embodiment of a gas turbine engine having combustionapparatus made in accordance with the present invention and using premixerapparatus variable exit geometry where the possible side effects of flash backs, flameinstability, and/or impingement due to uncontrolled mixing tube exit velocities can beminimized or eliminated. It will be evident from the succeeding discussion that whilethe methods and apparatus of the present invention can advantageously and preferablybe used with the previously described constructions that provide controlled fuel/airratio mixtures for single stage combustion for gas turbine engines and gas generators,the present invention is not limited to such use.
Specifically, Fig. 16 shows a sectional view throughgas turbine engine 1410having compressor section (not shown) andturbine section 1416 operativelyconnected for rotation aboutengine axis 1418.Engine 1410 includesannularcombustor chamber 1420, defined byliner 1422, withcombustion zone 1424 anddilution zone (not shown).Cooling shroud 1428 surroundsliner 1422 to provide flowpassageways for convection cooling ofliner 1422 particularly in the vicinity ofcombustion zone 1424. As with the other constructions discussed previously,combustion zone 1424 preferably is sealed off from the cooling air flowing throughpassageways 1462 and 1468 (seee.g., Fig. 14D) betweenshroud 1428 andliner 1420.Thus thecombustion zone 1424 receives air for combustion essentially only as part ofthe fuel/air mixture delivered tocombustion zone 1424 through premixer assembly 1430 (to be discussed in more detail henceforth) and thus constitutes a "single stage"combustion zone.
In accordance with the present invention, as broadly envisioned, a premixerapparatus for mixing fuel and compressed air from respective sources to provide afuel/air mixture comprises a premixer housing operatively connected to the sources ofcompressed air and fuel, a mixing tube disposed in the housing and having an entrancefor receiving fuel and compressed air, an axis, and an exit for delivering a fuel/airmixture, the mixing tube exit having a flow area, and a mixture valve for varying thefuel/air mixture velocity through the exit.
As embodied herein, and with continued reference to Fig. 16,premixerassembly 1430 includespremixer 1432 having venturi-type mixing tube 1434positioned to receive fuel from a source (not shown) viafuel valve 1435 andfuelnozzle 1436 and air frompremixer housing 1438 through venturi inlet 1440.Venturimixing tube 1434 is configured to deliver fuel/air mixture alongventuri axis 1442 andthroughmixture valve assembly 1444 intocombustion zone 1424.
With continued reference to Fig. 16,mixture valve 1444 is formed by thecooperation ofvalve member 1452 and anexit portion 1454 of mixingtube 1434, aswill be discussed hereinafter.Valve member 1444 includes elongatedstem 1446disposed substantially along the mixingtube axis 1442 and conically shapedplatemember 1448 disposed proximatemixing tube exit 1454.Valve actuator 1456engagesstem end 1450 through adrive 1458 configured to selectively movestem1446 andplate member 1452 along mixingtube axis 1442. A person having ordinaryskill in the art will appreciate thatvalve actuator 1456 can comprise a cam drive, ascrew drive, a rack and pinion drive, or a hydraulic/pneumatic drive, being located at aposition spaced fromcombustion zone 1424. As shown,drive 1458 includes acam1449 that interacts with a spring loadedfollower 1447 connected to stem 1446 toprovide an infinitely variable position and thus velocity control. A simpler, twoposition valve motion control using mechanical stops (not shown) can also be used atsome sacrifice in velocity control.Stem 1446 extends throughaperture 1460 inpremixer housing 1438. The effective exit flow area at mixingtube exit 1454increases or decreases asvalve stem 1446 is actuated in one or the other axialdirection, respectively because of the influence ofplate member 1448.
Portion 1434a of mixingtube 1434 proximate said entrance preferably iscurved away fromaxis 1442 whereinstem 1446 extends through anaperture 1462 inmixingtube 1434.Valve actuator 1456 is capable of engagingstem 1446 outside ofhousing 1438 and mixingtube 1434.
As further shown in Fig. 16,valve member 1452 preferably includesinterconnected cooling channels 1466 formed inplate member 1448 in flowcommunication withconduit 1468 instem 1446.Conduit 1468, in turn, is in flowcommunication withinlets 1470 operatively connected toconduit 1468 for admittingcompressed air fromhousing 1438. Preferably still,plate member 1448 is configuredin the shape of a hollow inverted cone with abase edge 1472, and multiple channelexits 1474 distributed about the base edge.Cooling channels 1466 serve to coolplatemember 1448. The compressed air admitted directly intocombustion chamber 1424throughchannels 1466 is small, and not an amount that would significantly affecteither the average or local fuel/air ratio. The hollow cone configuration provides arecirculation volume for the fuel/air mixture downstream of the premixer exit whichpromotes flame-holding and combustion stability.See e.g., discussion in relation toFig. 11.
In operation,valve member 1452 would be moved alongaxis 1442 bystem1446, which is affixed to a spring loadedfollower 1447 resting on acam 1449 that isrotated byactuator 1456 such as at the direction ofcontroller 1457.Controller 1457,which could be a microprocessor, would control the position ofvalve stem 1446 andthus the mixing tube exit flow area on the basis of engine power (actual or demand) ora related variable, as depicted in Fig. 16. Generally, high mixing tube exit mass flowrates associated with high power conditions could result in higher than desiredvelocities for fixed exit areas, thus prompting the need to increase the flow area todecrease the exit velocity to prevent flame instability and/or impingement. This would be accomplished by a left-ward movement ofvalve stem 1446 in the Fig. 16schematic. Conversely, for idle flow, minimum mixture mass flow rates, a decrease inthe flow area may be needed by right-ward movement ofstem 1446 to increase exitvelocities above the minimum to guard against flash backs.
Also in accordance with the invention, a sensor preferably is provided forsensing pressure upstream of mixing tube exit, in which a mixture valve actuator,operatively associated with the mixture valve, and a controller, operatively connectedto the pressure sensor and the mixture valve actuator, can varying the mixing tubeflow exit area in response to the sensed pressure. The controller controls the mixturetube exit flow area to provide mixture exit velocities greater than a predeterminedminimum value and less than a predetermined maximum value.
As further embodied in Fig. 16, asensor 1480 is provided havingsensingelement 1480a for sensing pressure upstream of the mixing tube exit area betweenplate 1464 and mixingtube exit 1454.Sensor 1480 is operatively connected tocontroller 1457, which is operatively connected to actuator 1456 which, in turn,engagesvalve stem 1446. Thus, in response to sensed pressure conditions alone, or inconjunction with a power level variable as discussed previously,controller 1457 cancontrolmixture valve 1452 to vary the mixing tube flow exit area to provide desiredfuel/air mixture exit velocities. Generally, the exit velocity only needs to becontrolled to a value or values greater than a predetermined minimum value to avoidflash backs and less than a predetermined maximum value that would cause flameinstability and/or impingement problems. This control could be provided by a two-positioncontrol scheme forplate member 1448. However, the infinitely variableposition control that can be achieved using the cam drive shown in Fig. 16 could beused to control velocity to a single target value,e.g., 30 m/sec, using an appropriateprogrammed microprocessor forcontroller 1457.
Fig. 16further shows controller 1457 being used to controlfuel valve 1435,and thus the engine power, and also actuator/valve 1486 controlling compressedairbypass 1488 frompremixer housing 1438 to a secondary set of dilution ports (not shown). The object ofbypass 1488 is to prevent undue pressure drops in thecoolantpassages 1468 leading to the primary dilution ports (not shown) for reasons givenpreviously in relation toe.g., the Fig. 13 construction.
As shown in Fig. 17A, which is a schematic detail of a variation of the Fig. 16embodiment, plate member 1448' can be configured in the shape of a hollow conewith abase edge 1472', in whichbase edge 1472' includes a fence 1476' positioned tostrip the boundary layer formed on plate member 1448' by the flowing fuel/airmixture. Also, premixer exit 1454' can be sharp edged to increase turbulent mixing.
Also, as is shown in Fig. 17B which is a schematic detail of another variationof the embodiment of Fig. 16,venturi tube 1434" can be spaced fromliner 1422" andcooling shroud 1428" bysleeve member 1478" which providescoolant channels1478a" to prevent excessive temperatures atventuri exit 1454". Due to thecompressed air flow throughcoolant channels 1478a" directly intocombustion zone1424" by passingventuri mixing tube 1434", the fuel/air ratio may not be controlledto the degree possible with the variations in Fig. 16 and Fig. 17A which may relay ona thermal barrier coating to prevent excessive mixing tube exit temperatures. Whilenot presently preferred, however, the variation depicted in Fig. 17B is considered partof the present invention in its broadest aspect and is expected to minimize flash backsand fuel residue due to impingement, as explained previously.
Figs. 18A-18C show other variations of the embodiment of Fig. 16. As shownin Fig. 18A,mixture valve 1552 is provided atexit 1554 of mixingtube 1534.Mixture valve 1552 includes avalve plate 1564 ofvalve member 1552 configuredpreferably in the shape of a hollow cone cooperating with mixingtube exit 1554. Anexit area is provided betweenvalve plate 1564 and mixingtube exit 1554 to allowfuel/air mixture into thecombustion zone 1524.Plate member 1564 is connected tostem 1546 and includescooling channels 1566.Valve stem 1546 is moved along axis1542 byactuator 1556 under the control ofcontroller 1557. The exit flow area willvary depending on the axial position ofplate member 1564 in relation to mixingtubeexit 1554, as discussed in relation to Fig. 16.
Importantly, as compared to the Fig. 16 embodiment, the embodimentsdepicted in Figs. 18A-18C include air valve/actuator assemblies that, in conjunctionwith respective fuel valves, determines the fuel/air ratio of the mixture in the mixingtube. With initial reference to Fig. 18A which shows a single premixer engineconfiguration, air valve/actuator assembly 1590 directly regulates the flow ofcompressed air topremixer 1530 under the control ofcontroller 1557. Through thecombined control of the fuel fromnozzle 1536 viafuel valve 1535 and compressed airviaair valve assembly 1590, a mixture with a controlled fuel/air ratio can be obtainedfor admission tocombustion zone 1524, inasmuch as essentially all the air forcombustion enters through the premixer as in Fig. 16. While the benefits of thepresent invention using a controlled mixing tube exit area are not confined toapparatus with controlled fuel/air ratio mixtures, the significant benefits attributable tocombustion with controlled fuel/air ratio mixtures discussed previously can beobtained while flash back, flame instability, and/or impingement phenomena areminimized.
It also should be remembered, however, that the Fig. 16 "air valve-less"embodiment can be used to achieve fuel/air ratio control in certain applications wherecompressed air flow is a function of power level, as discussed in relation to theconstruction shown in Figs. 5A and 5B.
Further,air valve assembly 1590 includes three-way valve 1592 for regulatingair flow topremixer housing 1538 and thus toventuri inlet 1540, and also tosecondary dilution ports (not shown) viabypass 1588, in a manner similar to thatshown in the Figs. 13A, 14C, and 15A, B constructions. However, the premixerapparatus of the present invention can be configured with a two-way air valve if thebypass feature is not used.
Moreover, the premixer apparatus can include multiple premixers as well asthe single premixer depicted in Fig. 18A. Fig. 18B shows an axial end view of a four-premixer-singleair valve/single fuel valve engine configuration that can achieve spacesavings for reasons explained in more detail in my copending application S.N. 60/081,465, the disclosure of which is specifically incorporated herein byreference. Specifically, the engine depicted schematically in Fig. 18B utilizes airvalve/actuator assembly 1590' to control combustion air flow to each of the fourpremixer 1532' of premixer assembly 1530' whilefuel valve 1535' controls fuel flowto the premixers 1532'. The axes of the mixing tubes of premixers 1532' generallyintersect axis 1518', similar to the configuration in Fig. 14B, being inclined less than90° relative to turbine axis 1518'. Fig. 18C, a schematic cross-section taken alongline AA of Fig. 18B, depicts premixer 1532' of premixer assembly 1530' at a positioncircumferentially spaced about axis 1518' from air valve/actuator assembly 1590'.Note in Fig. 18C that compressed air from the compressor is channeled to air valve1592' by circumferential seal 1594', as in the manner explained in relation to the Fig.15A construction, and air exiting valve 1592' is distributed to the individualpremixers 1532' via manifold 1598'. Manifold 1598' is positioned in the annularspace surrounding exhaust cone 1600', in the manner described in S.N. 60/081,465.
Alternatively, the premixer apparatus of the present invention can include aseparate air valve and fuel valve for each premixer, rather than the single air valve1592' andfuel valve 1535' used in the embodiment depicted in Figs. 18B and 18C.Still further, single interconnected mixture valve actuating systems could be usedrather than theindividual actuators 1556, 1556' shown in Figs. 18A and 18C. Also,although depicted in dotted lines in Figs. 18A and 18C, pressure sensors similar tothat shown in Fig. 16 as 1480, 1480a could be used to provide a further input tocontrollers 1557 and 1557' for use in controlling the respective mixture valvepositions viaactuators 1556 and 1556'.
Still further, it can be seen from Fig. 18C thatmixture valve 1552' includingstem 1546' and plate member 1564' is slidably mounted in fixture 1596' which isattached to premixer housing 1538'. Fixture 1596' advantageously provides anelongated bearing support for valve stem 1546', as one skilled in the art wouldappreciate.
Figs. 19A-19C depict a second embodiment of the present invention ofapparatus, combustor systems, and gas turbine engines utilizing a variable geometrymixing tube exit to control the fuel/air mixture velocity discharged into the combustorfrom a premixer. Specifically, Fig. 19A depicts agas turbine engine 1910 withcompressor section 1912,annular combustor 1920, andradial turbine 1916 situatedsimilarly to the engine layout in Fig. 8.Engine 1910 includes asingle premixer 1932supplied with a controlled flow rate of compressed air for combustion fromsingle airvalve 1990 via a pair ofmanifolds 1925, 1927 (only 1925 visible in Fig. 19A). Asdepicted in Fig. 19A,air valve 1990 is purposefully disposed at a diametricallyopposed angular position relative topremixer 1932, for reasons that will be discussedlater. While shown in Figs. 19A-19C with a single premixer, the present inventionnevertheless can be used with multiple premixers with a single or multiple air valves,and the premixers can be angularly inclined with respect to theengine axis 1918, suchas shown ine.g., Figs. 14-15 but using predecessor premixer combustor systems.
As best seen in Figs. 19B and 19C,premixer 1932 includes a venturi-typemixing tube 1946 including aninlet part 1946a and anoutlet part 1946b connected bya sliding joint 1947.Joint 1947 is configured to allow sliding relative movementbetweenventuri part 1946a, which is fixed relative topremixer housing 1938, andpart1946b which is movable along mixingtube axis 1974 by a pair of rack and piniondrives 1951,1953.Drives 1951,1953 are mounted internal to premixerhousing 1938but can be synchronously driven in turn by electric, hydraulic, or pneumatic actuators(not shown) mounted external tohousing 1938 and under the control ofcontroller1994 depicted schematically in Fig. 19B. For explanation purposes only, the portionof theventuri part 1946b to the left ofventuri axis 1974 in Figs. 19B and 19C isshown in a fully retracted (upward) position relative to the insertion depth intocombustion zone 1924 while the portion ofventuri part 1946b to the right ofaxis1974 is shown in a fully extended (downward) position.
As best seen in Fig. 19C,movable venturi part 1946b includes nozzleassembly 1972. Nozzle assembly 1972 includes hollowconical end cap 1903,sleeve extension 1907 connected to venturipart 1946b, and wall orrib sections 1905 whichdefine withsleeve 1907 andend cap 1903,nozzle exit ports 1909.Exit ports 1909together comprise a segmented, generally cylindrical-annular exit flow area geometry.Nozzle assembly 1972 is thus similar to the nozzle assembly construction depicted inuse with predecessor systems, particularly the asymmetric nozzle assemblyconstruction shown in detail in Figs. 10 and 11 adapted for use with annularcombustors. Nozzle assembly 1972 together withventuri part 1946b are slidablydisposed inco-axial skirt member 1949.Skirt 1949 is connected toengine pressurevessel 1914 and is therefore, like venturi part 14946a, "fixed" relative tomovableventuri part 1946b and attached nozzle assembly 1972. Fig. 19C also showscoolingholes 1967 formed inskirt 1949 to provide a small amount of cooling air which flowsaxially betweenskirt 1949 andmovable venturi part 1946b to reduce operatingtemperatures inskirt portion 1949a which extends intocombustion zone 1924.
Importantly, as can be appreciated from Fig. 19C, the degree of overlappingrelation ofskirt end 1949a and nozzleassembly exit ports 1909 act to limit theavailable flow area for the discharged fuel/air mixture. In this sense, movable nozzleassembly 1972 and fixedskirt member 1949 cooperate and act as a valve to increaseor decrease the effective flow area of the fuel/air mixture throughexit ports 1909depending upon the direction of movement ofventuri part 1946b. That is, for a givenfuel/air mixture mass flow rate throughpremixer 1932, decreasing the available exitflow area by withdrawingventuri part 1946b and nozzle assembly 1972 in an upwarddirection in Fig. 19C would act to increase the fuel/air mixture velocity, while adownward movement ofventuri part 1946b in the Fig. 19C construction would havethe opposite affect of increasing the available flow area and necessarily decreasing themixture exit velocity, as explained previously in relation to the embodiment of thepresent invention shown in Figs. 16-18.
The advantages afforded by nozzle assembly 1972 include distributing thefuel/air mixture within the annular combustor without undue wall impingement, asexplained in relation to predecessor constructions shown in Figs. 8-11. As in the Fig. 8 embodiment, nozzle assembly 1972 also can be configured with a reduced exitport area relative to a mixing tube area to accelerate the flow throughports 1909 andprovide a greater margin against flash back. Although not shown, the presentinvention clearly encompasses variations in the construction of the mixing tube andskirt components, such as a single piece movable mixing tube, or a fixed single pieceventuri mixing tube (and nozzle assembly) together with an axially movable skirtcomponent. As one skilled in the art would readily understand, it is the relativemovement between these components which provides the desired mixture valveeffect. Thus, the invention is to be limited only by the appended claims and theirequivalents in this respect, and not restricted to the actual embodiments shown.
In operation, and with reference to Fig. 19B, the fuel/air premixer 1932receives compressed air from the gas turbine engine compressor 1912 (not shown inFig. 19B) viacylindrical air valve 1990 andmanifolds 1925,1927. Manifolds1925,1927 can be separate conduits or, as shown in Fig. 19B, be formed frommembers cooperating with the outside surface ofpressure vessel 1914. As depicted inFig. 19B, the air fromcompressor 1912 flows generally axially betweenpressurevessel 1914 andcooling shroud 1928. Thereafter, a portion of the compressed airflows through impingement cooling holes 1981,1983 while the balance flowscircumferentially toair valve 1990. While depicted in Fig. 19B as a "two-way" airvalve,air valve 1990 can be configured as a three-way valve which can divert theportion of compressed air not required for combustion or impingement coolingdirectly to a second set of dilution ports (not shown) thereby bypassing the normalflow path for coolant air, namely axially, betweencombustor liner 1922 andcoolingliner 1928 to the primary dilution ports (also not shown). A full explanation of thebenefits and advantages of such a configuration is set forth in the discussion of thepredecessor systems such as the systems shown in Figs. 13A-13C.
The compressed air that is ducted from theair valve 1990 throughmanifolds1925,1927 to premixerhousing 1938 entersventuri 1946 via theinlet venturi part1946a, which is the fixed part of the venturi mixing tube. This air is mixed with fuel fromfuel nozzle 1985 as it flows alongpremixer axis 1974 until it reaches theend cap1903 of the nozzle assembly 1972. There the mixture is deflected away frompremixer axis 1974 and is distributed in opposing tangential directions designated bythe arrows F1,F2 in Fig. 19C, as well as in the direction of engine axis 1918 (notshown in the Figure). In Fig. 19C, the flow arrow F1 is depicted larger and longerthan the flow arrow F2 to represent the increased velocity throughnozzle exit ports1909 when partially restricted by skirt 1949 (left side ofpremixer axis 1974 inFig. 19C) relative to the fully extended and open nozzle exit ports on the right side ofFig. 19C.
Although the movement of theventuri mixing tube 1946 can be varied toprovide an intermediate opening area, it is expected that a two-position system (fullyretracted or fully extended) would suffice since the fuel/air ratio is controlled byairvalve 1990 as shown in Fig. 19B. However, the present invention is intended to coverconfigurations where the position ofmovable venturi part 1946b would controlled toan intermediate position such as bycontroller 1994.
Figs. 20A and 20B are schematic illustrations of a variation of the premixervariable geometry construction shown in Figs. 19A-19C suitable for can-typecombustors. Fig. 13 depicts such a can combustor application albeit with apredecessor fixed geometry premixer exit system. However, the specific applicationshown in Fig. 13 is not intended to restrict the application of the embodiment shownin Figs. 20A dn 20B, much less the scope of the present invention.
Specifically, Fig. 20A shows thelower part 2046b, of a venturi-type mixingtube to which is connectednozzle assembly 2072.Nozzle assembly 2072 includesopen-ended conical end cap 2035,sleeve extension 2037, and open-ended wedge-shapedribs 2039 interconnecting end cap 2035 andsleeve 2037. The upper conicalsurface of end cap 2035 together withwedge ribs 2039 andsleeve 2037 form aplurality ofnozzle exit ports 2033 for discharging the fuel/air mixture into the cancombustor.Nozzle ports 2033 define generally a segmented cylindrical-annular exitflow area fornozzle assembly 2072. Both open ends of end cap 2035 and the open ends ofwedge ribs 2039 provide recirculation of the fuel/air mixture (depicted bycurved arrows in Figs. 20A and 20B) and flame holding downstream ofexit ports2033 to enhance combustion stability.
Nozzle assembly 2072 is thus similar to theaxisymmetric nozzle assembly1132 in the Fig. 13 construction with the important difference thatnozzle assembly2072 can move up or down along mixing tube/premixer axis 2074 along with mixingtube part 2046b. As in the Fig. 19A-19C embodiment, this movement can beaccomplished using rack and pinion drives to movepart 2046b relative to a fixedmixing tube part (all not shown). Alternatively, a one piece mixing tube suitablymounted for sliding within a premixer housing (also not shown) carryingnozzleassembly 2072 can be used.
Importantly, as best seen in Fig. 20B, alower portion 2049a of co-axiallydisposedstationary skirt 2049 is configured to act withmovable nozzle assembly2072 as a valve to define the effective nozzle flow area throughexit ports 2033, toprovide mixture exit velocity control. The position depicted in Fig. 20B is the full-openposition, representing the maximum insertion depth ofnozzle assembly 2072into the combustor. Withdrawing mixingtube part 2046b and nozzle assembly 2072(upward) alongaxis 2074, such as during low power or idle conditions, will cause theaxial end 2049a to block a portion ofexit ports 2033 decreasing the effective flowarea and increasing the velocity, for constant mixture mass flow rate, as one skilled inthe art would understand.
With reference again to Figs. 19A-19C, the particular air valve and premixerorientation shown in has a further advantage. Due to the proximity to the premixernozzle assembly in the Fig. 19A-19C embodiment and the mixture exit velocitycontrol, theupper half 1924a ofcombustion zone 1924 inannual combustor 1920provides most of the reaction zone where combustion of fuel and air take place whilethelower half 1924b functions more like a transition duct. The cooling ofcombustor1920 is designed according to this requirement. At full power, more than 30% of theengine air massflow is used to cool the top half of the combustor, while only about 20% is required for bottom half cooling. The premixer massflow accounts for about45% of the air massflow, and about 5% is required for hot section cooling under theseconditions. Extracting the air from thebottom half 1924b of the combustor to supplythe premixer provides a more optimal split for the following reasons.
First, a smaller amount of air has to be diverted, than if the valve was at thetop. Because the compressor delivers the air uniformly distributed to thepressurevessel 1914 surroundingcooling liner 1928, only about 15% (20% + 45% - 50%) ofair has to flow from the top to the bottom half of the engine around the combustor inthe case of a top premixer and a bottom air valve placement. In the case of a top airvalveand top premixer arrangement, about 25% (30% + 45% - 50%) of air wouldhave to displaced from the lower half of the engine to the upper half. The availableflow areas are thus utilized more efficiently and available pressure drop is conservedwith a bottom air valve arrangement, because average velocities and thereforepressure losses are decreased.
The second reason for placing the valve at the bottom in the Fig. 19A-19Cembodiment is that the air traveling to the air valve experiences a static pressuredepression according to the equation ρ + ½ρv2= CONSTANT. As the static pressurebetween pressure vessel and cooling liner is decreased, the pressure differential acrossthe cooling liner decreases as well resulting in a decreased cooling mass air flow ratethrough a fixed size hole. Close to the valve, the amount and thus velocity of airtraveling towards the valve is the highest, resulting in the lowest static pressure andlowest impingement cooling flow. However, the impingement cooling flow decreaseswhere less cooling is required if the reaction zone is at the top. Therefore it isadvantageous to extract the premixer air in a zone of low cooling requirements, i.e., atthe bottom of the engine in the configuration depicted in Figs. 19A-19C.
In summary, extracting the premixer air from the region of the pressure vesselremote from the premixer exit is beneficial because:
- 1. Less air has to be displaced within the engine;
- 2. The biggest decrease in static pressure occurs where there is the leastcooling required.
In summary, in a premixer with a fixed geometry exit, the exit velocity of apremixed charge would vary with the position of the air valve. In a configurationsimilar to Fig. 19B and 19C but with no variable geometry exit, where the air valve isnearly closed during idle or low power operations, only a small amount of air passesthrough the venturi mixing tube with the velocity and the range of about 20m/s inorder to provide ample margin above the flame speed, somewhere below the 10m/s toavoid flashback. In such a construction, at full power the exit speed may exceed70m/s and lead to combustion instabilities. Also, at the high end there may beinsufficient available pressure drop to push the air through the venturi, leading toreduction in rated power, or to push sufficient cooling air through the cooling shroudto cool the liner and finally to exit the flow through the dilution ports. In order toconserve the pressure drop and yet avoid flash back under part power, it is thusadvantageous to use variable exit geometry premixer constructions such as are shownin Figs. 16-20. When the compressor flow varies the flow, for example, in a two-shaftengine or in any multi-spool engine, the idle mass flow could be very small at lowpowers, making the use of a variable exit premixer even more beneficial in order toavoid flashback with resulting internal premixer burning. In addition to preservingpressure loss in the system, an important additional advantage is to reduce the highexit velocity at full power of the fixed exit geometry system. This would substantiallyreduce the impingement and thermal load on the combustor liner. Furthermore, thecombustor volume would have a higher utilization by needing shorter distances fromthe premixer exit to reach the lower flame speed velocity required for stablecombustion.
With the above detailed description of the combustor system and fuel/airpremixer apparatus and method of operating same of the present invention, thoseskilled in the art would appreciate that modifications may be made to the inventionwithout departing from its spirit. Specifically, while the implementation of the invention is described above in relation to a radial gas turbine engine, the subjectinvention is not limited to this specific type of gas turbine-engine, but can be adaptedto axial and mixed axial-radial, as well. Similarly, while control of the mixture valveactuator such asactuator 1556 in Fig. 18A by a controller (e.g. 1557) or the actuator(not shown) fordrives 1951, 1953 under control ofcontroller 1994 in Fig. 19B, whichcontroller can be microprocessors is presently preferred for accuracy, it may bepreferred to use a more simplified and thus less costly construction.
For example, the movement of the movable mixture valve component could bemechanically or hydraulically/pneumatically activated by the air pressure in theventuri top box which changes with changing settings of the main air valve.Alternatively, the component can be moved mechanically or hydraulically inconnection with movement of the actuator which operates the main air valve. Ineither case, at high loads the annular air gap is the largest and at idle it is the smallest,keeping the velocity the same from idle to full load for continuous position control.Of course, as previously mentioned, a less expensive version could use two settings,low and high which nevertheless would constitute an improvement over the fixedgeometry in the predecessor constructions. All previous discussion about cooling andflame holding are still relevant.
Figs. 21A-21D depict further embodiments of the invention. Specifically, theembodiments of Figs. 21A-D provide a new, more simplified configuration, while stillproviding both the variation of the premixer exit flow area common to allembodiments and the control of the direction of the discharged fuel/air mixture toavoid or minimize impingement of nearby combustor liner surfaces typified by theFig. 19A-C embodiment. The embodiments of Figs. 21A-D essentially utilize amixture valve configured as an adjustable nozzle having a hollow, conical, centrallylocated, shaft-driven movable valve plate similar to that shown in the Fig. 16embodiment, but with a fixed surrounding skirt having a trailing end contoured toprovide flow ports for fuel/air mixture distribution within a combustor, such as anannular combustor.
With reference to Fig. 21A, there is shown the exit portion of venturi-typemixingtube component 2134 of fuel/air premixer 2132. Mixing tube configurationsother than a venturi-type could be used but a venturi-type is presently preferred. Asdepicted in Fig. 21A,mixture valve 2144 includes an inner valvemember includingplate 2148 and stem orshaft 2146, and a co-axial outer valve member, namelyskirt2149.Plate 2148 is generally conical in shape and hollow to improve flame holding,as discussed previously.Plate 2148 is coupled viatie bolt 2156 toshaft 2146 which,in turn, is mounted to the exit ofventuri 2134 viasleeve bearing 2158 and struts 2160for reciprocal, sliding movement alongventuri axis 1442. Fig. 21A shows two ofthreestruts 2160 contemplated, but fewer or a greater number of mounting strutscould be used.
Shaft 2146 can be driven by mechanical, hydraulic, or pneumatic actuator suchas, for example, through the cam and spring arrangements depicted in Fig. 16. Oneskilled in the art also would appreciate that the depicted construction could be adaptedto use the driving mechanism depicted in Figs. 19B,C. In such aconstruction plate2148 could be fixed tostruts 2160 using a truncated stem depicted in Fig. 21A byrounded shaft end 2146a (shown dotted), and bearing 2158 eliminated. The exitportion ofventuri 2134 to which struts 2160 are affixed would then be movable withrespect to a venturi entrance portion (not shown) using controlled actuators workingthrough gear and rack mechanisms, similar to the arrangement depicted in Figs.19B,C. In such a construction,skirt member 2146 would not be mounted toventuri2134 but would be fixed or at least movably connected to a premixer or combustormember such as the premixer housing (not shown),combustor liner 2122, or thesurroundingcooling shroud 2128. Shims 2150 (shown dotted in Fig. 21A) could beused to adjust the initial positions ofvalve plate 2148 at assembly.
In the Figs. 21A-D embodiments,skirt member 2149 is generally cylindricalbut has trailingend 2149a contoured to provide reliefs orports 2109 for channeling atleast most of the mixture flow in two generally opposed directions, such as theopposed tangential directions relative toannular combustor axis 2118 similar to the construction depicted in Fig. 19B. As with the mixture valve 1944 of the Fig. 19A-Cembodiment,mixture valve 2144 can be made asymmetric e.g. to provide somelongitudinal mixture flow along thecombustion chamber axis 2118 to better utilizethe volume ofcombustion zone 2124. In such a variation of the Fig. 21A-Dembodiments, this could be easily accomplished by providing suitable additional portsor reliefs in contouredskirt end 2149a angularly between the opposed depictedports2109.
In the Fig. 21A-D embodiments,skirt 2149 is fixed andplate 2148 is movableviashaft 2146 to provide an exit flow area in accordance with system requirements asdetermined by an appropriate control system (not shown) similar to those depicted inFigs. 16, 18A, and 19B. However, as stated previously, in alternate configurations inaccordance with the present invention,plate 2148 can be fixed andskirt 2149 can beconfigured to be movable, or both could be configured to be movable, although with asignificant increase in complexity and cost. Continuously variable or stepped (e.g.2-stop) movement could be provided by the control system as discussed previously.
The premixer is mounted withmixture valve 2144 protruding intocombustionzone 2124 through a sealed aperture inliner 2122.Seal 2162 is shown as a labyrinthseal, but could also be a piston ring, brush, or another seal type. As established duringtesting, excessive leakage flow through an unsealed opening can create a curtain of airsurrounding the mixture valve which can divert and destabilize the combustion undercertain operating conditions, particularly idle or low power operation.
Depicted in Fig. 21B are two of the components ofmixture valve 2144,namelyplate 2148 andskirt 2149. Under unfavorable operating conditions, forexample with inferior liquid fuels, "flash-back" into the exit portion ofventuri 2134may occur. In order to safeguard against heat damage through oxidation or meltdownof portions of these components,skirt 2149 and/orplate 2148, if constructed frommetal materials, could include appropriate cooling channels, such as those depictedschematically for plate 1648 in Fig. 16. See also the disclosure of Applicant'spending application S.N. 09/721,964 filed November 27, 2000, the disclosure of which is hereby incorporated by reference. Alternatively, or additionally,skirt 2149and/orplate 2148 could be provided with a thermal barrier coating (TBC) known tothose skilled in the art of gas turbine engine components.
However, one or bothplate 2148 andskirt 2149 are preferably formed from aceramic material which preferably includes dispersed ceramic fibers to ensureintegrity if cracking should develop during prolonged engine operation. It is expectedthat a ceramicmixture valve plate 2148 andskirt 2149 could be readily fabricated bycasting and then sintering. Also,lower portion 2146b ofshaft 2146 could be ceramicas well. While shrinking may occur during sintering, those skilled in the art offabricating shaped ceramic articles would be able to select appropriate "green" castingdimensions to yield near-net final (sintered) shapes without undue experimentation.Appropriate finishing can be used to provide desired final dimensions and shapes.
Due to the different expansion coefficients of ceramics and metals, the ceramicand metal parts could be flexibly clamped together using appropriate mountingarrangements. Those familiar with the engineering state of the art would know toconsider using such devices as Belleville washers or "wiggle strips" at these joints toprovide thermal expansion flexibility thereby reducing stresses and the chance ofcracking of the ceramic parts. For example, a Belleville washer (not shown) could beprovided at shaft/plate joint 2152 in Fig. 21A, or at the location ofshims 2150 if thelower part 2146b ofshaft 2146 was also formed from a ceramic material.
Most metals loose their strength at a level about 300°C below that of ceramics,allowing more margin for the effects of flash-back by the use of ceramics. Hence,appropriate cooling channels may be provided in struts 2160 (shown dotted inFig. 21A, in one strut only), in tie bolt 2156 (channel exit shown dotted in Fig. 21A),and/or in shaft 2146 (not shown) even when using ceramic materials forplate 2148andskirt 2149, if required.
Figs. 21C and 21D depict details of a variation of the embodiment of Figs.21A and 21B which has a bayonet-type clamping mechanism between ceramic skirt2149' and the metal exit portion of venturi 2134'. Axially directed slots 2164' and an annular groove 2166' are provided in the exit portion of venturi 2134' for receivingfingers 2168' cast intoskirt mating end 2149b'. In addition to compensating fordifferent expansion coefficients of the metal and ceramic components, metal wigglestrip 2170' provides an axially directed retaining force tending to seat fingers 2168' inrecesses 2172' in annular groove 2168' after skirt 2149' is inserted against wigglestrip 2170' and rotated to aligned fingers 2168' and recesses 2172'. Three sets ofslots 2164', fingers 2168', and recesses 2172' are contemplated but fewer or greatersets could be used. Additionalannular wiggle strip 2174' can be used to provide aradially directed centering force for skirt 2149'. Also, one skilled in the art couldprovide other clamping mechanisms including other bayonet or even screw-typemechanisms.
It should be understood that the premixer and combustor embodimentsdescribed above and depicted in the drawings can be used in various gas turbine gasgenerator and engine configurations including, but not limited to, the predecessor gasgenerator and engine configurations discussed previously as well as the configurationsdiscussed in relation to the variable exit geometry embodiments of Figs. 16 through21A-D. Also, the present invention can be used in engine configurations, bothannular and can combustor types, having multiple premixers, as well as engineconfigurations with a single premixer such as shown in the Fig. 19B embodiment.
Therefore, it is not intended that the scope of the invention be limited to thespecific embodiments illustrated and described above. Rather, it is intended that thescope of this invention be determined by the appended claims and their equivalents.
