(54) IMPROVEMENTS IN OR RELATING TO OPTICAL DEVICES ANDENGINE INSTALLATIONS INCLUDING SUCH DEVICES(71) We, SMITHS INDUSTRIES LIMITED, a British Company of Cricklewood,London NW2 6JN, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to optical devices and is especially concerned with such devices for use in gas-turbine engines.
According to one aspect of the present invention there is provided an optical device including a tubular passageway having an open front end, a sensor head positioned to extend a part-way along the rear of said passageway to define an annular recess between the passageway and the head, wherein said sensor head has an optical window towards its front end, said recess being vented into said passageway in front of the window, and wherein said device includes a first inlet for supplying air to said passageway at a point intermediate said sensor head and the open end of said passageway, and a second inlet for supplying air to said recess so that it vents into said passageway to the rear of said first inlet such that air supplied to said first and second inlets purges matter from the passageway that might otherwise cause fouling of the optical window.
The optical device may be for use in the visible, ultra-violet or infra-red regions of the electro-magnetic spectrum.
The invention is particularly, though not exclusively concerned with radiation pyrometers and flame detectors for use on gas-turbine engines.
Radiation pyrometers used on gas-turbine engines for sensing, for example, the turbine-blade temperature, conventionally have an optical head that is mounted to project through the engine by-pass duct to the turbine chamber. The head is directed to receive radiation from the turbine blades, and in this respect includes a lens or other optical window within a sighting tube that opens through the wall of the chamber. The use of the sighting tube ensures that radiation is received throughout a limited angle only, but also that the window is not exposed directly to the heat and combustion products within the chamber. Air is supplied to the sighting tube to ensure that it is purged of the combustion products or any other matter that might lead to fouling of the window during running of the engine.This air is in practice derived from the engine compressor being tapped off from the by-pass duct and supplied to the sighting tube either through a slot partway down the sighting tube or through one or more ports or apertures that open into the tube just in front of the window. In either case the air pressure is normally sufficient to purge the tube satisfactorily during normal running, but fouling still occurs to such an extent that it is necessary to clean the lens or window after only some 30 to 100 hours of operation.
The fouling of the lens or other window experienced, is believed to arise principally from the presence of fuel-air mixture within the sighting tube during starting of the engine. During starting the pressure supplied to the sighting tube from the compressor stage is low and fuel-air mixture becomes entrained in the sighting tube and then trapped there. As the engine speed rises, the air pressure increases correspondingly and much of the fuel is purged from the tube. That which is not purged is trapped and is initially oxidised, and then partially reduced as the environmental temperature increases. The reduction results in the formation of fine particles of carbon which settle on the cooler surroundings, in particular on the window to foul it and prejudice the integrity of the radiation-pyrometer output-reading.
Similar considerations apply to flame detectors as used, for example, to monitor the existence of flame in the combus tion chamber of a gas-turbine engine.
Here again the device has a head that includes a sighting tube which is conventionally purged with air from the compressor stage; fuel-air mixture can readliy be entrained into this tube during starting to result in fouling of the lens or other window of the detector.
The problem of fouling of the window within the sighting tube of a radiation pyrometer, flame detector of any other comparable optical device, by matter entrained during engine starting can be readily overcome or at least substantially reduced, in accordance with the present invention. More particularly, in accordance with the present invention it may be arranged that air supplied, for example, from the engine compressor-stage via the by-pass duct, is vented into the sighting tube at a location between the open end of the tube and the lens or other optical window of the optical device, and that air is also vented into the tube intermediate this location and the window from a separate source.The separate source may be a low-pressure source and would be arranged to be effective to supply the air to the tube throughout starting so as to purge the tube and maintain it free of the fuel-air mixture from the beginning.
This purging would continue until the pressure of air available from the compressor exceeded that from the separate source, whereupon the flow of air from this latter source would cease. The volume of air in the sighting tube in front of the window would now remain clean and stagnant, being contained by the curtain of air vented down the tube and flowing out of the front, open end. The separate source of air, which may be, for example, a pressurised reservior provided on the engine and arranged to be replenished during running, may be coupled to the optical device via a non-return valve.
a radiation pyrometer and its installation on a gas-turbine engine to respond to turbine-blade temperature, all in accordance with the present invention, will now be described, by way of example, with reference to the accompanying draw ings, in which: Figure 1 illustrates the installation; andFigure 2 is a sectional elevation of the radiation pyrometer.
Referring more especially to Figure 1, the radiation pyrometer has an optical head 1 that is enclosed by a metal sleeve 2 mounted on the turbine casing 3 of the engine. The sleeve 2 extends through the engine by-pass duct 4 to the turbine cham ber 5 for directing the head 1 towards the turbine blades 6. More particularly, the sleeve 2 incorporates a sighting tube 7 that opens into the chamber 5 from the duct 4 and serves to channel radiation from the blades 6 to a synthetic-sapphire lens 8 (Figure 2) in the head 1.
Referring now more especially to Figure 2, the lens 8 focusses radiation received from the blades 6 onto the end 9 of a fibre-optic cable 10. The fibre-optic cable 10 includes a bundle 11 of glassfibre or silica-fibre strands contained within a sheath 13 of woven glass-fibre or silica-fibre and is protected externally by stainless-steel convoluted tubing 14 and stainless-steel braid 15. The sheathed bun dle 11 is cusioned and supported within the tubing 14 by an elastic sleeve 16 formed by a double thickness of an openweave fibre material. The fibre material may be of glass or silica, or may be a mixture of silica-fibre and asbestos-fibre such as is sold, for example, under the nameRefrasil by the Chemical and InsulatingCompany Limited, Darlington, England.
Alternatively, in low-temperature applications, the sleeve 16 could be of an elastomeric substance such as, for example, silicone, flouro-silicone or flouro-carbon rubber and several sleeves could be provided coaxially of one another. The end 9 of the bundle 11 is enclosed by a glass tube 17 and is sealed hermetically into a metal ferrule 18 that serves to couple the cable 10 to the head 1. The opposite end 19 of the' cable 10 is similarly hermetically sealed, being enclosed by a glass tube 20 within a metal ferrule 21.
Referring again to Figure 1, the end 19 of the cable 10 is coupled into a detector unit 22 mounted at a comparitively cool location on the engine. The unit 22 includes a photoresponsive element 23 that serves to convert the optical signal transmitted via the cable 10 to the end 19 into an electrical signal. This electrical signal provides a measure of turbine-blade temperature and is supplied via an amplifier 24 to both an indicator 25 and and engine-control unit 26.
The accuracy of the electrical signal as a measure of turbine-blade temperature depends on the integrity of the optical signal derived by the head 1. Such integrity is much influenced by any attenuation of the radiation received by the head 1.
caused by fouling of the front, exposed surface of the lens 8 with fuel and combustion products within the sighting tube 7. Provision is accordingly made to purge fuel and combustion products from the tube 7 during running out of the engine, and in this respect the tube 7 has a slot 30 that opens into the bv-pass duct 4.
Cooling air as conventionally supplied to the duct 4 from the compressor stage of the engine, flows through the slot 30 into the tube 7 to be exhausted into the chamber 5 so as to block to a substantial extent entry of fuel and combustion products into the tube 7 during running.
The pressure of the cooling air supplied to the duct 4 is however low during starting of the engine and is in general insufficient to ensure that fuel-air mixture does not enter the tube 7 at this time.
To block entry of the fuel-air mixture in these conditions, purging air is supplied from a separate source 31.
The air-supply source 31 is coupled via a non-return valve 32 to an inlet 33 on the sleeve 2 to pass low-pressure air (for example, at 80 pounds per square inch) into an annular space 34 between the sleeve 2 and the casing 35 of the head 1. This air is vented from the space 34 across the front surface of the lens 8 into the tube 7 via circumferentially-distributed ports 36 in a lens-retaining ring 37 (Figure 2) of the casing 35. The flow of air across the lens 8 into the tube 7 purges the tube 7 free from the fuel-air mixture, and continues as starting proceeds, until the pressure applied to the inlet 33 is exceeded by that in the by-pass duct 4.
Air which now flows through the slot 30 from the duct 4 to maintain purging of the tube 7, creates a curtain trapping the volume of clean and virtually stagnant air remaining adjacent the lens 8, above the slot 30. The slot 30 is inclined forwardly to direct the flow downwardly of the tube 7 and thereby enhance the curtain effect.
With the installation described above there is the advantage that the tube 7 is purged adequately from the very beginning of engine operation, thereby minimizing fouling of the lens 8. In this respect it is of especial significance that an adequate purging of the tube 7 is effective during starting to inhibit entry of fuel-air mixture, since any such mixture which might be inadequately purged subsequently would be initially oxidized and then partially reduced as the environmental temperature increases to form a fine-particle deposit on, in particular the lens 8.
The air-supply source 31 may be an air-bottle or other reservoir that is mounted with the engine and is coupled to a pump to be replenished during engine running.
Sources of other gases could be used to provide the initial purging of the tube 7.
The construction of cable 10 involving the sheath 13 of woven fibre has advantage in relieving stress at the ferrules 18 and 21. Furthermore, the sheath 14 when tensioned acts as a braid tube in constricting the bundle 11 tightly. The cushioning of the sheathed bundle 11 provided by the sleeve 16 protects the glass fibres of the bundle 11 from shock and ensures good mechanical damping. The convoluted tubing 14 provides rigidity about its diameter to protect the fibre-bundle 11 from external pressures whilst it also enables the cable 10 to be flexed about its length.
Reference is directed to UK. patent application No. 5213/77 (Serial No.
1,589,532.
WHAT WE CLAIM IS:- 1. An optical device including a tubular passageway having an open front end, a sensor head positioned to extend a part-way along the rear of said passageway to define an annular recess between the passageway and the head, wherein said sensor head has an optical window towards its front end, said recess being vented into said passageway in front of the window, and wherein said device includes a first inlet for supplying air to said passageway at a point intermediate said sensor head and the open end of said passageway, and a second inlet for supplying air to said recess so that it vents into said passageway to the rear of said first inlet such that air supplied to said first and second inlets purges matter from the passageway that might otherwise cause fouling of the optical window.
2. An optical device according to Claim 1, wherein said sensor head has a plurality of apertures opening from said recess into said passageway in front of the window so that air supplied to said recess vents through said apertures into said passageway.
3. An optical device according to Claim 1 or 2, wherein said first inlet is a slot in the wall of said passageway, and wherein said slot is inclined to the axis of the passageway such as to direct a curtain of air between said window and said open end.
4. An optical device according to any one of the preceding claims, wherein said optical window is a lens.
5. An optical device according to any one of the preceding claims, wherein air is supplied to said first and second inlets from separate sources.
6. An optical device according to any one of the preceding claims for use in a gas-turbine engine, wherein said first inlet is arranged to receive air from the compressor stage of said engine.
7. An optical device according to any one of the preceding claims, wherein said second inlet is arranged to receive air from a pressurised air reservoir.
8. An optical device according toClaim 7, wherein air from said pressum
**WARNING** end of DESC field may overlap start of CLMS **.