SPECIFICATIONImprovements in or Relating to CombustionEquipment for Gas Turbine EnginesThis invention relates to combustion equipment for gas turbine engines and it is an object of the present invention to provide combustion equipment which will produce reduced quantities of objectionable exhaust emissions, such as nitrogen oxides.
The formation of nitrogen oxides is dependent upon a number of inter-related factors, including the temperature of combustion (the higher the temperature, the more nitrogen oxides are produced), the concentrations of nitrogen and oxygen in the fuel/air mixture, and the residence time of the combustion products in the combustion chamber. In the case of the residence time, low nitrogen oxide emissions can be achieved by having a short residence time with efficient combustion or by having a long residence time with less efficient combustion so that the temperature is maintained at a low value and is insufficient for significant quantities of nitrogen oxides to be formed.
Over the normal operating range of a gas turbine engine considerable variations occur within the combustion equipment including air and fuel flow rates, pressures and temperatures and thus it is very difficult to reduce the formation of nitrogen oxides at different engine speeds.
It is an object of the present invention therefore to provide combustion equipment for a gas turbine engine which will reduce the formation of nitrogen oxide emissions at different engine speeds.
In our prior Patent No. 1,427,146 a tubular primary intake containing a fuel injector is provided in the upstream wall of the flame tube.
An end cap is located at the downstream end of the tubular intake, to define an annular radially directed gap between it and the end of the tubular intake. This gap directs the fuel/air mixture radially into the flame tube creating a first toroidal vortex upstream of the gap, and a second toroidal vortex of opposite hand downstream of the gap.
This arrangement has the ability to achieve high combustion efficiencies at ground idling engine speeds without detriment to high speed performance.
According to an aspect of the present invention, combustion equipment for gas turbine engine comprises a primary air intake consisting of a tubular duct extending into the upstream end of a flame tube of the engine, a fuel injector located in the tubular duct, means for deflecting the fuel/air mixture into a radial flow from the tubular duct into the flame tube whereby first and second toroidal vortices are formed in the flame tube, the first being substantially upstream of the radial flow, and the second being substantially downstream of the radial flow and means for varying the size of the first toroidal vortex in accordance with engine speed, the size of the toroidal vortex being reduced with an increase in engine speed and increased with a decrease in engine speed.
Enlarging the first toroidal vortex at low speed enhances combustion efficiency by reducing gas velocities with a corresponding increased residence time, whilst reducing the first torodial vortex at high speeds reduces the mechanical risks due to high temperatures adjacent to the upstream wall of the flame tube.
The means for varying the size of the first toroidal vortex may comprise means for moving the tubular duct axially relative to the upstream end of the flame tube, the tubular duct projecting further into the flame tube at lower engine speeds.
Alternatively means for varying the fuel/air mixture deflecting means may be provided to vary the volume of the radial flow from the tubular duct.
Preferably the means for deflecting the fuel/air mixture into a radial flow comprises an end cap located at the downstream end of the tubular duct to define an annular radially directed gap between it and the end of the tubular duct.
According to a further aspect of the invention means for varying the flow of primary air into the flame tube is provided comprising a variable area annular duct surrounding the tubular duct. The rate of flow of air through the annular duct may also be varied in accordance with engine speed, the rate increasing with increase of engine speed, whereby to weaken the fuel/air mixture in the first toroidal vortex.
Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a partly sectioned side elevation of a gas turbine engine provided with combustion equipment in accordance with the invention;Figure 2 is a part cross-sectional view of the combustion equipment described and claimed inBritish Patent No.1,427,146, Figures 3 and 4 are cross-sectional views of combustion equipment according to the present invention in two different modes of operation,Figures 5 and 6 are cross-sectional views of another embodiment of the combustion equipment in two different modes of operation; andFigures 7 and 8 are cross-sectional views of a further embodiment of the combustion equipment in two different modes of operation.
With reference to Fig. 1, a gas turbine engine generally indicated at 10 comprises, in axial flow series, an air intake 1 compressor means 12, combustion apparatus 13, turbine means 14 and an exhaust nozzle 1 5. Combustion apparatus 13 comprises a plurality of separate, substantially cylindrical combustion chambers, one of which can be seen at 16, circumferentially mounted around the axis of the engine 10 in the manner which is generally described as a "cannular" or "Tubo-annular" array.
The upstream portion of a combustion chamber 1 6 constructed in accordance with our prior patent No. 1,427,146 is shown in Figure 2 This combustion chamber is provided with a combustor head 20, in the centre of which is located a fuel burner 22. The fuel burner 22 consists basically of a burner tube 24, at the upstream end of which is provided a fuel injector 31. At downstream end of the burner tube 24 is located a deflecting member or end cap 40 which defines a radially directed annular gap 41 between itself and the downstream end of the burner tube 24. An additional primary inlet consisting of a series of holes 26 and a shroud member 23 surrounding the burner tube 24 is provided to admit further primary air and to cool the burner tube 24.
In operation of the engine the air/fuel mixture issues radially of an annular gap 41 in the end of the burner 22 in the direction of arrows 99 to create a first toroidal vortex 100. This vortex is ignited, and burning air/fuel mixture is entrained in a second toroidal vortex 200 which 5 generated in part by the flow from the burner 22 and in part by dilution air entering the chamber 1 6. The burning mixture passes from the second vortex in a generally downstream direction to the downstream exit of the combustion chamber.
This arrangement has ability to achieve high combustion efficiencies at ground idling without detriment to high engine speed performance, although nitrogen oxide emissions are still produced.
By moving the burner axially relative to thecombustor head 20 (with or without the shroud 23), the size of the first toroidal vortex can be varied. Enlarging the first toroidal vortex 100 at low speed enhances combustion efficiency at low speed (by reducing velocities and increasing residence time) whilst reducing the first toroidal vortex at high engine speeds reduces the mechanical risks due to high temperatures adjacent to the combustor head 20.
Alternatively the end cap can be moved axially to vary the size of the gap 41 to permit a limited amount of variation in the flow of fuel/air mixture through the burner tube 24. Thus some adjustment of the first toroidal vortex is achieved.
Figures 3 and 4 illustrate substantially the same combustion chamber, but in the case the burner tube 24 the air shroud 23 and the end cap 37 are movably mounted relative to the combustor head 20. The burner 22 is supported on an axially movable tube 102 which in fact forms an upstream extension of the air shroud 23.
The fuel injector 31 does not move, and so an elongate hole 104 is provided in the tube 102 to permit movement thereof. Further holes 106 are provided to permit air to pass from the interior of the tube to the combustor head 20. The combustor head 20 is provided with a conical shroud member 108, the downstream end of which is adapted to be in sealing engagement with the air shroud 23 when the tube 102 is in its upstream position as shown in Figure 3.
As the tube 102 is moved in an upstream direction carrying the burner tube 24, the end cap 37 and the air shroud 23 with it, an annular gap is formed between the air shroud 23 and the conical shroud 108 permitting air to enter the combustion chamber from the tube 102 (SeeFigure 4) through holes 110. This extra air simultaneousiy provides more protection from heat for the burner tube 24 and weakens the fuel/air mixture in the first toroidal vortex 100 and reduces smoke formation.
Thus at low engine speeds the first toroidal vortex 100 is large and is enriched with fuel to improve low speed combustion efficiency and because of cooler inlet temperatures at low engine speeds, the burner 22 is not subjected to extreme condition and very little cooling is required.
In the embodiment shown in Figures 5 and 6 the air shroud 23 is extended radially at its upstream end and the combustor head 20 is provided with a much larger hole through which the burner 22 projects. At low engine speeds (Figure 5) the air shroud 23 extension abuts the combustor head 20 and at higher engine speeds the burner tube 23, the end cap 37 and the air shroud 23 move together in an upstream direction, a small distance (Figure 6). An annular gap 112 is formed between the combustor head 20 and the air shroud 23 to permit air to flow into the combustion chamber in the same direction as the primary toroidal vortex 100, and this air simultaneously cools the combustor head and weakens the mixture in the vortex 100 as well as the vortex 100 being reduced in size.
In the embodiment shown in Figures 7 and 8 the burner tube 24 and the air shroud 23 only are movable together, the end cap 37 being fixed. A conical shroud member 108 as in the embodiment shown in Figures 3 and 4 is. provided sq that as the burner tube 24 and the air shroud 23 are moved in an upstream direction the annular gap 41 is increased in size and more primary air Is admitted between the gap formed between the conical shroud and the air shroud 23. Thus with an increase in engine speed the first vortex 100 is weakened and at low engine speeds the first vortex 100 is enriched without variation in its volume.
The methods of moving the movable parts have not been described, but any suitable method could be used such as hydraulic or pneumatic rams mechanically connected to the tube 102 in the embodiments shown in Figures 3 and 4. A similar tube is used in Figures 7 and 8 whereas the air shroud 23 is shown as supported by a series of links in the embodiment shown inFigures 5 and 6. One or more of these links could be acted upon by a further mechanical connection to suitable rams. Such linkages of course must be capable of withstanding the high temperatures achieved in the combustion equipment.
Whilst the burner parts are moved in relation to engine speed, the relationship can be indirect, such as in accordance with fuel, pressure, compressor delivery pressure or temperature-ore combination of these parameters, all of which are to a greater or lesser degree engine speed responsive.
To avoid the possibility of weak mixtures causing flame extinction in these embodiments, such as by a rapid deceleration causing a sudden drop in fuel flow, but a slower reaction of the movable parts, the end cap 37 movement could be controlled independently of the burner tube 24 in accordance with fuel pressure. Thus a reduction in fuel flow would move the end cap 37 to reduce the size of the gap 41 causing local enrichment of the fuel/air mixture to avoid weak extinction, whilst the primary air flow would reduce at a slower rate to protect the combustor head and the burner from overheating.