GB1601218A - Combustion equipment for gas turbine engines - Google Patents

Description

(54} COBUSTI (54) COMBUSTION EQUIPMENT FOR GAS TURBINE ENGINES (71) We, ROLLS-ROYCE LIMITED, a British Company of 65 Buckingham Gate, London, SW1E 6AT, 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 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 emission such as nitrogen oxides.

The formation of nitrogen oxides is dependent upon a number of 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.

Nitrogen oxide emissions can be reduced therefore by lowering the combustion temperature, reducing the concentrations of nitrogen and oxygen in the mixture and/or varying the residence time of the combustion products in the combustion chamber.

One method is to introduce more air into the primary combustion zone as the pressure in the combustion equipment increases and it is an object of the present invention to reduce the nitrogen oxide emission from the combustion equipment of a gas turbine engine by this method.

Accordng to the present invention, combustion equipment for a gas turbine engine comprises a flame tube having a primary zone and a dilution zone, the flame tube being provided with primary duct means for the entry of air to the primary zone and dilution duct means for the entry of air to the dilution zone, obturating means being provided to control the effective area of the dilution duct means and to control the effective area of the primary duct means, said obturating means comprising valves adapted to control the flow of air into the dilution duct means and the primary duct means, said obturating means being provided to reduce the rate of flow of air through the dilution duct means and to increase the rate of flow of air through the primary duct means when the pressure of the air entering the combustion equipment exceeds a predetermined value.

Preferably the dilution duct and the primary duct means comprise holes.

The obturating meanis is adapted to reduce the effective area of the dilution holes when the pressure of air entering the combustion equipment exceeds a predetermined value.

The obturating means may comprise at least one valve which is adapted to move into and out of position adjacent to a dilution hole in which position the flow of air therethrough is reduced.

At least one of the dilution holes may be provided with a valve.

The combustion equipment may comprise a plurality of tubular flame tubes in which case a number of valves may be arranged substantially radially around each tube, or the combustion equipment may comprise a single annular flame tube with a number of valves arranged radially around the outer, the inner or both walls.

The obturating means may also comprise at least one valve movable into and out of a position adjacent to a primary hole in which position the flow of air therethrough is reduced.

The invention also comprises a gas turbine engine provided with combustion equipment as set forth above.

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 cut-away view of a gas turbine engine having combustion equipment according to the present invention, Figure 2 is a partial cross-sectional view of the combustion equipment of the engine shown in Figure 1.

In Figure 1 is shown a cut-away view of a gas turbine engine comprising an intake 10, a compressor 12, combustion equipment 14, a turbine 16, a jet pipe 18 and an exhaust nozzle 20. The engine is provided with an annular combustion chamber 22, a portion of which is shown enlarged in Figure 2.

The combustion chamber 22 consists of an outer annular wall 24 and an inner annular wall 26, and located between the walls, and spaced therefrom is an annular flame tube 28. The flame tube has a primary zone 30 and a dilution zone 32. Combustion takes place in the primary zone 30, a fuel and air mixture being injected into the primary zone from a burner 82 and further air for combustion being provided through primary air inlet 68 at the upstream end of the flame tube 28 and primary air holes 36 formed in the flame tube walls. Dilution air which cools the gases to a temperature which can be withstood by the turbine is provided through dilution air holes 38. It will be appreciated that since the flame tube is annular, a plurality of burners, primary air inlets, primary air holes and dilution air holes are provided, although only one of each are shown in the drawing.

Primary and dilution air holes are also provided in the inner wall of the flame tube (not shown).

Adjacent to each or a number of the dilution air holes is located a valve 40 which is adapted to be movable into and out of contact with the flame tube wall over the dilution hole 38. The valve 40 is shown in contact with the flame tube wall, but is spaced from it by a number of projections 42, conveniently three, so as to leave an annular gap between it and the holes 38. The valve is connected to a piston 44 which is urged by a spring 46 towards the closed end of a cylinder 48. The valve stem 50 is hollow and an orifice 52 connects the interior of the valve stem with the space between the flame tube 28 and the eombustion chamber wall 24. The pressure in the space 54 of the cylinder 48 is thus the same as the pressure in the space between the flame tube 28 and the combustion chamber wall 24.A restricted orifice 55 through the piston 44 connects the space 54 to a space 56 below the piston 44 and this pressure is communicated to a single pressure relief valve 58 serving all the dilution hole restriction valves 40, which consists of a spring urged piston 60 inside a cylinder 62. An orifice 64 connecting the interior of the pressure relief valve to atmosphere is provided in the wall of the valve 58.

A further valve 66 is provided at the upstream end of the flame tube 28, and is adapted to be movable into and out of contact with the flame tube wall over the primary air inlet 68 provided at the upstream end of the flame tube 28 by sliding over a tube 80 which provides uninterrupted flow of fuel to the fuel injector 82.

The valve 66 as shown is not in contact with the flame tube. The upstream end of the valve 66 is provided with suitable grooves 70 so that a smaller flow of air is permitted past the valve 66 when the valve 66 is in the position shown by the broken lines.

The pressure relief valve 58 is also connected via a duct 76 to a space 72 inside the valve 66 and a spring 74 positioned in side the valve 66 urges the valve 66 into contact with the flame tube wall over the primary air inlet 68.

One method of reducing nitrogen oxide emission from a combustion chamber is to increase the mass flow of primary zone air when the pressure in the combustion equipment increases. However, at idling speeds and start-up of the engine, maximum engine efficiency is achieved when the mixture in the primary zone is richer and there is a low primary zone air flow.

In operation therefore, when the engine is running at idling speed and the pressure of air entering the combustion equipment is low (say less than 100 p.s.i.), the valve 40 is arranged to be open, i.e. out of contact with the flame tube with the piston 44 urged to the end of the cylinder 48 by the spring 46, and valve 66 is arranged to be closed i.e. in contact with the flame tube with the valve 66 urged over primary air inlet 68 by the spring 74. In this state, air enters the combustion chamber, a portion mixing with the fuel in the burner 82, a portion entering the flame tube through the primary air inlet 68 and the grooves 70 on the valve 66 and the primary air holes 36 and a portion entering through the dilution air holes 38. The primary air flow is thus low and high engine efficiency is achieved.

When the pressure entering the combustion chamber rises (in this case to 100 p.s.i.) the pressure in the space 54 increases to typically 100 pss.i. and to typically 90 p.s.i. in the space 56. This pressure opens the pressure relief valve 58 permitting the piston 44 to move against the spring 46 and urging the valve 40 over the dilution hole 38 into the position shown in Figure 2, and permitting the valve 66 to move against the spring 74 due to the pressure acting on the upstream end of the valve 66 into the position shown in Figure 2. The dilution air flow is thus decreased and the primary zone air flow through the primary air inlet 68 and the holes 36 is increased by virtue of increased coefficient of discharge and pressure drop, thus reducing the nitrogen oxides emission. The valves 40 and 66 remain in these positions whilst the air pressure is 100 p.s.i. or above, but are retracted by the springs 46 and 74 respectively when the pressure drops below 100 p.s.i. due to the closing of the pressure relief valve 58, when the engine reverts to idling speed.

The arrangement in Figure 2 is particularly suitable for use in a combustion chamber having a plurality of tubular flame tubes where variation in coefficient of discharge of primary holes is difficult to achieve.

WHAT WE CLAIM IS: 1. Combustion equipment for a gas turbine engine comprising a flame tube having a primary zone and a dilution zone, the flame tube being provided with primary duct means for the entry of air to the primary zone and dilution duct means for the entry of air to the dilution zone, obturating means being provided to control the effective area of the dilution duct means and to control the effective area of the primary duct means, said obturating means comprising valves adapted to control the flow of air into the dilution duct means and the primary duct means, said obturating means being provided to reduce the rate of flow of air through the dilution duct means and to increase the rate of flow of air through the primary duct means when the pressure of the air entering the combustion equipment exceeds a predetermined value.

2. Combustion equipment as claimed in claim 1 in which the dilution duct means and the primary duct means comprises holes.

3. Combustion equipment as claimed in claim 2 in which the obturating means is adapted to reduce the effective area of the dilution holes when the pressure of the air entering the combustion equipment exceeds a predetermined value.

4. Combustion equipment as claimed in claim 3 in which the obturating means comprises at least one valve which is adapted to move into and out of a position adjacent to a dilution hole in which position the flow of air therethrough is reduced.

5. Combustion equipment as claimed in claim 4 in which at least one of the dilution holes is provided with a valve.

6. Combustion equipment as claimed in any preceding claim comprising a plurality of tubular flame tubes.

7. Combustion equipment as claimed in claim 6 in which a number of valves are arranged substantially radially around each tubular flame tube.

8. Combustion equipment as claimed in any of claims 1 to 5 in which said combustion equipment comprises an annular flame tube.

9. Combustion equipment as claimed in claim 7 in which a number of valves are arranged radially around the outer, the inner or both walls of the annular flame tube.

10. Combustion equipment as claimed in claim 2 in which the obturating means comprises at least one valve, movable into and out of a position adjacent to a primary hole, in which position the flow of air therethrough is reduced.

11. A gas turbine engine having combustion equipment as claimed in any preceding

Claims (1)

1. claim.

   12. Combustion equipment for a gas turbine engine constructed and adapted to operate substantially as hereinbefore described with reference to Figure 1 and Figure 2 of the accompanying drawings.