Combustor for a gas turbine engine

A combustor in a gas turbine engine has a wall which forms a gap with an outer nozzle guide vane platform. A closure mechanism is operable to open or close the gap. Opening of the gap, for example during cruise conditions, reduces the mass flow rate of air through an open end of the combustor, so enriching the air fuel ratio and improving flame stability and combustion efficiency. Under high power conditions, the gap is closed, causing an increase in air flow rate through the open end to achieve a lean burn air fuel ratio when the fuel flow rate is increased.

BACKGROUND

This invention relates to a combustor for a gas turbine engine, and particularly, although not exclusively, for a gas turbine aeroengine.

The thrust generated by a gas turbine engine is modulated by varying the flow of fuel to the combustor or combustors. Efficient combustion requires the air/fuel ratio of the air/fuel mixture to be maintained within close limits. Efficient combustion is desirable both because it minimises undesirable emissions, such as NOx and CO emissions, and because it improves specific fuel consumption (SFC).

It is known to provide air flow restricting mechanisms at the combustor inlet so as to vary the quantity of air available for mixture with the fuel. However, such mechanisms can be complex and are not suited to operation in the hostile environment which exists at the combustor inlet.

US 2005/095542 discloses a gas turbine engine combustor having a variable-geometry air inlet for supplying air to a pre-mixing zone of the combustor. US 2005/095542 also discloses dilution ports in a liner of the combustor, which ports have adjustable flow areas controlled by valves.

Since the dilution ports are at separate locations around the combustor liner, the air flow admitted through them to the combustor is not distributed evenly, and consequently temperature differentials around the axis of the combustor can arise owing to the introduction of low temperature air at discrete positions around the axis of the combustor.

SUMMARY

According to the present invention there is provided a combustor for a gas turbine engine, the combustor comprising a housing having a circumferential wall defining a circumferential gap which provides communication between the interior and the exterior of the housing, an annular closure mechanism being provided which is displaceable into and out of a closing relationship with the gap.

In the context of this specification, the expression “circumferential gap” is to be interpreted as meaning that the gap extends continuously, or substantially continuously, around the entire circumference of the circumferential wall.

The closure mechanism may comprise an actuator device which is displaceable radially with respect to an axis of the housing. The actuator device may comprise an array of arcuate actuator elements which extends circumferentially of the housing. The actuator elements may be displaceable by a common actuator. Adjacent circumferential edges of adjacent ones of the actuator elements may engage one another in a sealing manner.

An annular sealing element may be disposed between the housing and the closure mechanism. The sealing element may comprise an annular leaf spring.

The sealing element may comprise a retaining portion which is secured with respect to the housing, and a sealing lip which is displaceable relatively to the retaining portion for engagement with the closure mechanism. The sealing element may be disposed so that a pressure difference between the interior and the exterior of the housing biases the sealing lip towards the closure mechanism.

The sealing element may be disposed on one side of the gap, and a second sealing element may be disposed on the other side of the gap for sealing engagement with the closure mechanism throughout the full range of movement of the closure mechanism. In an alterative embodiment, the sealing element may be mounted on one side of the gap, for engagement with the other side of the gap under the action of the closure mechanism. The closure mechanism may engage the sealing element at a position nearer to the retaining portion than the sealing portion, to provide a mechanical advantage between the displacement of the actuator device and the sealing portion.

The closure mechanism may be controlled by modulating means to vary the flow area of the gap between a fully closed and a fully open position.

DETAIL DESCRIPTION

The combustor2shown inFIG. 1is an annular combustion chamber centred on the axis X of the engine. The axis X is shown only for purposes of reference inFIG. 1; in fact it is situated further than shown from the section of the combustion chamber2visible inFIG. 1.

The combustor2is situated downstream of the high pressure (HP) compressor of the engine. High pressure air issuing from the HP compressor is discharged into a region4of the engine which surrounds the annular combustor2. The forward end6of the combustor2is open to allow air from the region4to enter the interior8of the combustor2. A fuel delivery device8having a spray nozzle9is provided for delivering fuel to the interior of the combustor2.

The aft end of the combustor2opens at an array of nozzle guide vanes10at the entry to the turbine section of the engine. The nozzle guide vanes10extend between inner and outer annular vane platforms12,14. The combustor2comprises inner and outer annular walls16,18which terminate respectively at the inner and outer vane platforms12,14.

As shown inFIG. 2, the outer wall18of the combustor2terminates short of the outer vane platform14to leave a gap20.

The outer wall18terminates at a flange22having a rim24. The outer surface of the rim24carries a sealing element26in the form of an annular leaf spring. The leaf spring26is secured at one edge, to the outer surface of the rim24, so that the edge constitute a retaining portion of the leaf spring26. The leaf spring26extends obliquely away from the rim24in the forwards direction, and terminates at a sealing lip28.

As shown inFIG. 4, the leaf spring26is surrounded on its outside by a closure mechanism comprising an actuator device formed from an array of arcuate actuator elements30. Each actuator element30has an operating member32, and the operating members32are controlled by a common control mechanism to displace the actuator elements radially inwardly and outwardly in unison. Adjacent actuator elements30engage one another at an overlapping joint34, which provides a seal between the actuator elements30.

On the other side of the gap20from the flange22, the outer vane platform14is provided with a circumferential rib36which supports a second annular leaf seal38. The second leaf seal28is supported on the rib36by a series of pins40, and is biased into contact with the aft edges of the actuator elements30.

In the configuration shown inFIG. 2, the actuator elements30are displaced radially inwards, into a closing relationship with the gap20, in which they contact the sealing lip28of the leaf spring26. This prevents air from flowing between the rim24and the actuator elements30to the gap20. On the other side of the gap from the leaf spring26, the second leaf spring38provides a further seal between the region4and the gap20. Consequently, no flow can take place through the gap20to the interior of the combustor2from the outside region4.

In the configuration shown inFIG. 3, the actuator elements30are displaced radially outwardly from the closing relationship, and so lose contact with the sealing lip28of the leaf spring26. The second leaf spring38, however, remains in contact with the aft edges of the actuator elements30, so that flow past the second leaf spring38to the gap20is prevented. In the condition shown inFIG. 3, flow can take place, as indicated by an arrow F, from the region4through the gap20to the interior of the combustor2.

In operation of the combustor2, a proportion of the total output of the HP compressor flows from the region4through the open end6of the combustor to be mixed with fuel issuing from the spray nozzle9. This mixing occurs in a pre-mixing zone40within the combustor, and the air-fuel mixture is initially ignited by means of a spark to create a flame which is subsequently self-sustaining. Combustion continues in a reaction zone42in which secondary air is admitted through fixed apertures in the walls16,18. The secondary air mixes with the combustion products in a dilution zone44so as to cool the combustion products before they travel past the nozzle guide vanes10to the turbine section of the engine.

In a typical engine with the gap20closed by the actuator elements30, approximately 70% of the flow from the HP compressor into the region4will flow through the open end6of the combustor2, with the remainder of the air from the region4flowing into the combustor2through the apertures in the walls16,18.

During high power operation of the engine, for example during take-off and climb, the gap20is closed by the actuator elements30, which assume the position shown inFIG. 2. In this condition, the pressure drop from the region4to the interior of the combustor2will act on the leaf spring26to urge the sealing lip28into contact with the actuator elements30. With the gap20closed, maximum flow takes place through the open forward end6of the combustor2, to achieve the desired air/fuel ratio with the increased fuel delivery required for high power operation. Under cruise conditions, when the fuel flow rate is reduced, the actuator elements30are displaced radially outwardly, as shown inFIG. 3, to admit a proportion of the air from the region4to the interior of the combustor2immediately upstream of the nozzle guide vanes10. This reduces the proportion of the air from the region4flowing through the open end6of the combustor2, so as to match the reduced fuel flow rate.

In practice, it is usually desirable for the combustion process to take place with a relatively lean air fuel ratio under high power conditions, and with a relatively rich air fuel ratio under low power conditions. For example, when the actuators30are displaced to allow the flow F through the gap20, between 5 and 15% of the HP compressor exit mass flow may pass through the gap20. If 12% of the compressor exit mass flow passes through the gap20, the pressure drop between the compressor outlet (conventionally referred to as P30) and the combustor interior may be reduced by 25% from a value of, for example, 4.5% of P30to 3.6% of P30. The reduced air flow through the open end6of the combustor2enriches the fuel air mixture by approximately 12%, and this raises the flame temperature, so improving the stability of the flame and realising an improvement in combustion efficiency.

Furthermore, the distribution of pressure drop for the air entering the combustor through the gap20can be arranged so that the flow velocity can be optimised to the performance requirements of the nozzle guide vanes10.

The reduced fuel injector air flow (i.e. air flow through the open end6of the combustor2) not only increases the flame temperature, but also increases the combustion residence time, so promoting combustion efficiency. The ability to modulate the fuel injector air flow by opening and closing the gap20to the flow F means that combustion efficiency, and low emissions, can be achieved both under high power operation of the engine and during cruise at relatively low power. Consequently, both low NOx at high power, lean burn operation and low CO emissions under cruise conditions can be achieved.

Additionally, the ability to vary the mass flow rate of air through the open end6of the combustor by opening and closing the gap20enables a turndown ratio (the ratio between maximum and minimum fuel flow rates) of 6 to 8 to be achieved while maintaining desired emission and SFC levels.

Furthermore, the reduction in combustion pressure drop from 4.5% P30to 3.6% P30, i.e. a pressure drop difference of 0.9%, yields an improvement in SFC of, for example, 0.2%.

A further advantage of the ability to admit air to the combustor through the gap20is improved relighting of the engine following flame out at altitude. An engine configured for lean burn can be difficult to relight at altitude, owing to the high combustor loading (i.e. the high flow rate of air through the open end6of the combustor2). By opening the gap20to the flow F, the air flow velocity at the spray nozzle9is reduced, which can increase the altitude at which relighting can occur by, for example, approximately 600 metres.

FIG. 5shows an alternative embodiment in which the leaf springs26and38are replaced by a single leaf spring26which extends across the gap20. Thus, the leaf spring26is mounted at a retaining portion46to the rib36and extends across the gap20to a sealing lip28. In the condition shown inFIG. 5, the sealing lip28is spaced from the upper end of the flange22, allowing the flow F to pass through the gap20. An actuator device48comprising a circumferential array of pins (only one shown inFIG. 5), can be displaced radially inwardly so as to engage the leaf spring28until the sealing lip26contacts the outer end of the flange22, so closing the gap20.

By positioning the actuator pins48nearer to the retaining portion46than the lip28, a mechanical advantage can be achieved, so that a relatively small displacement of the pins48will be adequate to achieve a relatively large displacement of the sealing lip28.

In either of the embodiments ofFIGS. 2 to 4orFIG. 5, the motion of the actuator device comprising the actuator elements30and the pins48may be modulated in accordance with the operating condition of the engine, so that the NOx/CO trade at cruise conditions can be optimised across a range of cruise conditions.

It is also possible to control the radial temperature traverse into the turbine stage of the engine, by adjusting the mass flow rate of leakage air through the gap20.