GB2244098A - Variable configuration gas turbine engine - Google Patents

Abstract

A gas turbine engine suitable for supersonic aircraft uses a variable configuration to reduce jet noise at take-off without compromising engine efficiency at high cruise speeds. In the take-off configuration, supplementary air intakes (21) are opened to admit ambient air into bypass duct (20). The combined flow of bypass air and entrained ambient air is mixed with the engine core flow at the upstream end of jet pipe (30) to exhaust through a large common nozzle (32). Opening and closure of the intakes (21) is linked to the control of variable area nozzles (17, 22 and 32) such that nozzles (17 and 22) are reduced in area when intakes (21) are open whilst nozzle (32) is opened. This promotes efficient mixing as far upstream as possible, thereby attenuating the exhaust jet velocity and thus reducing its noise-generating capacity. In cruise operation the engine adopts the configuration of a conventional low bypass ratio engine: intakes (21) are closed, nozzles (17 and 22) are opened and nozzle (32) is reduced in area.

Description

VARIABLE CONFIGURATION GAS TURBINE ENGINE FOR SUPERSONIC AIRCRAFT This invention relates to a gas turbine engine suitable for supersonic aircraft and in particular to an engine having a variable configuration which permits a low-noise operating mode to be selected for the purposes of take-off, climb and overland flight.

Currently, there is a great deal of interest in the development of supersonic transport aircraft as possible replacements for both wide-bodied long-haul types and also the present generation of supersonic transports which utilise after-burning engines. It is recognised that future supersonic aircraft will need engines which exhibit high efficiency in the high speed cruise condition whilst meeting strict airport noise regulations as well as regulations governing flight overland. In addition, economic factors such as aircraft range and payload will play an important part in future designs.

The present invention is concerned with an engine having a variable configuration which offers a good compromise between high and low speed characteristics, ie high efficiency at high aircract speeds, when noise emission is not of prime importance and low noise at low aircraft speeds, when reduced efficiency can be tolerated. In order to achieve a cruise speed of around Mach 2 to 2.5 in an aircraft carrying 250 passengers over a range of between 5000 and 6500 nautical miles, economic considerations such as fuel consumption dictate the use of low by-pass ratio engines. However, the disadvantages of such engines are that they operate less efficiently at low aircraft speeds and also that they are noisy. The major sources of noise in gas turbine engines are the compressor stages, the turbine section and the jet efflux.In some engine designs, notably pure jet engines and low by-pass ratio types, the jet efflux is the dominant source of noise.

This is because noise is generated by turbulence created externally of the engine casing when fast-moving gases in the exhaust jet mix with ambient air. In engines having a low by-pass ratio this problem is exacerbated at low aircraft speeds because of the large velocity difference between the jet efflux and the surrounding air.

Under take-off conditions, the perceived engine would produce a very high jet velocity of around 800 metres per second. However, at a typical take-off speed of Mach 0.3, current international airport regulations require that the engine noise should be limited to a value which equates to a jet velocity of around 450 metres per second. Thus it is dear that measures are required to reduce in some way the noise which the engine would otherwise produce.

One way of overcoming this problem is to employ an engine system which allows different operating modes to be selected to suit the flight circumstances. A number of variable configuration engines have been proposed, but to date these suggestions have all suffered from the drawback of increased complexity, usually accompanied by extra weight which it is preferable to avoid. The present invention is also an engine with variable configuration, but is of a type which uses essentially unmodified turbomachinery. It has very little in the way of extra components, thus keeping additional weight to a minimum. The invention therefore presents an opportunity for reducing development costs through the use of a core portion from an existing engine design.

Conventional techniques for aircraft engine noise reduction often impose penalties on the performance of the aircraft which must be borne throughout the operating regime of the aircraft even though the benefit of reduced noise is required for only a small portion of that regime. This occurs for example, when shrouds equipped with sounddeadening material are used to extend the engine casing. Although such shrouds are effective in reducing noise, they incur weight and drag penalties through the entire flight envelope of the aircraft.

It has previously been proposed to reduce exhaust jet noise by variously increasing the contact area between the atmosphere and the emerging gas stream. For example, by using a corrugated nozzle, atmospheric air can be caused to flow along the external corrugations and into the exhaust jet to promote rapid mixing. Alternatively, using a lobe-type nozzle, the exhaust gases can be divided into a number of separate exhaust jets which issue through a series of lobes and a small central nozzle. The resulting increase in the surface area of the exhaust jet facilitates rapid mixing with the ambient air entrained by the lobes. Unfortunately, nozzle design alone is insufficient to give the degree of noise reduction required for the type of engine contemplated in future supersonic transport aircraft.

In our earlier UK Patent No 1 409 887 a modified jet pipe is described which acts as a noise shield in selected operating modes of the engine. The jet pipe is formed with the usual primary nozzle at its downstream end but is additionally provided with a plurality of moveable members located immediately aft of the turbine assembly.

These members are operable between a first position, where they sit retracted from the gas flow through the pipe, and a second position where they extend into the gas flow to create a secondary nozle. The secondary nozzle is of smaller diameter than the primary nozzle and its configuration is such that exhaust gases passing through it are divided into a number of gas streams. The jet pipe downstream of this secondary nozzle then acts as a noise shield. This method, too, would be inadequate to achieve the necessary reduction in noise required in the engine proposed for supersonic flight.

It is also known that noise can be reduced by throttling back engines during climb after take-off. Whilst this procedure can be effective in reducing the noise at some distance from the airport, it clearly does not alleviate noise during take-off itself, nor during the initial phase of climb.

It is therefore an aim of the present invention to provide a gas turbine engine suitable for supersonic aircraft which is capable of operating with reduced noise emission during take-off, climb and overland flight whilst avoiding the drawbacks and limitations of prior art engines.

The invention is a variable configuration gas turbine engine suitable for supersonic aircraft comprising: a low by-pass ratio turbo-jet engine of the type wherein air from the engine by-pass flow passes through a by-pass duct and recombines with air from the engine core flow to exhaust through a single jet pipe; an outer casing. for the engine, and occludable supplementary air intakes leading from the outer casing to openings in the by-pass duct said openings being configured such that ambient air is drawn into the by-pass duct by the by-pass flow even when the aircraft is stationary or moving at low speeds, in order to dilute the exhaust jet and attenuate the velocity of gases emitted from the jet pipe.

An engine constructed according to the invention allows a suitable operating configuration to be selected to match the particular flight circumstances. In the take-off or "quiet" configuration the engine is operated with its supplementary air intakes open to draw extra, ambient air into the engine by-pass stream. This boosted by-pass flow is caused to mix with the air from the engine core flow, thereby diluting it so that the exit velocity of the exhaust jet is reduced. The lower velocity of the exhaust jet creates less intense turbulence externally of the engine and thus less noise is generated. In its cruise configuration the engine is operated with the supplementary air intakes closed so that it performs as a normal low by-pass ratio turbo-jet engine. Engine core flow and engine by-pass flow are exhausted through a common jet pipe but no special provision need be made to promote their mixing, nor to reduce the noise which the engine generates under these operating conditions.

In a preferred form the jet pipe is equipped at its downstream end with a variable-area convergent-divergent nozzle. This is opened up when the engine operates in its take-off configuration in order to minimise the difference between ambient pressure and the static pressure in the jet pipe.

In an especially preferred form the engine is provided with an extensible/retractable nozzle for the engine core flow. This core nozzle is located at the upstream end of the jet pipe in the region where the core and by-pass flows recombine. Advantageously, it is a diffusing nozzle and may have a corrugated perimeter or may include holes or tubes for assisting in the division of the core flow into a jet of increased periphery so as to promote efficient mixing of gases in the jet pipe. The core nozzle is retracted for operation of the engine in its cruise configuration, when efficient mixing of core and by-pass flows is less important. Adjustment of the core nozzle position may also serve as a means of controlling the engine mix-plane conditions.

The supplementary air intakes may take the form of plain openings in the engine outer casing which are suitably configured to ensure entrainment. These communicate with the by-pass duct by means of passages which have inlets into the by-pass duct downstream of the fan assembly. When the supplementary air intakes are open, air is entrained into the by-pass duct by the by-pass flow but in normal flight, ie cruise configuration, they are kept dosed. Alternatively, scoops can be used which protrude from the engine outer casing in order to assist the intake of ambient air for boosting the by-pass flow. These, too, have associated passages which issue into the by-pass duct through inlets located downstream of the fan assembly.

The inlets into the by-pass duct are disposed such that ambient air can be drawn in by the by-pass flow, but they are configured to discourage flow of by-pass air to the exterior of the engine via the supplementary air intakes. This may be achieved, for example, by positioning the inlets such that they point in a generally downstream direction. An extensible/retractable supplementary nozzle is provided in the region where the supplementary air intakes discharge into the by-pass duct. This supplementary nozzle is extended when the supplementary air intakes are open in order to reduce the static pressure of the by-pass flow to a value near to that of the ambient air. Advantageously, the supplementary nozzle is a diffusing nozzle having a convoluted periphery which promotes mixing between the by-pass airflow and the entrained ambient air.When the supplementary air intakes are dosed the supplementary nozzle is retracted.

In order for the engine to operate efficiently at high aircraft speeds it is connected to a variable-geometry inlet duct. This is used to control the air velocity between the mouth of the inlet duct and the forward compressor section, since air entering the compressor must usually be slowed to subsonic velocity. As the aircraft approaches the speed of sound, shock waves develop which can give rise to high duct losses in both pressure and air flow if they are. not controlled. Poor air pressure and velocity distribution can lead to compressor stall. Various techniques are known for varying the inlet duct geometry, but since they do not form part of the present invention they will not be described in detail here.

The invention will now be described by way of example only with reference to the drawing. The upper half of the drawing is a section through an engine constructed according to the invention shown in its take-off configuration, whilst the lower half shows a section through the same engine in its cruise configuration.

At its left hand end as shown, the engine is provided with a variable-geometry inlet duct 10, details of which have been omitted from the drawing for the reasons given above. Air from the inlet duct is delivered at uniform pressure and velocity to the low pressure compressor or fan assembly 11 and is then divided into separate core and by-pass streams. The air in the by-pass stream passes downstream along annular by-pass duct 20 and is reunited with the core flow at the upstream end of jet pipe 30 in a manner to be described in more detail below. The air in the core stream passes through high pressure compressor 12, combustor array 13 and turbine assembly 14. At the downstream end of turbine assembly 14 is a bullet 16 which aids smooth diffusion of gas flow into the jet pipe 30.An intermediate portion of the jet pipe 30 is provided with sound-absorbing linings 31 which assist in absorbing some of the noise generated inside the engine casing. At its downstream end the jet pipe 30 is equipped with a variable-area convergent-divergent primary nozzle 32.

Referring now to the upper half of the drawing, this depicts the engine in its take-off configuration. At take-off, supplementary air intakes 21 are opened to admit ambient air into the by-pass duct 20.

These supplementary air intakes discharge into the by-pass duct 20 downstream of the fan assembly 11 and their operation is linked to that of a supplementary nozzle 22. When the supplementary air intakes are open, the supplementary nozzle 22 is extended into the by-pass duct to reduce the static pressure of the by-pass flow to a value near to that of the ambient air. This combined or boosted flow passes downstream to the end of the by-pass duct 20 and issues into the jet pipe immediately aft of the turbine assembly 14. In practice it is desirable to increase the engine by-pass flow with an approximately equal mass flow of entrained ambient air.

Still referring to the upper half of the drawing, an extensible/ retractable core nozzle 17 is provided at the point of entry of the by-pass duct 20 into the jet pipe 30. As shown, this nozzle is extended in take-off configuration to reduce the static pressure of the core flow and to encourage mixing between the core flow and the combined or boosted flow. The core nozzle is preferably a diffusing nozzle of convoluted shape and may have surface features such as holes, tubes or lobes to promote division of the core flow into a number of smaller streams or to create a stream of greatly increased periphery.The core nozzle thus aids mixing of the gas streams within the jet pipe 30 so that the noise which arises from any resulting turbulence is attenuated by the sound-absorbing linings 31: Moreover, because the exhaust jet which emerges from the primary nozzle 32 is diluted with low velocity air, its aggregate velocity is sufficiently low that turbulence occurring externally of the engine casing is reduced to a level such that the generated jet noise is within acceptable limits.

The upper half of the drawing also shows that the primary nozzle 32 is opened up to a relatively large area in order to minimise the difference between the static pressure in the jet pipe and the ambient pressure external to the engine.

Thus, for quiet operation, the exemplified engine requires the following conditions to be satisfied: i Supplementary air intakes open; ii Supplementary nozzle extended; iii Core nozzle extended, and iv Primary nozzle retracted.

In the bottom half of the drawing the engine is shown in its contrasting configuration for cruise operation, in which the features listed above assume their opposite modes. Accordingly, the primary nozzle 32 is extended to define a convergent-divergent passage in which the cross-sectional area of throat 33 is relatively small compared to the throat area in the engine at take-off. The reduction in throat area increases the pressure difference across the primary nozzle and has the effect of augmenting thrust. The core nozzle 17 is retracted so that the airflows pass from the fan 11 and from the turbine 14 to the throat 33 with minimal losses. The precise position of the core nozzle 17 may be varied in order to balance the conditions under which the core and by-pass flows meet.At cruise, the supplementary nozzle 22 is retracted because the supplementary air intakes are dosed and there is no need to balance the by-pass flow against boost air.

Unlike some prior art engines which attempt to control noise by employing an ancillary shroud attached to the jet pipe, the present invention works by shifting the mixing zone as far upstream as possible, both for take-off and during subsequent flight when quiet operation is required. Effectively, this is achieved by means of the core nozzle 17 and the supplementary nozzle 22 which together assume the role of throat 33 when the primary nozzle 32 is retracted. This has the effect of making the jet pipe 30 a large integral shroud which does not have to be moveable. No weight or drag penalties are incurred when the engine is modified to its cruise configuration because in these circumstances the jet pipe resumes its normal function.

The invention may therefore be regarded as combining two different engines within a single housing in such a way that additional weight and complication are kept to a minimum.

Although an engine constructed according to the invention is relatively quiet in the take-off configuration, it must be recognised that an engine which operates with a high by-pass ratio is likely to be more fuel efficient at low aircraft speeds.

Engines according to the present invention are primarily intended to work without after-burning. However, for certain aircraft designs it may be desirable to provide this capability in order to overcome the effects of transonic drag. Obviously it is preferable to omit after-burners if possible because they increase the weight and complexity of the engine for its entire operating cycle, even though after-burning is required for only a brief period as the aircraft passes through the sound barrier. Nevertheless, the perceived engine has a relatively long jet pipe which does make it suitable for the installation of an after-burning system if desired.

Claims (8)

1. A variable configuration gas turbine engine suitable for supersonic aircraft comprising: a low by-pass ratio turbo-jet engine of the type wherein air from the engine by-pass flow passes through a by-pass duct and recombines with air from the engine core flow to exhaust through a single jet pipe; an outer casing for the engine, and occludable supplementary air intakes leading from the outer casing to openings in the by-pass duct said openings being configured such that ambient air is drawn into the by-pass duct by the by-pass flow even when the aircraft is stationary or moving at low speeds, in order to dilute the exhaust jet and attenuate the velocity of gases emitted from the jet pipe.

2. A variable configuration gas turbine engine as claimed in claim 1 wherein an extensiblelretractable core nozzle is provided in the region where core and by-pass flows recombine at the upstream end of the jet pipe, such that when the supplementary air intakes are open the core nozzle is extended to promote mixing within the jet pipe between the core flow and the by-pass flow.

3. A variable configuration gas turbine engine as claimed in claim 2 wherein the core nozzle is a diffusing nozzle.

4. A variable configuration gas turbine engine as claimed in any preceding claim wherein an extensible/retractable supplementary nozzle is provided in the region where the supplementary air intakes admit ambient air into the by-pass duct.

5. A variable configuration gas turbine engine as claimed in claim 4 wherein the supplementary nozzle is a diffusing nozzle.

6. A variable configuration gas turbine engine as claimed in any preceding claim wherein the supplementary air intakes are scoops which are operable to protrude from the engine outer casing to assist the intake of ambient air for boosting the by-pass flow.

7. A variable configuration gas turbine engine as claimed in any preceding claim wherein the jet pipe is provided at its downstream end with a variable-area convergent-divergent nozzle.

8. A variable configuration gas-turbine engine as claimed in claim 1 and substantially as herein before described with reference to the drawing.