GB2041090A - By-pass gas turbine engines - Google Patents

Abstract

In a by-pass gas turbine engine operating on an unmixed cycle a core engine 10 drives a fan 12 to produce a by-pass flow in a by-pass duct 16, and a heat exchanger in the form of interconnected vanes 24, 26 spans both the by-pass duct 16 and the jet pipe 20 of the core engine, so that waste heat from the jet pipe 20 downstream of the last turbine stage can be transferred to the by-pass duct. The vanes 24, 26 are preferably formed at least in part as heat pipes. Alternatively, conventional vanes may be provided and may be interconnected by heat pipes. A modification of this type can increase the thrust of an unmixed engine cycle, but in particular, if the cycle is adjusted so that, upstream of the heat exchanger, the pressure of the by-pass flow is greater than the pressure of the hot gas exhaust from the core engine, the modified engine can produce more thrust than an equivalent engine operating on an optimised mixed cycle. <IMAGE>

Description

SPECIFICATION Improvements in by-pass gas turbine engines The present invention relates to by-pass gas turbine engines.

Two basic operating cycles are available to the engine designer concerned with by-pass engines. In the first, known as the mixed cycle, the by-pass flow and the hot gas exhaust from the turbine are mixed before being exhausted through a final nozzle. In the second, the unmixed cycle, the by-pass flow and the hot turbine exhaust are passed to atmosphere through separate nozzles.

From observations made during operation of both types of engines it is well known that the mixed cycle provides a lower specific fuel consumption (SFC) than the optimum unmixed cycle in spite of losses incurred during passage of the two flows through an appropriate mixing device. The improvement in SFC is often referred to as the "mixing gain A review of the theoretical performances of both cycles has shown us that an engine operating on the unmixed cycle can be modified by the inclusion of a heat exchanger so as to provide an increase in thrust. Also, provided that certain conditions of pressure within the two flows are met the thrust of the modified engine can be greater than that of an equivalent engine using an optimised mixed cycle, thus producing an engine cycle which is more efficient an engine than either of these two known cycles.

The use of heat exchanger in aircraft gas turbine engines is not new. For example, it is known to pass hot oil from the engine through a heat exchanger in the by-pass duct of a by-pass gas turbine engine thus slightly increasing the temperature of the by-pass air.

Such heat exchangers are generally bulky and heavy and provide pressure losses in the bypass flow so that they tend to be made as small as possible to minimise these detrimental effects.

Another example is in the cooling of turbine nozzle guide vanes in which a heat pipe is used to connect hot turbine vanes to vanes at the discharge end of the high pressure engine compressor, thus adding further heat to the compressed air delivered by the compressor to the combustion system and reducing the amount of fuel required. The amount of heat which can be taken from the vanes is limited however, to avoid cooling the working fluid flowing through the vanes, because this would adversely affect the efficiency of the downstream stages of the turbine.

It has not previously been proposed, however, in an engine operating on an unmixed cycle, to take significant quantities of heat from the hot turbine exhaust downstream of the last turbine stage, and put it into the fan exhaust. This significantly changes the engine performance and provides an increase in thrust of the engine. In particular our calculations show that provided the relationship between the pressures of the two flows is right, a worthwhile gain in thrust from the engine can be achieved compared to the best mixed or unmixed cycles using the same core engine and by-pass ratio.

According to the present invention a bypass gas turbine engine comprises a core engine arranged to drive a fan which produces a by-pass flow around the core engine, a by-pass nozzle through which the by-pass flow is exhausted, and a heat exchanger arranged to take heat from the core engine exhaust and put it into the by-pass flow between the fan and the by-pass nozzle.

In a particular embodiment, the engine cycle is arranged so that the pressure of the bypass flow upstream of the heat exchanger is greater than the pressure of the hot gas exhaust flow from the core engine upstream of the heat exchanger.

Conventional heat exchangers are bulky, heavy and produce significant losses in the fluid flows passing through them. An efficient form of heat exchanger which can be adapted for gas turbine engine use is a heat pipe, and in one embodiment of the invention a ring of vanes in the hot gas exhaust flow is connected to a ring of vanes in the by-pass flow by means of a heat pipe or heat pipes.

Support struts or guide vanes already spanning the respective flows in a conventional engine may be used to form the two ends of the heat exchanger, the heat pipe merely interconnecting the two. In an alternative form of the invention however, the vanes themselves may also be in the form of heat pipes to give more efficient heat exchange with the hot gas and by-pass flows.

The heat exchanger is preferably of the parallel flow type in which the two gas flows pass through it in the same direction, in which case the maximum amount of heat exchange which can take place is that which will leave the temperatures of the two flows equal downstream of the heat exchanger. It may be possible, in some engine configurations, however, to use a contraflow heat exchanger in which the by-pass flow and the hot gas exhaust flow pass through the heat exchanger in different directions. In this case the temperature of the by-pass flow could be further increased but any available gain in thrust would have to be offset against the increased losses in the flows due to reversals which the flow has to undergo.

The invention will now be more particularly described, by way of example only, and with reference to the accompanying drawing which illustrates a by-pass gas turbine engine embodying the invention.

Referring now to the Figure, there is shown a by-pass gas turbine engine having a core engine 10, which may be of any conventional form, and which drives a fan 1 2. The fan is surrounded by a fan cowl 14 to define an annular by-pass duct 1 6 through which air compressed by the fan passes, and which terminates in a by-pass nozzle 1 8. The core engine exhaust flow from the last stage of the turbine passes down a jet pipe 20 and is exhausted to atmosphere through a hot gas nozzle 22.

The fan cowl is supported from the core engine by means of aerofoil shaped vanes 24 which are formed at least in part as heat pipes, (indicated at 25), and these vanes extend completely across the by-pass duct so that heat may pass from the heat pipes into the by-pass flow.

A further set of aerofoil-shaped vanes 26, also made in the form of heat pipes, extend across the jet pipe 20 down-stream of the last stage of the turbine to pick-up heat, which is at this stage waste heat, from the jet pipe.

The vanes 26 act as the evaporator part of the heat pipe and the vanes 24 act as the condenser part so that waste heat from the jet pipe can be put into the bypass air to raise its temperature. By using heat pipes a very efficient heat exchange process can be used, and a large amount of heat can be transferred into the by-pass flow.

The arrangement produces a parallel flow heat exchanger in which maximum heat exchange occurs when the temperature in the two flows downstream of the heat exchanger vanes 24, 26 have become equal.

The use of a mixer in a conventional mixed cycle demands substantially equal fan and jet pipe pressures. By using a heat exchanger it is, in principle, possible to transfer heat from the hot gas exhaust flow in the jet pipe to the fan exhaust i.e. the by-pass flow, even when the two pressures are different. We call such a cycle a thrust recuperator cycle. The mixed cycle can then be regarded as a special case of the thrust recuperator cycle in which the equality of the pressures in the hot gas exhaust flow and the by-pass flow makes it possible to dispense with an actual heat exchanger.

Theoretical considerations can be used to show that loss free heat exchange can increase the total thrust provided that Ve VH ~ > ~ Tc TH (where c and H indicate the cold and hot conditions in the by-pass and hot gas exhaust flows respectively, and V and T represent the velocity and temperature of the respective flows). Since this condition is met by the optimum unmixed cycle it follows that a heat exchanger can increase the thrust of this cycle. It can also be shown that greatest benefit can be obtained from the thrust recuperator cycle when the pressure of the by-pass flow is greater than the pressure of the hot gas flow i.e. Pc > P,.

Sufficient heat can in fact be transferred into the bypass flow by means of a heat exchanger to provide a significant gain in thrust. While it may not be possible to equalise the temperatures of the hot and cold gas streams, it is possible by this means to exceed the thrust gains of a mixed engine without the disadvantages and losses of the use of a mixer. For example, the theoretical thrust gain, unlike that of the gain between the unmixed and mixed cycles, is unaffected by the efficiency level of the turbo-machinery.

Although there is bound to be some pressure loss by introducing the two sets of vanes, it is necessary to have vanes in the by-pass flow in any case to support the fan cowl from the core engine, so that efficient use can be made by these vanes by forming them into the required heat pipe configuration so reducing the weight and loss penalties of the proposed system. Thus it is possible to apply the invention to existing engines using an unmixed cycle with little modification to the engine.

In an alternative configuration (not shown) in which the vanes in the hot gas exhaust and in the by-pass flow are axially separated due to other design considerations, a series of heat pipes may be provided in the casing surrounding the core engine to interconnect individual vanes in the two flows.

Claims (5)

1. A by-pass gas turbine engine comprising a core arranged to drive a fan which produces a by-pass flow around the core engine, a by-pass nozzle through which the bypass flow is exhausted, and a heat exchanger arranged to take heat from the core engine exhaust and put it into the by-pass flow between the fan and the by-pass nozzle.

2. A by-pass gas turbine engine according to claim 1 and in which the engine cycle is arranged so that the pressure of the by-pass flow upstream of the heat exchanger is greater than the pressure of the hot gas exhaust flow from the core engine upstream of the heat exchanger.

3. A by-pass gas turbine engine according to claim 1 or claim 2 and in which the heat exchanger comprises vanes extending across the respective flows and interconnected by heat pipes.

4. A bypass gas turbine engine according to claim 1 or claim 2 and in which the heat exchanger comprises vanes extending across the respective flows and made at least in part in the form of heat pipes.

5. A by-pass gas turbine engine substantially as hereinbefore described with reference to the accompanying drawing.