GB2127099A - Improved gas turbine engine cycle including a heat exchanger - Google Patents

Abstract

A heat exchanger 20 is arranged to receive both the by-pass flow 14 and the exhaust 16 from the low pressure core turbine 10 in heat exchange relationship to heat the by-pass flow which is then passed through a low pressure power turbine 22 on shaft 26. Shaft 26 may be so arranged to drive an aircraft propeller (or other load) through gearing 28. The exhausts of the two low pressure turbines 10, 22 pass to atmosphere through separate final nozzles 18 and 24. <IMAGE>

Description

SPECIFICATION Improved gas turbine engine cycle including a heat exchanger The present invention relates to gas turbine engines which include heat exchangers.

It is known to use heat exchangers in gas tubine engines to improve the efficiency of the operating cycle. Conventionally the heat exchanger is positioned to receive heat from the exhaust of a turbine of the engine and to heat the compressed air leaving the last compressor stage to raise the temperature of the compressed air before it enters the combustion chamber of the engine. In this way less fuel is required to raise the temperature of the compressed air to the engine maximum temperatures.

The object of the present invention is to provide an operating cycle for a gas turbine engine including a heat exchanger which results in an engine having higher efficiency that the known cycles including heat exchangers.

According to the present invention a gas turbine engine cycle which produces excess power for driving a load comprises, a core system including a compressor, combustion equipment and a turbine and which produces a hot gas exhaust, a low pressure compressor which provides a by-pass flow which by-passes the core system, a low pressure power turbine through which the by-pass flow passes, and a heat exchanger which receives both the by-pass flow and the hot gas exhaust from the core system in heat exchange relationship and which heats the by-pass flow prior to the passage thereof through the low pressure power turbine, shafting being provided for drivingly connecting the turbines with the compressors and the load.

The low pressure compressor and power turbine may be entirely separate from the high pressure core system and mounted on separate shafts which may then drive into a suitable transmission system whereby they both contribute to driving the load.

Alternatively the low pressure compressor may be mounted concentrically with the core system and part of the flow compressed thereby may be used to supercharge the core system while the remainder of the flow forms the by-pass flow.

In a preferred form of the invention the low pressure compressor is mounted concentrically with the core system as described above and the core system includes a low pressure core turbine mounted on shafting drivingly connected to the low pressure compressor.

With this cycle the core system can be operated at high efficiency with maximum work being extracted from the core flow in the low pressure core turbine, and the overall efficiency of the cycle is then increased by using wast heat from the exhaust of the core system as the heat input to the otherwise inefficient low pressure part of the cycle.

In a preferred embodiment of the invention the exhausts from the two low pressure turbines are passed to atmosphere through separate final nozzles.

The shaft driven by the low pressure power turbine may be used to drive various loads, for example, an electrical power generator, an aircraft or ship's propeller, or a fan for a ducted fan gas turbine engine.

In a preferred arrangement the two low pressure turbines are both connected to a single shaft which drives the low pressure compressor and a load in the form of an aircraft propeller. In this case it may be necessary to introduce gearing between the shaft and the load.

The invention will now be more particularly described with reference to the accompanying drawings in which: Figure 1 illustrates a diagrammatic layout of a gas turbine engine according to the invention, and Figure 2 illustrates an alternative layout of a gas turbine engine according to the invention.

Referring now to Figure 1 of the drawings the engine comprises a core system having a high pressure compressor 2, combustion equipment 4, and a high pressure turbine 6 connected to drive the high pressure compressor by means of a high pressure shaft 8. Immediately downstream of the high pressure turbine 6 is a low pressure core turbine 10 which extracts further power from the core flow and produces a hot gas exhaust from the core.

A low pressure compressor 12 is provided upstream of the high pressure compressor 2, and flow delivered form the low pressure compressor is divided, a first portion of the flow passing to the high pressure compressor 2, and a second portion of the flow passing to a by-pass duct 14to provide a by-pass flow which flows around the core system.

The hot exhaust gases from the low pressure core turbine 10 pass into an exhaust duct 16 leading to a primary nozzle 18 which discharges the gases to atmosphere. In the exhaust duct 16 is positioned a heat exchanger 20 whereby the exhaust gases from the low pressure core turbine are arranged to flow in heat exchange relationship with a heat exchange fluid which becomes heated by the exhaust gases.

The heat exchange fluid is provided by the by-pass flow which is ducted through the heat exchanger.

After passage through the heat exchanger the heated by-pass flow passes to a low pressure power turbine 22 and is then exhausted to atmosphere through a secondary nozzle 24.

The two low pressure turbines 10 and 22 are connected by shafting 26 to drive the low pressure compressor 12 and a load 30 which, depending on the use to which the engine is to be put, may be an electrical power generator, an aircraft or ship's propeller, or a ducted fan.

In the layout shown, intended for a propeller driven aircraft, both of the low pressure turbines 10 and 22 are connected to a single low pressure shaft 26 which drives the low pressure compressor 12 and into a gear box 28 and the load 30 is provided by a propeller drive shaft.

The improvement in efficiency of this cycle arises as follows: The core system is arranged to run at as high a pressure ratio and temperature as possible, and the low pressure core turbine is designed to remove as much work as possible from the core system, to leave the hot exhaust of the core system with as nearly as possible only sufficient pressure in the exhaust duct 16 as is required to overcome the pressure loss through the heat exchanger and the nozzle 18. The final pressure ratio in the duct compared to atmospheric pressure may thus be of the order of 1.1:1 or 1.2:1.Thus the core system is highly efficient.

The low pressure compressor however, produces a flow having a pressure ratio of the order of 3:1 to 4:1 which would normally be too low to allow efficient burning of fuel therein before driving a turbine. However, the temperature of the exhaust gases from the low pressure core turbine is still high enough to provide a significant temperature difference between the two flows in the heat exchanger, and since the heat in these exhaust gases would normally be waste heat, use of this "free" energy increases the overall cycle efficiency because any net power produced by the low pressure power turbine is obtained without any increase in fuel flow.

The power derived from the low pressure core turbine 10 may be more than is required to drive the low pressure compressor, and the power derived from the low pressure turbine core 22 may be insufficient to drive the load. Thus in the preferred arrangement shown both low pressure turbines drive the shaft 26, which in turn drives the propeller shaft 30 (the load) and the low pressure compressor The gearbox 28 between shaft 26 and the propeller shaft allows for any mis-matching of power and speed between the two turbines. Clearly however, in alternative construction separate shafts may be used to connect the low pressure compressor 12 and the load shaft 30 to their respective turbines 10 and 22, or mis-matching of the turbines may be avoided so that the gearbox 28 may be eliminated.

Referring now to Figure 2 of the drawings an alternative layout is illustrated which allows the primary and secondary nozzles to be concentric.

Only the rear part of the engine is shown, it being understood that the core system, with its low pressure turbine, the low pressure compressor and the shafting may be arranged identically to Figure 1.

In this arrangement however, the by-pass flow is arranged to flow concentrically around the core system in an annular by-pass flow duct 40, and to flow radially inwardly through a heat exchanger 42 before passing to the low pressure power turbine 44 and the secondary nozzle 46. The exhaust from the low pressure core turbine 10 is turned radially outwardly to pass through the heat exchanger 42 in contraflow relationship with the by-pass flow to heat the by-pass flow, and then is passed to atmosphere through an annular primary nozzle 48 which surrounds the secondary nozzle 46.

Any form of heat exchanger may be used including a regenerative heat exchanger, the invention being concerned with the total cycle rather than individual parts thereof.

Although both embodiments described above have included separate primary and secondary nozzles for the exhausts of the two low pressure turbines, it is possible that the two exhaust could be mixed if their pressures can be matched and the mixed exhaust passed through a single nozzle.

Claims (10)

1. A gas turbine engine cycle which produces excess power for driving a load comprising, a core system including a compressor, combustion equipment and a turbine and which produces a hot gas exhaust, a low pressure compressor which provides a by-pass flow which by-passes the core system, a low pressure power turbine through which the by-pass flow passes, and a heat exchanger which receives both the by-pass flow and hot gas exhaust from the core system is heat exchange relationship and which heats the by-pass flow prior to the passage thereof through the low pressure power turbine, shafting being provided for drivingly connecting the turbines with the compressors and the load.

2. A gas turbine engine cycle according to claim 1 and wherein part of the flow compressed by the low pressure compressor supercharges the core system and part forms the by-pass flow.

3. A gas turbine engine cycle according to claim 2 and wherein the low pressure compressor is mounted concentrically with the core system and the core system includes a low pressure core turbine mounted on a shaft drivingly connected to the low pressure compressor.

4. A gas turbine engine cycle according to any preceding claim and in which the exhausts from the two low pressure turbines are passed to atmosphere through separate primary and secondary nozzles respectively.

5. A gas turbine engine cycle according to any preceding claim and in which the load is in the form of either an electrical power generator, an aircraft or ships propeller, or a fan of a ducted fan gas turbine engine.

6. A gas turbine engine cycle according to any preceding claim and wherein the load is driven at least by the low pressure power turbine.

7. A gas turbine engine cycle according to any preceding claim and in which the two low pressure turbines are mounted on a single shaft which drives both the low pressure compressor and the load.

8. A gas turbine engine cycle according to claim 7 and in which the load is connected to the said single shaft through gearing and a further drive shaft.

9. A gas turbine engine cycle according to claim 4 and in which the primary and secondary nozzles are concentric.

10. A gas turbine engine cycle substantially as hereinbefore described with reference to the accompanying drawings.