GB2088570A - Gas turbine engine test bed with exhaust heat recovery - Google Patents

Abstract

As a fuel-saving measure, a test bed (1) for gas turbine engines, particularly jet propulsion engines, is provided with a heat exchange system which exchanges heat between the exhaust efflux (B,C) of the engine (9) being tested and the test bed intake air (A), so as to produce warmer engine intake air (A min ). To take account of variable atmospheric temperatures and differing engine powers, the amount of heat discharged by the intake air heat exchanger (19) to the intake air can be varied by varying the mass flow rate of circulation of heat exchange medium around the system and changing the effective heat exchange area of the heat exchanger (17) which collects heat from the exhaust efflux. <IMAGE>

Description

SPECIFICATION Improvements in gas turbine engine test beds The present invention relates to test beds for gas turbine engines and methods of reducing the consumption of fuel whilst the engines are being tested.

Recent large increases in the cost of aviation turbine fuel have increased the cost of testing engines during their development and before delivery to the customer. It is therefore desirable to make savings in the use of fuel during testing.

Some savings could be achieved by testing engines only on hot days, since a higher temperature for the intake air means that less fuel needs to be expended to produce the optimum temperature and pressure for the combustion gases being delivered to the turbine. However such a scheme of testing would be commercially impracticable. Some means of heating the intake air is therefore required which does not require the expenditure of additional energy.

In the invention, this object is achieved by utilising the waste heat from the engine to heat the engine intake air.

According to the present invention a gas turbine engine test bed is provided with a heat exchange system for exchanging heat between the exhaust efflux of a gas turbine engine to be tested and the air which enters the intake of the engine, thereby to raise the temperature of said air. Conveniently, water is used as the heat exchange medium.

In order to facilitate variation of the rate of exchange of heat between the engine intake air and the exhaust efflux, the system is advantageously provided with means for circulating the heat exchange medium through the system at a variable rate.

The system may be provided with a first heat exchanger for collecting heat from the exhaust efflux and a second heat exchanger for discharging heat to the engine intake air; in order to vary the heat input to the heat exchange medium, the effective heat exchange area of the first heat exchanger is controllably variable, e.g. by varying the number of heat exchange elements through which the heat exchange medium is allowed to pass when picking up heat from the exhaust efflux.

The invention will now be described by way of example only with reference to the accompanying drawing, which is a schematic diagram showing an arrangement adopted to implement the invention.

The drawing is not to scale.

In the drawing, a test bed 1 comprises a test cell building 3 having at one end thereof an air inlet 5 and at the other end an exhaust efflux outlet 7. The air inlet 5 conventionally comprises a row of vertically oriented slats, 6, each one of which is rotatable about its own vertically extending axis X-X between a first position in which they effectively close off the air inlet 5, and a second position as shown in which they present their edges to the incoming air, thus effectively opening the air inletS. Situated within the test cell 3 to take in air through air inlet 5 is the engine 9 to be tested, which is fitted with an intake flare 10 as normal. In the present example engine 9 is a high by-pass ratio turbofan engine such as our Rub211 (Registered Trade Mark) aeroengine.However, the invention is of course applicable to the testing of other sorts of gas turbine engine, such as turbojets, low by-pass turbofans, and industrial or marine types of gas turbine.

When the engine 9 is being tested, it sucks in atmospheric air Athrough air inlet 5 by means of ducted fan 11. The fan 11 passes the air to the by-pass duct 13 and to the engine core 15, which contains compressor, combustor and turbine sections (not indicated). The exhaust efflux from the engine comprises the hot efflux of combustion gases C from the engine core 15 and, coaxial with and surrounding that, the relatively cool efflux of air B from the by-pass duct 13. The combined exhaust efflux E passes from the test cell via the outlet ducting 7.

In order to economise on fuel consumption during the test program me for the engine 9, the test bed 1 incorporates two heat exchangers 17, 19 located in the path of the exhaust efflux and the intake air respectively. A heat exchange medium such as water is circulated between the two exchangers through pipes 21,23 by pump 25. In this way heat is collected from the effluxes B and C in exchanger 17 and discharged to the test bed intake air A by exchanger 19 to produce warmer engine intake air A'.

In order to exchange heat with the heat exchange medium, the air or combustion gases flow through the heat exchangers 17,19, and come into direct contact with known types of heat exchange elements indicated diagrammatically at 18 and 20, such as finned tubes or tubes carrying the heat exchange medium through an array of conductive plates in a matrix structure.

Consider now an engine test programme being carried out on engine 9 on an average summer day in a temperate oceanic climate. The outside air temperature, and hence the temperature of test bed intake air A, is about 15"C, but the engine is designed to produce its full thrust with air temperatures up to about 29"C, so that the intake air can be heated to the latter temperature for, say, an endurance test at full take-off thrust, or cyclic take-off/landing thrust tests, whilst at the same time fuel consumption will be cut relative to similar tests using intake air at the prevailing outside temperature.

In order to exemplify the parameters of a practical system, a test will now be assumed in which the engine/heat exchanger system has reached constant working conditions throughout, whilst the engine (assumed to be an RB211) is working at full (i.e.

take-off) power. At this particular power setting (given the attainment of the 29"C temperature for the engine intake air A'), the mass flow of efflux B is about 472 kg/s at a temperature of 69"C and the mass flow of efflux C is about 98 kg/s at a temperature of 513"C. This gives a combined mass flow figure of 570 kg/s at a mean gas temperature of 218"C before entry to heat exchanger 17.

During this test the pump 25 circulates the heat exchange medium, which is a liquid, between ex changers 17 and 19 at a high mass flow rate. A high mass flow rate for the heat exchange liquid is chosen so that its change in temperature during its passage through exchangers 17 and 19 will be relatively small, and so that the exchangers can be designed with a low number of heat exchange elements per unit area and yet produce a large temperature change in the gases passing through them. Thus in order to raise a mass-flow of about 570 kg/s of intake air from a temperature of 1 5"C to the desired temperature of 29"C there is required a mass flow of 0.1 m3/s of water at 60"C from exchanger 17 to exchanger 19. After passage through exchanger 19 the water is at 40"C and is returned to exchanger 17 for re-heating.

Note that the use of heat exchangers with a low number of heat exchange elements per unit area gives low pressure drops in the gas flows across the exchangers, ease of installation through lighter weight, and lower cost.

Clearly, for different test conditions, such as different outside air temperatures or different engine power settings, either the temperature attained by the heat exchange medium after passage through exchanger 17 must be changed, or the mass flow rate of the heat exchange medium around the system must be changed; or both parameters may be changed at the same time. Such changes can be achieved by making pump 25 of the variable delivery type (e.g. in the case of an electrically driven centrifugal flow pump, utilising a variable speed electric motor) and altering the effective heat exchange area of heat exchanger 17, i.e. changing the heat input to the heat exchange medium other than by altering the temperature and mass flow of the engine exhaust efflux.

The effective area of heat exchanger 17 may be altered as desired by, e.g. changing the number of heat exchange elements through which the heat exchange medium flows, so that if it is desired to extract more heat from the efflux of the engine, a greater number of elements is utilised, whilst if it is desired to extract less heat from the effluxes, fewer elements would be utilised. Utilisation of elements could easily be controlled by a simple valving system (not shown).

In the case of by-pass engines with unmixed exhaust effluxes, such as the one illustrated, then as an elaboration of the above method of controlling the effective area of exchanger 17, it is contemplated that greater or lesser heat input to the heat exchange medium could be achieved by designing the exchanger so that if a very large heat input is required, most or all of the heat exchange medium will pass through elements which are impinged on by the combustion gas efflux C, whereas if a very low heat input is required, most or all of the heat exchange medium will pass through elements which are only impinged on by the by-pass air efflux B.This can be achieved by having two separate sets of heat exchanger elements which are arranged either so that one set of elements is in contact with onlyefflux B and the other set is in contact with only efflux C, or so that one set of elements is in contact with only efflux B over their entire lengths and the other set is in contact with both efflux B and efflux C over different portions of their lengths, the flow of the heat exchange medium through the elements in either case being again controlled by valves. Of course, in a case where some of the heat exchange medium is heated to a greater temperature than the rest, then constant delivery temperature of the medium to exchanger 19 must be assured by making adequate provision for thorough mixing of the medium in a plenum chamber or the like (not shown) after its passage through the two sets of elements.

On the occasions when the atmospheric temperature is the same as or higher than the required engine intake air temperature, then obviously no heat transfer from the exhaust efflux to the intake air is required, and the pump 25 must be switched off.

To take account of this and other conditions in which the flow of the water or other heat exchange liquid through exchanger 17 is insufficient to prevent overheating and boiling in those heat exchange elements subjected to efflux C, the elements may be provided with means for venting excess pressure, such as blow-off valves. These would be necessary in any case as a safety measure in case of failure of pump 25.

Overheating could also be prevented by ensuring that whenever the flow produced by pump 25 around the normal heat exchange circuit is insufficient of itself to prevent boiling in elements subjected to efflux C, the liquid is circulated around an auxiliary heat exchanger circuit through elements situated in the cooler by-pass exhaust efflux B, an auxiliary pump being used for this purpose. The auxiliary heat exchange circuit would be connected to the normal circuit through valves which would be controlled to open automatically when the heat exchange liquid in the hot elements reached a predetermined temperature. Blow-off valves as described above would still be necessary as a fail-safe measure.

In orderto accommodate changes in the volume of the heat exchange liquid due to changes in its mean temperature, the heat exchange system must clearly incorporate means such as expansion tanks (not shown) to allow excess heat exchange liquid to escape from the system at higher mean temperatures and to allow supply of extra heat exchange liquid to the system at lower mean temperatures.

It is anticipated that exhaust heat recovery systems such as those described above will be able to maintain an engine intake air temperature of 29"C at all engine power settings for outside air temperatures at least as low as 5"C, and probably lower.

In order to maintain the engine intake air temperature at any desired value against variations in engine power or outside air temperature, it is necessary to have a means of sensing the static temperature of the air after passage through exchanger 19, such as a quick-reacting resistance thermometer shielded from the airflow. The signal from this can then be compared with an optimum value indicating the desired temperature and a difference signal can then be produced for use as a feed-back signal in an automatic control system which adjusts the speed of pump 25 and the valves in exchanger 17 to change the amount of heat being transferred between the exhaust effluxes B and C and intake airA and so adjust the engine intake air temperature.

Claims (6)

1. A gas turbine engine test bed provided with a heat exchange system for exchanging heat between the exhaust efflux of a gas turbine engine to be tested and the air which enters the intake of the engine, thereby to raise the temperature of said air.

2. A gas turbine engine test bed according to claim 1 in which the heat exchange medium in the heat exchange system is water.

3. A gas turbine engine test bed according to claim 1 or claim 2 in which the heat exchange system is provided with means for circulating the heat exchange medium through the system at a variable rate.

4. A gas turbine engine test bed according to any one of claims 1 to 3, having a first heat exchanger for collecting heat from the exhaust efflux and a second heat exchanger for discharging heat to the air which enters the intake of the engine, the effective heat exchange area of the first heat exchanger being controllably variable so as to varythe heat input to the heat exchange medium.

5. A gas turbine engine test bed according to claim 4 in which the effective heat exchange area of the first heat exchanger is varied by varying the number of heat exchange elements through which the heat exchange medium passes.

6. A gas turbine engine test bed provided with a heat exchange system substantially as described in this specification.