GB2275738A

GB2275738A - Reducing combustion vibration

Info

Publication number: GB2275738A
Application number: GB8701570A
Authority: GB
Inventors: John Eirwyn Ffowcs Williams
Original assignee: Topexpress Ltd
Current assignee: Topexpress Ltd
Priority date: 1987-01-24
Filing date: 1987-01-24
Publication date: 1994-09-07
Anticipated expiration: 2007-01-24
Also published as: GB2275738B; GB8701570D0

Abstract

Combustion induced vibrations, which are liable to be caused in a combustion chamber, are reduced by introducing a second burner (13) downstream of the main burner (12), so as to neutralise a standing wave set up in the combustion chamber due to the flame instability of the main burner. In the case of a gas turbine aeroengine (1), the second burner may be an auxiliary reheat combustor (13) having its own control unit (40) to selectively supply fuel to the auxiliary combustor when reheat buzz is likely to arise. <IMAGE>

Description

Title: Reducing combustion vibration Field of invention The present invention relates to apparatus and methods for reducing combustion vibration caused by instability of combustion in a combustion chamber such as that of a furnace or gas turbine engine afterburner or reheat systems for gas tubine engines.

Rackground to the invention Vibration is generated by combustion-driven oscillations which occur both in a primary burner and in a reheat combustion system. The phenomenon when associated with reheat combustion systems is often referred to as reheat buzz, and includes vibration of flame gutters and other components of the reheat combustion system. The oscillations are liable to be unstable and can grow in amplitude until they pose a threat to the integrity of the system, either in the short term through excessive stresses in the components or in the longer term through excessive reductions in fatigue life.

It is already known to reduce reheat buzz by either redesigning the reheat combustor or by scheduling engine thrust to avoid combustion conditions known to produce a severe buzz problem for the particular system configuration being considered. However, such measures increase weight and expense or limit the rate of increase of thrust achievable on a reheated engine. Further, even the continuance of a less severe problem is undesireable because it still results in long term metal fatigue, albeit at a reduced level.

The present invention tackles the problem of reheat buzz by selectively changing the mode of combustion in the reheat system when conditions are such that reheat buzz would otherwise occur.

Summary of the invention According to one aspect of the present invention a method of reducing combustion induced vibrations in a combustion chamber which includes a first burner, comprises the step of introducing a second flame into the chamber at a position remote and downstream from the first burner, the position of the point of introduction and the size of the flame being selected so that the heat release of the combustion process is distributed within the chamber in a manner which reduces the occurrence of combustion induced vibration.

In a reheat combustion system for a gas turbine aeroengine which includes a jet pipe and at least a first main reheat combustor, a second, auxiliary reheat combustor is axially spaced from the first main combustor by a substantial distance within the jet pipe such that the heat release of the combustion process is distributed within said jet pipe in a manner which prevents the occurrence of reheat buzz.

Typically, the second, auxiliary combustor is positioned and its flame is adjusted so as to neutralise a standing wave set up in the chamber due to the flame instability of the primary combustor.

Preferably, the auxiliary reheat combustor is provided with its own control means selectively actuable to supply fuel thereto on occasions when reheat buzz is likely to arise.

A method of operating a reheat combustion system in a gas turbine aeroengine, in which the reheat combustion system comprises a jet pipe with reheat combustion means therein, comprises selectively burning fuel in at least two locations in the jet pipe substantially spaced apart from each other longitudinally thereof, whereby the boundary conditions of the reheat combustion process are modified to prevent the occurrence of reheat buzz.

Thus, for example the pressure fluctuation level in the combustion chamber may be monitored and if the fluctuation exceeds a given threshold, fuel may be supplied to the second burner.

Alternatively, compressed air and fuel may be supplied in a controlled manner to form the introduced flame, to produce more compact ignition and control the vibration damping effect of the introduced flame.

The axial separation of the main burner and the introduced flame is typically of the order of 1/3 the length of the chamber (eg jet pipe). However, the preferred spacing will vary according to circumstances, generally from little more than one jet pipe diameter up to most of the pipe length. In general, the separation required must be sufficient to cause a significant change in phase of the fundamental (standing wave) mode as between the two burner positions.

An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic part cross-sectional representation of a gas turbine engine fitted with a reheat combustion system and secondary burner according to the present invention; Figure 2 is a cross-sectional diagram of a conventional Rijke tube apparatus; and Figure 3 is a cross-sectional diagram of a modified Rijke tube apparatus.

Detailed description of the invention In Figure 1, a bypass gas turbine engine is shown fitted with a reheat combustion system with secondary burner in accordance with the present invention. The engine 1 comprises a core 3, a bypass duct 5 defined by an outer casing 7 which surrounds the core 3, and a reheat exhaust system 9 including a rear exhaust cone 11, main and auxiliary reheat combustors 12 and 13 within jet pipe 14, an exhaust mixer annulus 15 and a final variable area propulsive nozzle 17. The bypass duct 5 is supplied with bypass air 19 from a front fan 21, which also supplies core 3, the fan 21 being driven by a shaft (not shown) connected to a final turbine section 23 of core 3.The bypass air 19 passes through apertures 25 in mixer annulus 15 at the end of the bypass duct and mixes with at least the outer regions of the hot turbine exhaust gases 27 issuing from engine core 3 before the combined flow passes to atmosphere.

The engine core 3 comprises an intermediate pressure compressor section 29, which receives low pressure air from fan 21, a high pressure compressor section 31, an annular combustion chamber 33, a high pressure turbine section 35, an intermediate pressure turbine section 37 and a low pressure turbine section 23 already mentioned.

For a large proportion of operating time, all the thrust of the engine 1 is provided by burning fuel in combustion chamber 33. When more thrust is required than can be obtained by burning fuel in combustor 33 alone, the pilot selects reheat by throwing a switch and setting a lever to the power level required. A signal proportional to lever angle (power selected) is input to a reheat fuel control unit 39 to open a valve in unit 39 to feed fuel through feed pipe 41 to the primary reheat combustor 12. This has a circular fuel manifold 43 supported by struts 45 concentrically within the jet pipe 14, the feed pipe 41 being shrouded by a strut 45. Fuel is sprayed into the flame area from holes (not shown) in the downstream edge of the manifold. A flame retaining gutter or stabiliser 47 comprises a v-section annular ring located downstream of the manifold 43 to provide a region of reduced flow velocity to prevent the reheat flame being blown out by the turbine exhaust gases 27.

The basic task of the reheat fuel control unit 39 is to control the amount of fuel reaching the primary reheat combustor 12 as determined by the setting of the reheat control lever. It does this by scheduling the fuel flow with reference to the ratio between two further input signals, namely the compressor delivery pressure c and the pressure P. in jet pipe 14, the object being to "damp" 3 the reheat systems response to a movement of the control lever so that the increase in jet pipe pressure is not too rapid; such over-rapid increase would lead to excessive slowing down of the engine's turbine and compressor due to back-pressure from the jet pipe.

The ratio between pressures P and P. is also sensed by a c 3 nozzle control unit 49 which controls the area of the variable area propulsion nozzle by altering the position of pivotable flaps 51. Thus, when P. increases due to 3 reheat control unit 39 allowing more fuel to flow to the reheat combustor, the nozzle control unit 49 automatically pivots the flaps 51, through ram linkages 53, so as to increase the exit area of the nozzle 17 until the correct PC/Pj ratio has been restored. Similarly, when reheat c 3 power is reduced, the nozzle area is decreased.

For a more detailed explanation of a reheat combustor/variable area nozzle control system, see the Rolls-Royce Limited publication "The Jet Engine", revised third edition.

Reheat combustion systems generally are prone to suffer from combustion induced vibration known as reheat buzz, and the combustion regimes when buzz occurs are well known for each type of engine in terms of the relative magnitudes of the pressures P c and P. and the reheat power 3 level as set by the lever angle. As mentioned previously, it is known to avoid buzz conditions by scheduling the fuel flow to the reheat combustor so as to avoid the conditions known to give rise to vibration. However, this means reducing fuel flow rates relative to what would be allowed by scheduling the fuel flow merely by reference to P /P. values acceptable to the engine; this reduction of c3 fuel flow rates limits performance.However, use of the present invention should enable greater fuel flow rates to be utilised without the vibration problem arising.

Before further describing this embodiment of the invention, the inventive concept will be illustrated by reference to a single experiment.

Reheat vibrations is believed to be associated with dynamically unstable combustion in the reheat flame. The exact physical process whereby such instability arises can be likened to the conditions which occur in a so-called Rijke tube.

Referring to Figure 2, a Rijke tube consists simply of a vertically oriented tubular chamber 51, usually of a metal such as copper, which is fitted near its lower end with a metallic gauze 53 extending completely across the tube cross-section. If the gauze is heated, either by means of electrical resistance heating (not shown) or by means of a flame 54 from a gas burner 55, an acoustic resonance is excited in the tube. This is caused by a transfer of heat from the gauze to air 56 passing through the gauze due to convection in the tube.The combination of the upward convection current with acoustic oscillation of the natural modes of the pipe results in more heat being transferred to the air when it is compressed than when it is rarefield, and if the energy gained from the gauze is greater than the energy lost at the tube ends and elsewhere, the resonant motion accumulates more and more energy (ie the resonance is forced) until energy gains and losses are in balance. The pipe sounds mainly at its fundamental "organ pipe" frequency. It should be noted that a gauze is not essential to the working of this experiment: a bare flame can be enough.

Stated briefly, this phenomena illustrates that sound waves passing through a heat source, whether a hot gauze or a flame, can be amplified by the heat source, leading to intense acoustic disturbances if the heat source is located inside, or in proximity to, a resonator.

Since a reheat combustor (Figure 1) produces an intense flame within a jet pipe and in proximity to objects with their own resonance frequencies such as a flame gutter, it can be seen that there could be a strong parallel between a Rijke tube and a jet pipe which includes a reheat system.

The present invention arose from a consideration of the above factors. Stated broadly, we suggest that the worst resonance effects of unstable combustion in confined spaces may be avoided by ensuring that the heat-release of the combustion process is judiciously distrubuted within that confined space so as to provide an increase in the heating of the rarefaction phase of the oscillating air column. This can be achieved by the provision of two or more spaced-apart burners in which the spacing and flame size are selected so as to achieve the described heating of the rarefield regions of the oscillating air column.

The method of the invention has been successfully applied to reduce the effects of unstable combustion in a flamedriven Rijke tube as shown in Figure 3. In Figure 3, the tube 51 and optional gauze 53 are as described previously.

A primary gas supply was fed by pipe 61 to a point within tube 51 underneath gauze 53 and was caused to burn as a primary flame 63 distributed over the to surface of gauze 53. A secondary gas supply was fed by pipe 62 to a burner 64 positioned centrally within the tube 51 was caused to burn as a secondary flame 65, spaced apart from flame 63 longitudinally of the tube 57. In operation using the primary gas supply only, the tube 51 radiated sound mainly at its fundamental frequency, the flame 63 also manifesting visibly unstable combustion at this frequency.

It was found that the oscillation could be reduced or stopped at will by opening the secondary gas supply, lighting the burner 64 to give flame 65, and adjusting the gas flow rate appropriately.

This experiment demonstrates that the introduction of the further flame 65, spaced from the primary flame 63, changes the boundary conditions of the combustion process occurring within tube 51 and allows stable combustion of both flames. Viewed in another way the disturbance created by the first burner is largely cancelled by a complementing disturbing influence caused by the second burner provided the latter is sufficiently remote and the flame size is correct.

Returning now to consideration of Figure 1, the operation of the invention may now be more readily understood by analogy with the experiment just described. In order to ensure automatic operation of the invention, the engine 1 incorpoates an auxiliary reheat control unit 40 which, like main reheat control unit 39, has inputs to which signals Pc and P. and the lever angle signal, are supplied. These are ratioed with each other, and when the ratios are within a range of values known from observations to be associated with buzz a vibration condition, a valve in unit 40 is opened and fuel flows through feed pipe 42 to a circular fuel manifold 44 supported by struts 46 similar to struts 45. As in the case of main reheat combustor 12, the fuel is fed to the auxiliary reheat flame area from the manifold 44 and the flame is stabilised by V-section annular ring 44.

As a result of the additional distributed heat release occasioned by the burning of fuel at the location in the jet pipe spaced downstream from the main reheat combustor, the boundary conditions of the reheat combustion process within jet pipe 14 are changed and the reheat vibration phenomenon is avoided. This allows the thrust increase to be at the maximum rate compatible with acceptable backpressure in the jet pipe, rather than to be limited to a lower rate to suppress the generation of vibration.

It should be noted that the auxiliary reheat control unit 40 may be adapted to meter the fuel flow to the auxiliary reheat combustor according to a schedule, as for the main unit 40.

However, a simple on/off arrangement may also be adequate, the "on" fuel flow to the secondary reheat combustor being the maximum necessary to avoid combustion induced vibration at the most extreme conditions of combustion likely to be encountered. In this latter case, adjustment of total fuel flow to match engine back-pressure requirements is left to the main reheat control unit 39.

Although the two reheat combustors have been described as main and auxiliary combustors, combustors capable of equal power outputs could be utilised. In such a case it would be prefered always to have the two combustors in operation together, controlled by the same reheat control unit.

Claims

1. A method of reducing combustion induced vibrations in a combustion chamber which includes a main burner, comprising the step of introducing a second flame into the chamber at a position remote and downstream from the main burner, the position of introduction and the size of the flame being selected so that the heat release of the combustion process is distributed within the chamber in a manner which reduces the occurrence of combustion induced vibration.

2. A method according to claim 1 in which the combustion chamber is a reheat combustion system in a gas turbine aeroengine comprising a jet pipe, and the main burner is a reheat combustion means in the jet pipe, comprising the step of selectively burning fuel in at least two locations in the jet pipe substantially spaced apart from each other longitudinally thereof, whereby the boundary conditions of the reheat combustion process are modified to reduce the occurrence of reheat buzz.

3. A method according to claim 1 or claim 2 further comprising the step of monitoring the pressure fluctuation level in the combustion chamber, and if it exceeds a given threshold supplying fuel to the second burner (second flame).

4. A method according to claim 1 or claim 2 in which compressed air and fuel are supplied in a controlled manner to the second flame, to produce more compact ignition and control the vibration damping effect of the second flame.

5. A method according to any one preceeding claim in which the axial spacing of the main burner and the second flame is of the order of 1/3 of the length of the chamber (eg the jet pipe).

6. A method according to any one of claims 2 to 4 in which the preferred spacing varies from between one jet pipe diameter up to most of the pipe length.

7. A method according to any one preceeding claim in which the spacing is sufficient to cause a significant change in phase of the fundamental (standing wave) mode as between the two burner positions.

8. A reheat combustion system for a gas turbine aeroengine which includes a jet pipe and main reheat combustor, a second auxiliary reheat combustor axially spaced from the first main combustor by a substantial distance within the jet pipe such that the heat release of the combustion process is distributed within said jet pipe in a manner which reduces the occurrence of reheat buzz.

9. A system according to claim 8 in which the second auxiliary combustor is positioned downstream of the main combustor and its flame is adjusted so as to neutralise a standing wave set up in the chamber due to the flame instability of the main combustor.

10. A reheat combustion system for a gas turbine aeroengine substantially as herein described with reference to, and as shown in, the accompanying drawings.

10. A system according to claim 8 or claim 9 in which the auxiliary reheat combustor is provided with its own control means selectively actuable to supply fuel thereto on occasions when reheat buzz is likely to arise.

11. A method of reducing combustion induced vibrations substantially as herein described with reference to, and as shown in, the accompanying drawings.

12. A reheat combustion system substantially as herein described with reference to, and as shown in, the accompanying drawings.

Amendments to the claims have been filed as follows 1. A method of reducing reheat buzz in a jet pipe which constitutes the reheat combustion chamber downstream of a gas turbine in an aeroengine, comprising the step of introducing a second flame into the chamber at an auxiliary burner in the jet pipe downstream from a main burner producing a first flame in the jet pipe, , the position of introduction and the size of the second flame being selected so that the heat release of the combustion process is distributed within the chamber in a manner which reduces the occurrence of reheat buzz.

2. A method according to claim 1, further comprising the step of monitoring the pressure fluctuation level in the jet pipe, and if it exceeds a given threshold supplying fuel to the auxiliary burner.

3. A method according to claim 1 or claim 2 in which compressed air and fuel are supplied in a controlled manner to the auxiliary burner, to produce more compact ignition and control the vibration damping effect of the second flame.

4. A method according to any one preceeding claim in which the axial spacing of the main burner and the auxiliary burner is of the order of 1/3 of the length of the jet pipe.

5. A method according to any one preceeding claim in which the spacing is sufficient to cause a significant change in phase of the fundamental (standing wave) mode as between the two burner positions.

6. A reheat combustion system for a gas turbine aeroengine which includes a jet pipe constituting a reheat combustion chamber downstream of the turbine and main reheat combustor in the jet pipe, a second auxiliary reheat combustor in the jet pipe axially spaced downstream from the first main combustor by a distance within the jet pipe such that the heat release of the combustion process is distributed within said jet pipe in a manner which reduces the occurrence of reheat buzz.

7. A system according to claim 6 in which the second auxiliary combustor has its flame adjusted so as to neutralise a standing wave set up in the chamber due to the flame instability of the main combustor.

8. A system according to claim 6 or claim 7 in which the auxiliary reheat combustor is provided with its own control means selectively actuable to supply fuel thereto on occasions when reheat buzz is likely to arise.

9. A method of reducing reheat buzz substantially as hereinbefore described.