FR2657921A1 - - Google Patents

Abstract

The invention relates to an active control (3) for controlling instabilities due to combustion in a combustion chamber (17) having means for supplying atomized fuel mounted in the chamber. The active control includes pressure transducer means (43) for measuring pressure fluctuations in the combustion chamber and servo valve means (53) for modulating the amount of fuel entering the atomized fuel supply means. A phase shift means (47) connects the pressure transducer to the servo valve. The phase shift is adjusted so that the amount of fuel supplied for combustion during pressure peaks is reduced in order to decrease pressure fluctuations in the combustion chamber. Application to jet engines.

Description

The present invention relates to an active command to suppress the

  low-frequency instabilities caused by combustion in the engines, the chambers of

  combustion and afterburner chambers.

  Most aircraft engines, their afterburner chambers, and gas and liquid-fueled ground-based combustion chambers exhibit, under certain operating conditions, instability of some type or other, which is caused by combustion The instabilities in the main engines of airplanes and combustion chambers attached to the ground are generally in the low frequency range whereas those encountered in the afterburner chambers are

  oscillations of both low frequency and high frequency.

  It is conventional that instabilities in the high frequency range are susceptible to passive processing by acoustic gaskets and other such means. However, these devices are not very effective in suppressing oscillations in the range of frequencies. low

frequencies.

  The combustion chambers of the main aircraft engines and their afterburner chambers as well as the combustion chambers of the turbines present under certain conditions oscillations caused by unstable combustion following the coupling of unstable oscillations due to the release of heat. and fluctuations in the sound pressure Such oscillations, if not suppressed, can in extreme cases become very important and result in a physical disintegration of the material At worst, such oscillations result in

  an unacceptable fatigue of the metal and are therefore undesirable.

  Conventional means for suppressing such oscillations in practical systems are limited to passive devices such as acoustic gaskets or Helmotz resonators.

  type are relatively ineffective in suppressing

  low frequency (500 Hz or less) devices

  currents are limited in terms of their operating envelope.

  and do not act at their optimum to avoid

  potentially harmful oscillations.

  Laboratory research on active controls was done using loudspeakers

  as an actuating means for modifying the conditions

  acoustic limits in the combustion chambers.

  Active speaker-based control methods, while very attractive as search tools, are not a practical means for suppressing instabilities in the high frequency range because of the relatively low output energy of the speakers. and

  of their large physical dimensions.

  Work has been carried out in which active controls have been implemented by modulating a certain percentage of the gaseous fuel flow at a location very close to the flame stabilizer. A noise reduction of up to 1 Od B has been reported in a laboratory-scale combustion chamber by modulating the gas flow by using on-off actuators in a jet mode However, the use of a gaseous fuel is limited to the combustion chambers of gas turbines, and for most practical devices including combustion chambers and afterburner for aircraft using a liquid fuel, there is no known study of application of active control methods to suppress oscillations caused by combustion using modulation The present invention is applicable to both main and post-combustion chambers. mbustion in

  aircraft engines and in main combustion chambers

  In such devices, the fuel is generally injected just upstream of the flame stabilizer in the form of a fuel.

  spraying liquid with some sort of atomisa-

  The flame is stabilized in the recirculation wave or downstream vortices

flame stabilizer.

  The present invention is an active control to suppress instabilities in the low frequency range caused by combustion in engines, combustion chambers and afterburners without

impair performance.

  According to one aspect of the present invention, provision is made

  active control to control the instabilities caused by

  The active control comprises pressure transducer means for measuring the fluctuations of the pressure in the combustion chamber and a servo valve means. for modulating the amount of fuel supplied to the means providing the atomized fuel A phase shift means connects the pressure transducer to the servo valve The phase shift is adjusted so that the amount of fuel supplied for combustion during the pressure peaks is reduced so as to lower the fluctuations of the

  pressure in the combustion chamber.

  The following description refers to the figures

  4 - appended which represent respectively Figure 1, a sectional view of a jet engine, given by way of example, with an active command to suppress the instabilities caused by combustion in the low frequency range in the combustion chamber according to an embodiment of the present invention; FIG. 2 is a sectional view of a reaction engine, given as a representative, with active control to eliminate instabilities caused by combustion in

  the range of low frequencies in the post-chamber

  combustion, according to another embodiment of the present invention; FIG. 3 is a graph of the power spectrum measured on a logarithmic scale in volts by a

  dynamic pressure transducer according to the frequency

  quence in a simulated post-combustion chamber, an embodiment of the active control of the present invention being out of service; and FIG. 4, a graph of the power spectrum measured on a logarithmic scale in volts by a dynamic pressure transducer as a function of frequency in a simulated afterburner chamber, an embodiment of FIG.

  the active control of the present invention being in operation.

  Reference will now be made to the drawings in which the same reference numerals represent identical elements and in particular to FIG. 1, in which a jet engine 1 with an active control modulating the flow of the fuel supplied to the engine is shown. One of the fuel injectors 5 of the combustion chamber The engine 1 comprises a generally cylindrical housing 7 open at both ends A compressor 11 comprises a rotor on a common shaft with the rotor of a turbine 13 The compressor and the turbine are located in the housing From

  air is introduced into an intake port of the compres-

  through an admission diffuser and a -

  pipe system 15 located at one end of the housing.

  The air is then compressed by the dynamic compressor 11 and introduced into an annular combustion chamber 17. A multitude of fuel injectors 5, each located in the center of a swirl section 21, introduce fuel to be mixed. In the air and burned Downstream of the combustion chamber, the heated air expands in the turbine to obtain power to drive the compressor Air can be further heated by the additional combustion of a more The post-combustion chamber is located near the other end of the housing and comprises a diffuser cone 25, a multitude of fuel spray rods 27 and a large amount of fuel in a combustion chamber called an afterburner. extending to the interior of the housing A first set of spray bars surround the conical diffuser 25; another set of spray rods extends radially inwards, further downstream The spray bars each have orifices perpendicular to the air flow, which causes the atomization of the fuel and its vaporization by the current When the fuel exits the spray bar A short distance downstream of the fuel leaving the first set of spray bars is a first flame retardant stabilizer 31 having a V-shaped cross-section, so-called Also V-shaped gutter The top of the V is pointed upstream The boom 33 gives an indication of the direction of the air flow A second annular flame stabilizer 35 having a V-shaped cross section is downstream of the first flame stabilizer, a short distance downstream of the

  fuel leaving the second set of spray bars.

  The second flame stabilizer is held by support members 37 from the housing. The first flame stabilizer is held in place by elements of the flame.

  support 41 from the second stabilizer.

  An active control comprising a dynamic pressure transducer 43 is located in the combustion chamber downstream of the spray nozzle 5 to measure pressure fluctuations. The electrical signal supplied by the pressure transducer is applied to a filter and to a a pre-amplifier shown in a block 45 The filter eliminates the DC component of the signal as well as the high frequency noise The amplified and filtered signal is applied to a phase shifter represented in a block 47 which is adjusted with respect to the delay introduced by the position of the sensor with respect to the fuel injector, the delay caused by the servovalve and the delay due to the evaporation of the fuel and its combustion The adjustment of the phase-shifter can be carried out by adjusting the value of the phase shift required during the actual operation or an adaptive control can be used which provides the appropriate phase shift at different frequencies The phase-shifted signal is provided via an amplifier shown in a block 51 to a servo-valve 53 which is connected to a control line.

  fuel 55 supplying one of the fuel injectors 5.

  The servovalve 53 modulates a part of the flow of the fuel supplied through it The response of the reaction control is limited mainly by that of the servovalve 53. A servovalve with a response of 150 Hz can be obtained, for example, in the 73 series manufactured by Moog Inc., East Aurora, New York, by

example.

  The main mechanism responsible for instabilities in the high frequency range is thought to be the interaction between unstable oscillations due to heat and fluctuations in sound pressure and the fact that the resulting interaction represents a very delicate balance between energy supply to the system and the energy it loses Lord Rayleigh was the first to bring out -7-

  for the instability of combustion to continue

  In itself, unstable heat generation and unstable pressure must be on average "in phase" during a cycle. Most practical combustion chambers are very lightly damped and the balance between energy to the system as a result of phasing unstable heat and pressure release and energy loss through the system caused by acoustic losses and other viscous losses is very delicate A minimal phase shift of unstable heat release against the oscillations of sound pressure in the right direction can, therefore, be sufficient to counteract this delicate balance and remove such oscillations The active control modulates the flow of fuel (and consequently the unstable release of heat ) following a small percentage in a reaction mode to result in a sufficient phase shift between the unstable release of heat and the fluctuations of the unstable pressure to uncouple them

  effectively and thus suppress oscillations Modula-

  flow of fuel through the reduced servo-

  the flow of fuel available for the

  burning during pressure peaks.

  Now in conjunction with FIG. 2, there is shown

  A jet engine 1 of the type illustrated in FIG. 1 as well as an active control unit 3 is shown. A dynamic pressure transducer 43 is not placed in the combustion chamber 17

  as before, but is in the post-chamber

  23 A servo-valve 53 modulates the flow of fuel supplied to the spray bars 27 and not to the fuel injector. 5 In operation, liquid fuel is injected into the co-ordinated hot air stream through small orifices. in the fuel spray bars just upstream of the flame stabilizers 31 and 35 (in this case, V-shaped gutters). The fuel then mixes with the hot air and is atomized and 8-vaporized while downstream propagation While the partially premixed fuel-air mixture circulates on the lips of the flame stabilizers, vortices are caused in the span direction. The vortices have an axis of rotation perpendicular to the direction of flow. Vortices then propagate downstream, entrain hot circulating products, mate and grow, and after a certain period of time (depending on the speed of the fuel, etc.) br blow and release heat which then changes the dynamic pressure field in

  the afterburner chamber 23 The resulting fluctuations

  pressure of the lip of the V-shaped gutter in turn causes another set of vortices and the process repeats itself If the frequency with which this process occurs is paired with the acoustic resonance mode of the post-combustion chamber (depending on the geometry), a coupling takes place and instabilities develop in the range of the high frequencies The aim of the vortices is to mix the reagents relatively colder with the hot products in circulation and consequently play a determining role in the Flame Retention However, the same vortices result in oscillations caused by unstable combustion now, even if the fuel is injected upstream of the flame stabilizer

stably.

  The additive control 3 modulates the fuel injected upstream of the flame stabilizer in a small amount with the appropriate phase shift so that the vortices that burn in phase with the oscillations of pressure contain a little less fuel (and therefore release some energy. weaker) than the vortices which are out of phase This solution results in a release of the heat which is sufficiently "out of phase" with respect to the oscillations of the pressure, "on average" to result in the suppression of oscillations However, the phase shift 9 - appropriate that is necessary can not be calculated a priori because the delays are not known; therefore, the

  control operates in a jet mode.

  A 7.5 x 7.5 cm combustion chamber with instabilities in the high frequency range was used.

  quences to simulate post-combustion operation.

  The flame stabilizer was of the V-channel type (33% lock) and the fuel was injected at a distance of 15 cm upstream of the flame stabilizer using a conventional vortex atomization injector.

  preheated to a temperature of about 3150 C by a device

  tif electric preheating to help the atomization of liquid fuel (kerosene) In the absence of command

  active, the combustion chamber presented a mode of insta-

  longitudinal quarter wave with a dominant frequency

  130 Hz Figure 3 shows the power spectrum measured by a dynamic pressure transducer

  located 60 cm upstream of the flame stabilizer.

  The feedback loop of the active command was

  consisting of a transducer 60 cm upstream of the stabilizer

  flame tester which was used as a sensor, a high-order bandwidth filter of the Kronhite type, an amplifier of Tektronix, a blocking amplifier PAR as a phase-shifter, and a servovalve so-called Moog electronically controlled as an actuator The servo valve modulated fuel flow in response to the processed input signal With a suitable amplification and a suitable value for the phase shift in the feedback loop, a significant reduction in dynamic activity was observed Figure 4 shows the measured spectrum of power at the same location in the combustion chamber while the active control system is running As seen in the figure, there is a considerable drop in the dynamic activity at the dominant frequency of the oscillations The level of the overall sound pressure in the combustion chamber has been found to be reduced by about 4 dB If the active control system is "running" There is a minimal additional energy input for the system and it has been estimated that the fuel flow was modulated by only 5% of the average fuel flow. that higher-order modes have been significantly reduced even though they are outside the width of

actuator band.

  The reaction control shown in FIG. 1 and in FIG. 2 can be implemented in the form of digital and analog circuits.

  only a fuel injector that receives a fuel

  the fuel supplied to several injectors can be modulated In a similar way, the fuel supplied to several

also be modulated.

  The above description related to an order

  active to suppress instabilities due to low frequency combustion in engines, combustion chambers

  and combustion chambers without impairing performance.

he -

Claims (5)

  1 Active command (3) to control instabilities   Combustion chambers in combustion chambers (17) with means (31, 41) for supplying atomized fuel in the combustion chamber, characterized by comprising: pressure transducer means (43) for measuring the fluctuations of the pressure in the combustion chamber, a servovalve means (53) for modulating the quantity of fuel entering the atomized fuel supplying means, and a phase shift means (47) coupling the pressure transducer to the servo valve, the phase shifting means being adjusted so that the amount of fuel supplied for combustion during the pressure peaks is reduced in order to reduce the fluctuations of the pressure in the chamber of combustion.   Active control according to claim 1, characterized   in that the servovalve means (53) modulates about 5%   of all the fuel supplied to the combustion chamber tion.   Active control according to claim 1, characterized   in that it further comprises a filter means (45) located between the pressure transducer means (43) and the phase shift means (47) for removing the component   DC signal as well as noise.   Active control according to claim 3, characterized   characterized in that it further comprises a means of amplifying   (51) mounted between the phase shift means (47) and the servo-valve (53) to increase the gain of the signal provided by   the phase shift means for controlling the servovalve means.   A jet engine having reduced instabilities of combustion, characterized in that it comprises: a combustion chamber (17); Means (31,35) for supplying atomized fuel mounted in the combustion chamber; pressure transducer means (43) for measuring the fluctuations of the pressure in the combustion chamber; servovalve means (53) for modulating the fuel supplied to the atomized fuel supplying means, and phase shift means (47) coupling the pressure transducer means by servovalve, the servovalve means being adjusted so that the amount of fuel supplied for combustion during peak pressure is reduced to reduce pressure fluctuations in the combustion chamber.   6 jet engine according to claim 5, characterized in that the combustion chamber is a afterburner chamber (23).