FR2633370A1 - Process and device for active control of combustion instabilities - Google Patents

Abstract

The invention relates to a method and device for active control of combustion instabilities in a combustion chamber 1, in which the instabilities in the chamber are detected, and the flow 8, 6 of fuel injected into the chamber is modulated. chamber according to the instabilities detected.

Description

 The present invention relates to a method and a device for active control of combustion instabilities.

 Combustion instabilities are oscillations of large amplitude which appear in the combustion chambers of installations such as aircraft engines, rocket engines, ovens or industrial burners. These oscillations are extremely harmful and can lead to a reduction in system performance, an increase in the noise emitted, and even, in some cases, the complete destruction of the hearth as a result of the vibrations generated.

 There is already known a method for active control of these instabilities in which a microphone is used to detect variations in pressure in the chamber. The signal from this microphone is processed by a microcomputer and returned to compression chambers formed by loudspeakers installed in the flame supply air upstream of the fireplace.

 When the transfer function between the microphone signal and the loudspeaker is correctly chosen, the combustion instability can be eliminated.

 Although generally satisfactory, this method has the drawback of consuming a large amount of energy due to the large amount of air to be excited, and of being moreover difficult to implement in terms of control.

 The present invention aims to overcome these drawbacks.

 To this end, the invention firstly relates to a method for active control of combustion instabilities in a combustion chamber, in which the instabilities in the chamber are detected, characterized in that the flow is modulated of fuel injected into the chamber depending on the instabilities detected.

 The detection of instabilities can consist, for example, of detecting pressure variations or oscillations of the flame in the combustion chamber.

 As regards the modulation applied to the fuel flow rate, it may simply comprise a time delay with respect to the instabilities detected, or else a more complex transfer function obtained using a microcomputer, which may be better. adapted to the particular physical problem and also present a self-adaptation function.

 Modulating the fuel flow has a number of advantages over the excitation of air upstream of the fireplace.

 First, the flow to be modulated is much lower so that the energy required for control is reduced.

 Furthermore, the excited fluid is often liquid, which facilitates the implementation of the method.

 Finally, fuel flow control devices are easier to implement in industrial situations, because the speed regulation is already generally done on the fuel.

 The present invention also relates to a device for implementing the above method characterized in that it comprises detection means for detecting instabilities in the combustion chamber, means for controlling the flow rate of fuel injected in the room, and control means for controlling the control means according to the detected instabilities.

 The aforementioned detection means may include, for example, a microphone mounted in the combustion chamber, or an optical sensor mounted to detect the oscillations of the flame in this chamber.

 Control means can be constituted by a loudspeaker mounted on the fuel supply line of the combustion chamber.

 Finally, the control means can consist of a delay line, or comprise a microcomputer capable of implementing a more complex transfer function.

A particular embodiment of the invention will now be described by way of non-illustrative example with reference to the accompanying drawings in which
FIG. 1 is a schematic representation of a device according to the invention and,
- Figure 2 is a diagram showing the results obtained with a device of this type.

 FIG. 1 represents a combustion chamber I supplied with air by a suction pipe 2 and with fuel by a supply line 3 connected to a burner 4.

 In order to eliminate the combustion instabilities in the chamber 1, a microphone 5 is mounted in this chamber and connected to a delay line 6, possibly via an amplifier.

 The output of the delay line 6 is applied to the input of an amplifier 7, the output of which controls a loudspeaker 8.

 The diaphragm of the loudspeaker 8 forms one of the walls of a compression chamber 9 connected to the fuel supply line 3.

 The oscillations detected by the microphone 5 are therefore sent back to the supply line 3 via the delay line 6, so as to modulate the flow of fuel injected into the chamber by the burner 4.

 It has been observed that such a device makes it possible to completely eliminate the combustion instabilities in the case of a laminar burner.

 Figure 2 shows the results obtained in the case of a turbulent burner.

 This figure represents the noise level measured in Pascals as a function of the delay time in milliseconds applied by the delay line 6.

 In the absence of a control device, the noise level was 200 Pa (about 140 dB acoustic) and the frequency of the instability was 280 Hz (a period of 3.6 ms).

 On the other hand, when the method according to the invention is implemented, the noise is reduced by half, over a very wide range of phase shift values. This range (from 13 to 14.2 ms) corresponds approximately to 1/3 of the period of the phenomenon, and shows that the system is not very sensitive to variations in parameters.

 In fact, a spectral analysis not shown reveals that the residual noise is due to turbulent combustion but that the instability at 280 Hz has disappeared. In addition, an observation of the combustion chamber by ultrafast strioscopy (10,000 images / second) shows that the oscillations of the flames which were the source of the instability have also disappeared.

 Various variants and modifications can of course be made to the above description without departing from the scope or the spirit of the invention.

 Thus, in particular, other means of controlling the fuel flow could be used, for example a rotary valve or an adjustable restriction in the line 3.

 Likewise, the microphone 5 has been placed on the combustion chamber 1 but it could also be placed upstream of this chamber on the suction duct 2.

Claims (9)

 1 - Method for active control of combustion instabilities in a combustion chamber, in which the instabilities in the chamber are detected, characterized by the fact that the flow rate of fuel injected into the chamber is modulated as a function of the instabilities detected.  2 - Method according to claim 1, characterized in that, to detect instabilities, it detects pressure variations in the combustion chamber.  3 - Method according to claim 1, characterized in that, to detect instabilities, detecting the oscillations of the flame in the combustion chamber.  4 - Method according to any one of claims 1 to 3, characterized in that the modulation applied to the fuel flow comprises a time delay relative to the instabilities detected.  5 - Device for implementing the method according to any one of claims 1 to 4, characterized in that it comprises detection means (5) for detecting instabilities in the combustion chamber, control means (8) of the flow of fuel injected into the chamber, and control means (6) for controlling the control means as a function of the instabilities detected.  6 - Device according to claim 5, because terérized in that said detection means comprise a microphone mounted in the combustion chamber.  7 - Device according to claim 5, characterized in that said detection means comprise an optical sensor mounted to detect the oscillations of the flame in the combustion chamber.  8 - Device according to any one of claims 5 to 7, characterized in that said control means comprise a loudspeaker mounted on the fuel supply line of the combustion chamber.  9 - Device according to any one of claims 5 to 8, characterized in that said control means comprise a delay line.