Active Control Of Thermoacoustic Instability

In order to meet the stringent requirements of NOx emissions, Lean-Premixed combustion was widely used in modern gas turbines. Lean-Premixed combustion could effectively reduce the peak flame temperature, thus reducing the generation of NOx. But it often led to combustion instability called "thermoacoustic instability". Experimental test facility was built to study the controlling mechanism of the self-excited thermoacoustic instability in the Rijke combustor. Rijke combustor was a lean-premixed natural-gas combustor using nitrogen transverse jet and loudspeakers for active control.Using nitrogen as transverse jet for active control, the influence of jet velocity and jet flow on the control effect were analyzed. It was found that transverse jet was an effective method to control the thermoacoustic instability in the Rijke combustor. The control effect of the the thermoacoustic instability increased linearly with the increase of the jet velocity and jet flow. Under the experiment case with jet velocity of24.00m/s and the flow of4.52L/min, the pressure vibration amplitude reduction efficiency reached98.81%, accompanied by a slight vibration.Using loudspeakers for active control, loudspeakers were placed on the fuel inlet pipe and the open end of the Rijke tube. Loudspeaker outputted sound of different frequencies and sound pressure levels for open-loop active control of thermoacoustic instability. The influences of loudspeakers at the two different positions on the control effect were analyzed. It was found that loudspeaker placed on the fuel inlet pipe was an effective method to control the thermoacoustic instability in the Rijke combustor. Sound waves from the loudspeaker could attenuate the thermoacoustic pressure oscillations. At the same time, new pressure oscillation with the same frequency of sound waves was generated. In certain experiment cases, the amplitude of new pressure oscillation caused by the sound waves could be higher than the amplitude of thermoacoustic instability. Under the experiment case with sound frequency of50Hz and sound pressure level of100dB, the pressure vibration amplitude reduction efficiency reached38.55%. It was also an effective method that placing the loudspeaker on the open end of the Rijke combustor. Under the experiment case with sound frequency of300Hz and sound pressure level of115dB, the pressure vibration amplitude reduction efficiency reached34.58%.