Different Experimental Approaches for Characterization of Thermo-acoustic Instabilities

Abstract

The coupling between pressure fluctuations and heat release variations during the combustion process is responsible for occurrence of thermo-acoustic instability. These instabilities will cause many adverse effects in power-producing devices. To control thermo-acoustic instabilities, its characterization is essential for design and implementation of control technique. The characterization of thermo-acoustic instabilities can be done using pressure amplitude and its frequency. In this study, the authors have presented two different approaches for thermo-acoustic instability characterization. In the first approach, the wall pressure measurements were taken at eleven locations, along the longitudinal directions, using differential pressure transducer. In the second approach, acoustic pressure outside the Rijke tube is measured. To check the consistency in the results, measurements are taken at different equivalence ratios and total mass flow rates. The result shows that the sound pressure level is higher in case of wall pressure measurement as compared to acoustic measurement outside the Rijke tube. The peak frequency is similar in both the cases, over a wide range of equivalence ratios and total flow rates. The captured standing wave pattern is similar to third mode of instabilities, but it was affected by the pulling force due to use of suction pump at the other end of Rijke tube. The suggested approaches can be effectively used to design the control techniques for suppression of thermo-acoustic instabilities. The acoustic pressure measured outside the Rijke tube is able to give the exact frequency of thermo-acoustic instabilities and also free from disruptions of flow. The exact prediction of mode of instability is essential, to identify location for mounting the pressure transducer on the wall of combustor, which was the major drawback of wall pressure measurements. The study further concluded that the intensity of thermo-acoustic instability is dependent on equivalence ratio.