Experimental investigation on characteristics of thermoacoustic oscillations in a stagnation point reverse flow combustor

In order to investigate the thermoacoustic features of stagnation point reverse flow combustors, a laboratory scale experimental setup was built, and a series of experiment cases of CH4/air flames were carried out. The combustor was designed to be coaxial, the central tube was used as the fuel injector, and the surrounding tube was adopted as the air nozzle. The combustion chamber was a quartz tube with an inner diameter of 48 mm and a height of 300 mm, the bottom of which was sealed, whereas the upper side was open to the atmosphere. The combustor faced towards the open side of the chamber and the axis of the combustor was consistent with that of the combustion chamber. Results show that the stagnation point reverse flow combustion mode could not be established with low system input power. Once the expected combustion mode was obtained, strong thermoacoustic instability is excited inside the combustion chamber. The main peak amplitude and the corresponding effective amplitude of the pressure oscillating waves increases rapidly with the equivalence ratio, and the effective sound pressure level is found to be larger than 138 dB, whereas the equivalence ratio has no impact on the main peak frequency. On the other hand, the ratio of half-peak breadth of the frequency to the peak frequency lies between 4% and 10%, indicating that the sound energy disperses around the main peak frequency, and results in a large difference (over 10 dB) of sound pressure level between the effective sound pressure level and the main peak sound pressure level. Moreover, the temperature and the NOx concentration as well as the CO concentration increase with the equivalence ratio. The reason of that the CO concentration increases with the equivalence ratio lies in that the fuel diffuses in a harder way at high equivalence ratios, resulting in much more serious incomplete combustion. © 2011 Chinese Society for Electrical Engineering.