Experimental study of the beating behavior of thermoacoustic self-excited instabilities in dual swirl combustors

Abstract

The main objective of this work was to investigate the beating behavior of acoustically self-excited instabilities in dual swirl combustors. This was achieved by measuring fluctuations in sound pressure and heat release, and analyzing the resulting frequency maps to evaluate the propensity for thermoacoustic instability. The experiments were carried out while varying the combustor length and thermal power. Consequently, the frequency map representing the sound pressure of the flame instability can be divided into two modes exhibiting different behaviors. According to the resonance frequency equation, the frequency of the thermoacoustic instability increases with the thermal power and decreases with the combustor length. In the case of Mode 1, when the thermal power is relatively high, the propensity for predominant frequencies is consistent with the trend indicated by the resonance equation. In Mode 2, on the other hand, when the thermal power is low, the frequencies remain nearly constant even when the combustor length increases. In the case of lower thermal power and longer combustor lengths, a new wave pattern is observed, in which two signals of similar cycles merge to generate a new signal. Under these conditions, the combustion intensity is approximately 2.0–3.0 MW/m3 atm and the gradient of the combustion intensity decreases. This is caused by a beating phenomenon, which we verified by inputting the experimental results into the beating equation. We also analyzed high-speed images and measured the heat release fluctuations in the pilot flame, and confirmed that the pilot flame and main flame have different cycles, thus leading to beating.