On the spectral characteristics of a self-excited Rijke tube combustor-numerical simulation and experimental measurements

A computational fluid dynamics (CFD) study has been conducted to investigate the occurrence of combustion instabilities in a Rijke tube type of combustor. The purpose of the study has been to capture the reacting flow physics in the combustor using two-dimensional finite-volume solution of the conservation equations in order to predict the frequency and magnitude of the thermoacoustic instability. It has been observed from literature that past attempts at simulation of reacting flow inside Rijke tubes have not been successful at explaining the self-excited mechanism between unsteady heat release and acoustics. Instead, time-varying heat sources or instability triggering mechanisms (forced response) have been used. In the present study, time integration of the conservation equations enabled the capture of the self-excited three-quarter mode of the combustor. Results from associated experimental studies have been included to describe the acoustic signature of the combustor. Comparison between the computed and experimentally obtained results validate the theories proposed in an earlier study. The thermoacoustic instability frequency has been predicted at 190 Hz as compared to the experimental value of 182 Hz whereas the magnitude has been found to match the experimental value of 177 dB. The results confirm the ability of the CFD analysis in capturing the instability growth to a limit-cycle of pressure oscillation and its success in predicting the instability frequency as well as harmonics of the instability frequency.