Nonlinear pressure-heat release transfer function measurements in a premixed combustor

Abstract

This paper describes an experimental study of the response of a premixed combustion process to imposed pressure oscillations. These data were obtained to improve current understanding of the nonlinear phenomenon in unstable combustors that play an important role in their limit-cycle behavior. They were obtained by forcing oscillations in the combustor at discrete frequencies and measuring the resulting pressure and global CH* radical chemiluminescence oscillations. These data suggest that the nonlinear relationship between pressure and heat-release oscillations, as opposed to nonlinear gas dynamic processes, play an important role in saturating the amplitude of self-excited oscillations in premixed combustors. Specifically, it was found that the ratio of the magnitude of the heat-release response to pressure perturbations decreased at large amplitudes of driving: that is, that gain saturation plays a role in the combustor's nonlinear dynamics. The role of amplitude dependence of the phase between the pressure and heat-release fluctuations is less clear from the data, however, because it was found to be nearly independent and moderately dependent on the drive amplitude at different driving frequencies. Nonlinear interactions between a natural combustor mode (at 167 Hz) and those due to the imposed driving (at 157, 190, and 235 Hz) were also observed. Specifically, we observed a steady decrease in amplitude of the natural mode oscillations with increasing amplitude of forcing, even though they were at separate frequencies. This behavior appears to be due to frequency locking of the natural mode oscillations, a well known nonlinear oscillatory phenomenon. The amplitudes at which these nonlinear interactions began to be evident were lower than that where nonlinearities in the pressure-CH* amplitude relation were observed.