Independence and Dependence Ranges of Equivalence Ratio on System Responses for a Nonlinear Thermoacoustic System in a Rijke Tube

Abstract

Thermoacoustic phenomenon in a combustion system, which involves with the coupling effects between unsteady heat release and pressure fluctuations, remains challengeable in industry over decades such as Rich Burn, Quick-mix, Lean Burn combustor (RQL). Since the thermoacoustic combustion system presents nonlinear response at varied parameters, the investigations for system nonlinearity have been focused on in this study. A Rijke tube system with a premixed methane-air laminar flame was utilised to trigger the self-excited oscillations with varied equivalence ratios. The multiple system analysis method including Fast Fourier Transform (FFT), phase difference, Recurrence Plot (RP) and Recurrence quantitatively analysis (RQA) were applied to obtain the characteristics of system nonlinearities. Both independence and dependence ranges of equivalence ratios on system frequency responses have been discovered, as well the corresponding characteristics of system nonlinearity have been investigated. The analysis results from oscillation characteristics and system dynamics also validate the finding of both ranges. For the thermoacoustic oscillations at varied parameters, the similarity of oscillation systems has been observed in independence range. As effective analysis method, it is found that the phase difference analysis, RP and RQA utilised in this study are capable of characterising the nonlinearity of a combustion thermoacoustic system, as well multiple system properties.

Keywords: Thermoacoustics, Rijke tube, Nonlinear system, Time-domain analysis, Recurrence analysis