Large eddy simulation of combustion instabilities in a co-axial stratified model combustor under transient operating conditions

This work conducted large eddy simulation investigation on the effect of transient time and transient direction on the combustion instabilities in a co-axial stratified model combustor under transient operating conditions. Two transient times, which is greater than and less than the time delay, respectively, 0.1 s and 0.1 ms, were studied in different transient direction. The transient combustion process is analyzed by the global HRR, temperature, CH and CO. The time-frequency analysis of the pilot-stage and main-stage are carried out by FFT and SWT. The mode transition characteristics of the pilot-stage and main-stage are discussed based on the recurrence plots and Poincare plots. Finally, the bifurcations of the HRR and temperature caused by the transient time are analyzed. It was found that the shorter transient time is associated with larger oscillation amplitude of temperature, as well as the greater temperature and HRR gradient, and the maximum oscillation amplitude is larger. Furthermore, due to the shorter transient time in case III-, the changing rate of heat release rate is approximately 725 J center dot m(-3)center dot s(-2), which is significantly greater than the 72 J center dot m(-3)center dot s(-2) in case III+. In case III-, the maximum CH is 2.0e-12, which is approximately 11 % higher than the maximum value in other cases. Different modes were observed in different transient time and transient direction. Longer transient time is associated with both a greater number of modes and frequency of mode transitions, and higher possibility of excitation oscillation occurring. Under a specific global average mixture fraction of 0.0345, two different flame shapes were observed in separate cases: a stratified flame and an M-shaped flame.