Large-Eddy Simulation of High-Frequency Combustion Instability in a Single-Element Atmospheric Combustor

Large-eddy simulation is performed to simulate high-frequency combustion instability in a single-element atmospheric combustor. Simulations are conducted for corresponding combustion-instability experiments, and the self-excited combustion instability is successfully captured. The first tangential mode of the combustion chamber is excited in the large-eddy simulation, and the amplitude and frequency of the pressure fluctuations are consistent with the experimental observations. The first tangential mode in the large-eddy simulation was observed at 1 kHz, and the peak-to-peak amplitude was approximately 4% of time-averaged pressure. The higher-order modes were also observed in the large-eddy simulation at frequencies ranging from 2 to 4 kHz, although those amplitudes were approximately one-fourth of the first tangential mode. The coupling mechanism between the flame and acoustic mode is explored based on the large-eddy-simulation results. The periodic ignition of the unburntH(2)/O-2 mixture exhibits lifted combustion in a pulsating motion, and the frequency is similar to the first-tangential-mode frequency. This unsteady pulsating-flame behavior is caused by the coupling between the fuel injection and the first-tangential-mode oscillation. The Rayleigh index indicates that a primary driving factor for the instability is the acoustically coupled pulsating-flame motion. The present results demonstrate that large-eddy simulation can accurately capture the unsteady heat release and its coupling with pressure oscillations. The large-eddy-simulation results clarify details of flame structures that are not completely understood from the experimental measurements. These results are also valuable for understanding the coupling mechanism of the flame and acoustic mode in a combustion chamber.