Influence of Boundary Condition Treatment on Longitudinal-Mode Combustion Instability Predictions

摘要

Two inlet boundary conditions have been evaluated for an experimental longitudinal-mode combustor that is capable of sustaining high-amplitude combustion instabilities. These include the actual complex choked slot boundary used in the experimental configuration and a uniform flow boundary condition that approximates the choked flow inlet. A three-dimensional study using different inlet boundary conditions and identical operating conditions was used to elucidate the key differences between these two boundary treatments. The simulations for the two inlet conditions show good qualitative agreement in the overall predictions, but some notable quantitative differences are indicated. For instance, the acoustic-mode shapes of the first two longitudinal modes are in close agreement. However, the uniform flow inlet does not correctly predict the third (and higher) harmonic modes observed for the slots simulation and in the experiments. Moreover, the amplitude of the first longitudinal mode is overpredicted, whereas the third mode is significantly under-predicted for the uniform flow condition. The choked slots are found to produce a great deal of vorticity, which persists downstream into the combustion chamber, whereas the uniform flow boundary condition produces significantly less vorticity in the core oxidizer flow. These differences produce effects, which lead to different heat release distributions that cause a higher Rayleigh index for the uniform flow case. Consequently, quantitative differences in the amplitudes of the instabilities are evident for the first and third modes. It is therefore evident that the use of boundary condition approximations need to be carefully assessed for combustion stability predictions.