Flame Dynamics During Combustion Instability in a High-Pressure, Shear-Coaxial Injector Combustor

We present a large-eddy simulation based investigation of combustion instability in the Continuous Variable Resonance Combustor (CVRC); a high-pressure, shear-coaxial injector combustor studied experimentally at Purdue University. An important characteristic of the CVRC setup is the role that the oxidizer injector length plays in the stable/unstable combustion dynamics occurring within the combustor. We perform three simulations corresponding to different stability regions; one is a semi-stable case whereas the other two cases are unstable. The acoustic pressure fluctuation exhibits a limit cycle behavior in all cases. An abrupt change in the predicted amplitude between stable and unstable cases is also observed, consistent with the experimental trends. Further investigations to characterize the mechanism driving such abrupt change in the stability pattern when the oxidizer injector length is varied, show that acoustic interactions with the flow field lead to subtle changes in the vorticity (and mixing) dynamics, which significantly alters the distribution of species and heat release. In addition, aspects related to flame stabilization such as the mean flame anchoring location and the mode of burning, premixed or non-premixed, are also affected. The analysis of flame structures shows the presence of multi-mode burning regions where premixed and non-premixed flames appear to coexist. However, the dominant burning mode switches from premixed to non-premixed within an acoustic cycle, potentially playing an important role in flame stabilization.