Flame thickening and its consequences for the thermoacoustics of premixed flames

Prediction of thermoacoustic combustion instabilities via low-order modeling approaches requires a model for flame dynamics, which can be inferred from large eddy simulation (LES). Modeling errors associated with the thickened flame model for LES(TFLES), a combustion model widely used in this context, are benchmarked in the present study. In particular, the thickening factor—a fundamental parameter of the TFLES model—is varied firstly by changing the grid, while keeping the flame resolution constant. Secondly, the thickening factor is changed while using a constant computational grid. A fully flame-resolved simulation without the use of a turbulence–chemistry interaction model serves as a reference. A global chemical mechanism is optimized for use in LES of a partially cracked ammonia flame in order to facilitate cost-effective simulation. Its performance in canonical one-dimensional configurations is assessed versus a detailed chemical scheme. From the LES time series data, flame transfer functions (FTFs) are identified and compared to experiment qualitatively as well as to each other. It is demonstrated that the number of grid cells used to resolve the flame front, rather than the thickening factor, is the TFLES model parameter with the most impact on predictions when using a static modeling constant in the efficiency function. It is also demonstrated that thickening introduces damping of high-frequency heat release fluctuations in turbulent flames. However, the FTFs of the present configuration approach zero gain already at lower frequencies.