The coupling of turbulence and chemistry in a premixed bluff-body flame as studied by LES

In previous work a subgrid scale fractal model for large eddy simulation of turbulent combustion was developed and validated. In the present article the fractal model applicability is tested by simulating a bluff-body premixed flame anchored in a straight channel. The model assumes that chemical reactions take place only at the dissipative scales of turbulence, i.e., near the so-called "fine structures" (eddy dissipation concept). The model estimates the local spatial dissipative scale eta, considering also the growth effect due to heat release, and turns itself automatically off where the local spatial filter Delta equals eta. The premixed burner is simulated in 2 and 3 dimensions, both for cold flow and for reacting cases. Results are compared with experimental data and show three-dimensional vortex structures periodically shortening the recirculation zone downstream of the bluff body and entraining fresh mixture into the hot recirculating region. This physical mechanism is involved in flame anchoring. The effect of assuming periodic boundary conditions in the spanwise direction, instead of solid side walls, is also investigated. The analysis shows that periodic boundary conditions cannot capture various effects of side walls, such as the shortening of the recirculation zone and the flow acceleration downstream; furthermore, it also does not allow predictions of wall heat transfer. The 2D reactive case results are also compared with those using RANS kappa - epsilon and LES-Smagorinsky models. Finally, comparing kinetic energy spectral densities in the nonreacting and reacting cases it is shown that large-scale fluctuations are damped in the latter and that fast chemical reactions cause a high-frequency energy peak. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.