Numerical studies on vortex structures in the near-field of oscillating diffusion flames

Large-eddy simulations are used to investigate unsteady low-speed buoyant jet diffusion flames. The numerical method is based on a predictor-corrector approach for low Mach number compressible flows. The infinitely fast chemistry of "mixed-is-burned" is adopted as the combustion model. The dynamical phenomena of puffing and formation of large vortex structures are well captured. The entire flow oscillates and the dominating low-frequency is independent of locations in the flow field, characteristic of a self-excited global instability. The pulsation frequencies are found to be insensitive to the heat release rate and the coflow velocity. In general, the buoyancy-sustained sheer-layer instability can be best described by the Richardson number, as in the experimental observation of Cetegen. A quantitative description of the flow field is given by the instantaneous, mean and rms profiles of the axial velocity, temperature, major species and mixture fraction. Off-axis peaks exist in the radial profiles of the mean axial velocity, temperature and their corresponding rms quantities. The existence of inflection points in the mean velocity distributions, indicative of large-scale vortical structures, is consistent with the observation of Lingens et al.