Flow Dynamics in a Swirl Combustion System

Swirling flows provide intensive mixing in a small volume in addition to flame stabilization by the formation of a central recirculation zone which back-circulates heat. A numerical study of unsteady isothermal flow in a pilot scale pulverized- fuel swirl combustor has been performed using the standard k _ epsilon turbulence model and Reynolds stress model (RSM). The objective of this work is to compare the numerical predictions of the two turbulence models. The precession is induced by a tangential inlet, for primary air, and guide vanes, for secondary air. A sequence of CFD solutions on successively finer meshes is examined and the order of accuracy of the solutions is estimated by Richardson extrapolation. The results for a Reynolds number of 54 425 and 76 400, swirl number of 0.78 and 0.82 for primary and secondary air, respectively, show the presence of a large and asymmetric region corresponding to the precessing vortex core. Moreover, spiral and bubble type vortex breakdown have been observed and their frequencies analyzed by the monitored of velocities using Fast Fourier Transform (FFT).