Computational study of turbulent unconfined swirl flames

Abstract

This study investigates the performance of Reynolds Averaged Navier-Stokes technique in predicting the behavior of swirl flames as well as a parametric study on the effects of fuel jet velocity and swirl number on the structure of the swirl flames. Two turbulence models which are realizable k-e and standard k-? from RANS technique were chosen and applied as the closure model. Comparison of simulation results were done with the experimental evidence obtained from Sydney University database and it is found that both models show good performance in predicting the turbulent swirl flame near the vicinity of the burner exit plane. However, due to isotropic nature of the two-eddy viscosity model, turbulent swirl flows farther downstream were not accurately captured. Parametric studies on the effects of fuel jet velocity and swirl number on the swirl flame structure were done. Simulation of turbulent unconfined swirl flames shows that the structure of the swirl flames is consists of outer and inner recirculation zones. The outer recirculation zone occurs due to the bluff-body effect; meanwhile the inner recirculation zone occurs farther downstream due to reversed flow. For fuel jet velocity, an increase in the velocity causes suspension of the occurrence of secondary recirculation zone. In this case, for higher fuel jet velocity, longer time is required for the fuel jet to decay and recirculation zone to form. In addition to that, the flame height increases with increasing fuel jet velocity. On the other hand, an increase in swirl number causes an increase in flame width and flame height. Results from the simulation have also shown that for low swirl number of 0.3, recirculation zones were absent. As swirl number increases, the tangential momentum of air flow increase and therefore greater adverse pressure gradient will be produced. As a result, fuel jet is prohibited to travel farther downstream, and flow is reversed back into the recirculation zone.