Numerical Study of Stratified Flames Using Reynolds Averaged Navier Stokes Modeling

Reynolds averaged Navier Stokes technique was used to develop a validated numerical model for stratified flames. The validation was carried out with the experimental data of the non-swirl flames of the Cambridge dual annulus swirl burner. The RNG k-ϵ turbulence model along with the SG-35 skeletal chemical mechanism was found to give a good prediction of scalar and vector quantities while resulting in the reduction of computational time by 99.75% in comparison with that required for large eddy simulation techniques used in the literature. The effect of stratification at a constant input power, global equivalence ratio, and Reynolds number was examined. At stratification ratios (SRs = φin/φout) 1 and 2, intense burning, marked by the higher OH concentration, was observed close to the bluff body. Beyond SR = 2, the region of intense burning shifts downstream away from the bluff body. This is a result of the high equivalence ratio in the inner annulus, which is beyond the rich flammability limit of methane-air flames, and as a result, the primary flame region is shifted downstream after the mixtures from inner and outer annulus have mixed properly to produce a mixture with the equivalence ratio in the flammability limit. The maximum temperature was found to increase by 24.1% when the SR is increased from 1 to 2 and the combustion efficiency was found to significantly improve by 267%. The highest maximum temperature of 2249 K is observed for the mildly stratified flame at SR = 2. Beyond SR = 2, the maximum temperature decreases, while the combustion efficiency increases slightly.