Inverse diffusion and partially premixed flames with elliptical/swirling- and cross-flows

Abstract

The combustion performance of inverse diffusion and partially premixed flames was optimized by employing concentric elliptical ports with variable relative angular orientation, co-swirling, and cross-flow opposing jets of different inclination angles. The flexible and favorably active control of combustion was pronounced by changing the nozzle relative angular orientation. The optimum angular shift was found to be 20° where the flame length was shortened by 39% from the case of 0° and the NOx levels were reduced to about 0.51 g/kg of fuel for the inverse partially premixed flames. At such optimum angular shift, the minimum CO and NOx emissions respectively decreased to 17.1 and 1.52 g/kg of fuel for the inverse diffusion flames. The elliptical jets increased the size of the swirling flow recirculation zone by about 28%. Increasing the inner aspect ratio consistently increased the combustion efficiency, where the CO emissions reach minimum values of 12.3 and 6.22 g/kg of fuel respectively for the inverse diffusion and partially premixed flames. The correspondingly minimum NOx emissions were thus 0.89 and 0.51 g/kg of fuel at a cross-flow momentum flux ratio of 2.0. Splitting the opposing cross-flow air jets by increasing their inclination angle above 0° duplicated the stagnation zone vortical impact to enrich the average turbulent energy at the major axis tips. The inclination angle of 15° was the optimum at no swirl while increasing the outer swirl angle shifted the optimum inclination angle to 25°. The inner/outer co-swirl of 45°/60° thus provided a minimum CO emission of 11.1 g/kg of fuel for the inverse diffusion flame and 2.65 g/kg of fuel for the inverse partially premixed flame.