Inlet temperature effects on unsteady hot spots in swirling spray flame

Abstract

Effects of inlet-air temperature on the hot-spot dynamics in a dual-stage swirl spray combustor were investigated using large-eddy simulation with a flamelet-generated manifold combustion model. The mesh resolution was tested with the Pope criterion, and the predicted velocity was validated with particle image velocimetry measurements. Two kinds of flame conditions were designed with improved inlet-air temperature: the constant-velocity strategy from 403 to 473 K, and the constant-flow-rate strategy from 473 to 543 K, featured by increased inlet velocity. The results show that increased inlet-air temperature consistently improves the exit-temperature uniformity for the two strategies. The hot-spot formation is found to be sustained by diffusion-flame-dominated heat release. The formation and dissipation of the hot spots are systematically revealed. With a lower inlet temperature of 403 K, the primary recirculation zone (PRZ) is the weakest, which permits deep spray penetration and strong fuel accumulation, leading to aggregation-dominated hot spots. These hot spots are trapped within the PRZ and persist downstream, thereby deteriorating the exit temperature uniformity. With elevated inlet temperature for both strategies, the hot-spot dissipation is enhanced following a similar mechanism, i.e., with strengthened vortices and turbulent mixing promoting the hot-spot stretching and breakup, which lead to better temperature uniformity. Especially at 543 K, the hot spots dissipate over a short axial distance rapidly before reaching the exit. These findings provide new insights into the formation, evolution, and dissipation of hot spots in swirl spray combustors through detailed analysis of flow-flame dynamics.