Velocity nonuniformity and wall heat loss coupling effect on supersonic mixing layer flames

Abstract

With urgent demand for reliable flame stabilization in air-breathing high-speed aircrafts' combustors under a wide range of operational scenarios, understanding flame characteristics under various inflow and wall thermal conditions is crucial for mixing layer-stabilized flames. In this paper, the effect of inflow velocity nonuniformity, wall heat loss and their coupling on flame structures, flame lift-off and stabilization mechanisms in the supersonic mixing layer are analyzed. An approach based on the Beta distribution model is innovatively proposed to generate the pseudo nonuniform inflow velocity profiles. Different flame modes, the regime diagram and key controlling parameters are quantitatively analyzed via the active subspace method. The results show that the coupling of velocity nonuniformity and wall heat loss can lead to three flame modes, i.e., the heat loss-dominant flame, the intermediate flame, and the nonuniformity-dominant flame, as the flame stabilizes from upstream to downstream regions with the normalized flame lift-off length Lnorm in the range of [0,0.50], (0.50,0.58], and (0.58,1.0], respectively. This finding is consistent with the autoignition flame stabilization mechanism. In addition, the fuel burnt-out ratio of the flame is found to be nonlinearly correlated with flame lift-off length and peaks approximately with Lnorm=0.8, owing to the subtle interactions of mixing and chemical reactions in the mixing layer. The viscous heating in the boundary layer may have a pronounced impact on the flame lift-off depending on the level of wall heat loss, and the predicted Lnorm variation could be approximately 26 percent of the combustor length with/without viscous effect under the isothermal wall condition.