Investigation of Kerosene Combustion Characteristics in a Variable-Cavity Scramjet Combustor

The effects of cavity depth on kerosene combustion characteristics during mode transitions in a variable-cavity scramjet combustor at Mach 2.5 are investigated numerically by improved delayed detached eddy simulation coupled with a dynamic zone flamelet model. A mode transition control method based on the variable cavity for the variable-geometry scramjet combustor is proposed. Depth-increasing (from 28 to 34 mm) and depth-decreasing (from 34 to 28 mm) paths are reproduced numerically by dynamic meshing to reveal the influence of the variable cavity on the mode transition under an equivalence ratio of 0.4. Mode transition occurs because of reduced incoming flow velocity and increased participation of kerosene droplets in the reaction. A critical cavity depth for mode transitions is identified as 32 mm. The scramjet and ramjet modes assume cavity stabilization and jet wake flame stabilization modes, respectively. Mode transition hysteresis occurs during the ram-to-scram transition. The increased kerosene vaporization rate due to the intensified interaction between the cavity shear layer and the kerosene jet is the underlying reason for the flame upstream propagation. The flame upstream propagation creates a high-temperature but low-speed reaction zone, which intensifies the reactions and eventually contributes to hysteresis.