Numerical investigation of mode transition and hysteresis in a cavity-based dual-mode scramjet combustor

The effect of ethylene fuel equivalence ratio (ER) variation directions on combustion states in a dual-mode scramjet combustor was numerically investigated. The combustor employed transverse wall fuel injectors and downstream cavity flameholders without pilot fuel, which are fundamental components in many practical combustors. The isolator inflow Mach number was 3.1, and static pressure, stagnation pressure and stagnation temperature were 53 kPa, 2622 kPa and 1656 K, respectively. The ER was regulated abruptly in a piecewise constant manner, from 0.10 to 1.02, and then back to 0.10. A 3-D URANS method with a recognized two-step kinetics model was adopted. Results exhibited two combustion hysteresis loops, which indicated that different types of combustion mode transitions could result in hysteresis. The first was a hysteretic phenomenon between separated and shock-free scramjet modes based on steady quasi-one-dimensional combustor flow assumptions, and the second was between two different patterns of separated scramjet modes. Hysteresis mechanisms are elucidated from the viewpoint of combustion flow structures. The first hysteresis was attributed to flame stabilization mode transitions between the cavity shear-layer stabilized mode and the jet-wake stabilized mode, along with the transition hysteresis of a pre-combustion shock train's establishment and vanishment. The flame stabilization locations were greatly influenced by the flow separation states ahead of the fuel injectors, and the flow separations were in return determined by the flame distributions. The second hysteresis was attributed to transitions between weak-oscillation mode and intensive-oscillation mode with the transition hysteresis of shock reflection amount increase and decrease of the pre-combustion shock train structure, which were both in the jet-wake stabilized location. Flame in the low-speed region beside the separation bubbles ahead of the fuel injectors provided heat and hot radicals for downstream flame stabilization, and the pre-injector flame intensity greatly influenced the combustion oscillation states. (C) 2019 Elsevier Masson SAS. All rights reserved.