Inhibition of the oblique detonation wave detachment in two-phase n-heptane/air mixtures

In this work, oblique detonation wave propagations in n-heptane droplet/vapor/air mixtures induced by a wedge at high altitude and Mach number are simulated by the Eulerian-Lagrangian method with a skeletal chemical mechanism. This work is a first attempt to inhibit the detachment of oblique detonation wave (ODW) and expand the operation range of thrusters based on oblique detonation combustion in partially pre-vaporized n-heptane/air mixtures. Effects of gas/liquid equivalence ratios and droplet diameters are considered, and the ODW morphology is analyzed. Standing windows, ODW characteristics and predictability of detachment inhibition are discussed. First, the detonation theoretical calculation is performed at 25–40 km operation conditions for pure gas n-heptane/air mixtures. The results show that the wedge angle range for successful ignition, i.e., standing window, increases with the increase of flight altitude and Mach number. The internal energy and kinetic energy of mixtures are affected by the wedge angle and flight Mach number, so that the maximum angle of ODW with flight conditions presents an opposite trend over or below Mach 10. The two-dimensional (2D) ODWs in our simulation scope, which are triggered by the collision of the initiation zones formed from the wedge, show a multi-wave head structure similar to normal detonation. Detachment is inhibited in partially pre-vaporized n-heptane/air mixtures, due to the non-explosive mixtures near triple-point and the post-ODW inter-phase mass and energy transfers by the latent heat absorption and vapor addition from droplet evaporation. Moreover, the predictability of pre-vaporized n-heptane on inhibiting ODW detachment is explored through the relationship of chemical and evaporation timescales. It is demonstrated that the excitation time is very small approaching the detachment condition where an abrupt ODW tends to form. Accordingly, when the ratio of droplet heat absorption time to evaporation time is relatively high, i.e., τ_h / (τ_ev)^1/2 > 0.01, the ODW detachment is more likely to be inhibited.