Effects of thermal stratification on detonation development in hypersonic reactive flows

Gaseous detonation waves in a uniform mixture have been studied widely, but uniformity is seldom realized in practical applications such as detonation-based engines. Nonideal scenarios involving incomplete mixing and curved intake compression lead to the thermal stratification of reactants. Local high-temperature regions first trigger the reactant autoignition and even result in the untimely formation of a detonation wave. Using the two-dimensional Euler equations and a detailed H2-air reaction mechanism, we examine the effects of reactant thermal stratification on the autoignition wave morphology in hypersonic reactive flows. Three flow regimes, namely, the autoignition-driven reaction front, detonation wave, and decoupling shock/reaction front, are observed. These flow regimes are determined by the temperature gradient, and only a moderate temperature gradient can trigger an oblique detonation wave. The oblique detonation can stabilize in hypersonic inflows, primarily because the upstream autoignition region acts as an anchorage point. Comparisons of one- and two-dimensional autoignitions confirm that supersonic flow enhances the formation of pressure waves. An analysis of the reaction front propagation speeds reveals that the detonation development is determined by two aspects. One is the convergence of compression waves originating from thermal expansion and supersonic flows, and another is the reaction front propagation speed at the early stage, which must exceed the local sound speed to promote positive feedback between pressure waves and heat release.