The diffraction and re-initiation behavior of detonation wave in premixed H2–O2–Ar mixture

Abstract

Previous studies have demonstrated the effective enhancement of initiation stability in the combustor of a rotating detonation engine through the use of a pre-detonator. However, when a stable detonation propagates from a small-diameter pre-detonator into a larger-diameter combustor, a brief failure occurs, followed by the re-initiation of the detonation wave. This failure is attributed to detonation diffraction as the wave passes through a sudden expansion section. This study systematically investigates the phenomenon of detonation diffraction and re-initiation through 2D numerical simulations. The effect of the diameter ratio between the pre-detonator and combustor on detonation propagation is examined by adjusting the diameter ratio D/d in the range of 1.1–2.7, where D and d represent the diameters of the larger (corresponding to the combustor) and smaller (corresponding to the pre-detonator) tubes, respectively. Various modes of detonation propagation and the corresponding evolution of the detonation wave are observed. The results reveal that as the detonation wave propagates through the sudden-expansion section, it diffracts at the corner and then rapidly decouples at the boundary. For D/d less than 1.3, the detonation wave can maintain its propagation by adjusting the structure of the cell distribution, referred to as the supercritical mode. As D/d increases, the detonation wave experiences initial failure followed by re-initiation, corresponding to the critical mode. When D/d exceeds 2.6, complete failure of detonation occurs and cannot be re-initiated, resulting in the sub-critical mode. Notably, under the critical mode, the distance LDDT required for re-initiation linearly increases with D/d (LDDT = 41.48(D/d) + 4.30). Additionally, the pressure of the triple point near the wall, formed by the convergence of the reflected shock wave, transmitted wave, and precursor shock wave, determines the possibility of local detonation initiation, which induces re-initiation in the testing tube. It is proposed that when the pressure of the triple point exceeds the theoretical pressure of post-shock (Ppost-shock), a local overdriven detonation occurs. Furthermore, when the convergent point of the transmitted shock wave propagates along the central line and passes through the flame front, local detonation can be induced as well. This study systematically illustrates the details of diffraction and re-initiation when the detonation wave passes through a sudden-expansion section.