Initiation characteristics of wedge-induced oblique detonation waves in a stoichiometric hydrogen-air mixture

Abstract

The initiation features of two-dimensional, oblique detonations from a wedge in a stoichiometric hydrogen-air mixture are investigated via numerical simulations using the reactive Euler equations with detailed chemistry. A parametric study is performed to analyze the effect of inflow pressure P 0 , and Mach number M 0 on the initiation structure and length. The present numerical results demonstrate that the two transition patterns, i.e., an abrupt transition from a multi-wave point connecting the oblique shock and the detonation surface and a smooth transition via a curved shock, depend strongly on the inflow Mach number, while the inflow pressure is found to have little effect on the oblique shock-to-detonation transition type. The present results also reveal a slightly more complex structure of abrupt transition type in the case of M 0 = 7.0, consisting of various chemical and gasdynamic processes in the shocked gas mixtures. The present results show quantitatively that the initiation length decreases with increasing M 0 , primarily due to the increase of post-shock temperature. Furthermore, the effect of M 0 on initiation length is independent of P 0 , but given the same M 0 , the initiation length is found to be inversely proportional to P 0 . Theoretical analysis based on the constant volume combustion (CVC) theory is also performed, and the results are close to the numerical simulations in the case of high M 0 regardless of P 0 , demonstrating that the post-oblique-shock condition, i.e., post-shock temperature, is the key parameter affecting the initiation. At decreasing M 0 , the CVC theory breaks down, suggesting a switch from chemical kinetics-controlled to a wave-controlled gasdynamic process. For high inflow pressure P 0 at decreasing M 0 , the CVC theoretical estimations depart from numerical results faster than those of low P 0 , due to the presence of the non-monotonic effects of chemical kinetic limits in hydrogen oxidation at high pressure.

Numerical investigation of oblique detonations induced by a finite wedge in a stoichiometric hydrogen-air mixture

“…The computational domain is shown in region enclosed by the dashed line, and the Cartesian grid is aligned with the wedge surface. Similar to previous numerical studies on oblique detonations [28], the present analysis is based on the two-dimensional multi-species Euler equations written as follows:̃̃∂…”

“…To study the ODW influenced by the fuel injection, the inflow inhomogeneity is modeled and simulated, demonstrating the distorted reaction surfaces and morphology variation of the ODW structures [26,27]. Our recent study demonstrated that there are two initiation mechanisms of ODW in the wedge-induced structures in hydrogen-air mixtures [28]. In the case of high incident Mach number, the temperature behind the oblique shock is high enough to achieve the self-ignite, so it is called the kinetics-controlled mechanism.…”

“…To deepen our understanding on the ODW initiation, this study simulated the ODW using a finite-length wedge, replacing the semi-infinite wedge in the previous study [28]. Due to the effects of expansion waves, the initiation features of ODW structures may change and induce complicated phenomena.…”

The Effect of Chemical Reactivity on the Formation of Gaseous Oblique Detonation Waves

“…While numerical simulations using complex chemistry with detailed chemical reaction rates are nowadays possible with increasing computational resources, it remains challenging to fully synthesize and explain the tremendous amount of chemical kinetic and flow field information that are generated from the computation. In fact, although recent studies have shown that detailed chemical kinetics plays an important role in detonation dynamics and its use may introduce additional transient and multiscale effects [52][53][54][55], qualitative comparison with simulations using a detailed reaction mechanism has shown the simplified chemistry models, e.g., one-step or two-step chain-branching type kinetics, correctly elucidate the underlying physics of the ODW dynamics, importantly the two transition processes this study is focusing on, e.g., References [38,39,56]. In addition, the two-step induction-reaction kinetics, consisting of a thermally neutral induction step followed by a main heat release reaction layer, provides a compromise on the detailed chemistry model.…”

“…To this end, the dynamics of ODW formation has recently drawn significant research attention.With the advance in scientific computing using parallel Central Processing Units (CPUs) and Graphics Processing Units (GPUs) computing technology [32][33][34], there has been significant advance in investigating in detail the unsteady oblique detonation waves. Using high-resolution numerical simulations, various formation structures of wedge-induced oblique detonation waves, i.e., the transition from the oblique shock wave (OSW) to the oblique detonation wave (ODW), have been revealed in recent investigations, e.g., References [35][36][37][38][39][40]. Hysteresis phenomenon of the ODW structure related to initial condition and ODW responses subject to inflow non-uniformities and turbulences have also been investigated [41][42][43][44][45].…”