A gaseous detonation wave that emerges from a channel into an unconfined space is known as detonation diffraction. If the dimension of the channel exit is below some critical value, the incident detonation fails to re-initiate (i.e., transmit into a self-sustained detonation propagating) in the unconfined area. In a previous study, Xu et al. ["The role of cellular instability on the critical tube diameter problem for unstable gaseous detonations," Proc. Combust. Inst. 37, 3545-3533 (2019)] experimentally demonstrated that, for an unstable detonable mixture (i.e., stoichiometric acetyleneoxygen), a small obstacle inserted in the unconfined space near the channel exit promotes the reinitiation capability for the cases with a sub-critical channel size. In the current study, numerical simulations based on the two-dimensional, reactive Euler equations were performed to reveal this obstacle-triggered re-initiation process in greater detail. Parametric studies were carried out to examine the influence of obstacle position on the re-initiation capability. The results show that a collision between a triple-point wave complex at the diffracting shock front and the obstacle is required for a successful re-initiation. If an obstacle is placed too close or too far away from the channel exit, the diffracting detonation cannot be re-initiated. Since shot-to-shot variation in the cellular wave structure of the incident detonation results in different triple-point trajectories in the unconfined space, for an obstacle at a fixed position, the occurrence of re-initiation is of a stochastic nature. The findings of this study highlight that flow instability generated by a local perturbation is effective in enhancing the reinitiation capability of a diffracting cellular detonation wave in an unstable mixture.

No abstract

High-fidelity numerical simulations using a Graphics Processing Unit (GPU)-based solver are performed to investigate oblique detonations induced by a two-dimensional, semi-infinite wedge using an idealized model with the reactive Euler equations coupled with one-step Arrhenius or two-step induction-reaction kinetics. The novelty of this work lies in the analysis of chemical reaction sensitivity (characterized by the activation energy E a and heat release rate constant k R ) on the two types of oblique detonation formation, namely, the abrupt onset with a multi-wave point and a smooth transition with a curved shock. Scenarios with various inflow Mach number regimes M 0 and wedge angles θ are considered. The conditions for these two formation types are described quantitatively by the obtained boundary curves in M 0 -E a and M 0 -k R spaces. At a low M 0 , the critical E a,cr and k R,cr for the transition are essentially independent of the wedge angle. At a high flow Mach number regime with M 0 above approximately 9.0, the boundary curves for the three wedge angles deviate substantially from each other. The overdrive effect induced by the wedge becomes the dominant factor on the transition type. In the limit of large E a , the flow in the vicinity of the initiation region exhibits more complex features. The effects of the features on the unstable oblique detonation surface are discussed.Aerospace 2019, 6, 62 2 of 17 the conditions and interpret the observed flow field regimes in terms of competing reactions and flow-quenching effects.Alternatively, an oblique detonation wave can be induced by a wedge in an incoming reactive flow [20][21][22]. A standing oblique detonation wave attached to the wedge tip presents a more practical configuration for engine operation [23][24][25]. There has been indeed a remarkable progress in understanding the fundamental aspects of oblique detonation waves induced by a semi-infinite, two-dimensional wedge. Analytical solutions such as wave angles and steady structures as the basic foundation were sought in a number of pioneering works using detonation polar analysis by approximating the ODW as an oblique shock wave (OSW) coupled with an instantaneous post-shock heat release [26][27][28][29]. In later studies, it has been demonstrated that the ODW formation structure induced by the wedge is more complex. In many cases, an oblique shock wave first forms upon the flow interaction with the wedge igniting the combustible flow mixture, and subsequently transits into an oblique detonation wave [30]. Due to the strong coupling sensitivity between fluid dynamics and chemical reactions, as well as the inherent unstable nature of detonation waves [31], it remains technically challenging to establish steady oblique detonations in high-speed combustible mixtures for practical propulsion applications, and such success requires fundamental understanding of the initiation structure of oblique detonation waves. To this end, the dynamics of ODW formation has recently drawn significant research attention...

No abstract

The duty cycle for C| 0 H 19 /oxygen injection and time sequence for injection, mixing and ignition to produce two-phase, fully-developed detonations are investigated in a pulse detonation rocket engine (PDRE) model. The The PDRE test model is 25mm in inner diameter and 1.1m long. The solenoid valves are employed to control intermittent supplies of propellants and purge gas. The spark igniter has ignition energy of around 50mJ. The solenoid valves and igniter are controlled by a control system. Under some given supply conditions, the detonability limits in terms of injection duty cycle and the controlling method are obtained. From fundamental analysis, these factors influence the operation of PDRE essentially by appropriate equivalence ratio and mixing. The experimental results show that to gain reliable detonations, first of all, the injection duty cycles for Ci 0 H ]g and oxygen should ensure sufficient filling and then the injection time phases and duty cycles should be kept as close as possible to attain effective mixing.