Numerical study on detonation reflections over concave and convex double wedges

The solver DCRFoam is used to analyze the characteristics of detonation reflection over concave and convex double wedges. Seven reflection processes are observed depending on the wedge angles (θ₁ and θ₂), the critical angle of a single surface (θ_c,single), and the angle difference (Δθ). For concave wedges with small θ₁ and Δθ, a Mach reflection is established along the first surface and the leading edge of the second surface. At the end of the second surface, either a Mach reflection or a regular reflection could be observed depending on the θ₂. For concave wedges with large θ₁ and Δθ, the Mach stem formed over the first surface regularly reflects at the leading edge of the second surface. Then the incident wave regularly reflects over the tail of the second surface. For concave wedges, the actual critical angle of the second surface is found to be larger than the θ_c,single. While the second surfaces of convex double wedges are found to have the same as a single surface. The detonation reflection processes over convex wedges are much simpler.     

Numerical simulation of detonation wave propagation through a rigid permeable barrier

The results of calculation of the detonation propagation in a porous medium for hydrogen-air mixture are presented. The porous medium was specified explicitly and consisted of sets of individual obstacles in the form of solid walls or the sets of finite-size plates. Various modes of detonation propagation depending on obstacle parameters are obtained: propagation in a cellular mode, stationary propagation with destruction of the cellular structure of the detonation front, propagation of a monotonically attenuating detonation wave with destruction of the cellular structure of the front. The possibility of reducing the detonation propagation velocity by replacing solid plates with finite-size ones was shown. The effect of the geometrical parameters of the plates and the step of it installation on the degree of detonation attenuation was estimated. It was determined that an increase in the number of plates leads to a stronger attenuation of the detonation.     

Self-organized generation of transverse waves in diverging cylindrical detonations

Self-organized generation of transverse waves associated with the transverse wave instabilities at a diverging cylindrical detonation front was numerically studied by solving two-dimensional Euler equations implemented with an improved two-step chemical kinetic model. After solution validation, four mechanisms of the transverse wave generation were identified from numerical simulations, and referred to as the concave front focusing, the kinked front evolution, the wrinkled front evolution and the transverse wave merging, respectively. The propagation of the cylindrical detonation is maintained by the growth of the transverse waves that match the rate of increase in surface area of the detonation front to asymptotically approach a constant average number of transverse waves per unit length along the circumference of the detonation front. This cell bifurcation phenomenon of cellular detonations is discussed in detail to gain better understanding on detonation physics.     

Numerical simulation of interaction of cellular detonation wave with systems of inert porous filters

Numerical modeling of the interaction of cellular detonation in a hydrogen-air mixture with several systems of porous filters covering part of the channel was carried out. The main regimes of detonation propagation and the critical conditions for detonation failure in the filter systems were obtained for each system. A map of detonation regimes was constructed, from which it follows that with an increase in the concentration of particles in the filters, it is possible to increase the distance between the filters or reduce the number of filters in the system necessary to successfully suppress detonation. A comparison of various filter systems in terms of blockage ratio and the total surface area of particles was made, from which it was concluded that the system of two filters was the most efficient to suppress detonation.     

Numerical simulation and validation of flame acceleration and DDT in hydrogen air mixtures

Combustion of hydrogen can take place in different modes such as laminar flames, slow and fast deflagrations and detonations. As these modes have widely varying propagation mechanisms, modeling the transition from one to the other presents a challenging task. This involves implementation of different sub-models and methods for turbulence-chemistry interaction, flame acceleration and shock propagation. In the present work, a unified numerical framework based on OpenFOAM has been evolved to simulate such phenomena with a specific emphasis on the Deflagration to Detonation Transition (DDT) in hydrogen-air mixtures. The approach is primarily based on the transport equation for the reaction progress variable. Different sub-models have been implemented to capture turbulence chemistry interaction and heat release due to autoignition. The choice of sub-models has been decided based on its applicability to lean hydrogen mixtures at high pressures and is relevant in the context of the present study. Simulations have been carried out in a two dimensional rectangular channel based on the GraVent experimental facility. Numerical results obtained from the simulations have been validated with the experimental data. Specific focus has been placed on identifying the flame propagation mechanisms in smooth and obstructed channels with stratified initial distribution. In a smooth channel with stratified distribution, it is observed that the flame surface area increases along the propagation direction, thereby enhancing the energy release rate and is identified to be the key parameter leading to strong flame acceleration. When obstacles are introduced, the increase in burning rate due to turbulence induced by the obstacles is partly negated by the hindrance to the unburned gases feeding the flame. The net effect of these competing factors leads to higher flame acceleration and propagation mechanism is identified to be in the fast deflagration regime. Further analysis shows that several pressure pulses and shock complexes are formed in the obstacle section. The ensuing decoupled shock-flame interaction augments the flame speed until the flame coalesces with a strong shock ahead of it and propagates as a single unit. At this point, a sharp increase in propagation speed is observed thus completing the DDT process. Subsequent propagation takes place at a uniform speed into the unburned mixture.     

Numerical simulation of radial-stratified rotating detonation flow field structures with different injection patterns

Rotating detonation engine has been widely studied in recent years because of its high theoretical efficiency and heat release rate. In many numerical simulations, the combustible mixture is injected and fully filled at the head of the combustor. In this paper, annular injection slits are proposed and three representative injection patterns are simulated by changing the injection directions. Stable single-wave modes are formed in all three patterns and two kinds of combustible mixture layer structures are found, namely “L-shape” and “T-shape” structures. Following the combustible mixture layer, the detonation wave is not fully filled in the radial direction, thus radial and circumferential shock waves are induced from the detonation wave, forming more complex wave structures. After the radial shock wave, velocity vortex and significant deflagration are found and propagate with the shock wave, thus maintaining a higher pressure and temperature there.     

Numerical simulation on the operating characteristics of rotating detonation ramjet engines at high flight mach number

For high-Mach-number incoming flow circumstances, a rotating detonation ramjet engine configuration is proposed in this research. By installing supporting blocks at the rear of the combustor, this configuration achieves continuous rotating detonation operation. Based on the Comparison of the flow structures obtained from the engine configuration with and without the supporting block before and after detonation ignition respectively, we obtain the intrinsic mechanism of detonation wave's propagation and re-initiation under the action of the supporting block. The supporting block creates a deflagration wave that is almost stationary before detonation ignition. In the detonation-ignited state, the deflagration wave is continually formed and traveling upstream under the influence of the supporting block, which is analogous to the periodical before detonation ignition of a transverse wave structure. The dynamic deflagration wave will cause the incomplete reactants behind the detonation wave to burn as the intensity of the detonation wave decreases. As a result, the incident shock wave is transformed into a Mach stem to achieve the re-initiation of the detonation wave.     

Numerical simulation of flow and heat transfer from slot jets impinging on a cylindrical convex surface

Flow and heat transfer characteristics of slot jets impingement to a cylindrical convex surface are numerically investigated.Suitable turbulence models have been determined through comparison with the experimental data.Flow structures are described and impingement heat transfer characteristics are discussed.The effects of Re,H/B and D/B on single-slot jets impingement heat transfer are analyzed and heat transfer characteristics of multiple-slot jets are investigated.The results show that:Gas flows along the convex surface and boundary layer separation occurs in both single and multiple-slot jets impingement.A maximum stagnation Nu appears at H/B=8 and the local Nu decreases with increasing H/B in the region far away from the stagnation.The Nu in the stagnation region decreases with increasing D/B but the Nu is nearly the same in the region far away from the stagnation.Pressure gradient is an important factor on heat transfer enhancement.Correlations of the Num for single-slot,double-slot and quadric-slot jets impinging on a convex surface are obtained.It indicates the effects of Re and D/B on Num could become more important in less slot jets impingement.