Two skeletal kinetics mechanisms for reactive CH 4/O2 and H2/O2 ram accelerator e owe elds are presented. Both models were derived from a 190-reaction, 38-species kinetics mechanism (RAMEC or RAM accelerator MEChanism) that successfully reproduces the high-pressure (>50 atm), low-dilution (<70%), fuel-rich chemistry of ram accelerator mixtures. The reduction procedure for the CH 4/O2 mechanism utilized a detailed-reduction technique with ignition delay time and heat release as the selection criteria. The methane-based mechanism (REDRAM or REDuced RAM accelerator mechanism ) contains 34 reactions and 22 species and predicts ignition times to better than 5% and postcombustion temperatures to within 10 K of the full mechanism for a representative range of ram accelerator mixtures and conditions. This CH 4/O2 mechanism is an improvement over existing reduced methane-oxidation mechanisms that arebased on lower-pressure, higher-temperature chemistry. An 18-step, 9-species mechanism is presented for hydrogen-based ram accelerator combustion that is based on the H2/O2 submechanism of the RAMEC/Gas Research Institute GRI-Mech 1.2 methane-oxidation mechanism. The H2/O2 kinetics model includes HO 2 and H2O2 chemistry near the second and third explosion limits, necessary for ignition at ram accelerator pressures but lacking in certain e nite rate chemistry models currently in use.

Reduced Kinetics Mechanisms for Ram Accelerator Combustion

Stabilization of an oblique detonation wave by a wedge: a parametric numerical study

Oblique detonation waves are simulated to study the evolution of their morphology as gasdynamic and chemical parameters are varied. Although two kinds of transition pattern have previously been observed, specifically an abrupt transition and a smooth one, the determining factors for the transition pattern are still unclear. Numerical results show that the transition pattern is influenced by the inflow Mach number, chemical activation energy and heat release. Despite the fact that these parameters were known to influence the detonation instability, the transition pattern variation cannot be predicted according to the instability criterion. In this study, the difference in the oblique shock and detonation angles is proposed as the criterion to determine the transition pattern with the aid of shock-polar analysis. It is found that the smooth transition will appear when the angle difference is small, while the abrupt transition will occur when the difference is large. The shift from the smooth transition to the abrupt transition occurs when the angle difference is about 15 degrees-18 degrees. The previously proposed criterion using the characteristic time ratio is also examined and compared with the present angle difference criterion, and the latter is proved to provide better results.

On the transition pattern of the oblique detonation structure

Evolution of cellular structures on oblique detonation surfaces

In this study, the onset of cellular structure on oblique detonation surfaces is investigated numerically using a one-step irreversible Arrhenius reaction kinetic model. Two types of oblique detonations are observed from the simulations. One is weakly unstable characterized by the existence of a planar surface, and the other is strongly unstable characterized by the immediate formation of the cellular structure. It is found that a high degree of overdrive suppresses the formation of cellular structures as confirmed by the results of many previous studies. However, the present investigation demonstrates that cellular structures also appear with degree of overdrive of 2.06 and 2.37, values much higher than similar to 1.8 suggested previously in the literature for the critical value defining the instability boundary of oblique detonations. This contradiction could be explained by the use of differently shaped walls, a straight wall used in this study and a custom-designed curved wedge system so as to induce straight oblique detonations in previous studies. Another possible reason could be due to the low and possibly insufficient resolution used in previously published studies. Hence, simulations with different grid sizes are also performed to examine the effect of resolution on the numerical solutions. Using the present results, analysis also shows that although the characteristic lengths of unstable surfaces are different when the incident Mach number changes, these length scales are proportional to tangential velocities. Hence, the interior time determined by the overdrive degree is identified, and its limitation as the instability parameter is discussed.

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Numerical study on unstable surfaces of oblique detonations

Stable delayed oblique detonation waves have been observed both numerically and experimentally in hydrogen-air stoichiometric mixtures for Mach numbers 6 and 7.5. The experimental results obtained using an oblique shock tube facility are compared to calculations made by solving the full conservation equations for a reactive gas. The overall flow structure obtained experimentally is compared to detailed computations of an oblique shock wave to oblique detonation-wave transition over a wedge, and resulting flow fields are analyzed on the basis of shock and detonation polars.

Daniel Desbordes

Papers

Onset of oblique detonation waves: Comparison between experimental and numerical results for hydrogen-air mixtures