Modeling of a Reduced Hybrid H2–Air Kinetic Scheme Integrating the Effect of Hypersonic Reactive Air with Supersonic Combustion

A hybrid H2–air kinetic scheme of 11 species and 15 reactions is developed, which is capable of simulating the high-temperature air reaction flows and H2–O2 combustion flows respectively or simultaneously. Based on the Gupta scheme, the mole fraction varying with a Mach number at specific conditions is analyzed, and the weakly-ionized 7-species 7-reaction scheme is selected. The effect of nitrogenous species on the H2–O2 combustion is analyzed by a zero-dimensional simulation of steady-state and unsteady-state combustion under specified conditions, and the selected dominant nitrogenous reaction N + OH = NO + H is distinguished by the production rate of the nitrogenous species. The thermodynamic properties are verified by comparison using the NIST–JANAF database. The reaction rate coefficients of the dominant reaction of the hybrid kinetic scheme distinguished by a sensitivity analysis are corrected. The proposed kinetic scheme is validated by a zero-dimensional calculation of the ignition delay time and two-dimensional computational fluid dynamics (CFD) simulation with finite-rate chemistry on the shock-induced sub-detonative and super-detonative combustion. The ignition delay time of the hybrid kinetic scheme is almost in the middle between the Shang scheme and Jachimowski scheme, and all the calculated ignition delay times are acceptably greater than the experiments due to the errors of the experiments and numerical models. The clearly captured bow shock wave and combustion front using the hybrid kinetic scheme and Shang scheme are almost the same, which is strongly consistent with the schlieren image. In addition, a good agreement of the flow characteristics and mass fraction of the species along the stagnation line is also obtained, which indicates the accuracy and reasonableness of the hybrid kinetic scheme to simulate hybrid H2–air reactive flows.

An Experimental Study on the Effects of Burst Pressure on Air Blast Development in a Blast Wave Simulator

Due to the extensive use of explosive devices in military conflicts, there has been a dramatic increase in life-threatening injuries and resultant death toll caused by explosive blasts. In an attempt to better understand the blast waves and mitigate the damages caused by such blast waves, various devices/systems have been developed to replicate the field blast scenarios in laboratory conditions. The East Carolina University Advanced Blast Wave Simulator (i.e., ECU-ABWS) is one such facility that can reproduce blast waves of various shapes and profiles. The peak overpressure of a blast is the key factor that causes the greatest number of damages, and it is essentially determined by the burst pressure of the blast. Therefore, a better understanding of the effects of burst pressure on blast generation and development is strongly desired to develop safer and more effective blast mitigation technologies. In the present study, a series of experiments were carried out in the ECU-ABWS to characterize the blast waves generated under different burst pressure conditions. While the incident (side-on) pressures at multiple locations along the blast propagation direction were measured using a temporally-resolved multi-point pressure sensing system, the time-evolutions of blast wave profiles were also qualitatively revealed by using a high-speed Schlieren imaging system. The synchronization of pressure sensing and Schlieren image acquisition enables us to extract more physical details of the dynamic blast wave development under different burst pressure conditions by associating the incident pressures and shock wave morphologies. In this study, the different burst pressures were achieved by altering the thickness of the membrane separating the driver section of pressurized gas and the driven section of air at atmospheric pressure. It is found that there is a linear relationship between the burst pressure and the peak overpressure. As the burst pressure increases (by increasing the membrane thickness), more clearly defined shock wavefronts are also observed along with the peak overpressure increase.

Influence of nozzle exit geometrical parameters on supersonic jet decay

Experiments and simulations were carried on C-D nozzles with four different exit geometry aspect ratios to investigate the impact of supersonic decay characteristics. Rectangular and elliptical exit geometries were considered for the study with various aspect ratios. Numerical simulations and Schlieren image study were studied and found the agreeable logical physics of decay and spread characteristics. The supersonic core decay was found to be of different length for different exit geometry aspect ratio, though the throat to exit area ratio was kept constant to maintain the same exit Mach number. The impact of nozzle exit aspect ratio geometry was responsible to enhance the mixing of primary flow with ambient air, without requiring a secondary method to increase the mixing characteristics. The higher aspect ratio resulted in better mixing when compared to lower aspect ratio exit geometry, which led to reduction in supersonic core length. The behavior of core length reduction gives the identical signature for both under-expanded and over-expanded cases. The results revealed that higher aspect ratio of the exit geometry produced smaller supersonic core length. The aspect ratio of cross section in divergent section of the nozzle was maintained constant from throat to exit to reduce flow losses.

Influence of nozzle exit geometrical parameters on supersonic jet decay

Experiments and simulations were carried on C-D nozzles with four different exit geometry aspect ratios to investigate the impact of supersonic decay characteristics. Rectangular and elliptical exit geometries were considered for the study with various aspect ratios. Numerical simulations and Schlieren image study were studied and found the agreeable logical physics of decay and spread characteristics. The supersonic core decay was found to be of different length for different exit geometry aspect ratio, though the throat to exit area ratio was kept constant to maintain the same exit Mach number. The impact of nozzle exit aspect ratio geometry was responsible to enhance the mixing of primary flow with ambient air, without requiring a secondary method to increase the mixing characteristics. The higher aspect ratio resulted in better mixing when compared to lower aspect ratio exit geometry, which led to reduction in supersonic core length. The behavior of core length reduction gives the identical signature for both under-expanded and over-expanded cases. The results revealed that higher aspect ratio of the exit geometry produced smaller supersonic core length. The aspect ratio of cross section in divergent section of the nozzle was maintained constant from throat to exit to reduce flow losses.