Том 54, № 6 (2018)

Results of numerical and experimental investigations of a high-velocity flow in a plane channel with sudden expansion in the form of a backward-facing step, which is used for flame stabilization in a supersonic flow, are presented. The experiments are performed in the IT-302M high-enthalpy short-duration wind tunnel under the following test conditions: Mach number at the combustor entrance 2.8, Reynolds number 30 · 10⁶ m⁻¹, and total temperature T₀ = 2000 K, i.e., close to flight conditions at M = 6. The numerical simulations are performed by solving full unsteady Reynolds-averaged Navier–Stokes equations supplemented with the k–ω SST turbulence model and a system of chemical kinetics including 38 forward and backward reactions of combustion of a hydrogen–air mixture. Three configurations of the backward-facing step are considered: straight step without preliminary actions on the flow, with preliminary compression, and with preliminary expansion of the flow. It is demonstrated that the backward-facing step configuration exerts a significant effect on the separation region size, pressure distribution, and temperature in the channel behind the step, which are the parameters determining self-ignition of the mixture. The computed results show that preliminary compression of the flow creates conditions for effective ignition of the mixture. As a result, it is possible to obtain ignition of a premixed hydrogen–air mixture and its stable combustion over the entire channel height.

A modified Fourier law with account for heat flux relaxation and scalar value of the temperature gradient serves as a basis for the mathematical model of the locally nonequilibrium process of thermal ignition of systems with a hear source exponentially changing due to temperature. It is shown by the studies performed under the boundary conditions of the first kind that accounting for the space–time nonlocality increases the time delay of thermal ignition. Moreover, it is shown that, when the relaxation properties of the material are considered, the boundary conditions can only be accepted after a certain time rather than instantaneously. Consequently, the amount of heat fed to the system has a limit that depends on the physical properties (including relaxation properties) of the medium.

A physicomathematical model is developed for ignition and combustion of a methane–air mixture containing coal microparticles. The model takes into account the detailed kinetics of oxidation of the gaseous methane–hydrogen–air mixture and the processes of thermal destruction of coal particles with release of volatiles (methane and hydrogen) to the gas phase, ignition and combustion of these volatiles in the gas phase, and heterogeneous reaction of carbon oxidation. It is demonstrated that addition of coal particles to the methane–air mixture in the temperature interval from 900 to 1450 K reduces the ignition delay time. Moreover, addition of coal particles to the methane–air mixture leads to shifting of the ignition limit of the gas mixture toward lower temperatures. The calculated delay time of coal ignition in the air–coal mixture and the predicted delay times of methane and coal ignition in the methane–air–coal mixture are found to be in reasonable agreement with experimental data obtained in a rapid compression machine.

The effects of the strain rate, equivalence ratio, and particle diameter on the combustion of a mixture of aluminum microparticles with air under fuel-lean conditions are studied in the counterflow configuration with an approximate analytical perturbation method. The flame structure is assumed to consist of three zones: preheating, flame, and post-flame zones. Reasonable agreement between the current results and experimental data is obtained in terms of the flame temperature. The dimensionless ignition and ultimate flame temperatures, place of the flame starting point, and flame thickness are obtained as functions of the strain rate for different particle diameters and equivalent ratios. The results indicate that the ignition and ultimate flame temperatures and also the flame thickness decrease with increasing strain rate. With a decrease in the strain rate, the length of the preheating zone increases. With increasing particle diameter, the flame thickness increases, whereas the ignition and ultimate flame temperatures decrease. An increase in the equivalence ratio causes an increase in the ultimate flame temperature and reduction of the preheating zone and flame thickness.

The dependence of the specific impulse of metal-free energetic compositions based on high-enthalpy organic oxidizers on the elemental content and enthalpy of formation of the oxidizer and the nature and volume content of the composite binder consisting of hydrocarbon and active components has been studied. At a given volume content of the binder, the specific impulse of compositions based on oxidizers with an oxygen saturation coefficient of 0.6–1.3 can be increased by finding the optimal mass ratio of the hydrocarbon and active components in the binder. The optimal content of the hydrocarbon component increases from 0 to 100% as the oxygen ratio of the oxidizer increases from 0.6 to 1.3.

Thermal radiation from a water layer compressed by an incident shock wave and a shock wave reflected from a LiF window was recorded in the range of incident-wave intensity of 28–36 GPa. Losses of radiant flux at the interfaces were estimated. The temperature of water compressed by one and two shock waves was calculated, and the calculation results are in good agreement with experimental data.

A three-dimensional numerical simulation of interaction between an impactor directed at an angle and a double-layer target with an outer ceramic layer is performed. It is shown that the presence of a ceramic layer normalizes the penetration process: the main deformation and displacement of the target elements occur so as if the action of the impactor is directed along a normal to the target surface. A quite intense rotation of the impactor remains at the final stages of interaction is observed. Experimental data and propositions on the transformation of the impact at an angle into an equivalent impact along the normal are used to develop an approximate analytical technique for calculating the limiting rate of penetrating the double-layer ceramic–metallic target under the action of the impactor at an angle.

The natural fracture of rock has a strong effect on the law of explosion stress wave transmission and crack propagation during blasting. Based on the stress wave theory, the influential mechanism for both the law of transmission of the stress wave and of crack propagation due to natural fracture and water jet slot are analysed. Next, an experiment is conducted to understand the crack propagation law because of the effect of an explosion shock wave, and the evolution law of the blast stress wave and blast-induced crack propagation is simulated by ANSYS/LS-DYNA. The results indicate that the existence of the water jet slot not only promotes the generation of the main crack along its direction, but also promotes the generation of the secondary crack near the water jet slot because of the explosion shock wave. The direction of propagation of the secondary crack and the main crack are seriously affected by the natural fracture. In addition, if the distance between the blast hole and the natural fracture is too small, a smash area is formed; and with an increase in the distance between the blast hole and the natural fracture, the smash area becomes smaller, and the effect on the blast-induced crack becomes weaker.