Том 15, № 3 (2022)

Steady-state theory of ignition by a heated flat surface formulated by Ya. B. Zeldovich in 1939 played a pioneering role in the successful development of ignition of various combustible materials. The analytical solution obtained later for a cylindrical surface opened the opportunity of comparison with experimental data. In the work performed by Filippov et al., a discrepancy was found between experiments on the ignition of methane–air mixtures with heated wires and an analytical solution. Based on it, Filippov et al. expressed doubts about the correctness of the model. However, this discrepancy may be due to the fact that the experiment used for comparison with the model does not fully satisfy the limitations that follow from the simplifying assumptions made when formulating the model. These assumptions and the limitations that follow from them are analyzed in this paper in relation to Kumagai’s experiments on the ignition of a methane–air mixture. Key assumptions of the steady-state model: a simplified description of the kinetics of chemical heat release using a global one-stage Arrhenius-type reaction without burnup; the condition that the thickness of the reaction zone must be substantially less than the thickness of the dynamic boundary layer; and the cylindrical symmetry of the thermal field around the heated body. Based on the analyses of the heat release kinetics, a numerical solution for two nonsteady problems of gas ignition with a heated body and gas self-ignition in a plug flow reactor with detailed reaction kinetics has been obtained. The solutions showed that the dependence of the heat release rate on temperature constructed for specific calculation options has a complex shape that cannot be even approximately described using the heat release law in the Arrhenius form. Nevertheless, it turned out that the critical Nusselt numbers delimiting the ignition region and the region of steady-state temperature profiles, which were calculated using the formulas of the analytical model with the appropriate procedure for calibrating the heat release characteristics, are in good agreement with the experimental data in the entire range of diameters and wire heating temperatures and gas flow rates. Also, good agreement with the experiment and the analytical model for critical ignition conditions was obtained in calculations using the unsteady ignition model, despite noticeable differences in the heat release rate depending on temperature. The condition of a small thickness of the reaction zone in relation to the size of the boundary layer is generally satisfied quite well, although at high gas flow velocities (at the level of 10 m/s), the mathematical rigor of this condition becomes insufficient for avoiding the convective heat transfer at the reaction zone. Due to separation of the flow from the body surface and formation of eddies, a region of reduced heat transfer is formed at the body surface near the azimuth angle of 90. The value of this reduced heat transfer can be noticeably less than the average one. It is in these areas of the surface of a heated body that conditions favorable for ignition are created. If a correction is made with a corresponding reduction in the critical Nusselt numbers in these experiments, then this would weaken the dependence of the critical Nusselt number on the wire diameter observed in the experiment bringing it closer to the approximately proportional dependence that follows from the analytical solution.

Calculations of the interaction of a cellular detonation wave (DW) with an inert porous filter were carried out based on the developed physical and mathematical model of the mechanics of heterogeneous medium which takes into account the detailed chemical kinetics of chemical reactions in the gas phase. Under an inert porous filter, a motionless lattice of inert particles is considered. The following flow regimes were revealed: propagation of attenuated cellular DW at velocities less than the Chapman–Jouguet velocity and detonation failure with the destruction of the cellular structure. Critical volume concentrations of filter particles corresponding to the detonation failure regime were calculated. Dependences of the normalized DW velocity and detonation cell size on the volume concentration of particles in filters were calculated. The concentration limits of detonation (critical volume concentrations of particles lead to detonation failure) in filters with diameters of 50, 100, and 200 m were determined. The dependences of the normalized DW velocity and size of detonation cell on the volume concentration and diameter of particles in filters were calculated.

In the study of combustion processes in the combustion chambers of air-jet engines, the main attention is paid to determining their thrust characteristics. The main way to determine the thrust characteristics is the direct measurement of the thrust generated by the combustion chamber or a charge of solid fuel. Another way to determine the thrust characteristics is to define the momentum of the jet flowing through the sonic nozzle according to the known flow parameters at the nozzle exit (pressure, temperature, and gas composition). Depending on the task at hand, it is convenient to use one of the indicated methods. In this paper, the results of determining the thrust characteristics of a model combustion chamber have been compared in two ways under the conditions of one experiment.

A physical and mathematical model in one- (1D) and two-dimensional (2D) formulation that describes the process of interaction of a heterogeneous detonation wave (DW) with a semi-infinite porous medium is proposed. The following detonation regimes are revealed: propagation of the attenuated cellular DW at velocities less than the Chapman–Jouguet velocity at concentrations of cylinders less than critical and detonation failure with the destruction of the cellular structure at concentrations of cylinders equal or greater than critical. A map of the propagation regimes of heterogeneous detonation of the stoichiometric mixture of aluminum particles with a diameter of 1, 2, and 3.5 µm in oxygen in a cylindrical porous zone with a diameter of 100 µm was constructed. The critical conditions for the propagation of heterogeneous detonation in the stoichiometric mixture of 1, 2, and 3.5 µm are obtained. It is shown that with an increase in the size of burning particles from 1 to 3.5 µm, the critical volume concentration decreases. Comparison of the results of 1D and 2D calculations shows that they are similar to each other.

The present paper discusses the design of a pulse-jet engine in which a new mechanism of deflagration-to-detonation transition and establishment of a mode close to the detonation combustion or quasi-detonation mode is realized. It is found that the possibility of such a transition is provided by a special design of the gas duct which has a double bend leading to the formation of a specific vortex structure of the internal flow allowing the acoustic tuning of the gas duct to reach such combustion modes.

Previously, it was found that a stoichiometric mixture of ammonium perchlorate and sevilen (a thermoplastic adhesive, a copolymer of ethylene, and vinyl acetate) with the addition of 20% micron-sized aluminum powder in pressed dense samples gives an explosion when initiated by an electrical high-voltage discharge. In the facilitates high-voltage initiation of an explosion: explosions were detected on samples from a mixture with a 10% aluminum addition and in the case of mixtures with a 20% aluminum addition, the threshold value discharge voltage at which an explosion is initiated is significantly reduced. When aluminum was replaced by copper and zinc powders of similar fineness, there were no explosions. This confirms that it is aluminum that has the set of qualities necessary for high-voltage initiation of an explosion. The minimum discharge energy required for high-voltage explosion initiation has been determined. The dependence of the minimum energy on the method of preparing mixtures (with or without a solvent) and the amount of aluminum introduced into the mixture is obtained. The unique combination of low impact sensitivity of mixtures of ammonium perchlorate with sevilene and 20% aluminum which was obtained in a pile driver test, high susceptibility to explosion at a high-voltage discharge, and excellent adhesion of sevilene to the dispersed components of the mixture in pressed samples opens up opportunities for using this composition in devices initiation.