Home – Home – Jornals – Combustion, Explosion and Shock Waves 2015 number 6

The combustion chemistry of formaldehyde in fuel-rich flames has been studied by numerical modeling and sensitivity analysis. It has been shown that the wide flammability limits of CH2O air mixtures are due to features of the combustion chemistry of formaldehyde at high equivalence ratios rather than to the superadiabatic temperature effect. In this case, the thermal decomposition reaction of hydrogen peroxide H₂O₂ plays a key role in the conventional branching reactions.

This paper is devoted to methods of estimating the combustion regimes of gaseous fuel in vortex chambers of various designs in the presence of a bed of inert particles in the chamber, which rotate with the gas flow. A simple geometric criterion for determining the possibility of fuel combustion in the particle bed without the formation of a flame above the bed is proposed.

This work is an experimental and theoretical study of the dependence of normal flame velocity in polydisperse aerosuspensions of aluminum particles on the parameters of the particle size distribution function. It is shown that the velocity decreases with increasing dispersion of the distribution function as a result of reduction of the specific reaction surface.

Self-heating processes in binary homogeneous mixtures are studied based on the analysis of phase trajectories on the heating rate–temperature plane. The proposed approach allows determining the characteristic regions of exothermic reactions in the parametric diagram of the Todes criterion versus the Semenov criterion. Generalized strict criteria for the degeneration of thermal explosion are determined for reactions with low activation energy and reactions with a low thermal effect for any reaction orders. An approximating formula describing the dependence of the Todes criterion on the Semenov criterion is derived for calculating the critical conditions of thermal explosion up to its complete degeneration for reaction orders higher (or equal to) one. A criterion for the applicability of the classical thermal explosion theory as a special case of the proposed model is found. A diagram of the critical parameters in the introduced new variables is considered to predict and control the self-heating kinetics of mixtures.

The effect of sodium hydrogen carbonate and ammonium chloride on the combustion of iron aluminum thermite in air at atmospheric pressure is experimentally studied. The effect of salt additives on the average linear velocity of combustion, the relative yield of the metallic phase in the ingot, and the relative weight loss during thermite combustion are investigated. The concentration flammability limits of the mixtures under consideration are determined.

This paper presents the results of a study of the transition from layer-by-layer to convective combustion and to explosion for charges of advanced high-density explosives in a constant-volume chamber at a pressure of up to ≈60 MPa. Approximation of the results of the experiments with good correlation revealed a common parameter for explosives that characterizes the kinetics of convective combustion and made it possible to rank explosives according to their capability of transition from combustion to explosion. The results can be useful for computational modeling of explosion under accidental heating conditions.

This paper presents a physicomathematical model for the combustion of N powder which takes into account chemical reactions in the condensed and gas phases. On the burning surface, boundary conditions (coupling conditions) are specified. Results of calculation of the burning rate of N powder at constant pressure are in good agreement with available experimental data. The extinction of combustion of N powder by a sharp pressure drop pressure is simulates. Calculations of the boundary values of the degree and rate of pressure drop at which extinction of N powder occurs are in good agreement with published data of measurements.

Three types of coal gangue are compared systematically. The burnout index and the comprehensive combustibility index are found to increase significantly as the ash content decreases. The Arrhenius activation energy is obtained by using the Ozawa–Flynn–Wall and Vyazovkin models.

The shock wave motion of a mixture of a gas and fine solid particles is considered with allowance for the difference in velocities and the particle phase pressure, which is described by Anderson-type and other equations. Various forms of the equation of state for the particle phase are described. Graphical illustrations are given for the equation determining the composite type of this model with the particle phase pressure being neglected. Under certain assumptions, the complete model can be reduced to a hyperbolic system of equations. Types of shock waves formed in such a mixture are determined for this system of equations. The statements are illustrated by results of numerical simulations.

Regimes of continuous detonation burning of syngas–air mixtures in transverse (spinning) detonation waves in a flow-type annular cylindrical combustor are considered. Mixtures of carbon oxide and hydrogen in proportions of 1/1, 1/2, and 1/3 are used. The varied parameters are the combustor geometry and the fuel injection system, as well as the flow rates of air and syngas. The influence of additional supply of air to the products on the detonation wave parameters, pressure in the combustor, and specific impulse is determined. The range of realization of continuous spin detonation of the syngas–air mixture in terms of specific flow rates of the mixture is expanded from 25 to 786 kg/(s⋅m²). It is shown that additional supply of air increases the pressure in the combustor, the thrust, and the number of detonation waves, but decreases the detonation wave velocity. The flow structure in the domain of detonation waves is studied. For some values of the combustor expansion coefficient, a chart of detonation regimes in the coordinates of the fuel–to–air equivalence ratio and specific flow rate of air is constructed, and the specific impulse of the thrust force is calculated.

Injection of a hot gas flow produced by an external source into a powdered explosive allows very fast (within a few microseconds) deflagration-to-detonation transition. Under this high-enthalpy initiation conditions, the process begins with the stage of convective combustion, and the initial velocity of the wave is about 1 km/s. The combustion kinetics known from the literature does not provide the observed rapid development of the process. Various mechanisms of acceleration of the reaction are considered. Results of calculation for a two-phase gas-dynamic model are compared with data of synchrotron diagnostics (set of density profiles in the wave).

Some important aspects of rotating detonation waves are discussed: specific features of recording and interpretation of trajectories of rotating transverse waves onto a moving film, relationship between the acoustic velocity of reaction products and the velocity of rotation of transverse waves, energy release and deficit of velocity of a rotating detonation wave, and multifront structure of rotating transverse waves.

An experimental study is conducted to investigate the effect of the diaphragm thickness on the transmission of an incident detonation wave from a propane–oxygen mixture (donor) to a propane-air mixture (acceptor). Quenching of the incident detonation wave near the interface is observed for all test cases. The presence of a diaphragm results in a longer distance required for re-initiating the detonation wave in the acceptor. In the presence of a diaphragm with a thickness smaller than 50 μm, the velocity of detonation wave propagation approaches that of the test case with a slide gate valve (no diaphragm).

The structure of unsteady shock waves and bubble detonation waves generated by electrical explosion of a wire in water and chemically inert and reactive bubbly media was studied at a capacitor energy storage of 8÷81 J. The formation and expansion of a plasma bubble after wire explosion in water and bubble media was investigated using optical recording. The formation of cavitation zones and the breakdown characteristics of the liquid and bubbly media in short strong shock waves were established. It is proved that the initiation of bubble detonation by wire explosion occurs by a resonance shock–wave mechanism. A comparative frequency Fourier analysis of shock waves in an inert bubble medium and bubble detonation waves was performed.

The amplitudes of the elastic precursor in LY12 Al-based alloy samples under different pre-stress states are obtained. Even though the material does not undergo a brittle-to-ductile transition, the amplitude of the elastic precursor still increases with pre-compression. It is demonstrated that the amplitude of the elastic precursor is not only related to the yield stress, but also to the stress state. A method for obtaining a more accurate yield stress of materials under both uniaxial strain and uniaxial stress conditions is presented.