Home – Home – Jornals – Combustion, Explosion and Shock Waves 2014 number 1

The effect of SO₂ on the chain reaction of hydrogen oxidation in the autoignition region (T = 470–510 ^oC and p < 200 torr) is studied. In the absence of SO₂ under flow conditions, the process occurs in a low-temperature flame mode with a characteristic pale blue glow. With the addition of sulfur dioxide, SO₂ is transformed to elemental sulfur and a new phenomenon is observed. At a small contact time (less than 4–5 s), the process enters the mode of intermittent flames. Dependences of the rate and intensity of light flashes on pressure, temperature, and contact time are studied. An interpretation of the observed phenomenon is given.

A kinetic mechanism for combustion of hydrogen azide (HN₃) comprising 61 reactions and 14 flame species (H₂, H, N, NH, NH₂, NNH, NH₃, HN₃, N₃, N₂H₂, N ₂H₃, N₂H₄, N₂, and Ar) was developed and tested. The CHEMKIN software was used to calculate the flame speed at a pressure of 50 torr in mixtures of HN₃ with various diluents (N₂ and Ar), as well as the self-ignition parameters of HN₃ (temperature and pressure) at a fixed ignition delay. The modeling results of the flame structure of HN₃/N₂ mixtures show that at a 25–100% concentration of HN₃ in the mixture, the maximum temperature in the flame front is 25–940 K higher than the adiabatic temperature of the combustible mixture. Analysis of the mechanism shows that burning velocity of a HN₃/N₂ mixture at a pressure of 50 torr is described by the Zel'dovich–Frank-Kamenetskii theory under the assumption that the burn rate controlling reaction is HN₃ + M = N2 + NH + M (M = HN₃) provided that its rate constant is determined at a superadiabatic flame temperature. The developed mechanism can be used to describe the combustion and thermal decomposition of systems containing HN₃.

Specific features of the unsteady flame in a microchannel with a controlled wall temperature are theoretically studied within the framework of a one-dimensional diffusion-thermal model. The case with the channel wall temperature increasing in the gas flow direction and the channel size being smaller than the critical value determined on the basis of the ambient temperature is considered. Depending on the flow rate of the combustible mixture of gases through the channel, either flame stabilization or alternation of flame repetitive extinction/ignition is possible. The influence of the characteristic length of channel wall heating on the domains of existence of various combustion modes is studied for the first time. The theoretical study shows that there exists a critical value of the temperature gradient in the channel walls, below which the regime of flame repetitive extinction/ignition is no longer observed. At small values of the temperature gradient, a hysteresis phenomenon is found, which is associated with different changes in the flame position in the cases with increasing and decreasing flow rates of the gas.

In this paper, the effect of the flame holder geometry on the flame structure of a mixed hydrogen–hydrocarbon fuel is numerically studied. The fuels used in this study are 100% H₂, 50% H₂ + 50% CH₄, and 100% CH₄. Numerical results obtained by using the κ–ε and β-PDF models show good agreement with experimental data. The results show that increasing both the flame holder length and hydrogen percentage in the fuel decreases the flame length. The flame temperature decreases with decreasing flame holder length. Adding hydrogen to methane increases the peak temperature of the flame and moves its location toward the burner inlet. It is observed that the dependence of the flame length as a function of the flame holder diameter has a minimum at a certain value of the latter. The flame temperature is higher for smaller flame lengths.

External manifestations of the heterogeneous combustion front are characterized. Gas-dynamic aspects of the phenomenon are considered, and experimental data on the change of combustion modes during natural filtering of an air mixture are analyzed. Results of an investigation of the bifrontal structure of the combustion zone in a horizontal plane layer during convective gas transfer are presented.

Combustion of ammonium perchlorate (20–90%) mixtures with ferrocene is studied. It is demonstrated that, depending on the ratio of the components in the examined compositions, in addition to the usual gas-phase combustion model, another possible combustion mechanism exists. A somewhat unusual condensed-phase (c-phase) model may be realized, in which the heat-generating reaction occurs in a foam/aerosol layer at the temperature of evaporation of the less volatile component, which the surface temperature is defined by evaporation of the more volatile component. The efficiency of ferrocene depends on the propellant combustion mechanism: in systems that obey the gas-phase combustion mechanism, the influence of ferrocene addition is higher than the influence of addition of a hydrocarbon fuel; in systems with the c-phase mechanism of combustion, ferrocene addition produces a significant effect. Depending on the ratio of the components, ferrocene first acts in these compositions simply as a highly reactive fuel; it is only in fuel-enriched compositions with a high equivalence ratio that the burning rate increases owing to catalysis of the combustion process by the ferric oxide on the soot skeleton.

The effects of the particle size of powder components, the relative density of the sample, and the degree of dilution of the mixture by thermally inert materials on the pore structure of self-propagating high-temperature synthesis products obtained in the combustion process involving melt were experimentally studied using the (Ti + 26% Si)–Al₂O₃ system as an example. Special techniques of quantitative metallographic analysis allowing for the analysis of materials with complex pore space structure were used.

It is found that the main factors determining the synthesis parameters of tungsten carbide by mechanically stimulated thermal explosion are the structure of carbon modifications and the degree of their aromaticity. The prospects of using carbon modifications obtained from plant raw materials for the synthesis of tungsten carbide with a low content of sulfur are discussed.

Physical and chemical processes of gasoline combustion in internal combustion engines are considered. A model of combustion evolution in gasoline-driven engines, which explains some specific features of the processes in internal combustion engines, is proposed.

Experimental studies were performed to investigate the dependence of the laminar flame velocity in dust clouds of Al, Mg, Zr, Fe, and B particles on the physicochemical parameters (fuel concentration and composition, particle size distribution) and hydrodynamic conditions of the combustion process (semi-open tubes, free clouds of particle–air mixtures). Heat conduction was found to make a predominant contribution to the overall heat transfer in the combustion wave. The main causes of instability of laminar flames (acoustic disturbances, interfacial exchange, forced and natural convections), transient phenomena, and vibrational and turbulent combustion of dust were studied experimentally.

A reduced two-stage model of detonation combustion of methane in oxygen and air for equimolar and fuel-lean mixtures is proposed. One-dimensional structures of the detonation wave are calculated for different ratios of the fuel and oxidizer corresponding to the overdriven and Chapman–Jouguet regimes. A comparison of the calculated dependences of the detonation velocity on the methane concentration in the methane–air mixture with available published data reveals their reasonable agreement.

Results of measurements and processing of sizes, energy, and power of radiation of a cloud formed after an explosion of 50/50 TNT/RDX and TNT cast charges with masses ranging from 0.01 kg to 1000 tons on the ground surface and at different heights in air are presented; the measurements and data processing are performed within wide temporal (up to 10 s/kg^1/3) and spectral (up to 28 m m) intervals. The results are compared with available published data. These explosives have the maximum radiative characteristics owing to the high content of carbon in explosion products. Under conditions of explosions in air, the measured emitted energy approaches 50% of the explosion energy. In the case of ground explosions, the radiation is anisotropic because of screening by ejected soil, and the ratio of energies emitted upward and along the ground surface can exceed the order of magnitude.

A numerical simulation of the initiation of PETN by a laser pulse was performed. The heat-conduction equation was solved in a cylindrical coordinate system taking into account multiple reflection of the light beam, zero-order exothermic reaction, and melting. A criterion for the ignition of explosives by a laser pulse with multiple reflection of the light flow was obtained. The calculation results are in satisfactory agreement with experiment and the ignition criterion. Dependence of the critical energy density on the light beam radius is due to radial heat transfer. The ignition threshold can be controlled by changing the reflection coefficient of the back surface of the sample.

The explosion of PETN of density 1.73 g/cm³ was first initiated by the second-harmonic pulse of a Q-switched neodymium laser. It is shown that the primary process of energy absorption in this case is PETN molecule ionization involving two-photon absorption. The critical initiation energy density corresponding to a 50-% probability of explosion is 12.3 J/cm².

A new method for determining the dynamic friction coefficient of explosives is presented. The method combines the physical model of friction sensitivity with theoretical analysis and numerical calculations. Experimental measurements of the dynamic friction coefficient of steel indicate that the proposed method is effective, provides secure and reliable data, and can be used to calculate the dynamic friction coefficient between cyclonite (RDX) and steel.