Home – Home – Jornals – Combustion, Explosion and Shock Waves 2016 number 2

Phenomena inherent in degenerate branched and completely branched chain reactions are considered from a unified viewpoint. In the case of degenerate branched chain reactions, such phenomena include a negative temperature coefficient, cool flames, and oscillations arising in slow combustion of hydrocarbons. Another phenomenon (intermittent flames) is inherent only in completely branched chain reaction of low-temperature combustion of hydrogen at reduced pressures in the presence of SO₂ additives. These kinetic manifestations of chain branching processes are characterized by a variety of elementary reactions with participation of intermediate compounds and free radicals with different structures. A specific kinetic feature of reactions of both types is simultaneous participation of the active center responsible for chain branching in the branched reaction and in the reaction of propagation of an ordinary chain.

Normal burning velocities in methanol-air mixtures and in the same mixtures with added 4.5 and 7.2% hydrogen as a second fuel were measured over a wide range of equivalence ratio and for initial conditions of 0.16 MPa and 354 K. It has been shown that the mechanism previously proposed for the combustion of mixtures of CO, CH₂O and CH₃OH with air is applicable to multicomponent mixtures containing hydrogen and methanol.

Molecular beam mass spectrometry was used to measure mole fraction profiles of the reactants, major reaction products and intermediates, including precursors of polycyclic aromatic hydrocarbons, in a premixed fuel-rich (equivalence ratio of 1.75) n -heptane/toluene/O2/Ar flame stabilized on a flat burner at atmospheric pressure. The ratio of the liquid volumes in the n -heptane/toluene mixture was7:3. The chemical structure of the flame was modeled using a detailed mechanism of chemical reactions tested against experimental data of other authors on n -heptane/toluene flames and comprising the reactions of formation of polycyclic aromatic hydrocarbons. The mechanism was extended with cross-reactions involving derivatives of n -heptane and toluene. Overall, the new experimental data are in satisfactory agreement with the numerical simulation results; however, there are differences between the measured and calculated mole fraction profiles of some species. Analysis shows that in the n -heptane/toluene flame, the main reactions leading to the formation of low-aromatic compounds (benzene and phenyl) are reactions typical of the pure toluene flame.

A PIV study of a conical premixed methane-air Bunsen flame has shown that the inside of the cone has a complex gas-dynamic structure. In this system, the velocity of the gas flow entering the flame front varies in different parts of the flame cone and the stream tubes are not straight. The Landau-Markstein effect is discussed in the interpretation of the experimental data. A method of processing PIV measurement results is proposed that improves the accuracy of determining the burning velocity and allows a quantitative determination of the catalytic effect of submicron TiO2 particles, which is proportional to the particle surface area. The relative increase in the burning velocity is 2% per each »0.01 cm2/cm3 (particle surface/gas volume) of the total specific surface area of the particles. The experimental data are well described by modeling using well-known literature data on the detailed mechanism of chemical reactions and the mechanism of catalytic oxidation of methane with oxygen on metal oxides.

The influence of an electric field whose intensity vector rotates around the flame axis on the shape of the diffusion flame of propane is experimentally studied. Application of spectrozonal registration makes it possible to obtain information about the radiation intensity distribution at wavelengths of intermediate reaction products (OH, CH, and C2). Different positions of the peak intensity of the own radiation of the flame at different wavelengths testify to the influence of such an electric field on the mixing processes, namely, mixing is more intense than that in the regime without application of the electric field. This feature may turn out to be useful for increasing the efficiency of combustion of gaseous hydrocarbon fuels.

Experimental data on the critical pressure of ignition of titanium alloy fragments in gaseous oxygen are analyzed. The fragments are obtained after fracture of alloy samples in the dynamic mode (p₂*) and under natural convection conditions (p₁*). The results are analyzed with allowance for the heat transfer coefficients from material ignition initiators under similar conditions. Based on the shape of the experimental thermograms of plate cooling, the coefficient of heat transfer from microcraters with a juvenile surface formed due to knockout of metal particles from the plate by the high-velocity flow is found: α₂ ~ 11 W(m² × K). The value of α₂ is close to the value of this coefficient calculated with the use of the coefficient α₁ ~ 5 kW/(m² × K) of heat transfer from titanium rod microfragments (with the size of the order of metal grains) formed during titanium rod fracture in oxygen under conditions of natural convection with allowance for the ratio p₂*/p₁*.

A mathematical model of combustion of a composite solid propellant called ALICE (frozen suspension of nanosized aluminum in water) is presented. The model takes into account the combustion of aluminum nanoparticles in water vapor, the motion of combustion products, and the smaller velocity of particles as compared to the gas. The calculated burning rate is consistent with available experimental data on the burning rate of ALICE as a function of pressure.

The aluminothermic reduction of some oxide systems in the solid-state combustion mode in nitrogen in a high-pressure reactor has been studied for the purpose of obtaining nitride-containing composites. The properties of the synthesis products obtained at various nitrogen pressures were determined.

This paper describes the numerical simulation of combustion with the manifestation of the Vilyunov-Dvoryashin effect that comes down to reduction of the burning rate in the case of blowing of gaseous combustion products past the propellant gasification surface. The cases of endothermic and exothermic reactions of gasification of the solid propellant are considered. The Vilyunov-Dvoryashin effect can terminate combustion even before the erosion coefficient reaches a minimum value of 0.61. Self-oscillating combustion may also occur. The simulation of propellant combustion similar in its properties to the propellant N shows qualitative agreement between the theoretical and experimental results. However, it also reveals the need for more accurate data with regard to performance conditions and experimental results. The existing models of solid-propellant combustion require significant updates as well.

The oxidation kinetics of a diamond powder, P-803 carbon black, and the OSUNT raw material (obtained by electric arc synthesis of single-walled carbon nanotubes) in a stream of steam in the temperature range 600-1315 K was studied. The parameters of the kinetic equations were determined. The temperature dependence of the oxidation rate was found to consist of three regions with different activation energies.

Based on numerical simulations of two-dimensional unsteady flows of gas suspensions, the contribution of particle collisions to dispersion processes during interaction of shock waves with dense dust layers is analyzed. A model of collision dynamics of the two-phase medium based on molecular-kinetic approaches is used. The model is tested by using a problem of a shock wave passing along a dense layer of particles; the model predictions are found to agree well with available experimental data. The problem of interaction of a blast wave with a dense layer on a flat surface is also considered. A comparative analysis of various mechanisms acting on particles and the influence of the initial parameters of the layer on the particle lifting dynamics is performed. A weak effect of the Saffman force and inhomogeneity of the layer surface (waviness) and a significant effect of the Magnus force on dispersion of the layer directly behind the shock wave are demonstrated. In some cases, the contribution of the particle collision dynamics is found to be comparable with the Magnus force effect. Dust lifting due to the development of the Kelvin-Helmholtz instability occurs at late stages of the process.

A mathematical model of synthesis in a mechanically activated SiO₂ + Al mixture is constructed in the macroscopic approximation. It is demonstrated that preliminary mechanical activation makes it possible to obtain solid-phase ignition and to ensure synthesis of Al₂O₃ and Si products in the thermal explosion regime. Based on experimental data, thermophysical and thermokinetic constants of the process are determined.

A superadiabatic regime of the thermal explosion in mechanically activated stoichiometric mixtures of tungsten and carbon black is obtained. Regimes of preliminary mechanical activation of mixtures and the subsequent thermal explosion that allow obtaining a single-phase carbide WC with a submicron grain size are determined. The mechanical energy accumulated in the sample as a result of preliminary activation is estimated. Results of the x-ray diffraction analysis and electron microscopy of mechanically activated samples and thermal explosion products are reported.

The dependences of the critical energy density required to initiate the explosive decomposition of lead azide and the radius of the most heated nanoparticle on the pulse duration of the first harmonic of neodymium laser (1064 nm) are calculated within the framework of the micro-hotspot model of thermal explosion. The calculations are carried out with account for the dependence of the absorption efficiency factor of the laser pulse on the lead nanoparticle radius. With the maximum value of the absorption efficiency factor (1.18), the lead nanoparticle radius (in lead azide) becomes 74 nm. If the pulse duration is short (smaller than 40 ns), the radius of the most heated lead nanoparticle in the lead azide matrix varies slightly (less than 15%) and equals 63.5 nm within the range of short pulse durations. Accounting for the dependence of the absorption efficiency factor of the laser pulse on the nanoparticle radius makes it possible to resolve the paradox of small particles.

Mass velocity profiles of detonation waves in mixtures of nitromethane with acetone and methanol with added diethylenetriamine sensitizer were measured using a VISAR laser interferometer. It was found that even small, about 1%, concentrations of acetone and methanol, inert diluents, led to instability of the one-dimensional detonation front in nitromethane. The results of the experiment show that the use of the sensitizer is an effective method of flow stabilization and if the concentration of the inert diluent does not exceed 10%, the detonation front becomes stable with the addition of 1% diethylenetriamine. At a higher diluent concentration, the sensitizer does not suppress the instability but decreases the oscillation amplitude by several times. The addition of diethylenetriamine to the mixture has been found to increase the detonation velocity.

The temperature of a shock-compressed invert emulsion based on an aqueous solution of ammonium and sodium nitrates was measured using two experimental techniques: using planar thermocouples at pressures of 3.4-12.0 GPa and optical pyrometry at pressures of 9-22 GPa. The experimental data obtained using the thermocouple method are consistent with the calculated values. The optical measurement results are significantly higher than the calculated data and indicate the presence of a spatially inhomogeneous temperature field behind the shock front in the emulsion due to the structural inhomogeneity of the medium.