Home – Home – Jornals – Combustion, Explosion and Shock Waves 2020 number 1

Chain branching and heat release processes and their influence on the burning velocity of pre-mixed rich and near-stoichiometric flames of methanol with air were studied by numerical simulation and sensitivity analysis. The phenomenon of super-adiabatic temperatures in these flames due to the kinetic mechanism of methanol combustion was first detected. Comparison of the results of simulation of the structure of methanol and formaldehyde flames showed that the formation of water in super-equilibrium concentrations in flames does not necessarily lead to superadiabatic temperatures, as believed earlier. It was first found that decreasing the dilution of the CH₃OH/O₂/N₂ combustible mixture with nitrogen at a constant equivalence ratio enhances the superadiabatic temperature effect. According to simulation results, in a rich near-limit methanol flame, the role of the H + O₂ = О + OH and О + H₂ = H + OH is negligible due to their low rate. At relatively low temperatures, branching occurs mainly in reactions involving HO₂ and H₂O₂ peroxide compounds, whose concentration is orders of magnitude higher than the concentration of the main carriers of the chain H, O, and OH. From the sensitivity analysis it follows that the methanol flame speed positively affects mainly the reactions of the formation of chain carriers and negatively affects the reactions in which chain carriers are consumed. The stages introducing the main contribution to heat release, but not involved in the formation and consumption of radicals have small sensitivity coefficients.

A kinetic analysis of the model of oxidative conversion of methane to useful products based on a set of many possible elementary reactions that could most fully reflect the chemistry of the process and for which data on rate constants are available. The analysis was carried out numerically. Key stages of the mechanism were identified and used to predict conditions under which the process can be effectively carried out in order to obtain a mixture of methanol and formaldehyde or mainly methanol. It has been shown that high efficiency can be achieved at increased pressures of the reactive mixture due to the high methane content. The directionality of the process increases with decreasing temperature.

This paper describes the development of two simplified reduced kinetic models for high-temperature oxidation of propane, which can be incorporated into complex turbulent flame simulations. Equilibrium, 0D or 1D propagating premixed flames and 2D co-flowing laminar jet flames, with or without preheating, attached or lifted, are computed during the iterative optimization process. Accompanying computations with the USC-II mechanism, as well as available experimental data are exploited for validation. Comparisons demonstrate that these reduced kinetic models ensure satisfactory agreement with data over the investigated parameter space.

A new method of initiation and effective burning of kerosene at supersonic velocities of the flow in a constant-section channel is considered as applied to combustion chambers of variable geometry. The regime of intense combustion occurs due to an external action of a package of gas-dynamic pulses with the energy varying with time. It is experimentally observed that this regime is retained if the action is terminated after the combustion process is organized in the expanding part of the combustor. It is demonstrated that the proposed method can be applied at flow temperatures and high velocities at which kerosene combustion is impossible without special tools for flame stabilization.

The boundaries of stable combustion in a flow of an air suspension of aluminum particles in a wide range of changes in an excess air coefficient (0.1-2.0) are experimentally determined. A characteristic feature of the dependence of a flame blowoff velocity on the excess air coefficient is revealed: the dependence has two maxima. Taking into account the principle of the flame stabilization mechanism, based on the equality of the flow velocity at which the flame is blown off to the flame front propagation velocity, it is concluded that there are two maxima in the dependence of the front velocity on the excess air coefficient, which correspond to the maximum values of heat release and combustion temperature of the air suspension of aluminum particles with an air excess coefficient of 0.2 and 1.0. Previously, these maxima were obtained by other researchers in thermodynamic calculations.

Combustion of a Ca-FeF₃-Fe model system is under study. It is shown that the combustion front propagation in the Ca-FeF₃-Fe system is focal. The foci of ignition of calcium particles arise ahead of the front, followed by the expansion of the reaction zone until the foci completely merge with each other and with the formation of new centers of ignition in the heating zone. The dependences of the temperature and the burning rate on the degree of dilution with iron and pressure in the system are presented. Thermodynamic calculations are performed, and the calculated and experimental data are compared. Under the general tendency of an increase in the burning rate with increasing pressure, its decrease was found to be associated with the vapor - calcium metal liquid phase transition.

The critical ignition parameters of an air suspension of promising energy-intensive compounds of MgB₁₂, AlB₂, ZrB₂, TiB₂ metal borides and the mechanical mixtures of initial components, which simulate the composition of compounds, are experimentally studied. It is established that magnesium borides ignite at temperatures much lower than those for boron, which indicates the active role of magnesium in the ignition process. For other borides, the ignition temperatures are close to those for boron, i.e., the governing factors are the diffusion of oxygen through a liquid boron oxide film and the oxide evaporation rate.

The standard enthalpies of formation were experimentally measured for four compounds: 4,6-diazido-N-(2,2,2-trinitroethyl)-1,3,5-triazine-2-amine, 6-azido-N², N⁴-bis(2,2,2-trinitroethyl)-1,3,5-triazine-2,4-diamine, 6-azido-N², N⁴-dinitro-N²,N⁴-bis(2,2,2- trinitroethyl)-1,3,5-triazine-2,4-diamine, and N²,N⁴, N⁶-trinitro-N², N⁴, N⁶-tris(2,2,2-trinitroethyl)-1,3,5-triazine-2,4,6-triamine - 690.1 ± 5.9, 326.2 ± 13.6, 630.1 ± 6.1, 415.9 ± 9.3 kJ/mol, respectively. Based on these values, the energy equivalent of replacing the hydrogen atom and the azido group by trinitroethylamine and trinitroethylnitramine groups in triazine and azido-substituted derivatives of triazine. Calculations have shown that 6-azido-N², N⁴-dinitro-N²,N⁴-bis(2,2,2-trinitroethyl)-1,3,5-triazine-2,4-diamine, and N²,N⁴, N⁶-trinitro-N², N⁴, N⁶-tris(2,2,2-trinitroethyl)-1,3,5-triazine-2,4,6-triamine with an oxygen supply ratio greater than 1.0 can be used as the basis for designing compositions with a low aluminum content and with a specific impulse of 257-260 s.

A comparative assessment of the use of various metals and non-metals as combustible components of a solid propellant containing ammonium perchlorate as an oxidizer and rubber as a binder was performed using the range criterion of an aircraft with a ramjet engine taking into account the expected completeness of combustion of individual components. The influence of the energy properties of propellants on the technical and economic indicators and flight performance of the aircraft was also taken into account. Range calculation was carried out by numerical integration of flight dynamics equations. Based on the results, a more detailed study of a number of combustible components is recommended, in particular, for the purpose of the full or partial replacement of the currently used boron in solid propellants of ramjet engines.

Results of an experimental study of heat and mass transfer and phase transitions during suppression of thermal expansion and flaming combustion of condensed matter (CM) by an example of typical combustible materials in the forest (mixtures of leaves, conifer needles, and twigs) by means of the action of a non-atomized water array at different stages of its transformations are reported. The liquid volume and the height of its discarding with respect to the CM surface are varied with allowance for the known stages of water array transformations to illustrate possible differences in the cross-sectional areas of these arrays, which determine the water-CM contact area. The characteristics of CM burning termination are obtained as functions of the height of the beginning of water array motion.

Stability of detonation waves (DWs) propagating in a plane or rectangular channel with respect to two-dimensional and three-dimensional disturbances is considered. Accepting a simple hypothesis that the most unstable mode of the linear theory continues to dominate even in the nonlinear regime, one can derive a number of fairly definite predictions of the developed DW structure from the linear stability theory. In particular, the theory predicts the number of detonation cells formed in a channel of a specified size and the cell size, the minimum size of the channel in which the multifront DW structure can still exist, and the parameters at which the number of cells changes in a jump-like manner. All these predictions are qualitatively consistent with available experimental data and numerical results.

The results of analyzing experimental data on the conditions for initiation of explosive transformation in phlegmatized HMX samples on impact with an indenter with a spherical end are presented. It is shown that the conditions for initiation of explosive transformation in a wide range of variation of parameters of the indenter and impact velocities are described by empirical equations that relate the mass-geometric parameters of the projectile with its minimum velocity, thereby initiating the explosive transformation. The resulting dependences can be used for practically analyzing the outcomes of the impact on an explosive composition, in the construction of adequate physical models of initiation, and their verification.

Examples of calculating the shock compression of liquid water and water vapor using a slightly adjusted simplified Nigmatulin-Bolotnova equation of state of water are considered. The main attention is paid to the achievement of those numerical simulation parameters (primarily, water temperature) that may be useful for synthesizing diamond from graphite. It is shown that a favorable factor for achieving a required temperature is the shock compression of a preheated layer of liquid water. It seems even more promising to use a layer of heated water vapor instead of liquid water. In the latter case, reaching the required temperatures and pressures requires a significantly lower velocity of the impactor.

The blast wave mitigation efficiency of open-cell natural rubber foam is compared in laboratory and field test setups. Blast wave mitigation is evaluated in terms of the peak overpressure and positive phase impulse. Piezoelectric pressure sensors are flush-mounted with the top surface of the base plate of the composite material in field tests and at the end plate of the blast wave generator (BWG) in laboratory tests to measure the incident pressure and pressure transmitted through the foam. The blast wave is generated by means of detonating high explosives in field tests and by bursting the diaphragm in the BWG in laboratory tests. It is observed from the test results that the pressure transmitted through foam and the corresponding impulse values depend on the shape of the loading wave and also on the test configuration used in the laboratory method. Therefore, it can be concluded that laboratory tests using the BWG only give a trend of the transmitted pressure and so can be used, at the best, for comparative evaluation of mitigation of the blast wave passing through different materials. However, actual values of the transmitted pressure and impulse can only be obtained in field tests with blast loads generated by using explosives.