Home – Home – Jornals – Combustion, Explosion and Shock Waves 2023 number 6

Results of studying stabilization of a homogeneous hydrogen-air flame on an optical discharge plasma in a high-velocity flow are reported. The main aspect of experiments is providing stable combustion behind the region of laser beam focusing without any mechanical flame holders. The laser radiation parameters are sufficient for creating a quasi-steady plasma in the flow. It is shown that the optical discharge stabilizes the flame front in a wide range of equivalence ratios for flow velocities up to u = 200 m/s. The laser radiation parameters within the range of their variation from one experiment to another exert a minor effect on the turbulent flame velocity. Flame stabilization behind the optical discharge region has some specific features. An important parameter is heat release due to hydrogen combustion. A dimensionless criterion is derived; the turbulent flame velocity is a linear function of this criterion.

Injection of inert particles into a stoichiometric hydrogen-air mixture with velocities ranging from 0.1 to 1.0 of the Chapman-Jouguet detonation velocity is calculated. Resultant flow regimes are analyzed. It is found that an increase in the particle temperature leads to ignition of the mixture, while an increase in the particle velocity leads to detonation wave initiation. Critical conditions of detonation initiation in terms of the particle concentration, particle size, and injection velocity are determined. Various possible scenarios of detonation initiation are demonstrated, depending on the particle diameter and concentration, including regimes with multiple initiation sites. Flow charts are constructed in the plane of the parameters “injection velocity, temperature of particles of various sizes”.

This paper first presents chemical time scale values measured by the PIV method for premixed methane-air and dimethyl ether-air flames depending on the equivalence ratio and the concentration of the inhibitor trimethyl phosphate at atmospheric pressure. Comparison of the experimental results with theoretical estimates made based on the Zeldovich-Barenblatt hypothesis shows their qualitative agreement. The chemical time scale within the accuracy of the experiment depended only on the burning rate, rapidly decreasing as it increases. At fuel-air flame speeds close to and above 0.6 m/s, the results of the experiments shown high accuracy of theoretical estimates based on the Zeldovich-Barenblatt hypothesis.

Heat and mass transfer processes and the rate of fuel oxidation are the determining parameters of combustion of pre-mixed fuel gas streams and an oxidizer and combustion of condensed fuels in a gaseous oxidizer. A correct description of these processes is of both scientific and practical interest. The influence of the kinetics of chemical reactions and diffusion of fuel molecules on the thermal and chemical structure of the flame forming around a polymethyl methacrylate (PMMA) sphere under f natural convection in the air has been studied by numerical simulation. The three-dimensional gas flow around the solid body was calculated on the basis of the system of full Navier-Stokes equations for a multicomponent mixture taking into account diffusion and heat exchange between the surface and gas, convection, and radiation heat transfer. The kinetic model represents conjugate reactions both on the surface of the condensed material and in the gas phase. The formation of gaseous fuel methyl methacrylate (MMA) on the surface is described by a one-step efficient PMMA pyrolysis reaction. The oxidation of MMA in the gas phase is described by the global reaction C₅H₈O₂ + 6O₂ → 5CO₂ + 4H₂O. It has been found that the temperature and flame species concentration profiles practically do not depend on the rate constant of this reaction provided that the characteristic reaction time is much less than the characteristic time of MMA diffusion. It has been shown that varying the diffusion coefficient of MMA has a significant effect on the thermal and chemical structure of the flame. An increase in the diffusion coefficient of MMA leads to an increase in the maximum flame temperature. The results of the study show that the transport properties of compounds required to calculate their transpot coefficients are one of the most important parameters for accurate CFD simulation.

This paper presents the results of a chromatographic analysis of gaseous products formed during the laser synthesis of catalytic Cr/Al₂O₃ nanoparticles in a methane-argon environment. The main difficulties of such studies are noted. Methods for solving this problem and ways to optimize the methane pyrolysis accompanying the laser synthesis of nanoparticles are proposed. The fundamental possibility of simultaneous synthesis of catalytic nanoparticles and their use for methane pyrolysis is demonstrated. The main products of pyrolysis in this process are hydrogen and amorphous carbon. The maximum hydrogen yield is 4% (vol.). It is shown how the process can be optimized to increase the yield of hydrogen and expand the range of reaction products for unsaturated hydrocarbons.

The properties of azidoacetylene derivatives of s-triazine containing various combinations of propynyloxy, propynylamino, methylpropynylamino and azido groups have been studied, and their enthalpy of formation in the condensed phase, density, and impact and friction sensitivity have been determined. Based on these data, the energy parameters of the detonation, combustion, and adiabatic transformation of both individual compounds and their compositions with SKI-3 binding isoprene rubber have been calculated. The results of the integrated experimental and theoretical studies lead to the conclusion about the high calorific value of the investigated individual compounds and compositions based on them.

The ignition and combustion characteristics of a high-energy material containing ammonium perchlorate, butadiene rubber, and an ultrafine powder mixture of aluminum, titanium, or iron with amorphous boron are presented. An experimental testbed, a CO₂ laser, and a constant-pressure bomb are used to measure the ignition delay time and burning rate of the high-energy material while varying the heat flux density and pressure in the chamber. It is shown that replacing amorphous boron with ultrafine Al/B, Ti/B, or Fe/B in the material reduces the heating time and the moment of flame appearance on the propellant surface due to an increase in the reaction rate and a decrease in the oxidation temperature of these mixtures on the surface of the reaction layer. In this case, the burning rate of the high-energy materials with Me/B at excess pressures increases significantly (up to 240% for Al/B-HEM and up to 120% for Ti/B-HEM at a pressure of 5.0 MPa).

This paper presents combined metal oxide catalysts for the decomposition of ammonium perchlorate, combining two transition metal oxides (iron and cobalt) deposited on the surface of a carbon support. Combined catalysts are obtained by impregnation and chemical precipitation methods. Catalyst samples containing various phases of iron and cobalt oxides are obtained by varying the calcination temperature. The structural and morphological features of the synthesized catalysts are studied using XRD, SEM, and BET methods. As shown by the study performed using differential scanning calorimetry, the synthesized combination catalysts manifest high catalytic activity during the thermal decomposition of ammonium perchlorate, reducing the peak temperature of the high-temperature stage of decomposition by more than 60oC.

The rheological characteristics of struvite-based fire-extinguishing powder mixtures are comparatively analyzed when using hydrophobic silicon dioxide as a functional filler, obtained during a single-stage synthesis by various methods. Infrared spectroscopy, scanning electron microscopy, low-temperature nitrogen sorption-desorption, and other methods are used to investigate the influence of the synthesis method on the textural and structural properties of hydrophobic functional fillers of fire-extinguishing powder mixtures. It is revealed that the key factor affecting the rheological properties of such mixtures is the uniform distribution of the functional filler over the surface of the particles of a fire extinguishing component (struvite). It is proven that the struvite-based powder composition and the developed functional filler are highly effective for fire extinguishment.

Due to the total lack of reliable experimental data on the kinetics of solid-phase transformations at high temperatures, adequate estimates of the ignition and combustion characteristics of real energetic materials are currently unavailable. In combustion theory, balance relations in the form of ignition criteria and in the form of the equivalence principle of the increase in burning rate under the action of a radiation flux corresponding to an increase in initial temperature are used in most cases without sufficient theoretical justification, what can lead to incorrect results. Numerical simulation of the ignition and combustion of model energetic materials can provide the basis to determine the conditions for the correct use of balance relations. In this work, ignition and combustion under the action of a radiant flux have been numerically studied using a model of unsteady combustion of melting energetic materials and the matching coefficients in the balance relations were obtained. It is shown that the values of these coefficients depend on the kinetic parameters of solid-phase transformations and the intensity of the external heating source. It is concluded that it is necessary to continue the theoretical research aimed at developing valid approaches to determining the parameters of global reactions in the condensed phase using data on ignition delay by heat flux and to determining the correct matching coefficients when using the equivalence principle.

A tetrahedral structure model has been proposed to estimate the size of metal agglomerates during combustion of a composite solid propellant. According to this model, oxidizing particles are located at the vertices of a regular tetrahedron, and the internal volume of the pyramid is occupied by a mixture of binder fuel and metal - the so-called pocket. Experimental data are compared with the results of calculations using the tetrahedral model, the Cohen model and the empirical correlations of Hermsen, Salita, Beckstead, Grigoriev, and Duterque. The comparison was carried out for a composite propellants containing ammonium perchlorate, binder and aluminum as an example. It has been shown that in some cases the tetrahedral model predicts the diameter of agglomerates better than the other models.

An attempt to understand the relation between the burning characteristics and thermal decomposition in a wide range of pressure is made in the present investigation based on solid propellants with ammonium perchlorate as an oxidizer and 3,3-diazomethylepoxybutane and tetrahydrofuran as a fuel binder. The burning rate measurement is carried out in a wide range of pressure: 1.0, 3.0, 7.0, 13.8, 15.0, and 20.0 MPa. The inflection point of the pressure exponent for ammonium perchlorate with and without oxalate and the flame extinguishing point both appear at 13.8 MPa. Various mechanisms of burning rate reduction by the quaternary ammonium salt and oxalate are analyzed by theoretical analysis, thermogravimetric analysis, and differential scanning calorimetry analysis. The burning rate and decomposition of the oxidizer and fuel binder with combustion modifiers and their overall impact on propellant combustion are studied. Due to modifiers, a transition between kinetically controlled combustion and diffusion controlled combustion is found to occur.

This study describes a model for initiating an explosive decomposition of composite materials based on high explosives that weakly absorb radiation and ultradisperse metal inclusions under the influence of nanosecond laser pulses. The model is based on experimental results obtained from studying the explosive decomposition of PETN with inclusions of ultrafine metal particles (Al, Ni, Fe). The model serves as a basis for constructing a scientifically grounded algorithm for determining the composition of a material with minimal thresholds for laser initiation of explosive decomposition, which makes it possible to replace most experiments with theoretical calculations and optical-acoustic measurements. The algorithm is verified using data from laser initiation of RDX with inclusions of ultrafine iron particles.

This paper describes a study of explosive parameters of a Bi₂O₃/Al nanothermite mixture with the addition of 1-methyl-3-nitro-1,2,4-triazole (1Me-3H) depending on the content of the latter and the component ratio of a Bi₂O₃/Al basic nanothermite pair. Adding 1Me-3H to the mixture increases the explosive force, but it begins to decrease as soon as the additive content reaches over a certain limit. Depending on the mixture formulation, it is possible to increase the explosive force by 22-29% relative to Bi₂O₃/Al nanothermite. Changing the mixture composition makes it possible to vary the burning rate of Bi₂O₃/Al/1Me-3H within a range of 400-690 m/s in charges 2 mm in diameter and within a range of 120-430 m/s in a 0.1-mm thick layer.

Formal transposition of kinetic data obtained in studying the processes of ignition and low-velocity combustion to supersonic detonation processes most often leads to noticeable underestimation of the critical initiation energy, detonation cell size, and other dimensional parameters of detonation as compared to experimental data. Thus, numerical predictions of the combustible system behavior become less reliable. However, because of the instability-induced non-one-dimensional, nonhomogeneous, and oscillating character of the multifront detonation wave, it is next to impossible to perform reliable experimental measurements of the kinetic parameters of combustible mixtures under the detonation conditions. In the present paper, we propose and approve a method that allows one to get over the above-mentioned limitations by using a technique as close to the detonation conditions as possible. The technique is based on using a decaying shock wave for combustible mixture initiation instead of the classical steady shock wave. Such a decaying wave is formed in the case of reaction failure behind a steadily propagating detonation wave due to its propagation in a channel with sudden expansion (so-called detonation wave diffraction). The basic issues of the technique are discussed, required estimates are made, experimental verification is performed, and results obtained are reported.

Measurements of the electrical resistance of shock-compressed aluminum is used in the present study to estimate the concentration of point defects generated by the shock wave front. The parameters of the physical state of a thin metal sample are found by means of modeling the shock wave processes in the measurement cell. Experimental values of the specific electrical resistance of aluminum are compared with predictions of the equilibrium electrical resistance model. The proposed model ensures an adequate description of currently available reference data on equilibrium isothermal compression and isobaric heating of aluminum. At the same time, the shock wave experiment yields a higher specific electrical resistance than that predicted by the model of the electrical resistance of an equilibrium defectless crystal. The detected difference in the specific electrical resistances testifies to generation of defects of the crystal structure of the metal subjected to dynamic compression. Under the assumption of predominant formation of vacancies, the concentration of defects in aluminum is estimated as a function of the shock wave pressure. The number of defects in the metal increases with an increase in the shock wave pressure. The data obtained are qualitatively consistent with available results for copper and silver, which allows one to claim that generation of defects under shock compression has common specific features for these metals. The physical state of shock-compressed aluminum is thermodynamically nonequilibrium and includes numerous defects.