Home – Home – Jornals – Combustion, Explosion and Shock Waves 2018 number 2

The structure of premixed ethyl butanoate/O2/Ar flames stabilized on a flat burner at atmospheric pressure was studied by molecular beam mass spectrometry. Mole fraction profiles of the reactants, stable products, and basic intermediates and temperature profiles were obtained in flames of a stoichiometric φ = 1) and rich φ = 1.5) combustible mixtures. Experimental data and their analysis are presented in comparison with experimental and numerical data obtained earlier in methyl pentanoate flames. A simulation of the structure of ethyl butanoate flames was modeled using a detailed literature chemical-kinetic mechanism for the oxidation of esters fatty acids. The experimental profiles are compared with the calculated ones, and the transformation pathways of ethyl butanoate were analyzed. Based on a comparative analysis of experimental and calculated data, the main shortcomings of the model presented in the literature are identified and possible ways to improve the model are suggested. Features of decomposition of ethyl butanoate and methyl pentanoate are discussed based on an analysis of their transformation pathways; similarities and characteristic differences between their oxidation processes due to the different structure of the molecules of the original fuels are outlined.

Obtaining of hydrogen during pyrolysis and partial oxidation of hydrogen sulphide is analyzed on the basis of a detailed kinetic model of H2S oxidation. It is shown that the H2 output in the case of H2S pyrolysis in adiabatic flow reactor with a residence time of ≈1 s. Even for the initial temperature of the mixture T₀ = 1400 K, the molar fraction of H₂ is only 12%, though the equilibrium value is reached within the reactor. At T₀ <1200 K, there is no enough time for the chemical equilibrium inside the reactor to be established, and the H₂ concentration is lower than the equilibrium value. At T₀ <1000 K, there is practically no pyrolysis reaction in the reactor. Addition of a small amount of air to H2S leads to energy release, to an increase in temperature, and, as a consequence, to acceleration of H2S conversion. The normalized output of H₂ can be increased by several times. For each value of T₀, there exists an optimal value of the fuel-to-air equivalence ratio φ that ensures the maximum H₂ output in the H₂S-air mixture. The process of partial oxidation at high values of φ > φ_b and low values of T₀ is essentially nonequilibrium; as a result, the H₂ concentration at the exit from a finite-length reactor can be higher than its equilibrium value, e.g., by 30-40% at T₀ = 800 K and φ = 6-10. The reasons responsible for reaching a “superequilibrium” concentration of H₂ at the flow reactor exit are determined.

Bis(4-nitrofurazan-3-yl)furazan (DNTF) and 3,4-bis(4-nitrofurazan-3-yl)furoxan (DNFF) have been studied as potential components of composite solid rocket fuels. Their heats of combustion and enthalpies of formation were determined experimentally. X-ray diffraction analysis showed that the DNTF and DNFF crystals were orthogonal with the same space group P2₁2₁2₁. DNTF and DNFF were found to be ineffective in rocket fuels with a hydrocarbon binder; however, for compositions without aluminum and with an active binder, the use of DNTF provided a specific impulse of 254.5 s at combustor and nozzle exit pressures of 40 and 1 atm, respectively, and at a density of 1.77 g/cm³, and in the case of DNFF, 258 s at a density of 1.79 g/cm³.

This paper describes the optimal modes of initiation of self-propagating high-temperature synthesis with the help of electron beam on the example of a Ti–Al–C powder mixture. A pulsed electron beam with a particle energy of tens of kiloelectronvolts and a duration of hundreds of microseconds is used. Morphology, structure, and elemental composition of formed products in the form of Ti₃AlC₂ and TiC are studied.

The possibility of burning of a mixture of chromium peroxide with aluminum nitride is shown experimentally. The effect of initial pressure of nitrogen and ratio of reagents on an average linear burning rate, as well as for the relative mass loss in the burning of a CrO₃/AlN mixture. The concentration limits of the burning rate of the test mixture are determined. The microstructure and phase and chemical compositions of the combustion products of the chromium- and nitride-aluminum mixture are studied.

This paper presents an approach to the generalization of the combustion regularities of solid propellants in gas generators of propulsion systems based on a one-dimensional differential model. Identification of the heat release rate function represented by mutually orthogonal Laguerre polynomials in the differential model is performed by solving the inverse problem of combustion theory and using measured gas parameters in the gas generator. This makes it possible to establish the relationship between the combustion features of solid propellant and the operating modes of the gas generator and formulate the boundary conditions for numerical studies of solid propellant gas generators.

This paper describes the development of a physico-mathematical model of spark ignition of an air-borne powder dust, which is based on a two-phase two-speed model of reacting gas-dispersion medium. There are the results of numerical solution on the problem of spark ignition of an air-borne powder dust with allowance for its movement caused by gas expansion during heating. The relationships between the minimal energy of spark ignition of the air-borne powder dust and the mass concentration and particle size of powder dust are obtained. The particle size increases along with the minimal energy of spark ignition. There is mass concentration of powder dust particles with which the energy of spark ignition is minimal. The comparison of the results of calculations of the minimal energy of spark ignition of air-borne powder dust with known experimental data yields their satisfactory agreement.

A physicomathematical model of detonation of a gas suspension of nano-sized aluminum particles with allowance for the transition from the continuum to free-molecular flow regime and heat transfer between the particles is proposed. A formula for logarithmic interpolation for the thermal relaxation time in the transitional regime is derived. A semi-empirical model of Arrhenius-type reduced kinetics of combustion is developed, which ensures good agreement with available experimental data. Steady (Chapman-Jouguet and overdriven) structures and also attenuating detonation waves in suspensions of nano-sized particles are analyzed. Typical features of detonation in nano-sized particle suspensions are found: the normal detonation regimes correspond to the solution in the Chapman-Jouguet plane with a sonic final state in terms of the equilibrium velocity of sound; combustion occurs in an almost equilibrium mixture in terms of velocities and temperatures; a strong dependence of the combustion region length on the amplitude of the leading shock wave is observed.

Physicomathematical models are proposed to describe the processes of detonation propagation, attenuation, and suppression in hydrogen-oxygen, methane-oxygen, and silane-air mixtures with inert micro- and nanoparticles. Based on these models, the detonation velocity deficit is found as a function of the size and concentration of inert micro- and nanoparticles. Three types of detonation flows in gas suspensions of reacting gases and inert nanoparticles are observed: steady propagation of an attenuated detonation wave in the gas suspension, propagation of a galloping detonation wave near the flammability limit, and failure of the detonation process. The mechanisms of detonation suppression by microparticles and nanoparticles are found to be similar to each other. The essence of these mechanisms is decomposition of the detonation wave into an attenuating frozen shock wave and the front of ignition and combustion, which lags behind the shock wave. The concentration limits of detonation in the considered reacting gas mixtures with particles ranging from 10 nm to 1  m in diameter are also comparable. It turns out that the detonation suppression efficiency does not increase after passing from microparticles to nanoparticles.

Detonation in mixtures of acetylene and propylene with oxygen in the range of fuel component concentrations with possible formation of carbon condensate in detonation products is studied both experimentally and theoretically. In contrast to the traditional method of studying detonation in a quiescent mixture located in a closed tube, the present investigations are performed in a tune with an open end (for exhaustion of detonation products) under the conditions of separate injection of the components and their mixing after injection into the detonation tube through the ignition chamber. The components are injected into the tube from a computer-controlled multichannel system of gas injection of the CCDS2000 detonation sputtering setup. The detonation cell size and detonation velocity are measured; these parameters are also calculated by the BEZOPASNOST (SAFETY) computer program. A comparison of the computed and experimental dependences testifies to a complicated character of transformation of detonation products from a purely gaseous to heterogeneous state and to its effect on the detonation wave.

Experimental data on single and double shock compression of initially liquid and gaseous (compressed by initial pressure) hydrogen isotopes (protium and deuterium) at pressures of ≈10-180 GPa and temperatures of ≈ 3000-20000 K are considered. The mean values of the measured variables (pressure, density, internal energy, and temperature) show that hydrogen at a pressure of ≈ 41 GPa in the temperature interval of ≈ 3500-5700 K and at a pressure of ≈74 GPa in the temperature interval of ≈5000-7500 K is characterized by a negative value of the Gruneisen coefficient. Such an anomaly may play a key role in some processes, including those proceeding in the Jupiter gas shell, which mainly consists of protium (≈ 90%) and helium (≈ 10%). In the range of pressures (depths) of its manifestation, convection in the protium shell is forbidden with an increase in temperature in the shell with increasing pressure. Possibly, a comparatively moderate fraction of helium does not suppress the anomaly, and it serves as a barrier for large-scale convection in the Jupiter shell. Experiments are required to confirm this anomaly.

Joint of steels of different hardness through a plastic layer was obtained by explosive welding using an emulsion explosive. In the weld zone, two types of waves were found: large waves and small waves which have not been observed in previous experiments. Empirical relations for calculating the wave dimensions are proposed that take into account the influence of the strength and density of the colliding materials on them. Cracking in the weld zone can be avoided by reducing the wave dimensions.

Explosively welded metal plates are characterized by the formation of local microvolumes at the interlayer boundaries within which there is a mixing of interacting materials. These microvolumes can be located discretely along wavy boundaries or continuously in the form of thin interlayers along planar boundaries. Based on the results of many published works, it has been shown that the material in these zones is melted, and its subsequent solidification occurs at a high rate leading to the formation of metastable phases. In this paper, the formation of metastable phases in steel-steel, Ta-steel, Nb-Al, and Zr-Cu joints is analyzed. The cooling rates of these materials in mixing zones is estimated. Calculations show that the cooling rate of the melts formed in the welded zones of the investigated composites is in the range 10³-10⁶ K/s. Cooling of mixing zones at such high rates results in the formation of metastable structures. In some cases, the crystallization of materials is suppressed and metallic glasses and quasicrystalline phases are formed in the melt zones.

To understand the complex dynamic response of cylindrical metal shells under high-strain-rate loading, a mid-explosion recovery device is designed to recover cylindrical shells at an intermediary phase, chosen in this study to be the phase wherein cracks penetrate through the entire casing wall thickness. The surface dynamic processes of the expansion, fracture propagation, and rupture of a 40CrMnSiB steel cylindrical shell are measured with a high-speed framing camera for determining the appropriate inner diameter of the mid-explosion recovery device. The numerical simulation software Autodyn-3D is used to predict the influence of the device wall thickness and the maximum radial deformation of the device inner wall upon the outer fracture diameter of the casing. The casing at the desired phase is recovered by the device, and the outer diameter of the shell is found to increase by 1.77 times, while the radial deformation of the device is 5 mm. The crack distributions, the distance between the adjacent penetrating shear cracks, and the number of circumferential divisions are found to vary along the axis of the cylindrical casing.