Home – Home – Jornals – Combustion, Explosion and Shock Waves 2019 number 6

An intense effect of acoustic waves on the structure and shape of the burner flame was discovered by Rayleigh. The present paper deals with acoustic waves in the case of flame propagation in a tube. The flame emits acoustic waves generating the flow in the tube. Being actually a wave beam, the flow bounded by the walls experiences friction and also diffraction divergence, resulting in the emergence of secondary waves, i.e., waves of flow perturbations. They induce the formation of a cellular structure of the flame; at high velocities, the flame becomes turbulent. All these processes play important roles in the deflagration-to-detonation transition in tubes. Emission of acoustic waves by the flame is also responsible for the formation of spherical detonation. Spin and pulsed detonation can be interpreted on the same basis.

The standard enthalpy of formation of 1,4-diethynyl)benzene was determined experimentally to be 500.6 ± 6.7 kJ/mol. Calculations showed that 1,4-di(ethynyl)benzene has a relatively high adiabatic combustion temperature (about 1970 K at a pressure of 5 atm) and very high heat of combustion in oxygen (42 MJ/kg); therefore, 1,4-di(ethynyl)benzene can be used as the basis to develop an effective dispersant of solid fuel providing an adiabatic combustion temperature of up to 2500 K at a heat of combustion far exceeding the values that provide the HMX and other azides of N-heterocycles previously proposed for this purpose.

The passage of the combustion wave of a titanium powder layer in air through an inert shaped obstacle was studied experimentally. The study showed the possibility of the formation of an inhomogeneous wave-like structure of the front associated with a limited supply of gaseous reagents and the thermal instability of combustion of the titanium powder layer. The conditions of the optimal transition combustion in the presence of gaseous impurities affecting gas exchange in the reaction zone and the stability of the front in the layer system were obtained. It is shown that there exists a phenomenological criterion for assessing the nature of the transition combustion mode of titanium powder depending on the shape of the inert obstacle and the front.

The formation of a gas filtration combustion wave by a heated region of an inert porous medium is studied by numerical simulation. The mechanism of formation of the combustion wave is described. Dependencies of the time of wave formation on the feed rate of the gas mixture, the temperature of heated region of the porous medium, and its length were obtained. The ignition limits of the filtration combustion wave were found.

The conditions of occurrence, nature, size, and shape of combustion hot spots of energy condensed systems and the spatial dynamics of the hot-spot combustion wave are discussed. A model for the propagation of the hot-spot combustion wave in condensed systems is briefly described based on the cellular automata method. A possible explanation of the occurrence of unsteady effects in the combustion of solid rocket propellants is proposed.

The laws and mechanism of combustion of TiO₂ based termite systems with a complex reducing agent (Al and Ca) under the influence of overload are revealed. The termite system includes a basic composition whose combustion products are target elements (Ti, Al, Nb, and Cr) and a high-energy additive (CaO₂, Al, and Ca) for realizing a high combustion temperature. With the introduction of an energy additive, the system acquires the ability to burn, and the combustion products (Ti _x Al _y and oxide solutions Al₂O₃ and CaO) are capable of melting with a sufficient content of the additive. With an increase in the proportion of Ca in the base mixture composition, the burning rate drops, and the reduction completeness of the target oxides increases. With an optimal ratio of Ca and Al in the mixture, the yield of the target elements in an ingot is close to the calculated value

This paper presents a study of direct conversion of chemical energy into electrical energy during the combustion of a (80Zr + 20CuO) - (LiF + CaF₂ + MgF₂) - (15Zr + 85CuO) thin three-layer condensed energy system, which is a high-temperature galvanic cell. It is determined that this cell during combustion generates an electric signal with an amplitude of 1.6 V and a half-width of 15 s. Its formation mechanism is proposed. A time-resolving X-ray diffraction method is used to identify the phases formed.

Effect of carbon content in a Ni-Al-C system and preliminary mechanical activation on a burning rate, sample elongation, and yield of the mixture after mechanical activation, as well as on the structural features of combustion products is under study. Combustion in pressed samples from a mechanically preactivated Ni + Al mixture at room temperature could not be activated. The introduction of carbon (2, 4, 6% (wt.)) Into the Ni + Al reaction mixture allows for combustion in Ni - Al - C based pressed samples. Preliminary mechanical activation of the Ni + Al + x C reaction mixture expands a carbon content limit in the mixture at which the combustion of pressed samples is possible. In addition, mechanical activation, as well as an increase in the carbon content in the Ni - Al - C system reduces the burning rate. An explanation of the observed relationships is proposed.

The results of diagnostics of thermal conditions of an electric thermal explosion (ETE) of a mixture of titanium and soot powders under quasistatic compression are presented. The effect of quasistatic compression pressure on the thermal and electrical parameters of an ETE is studied. It is shown that nonuniform heating occurs at low pressure, while uniform heating occurs at high pressure. A criterion is proposed for determining the thermal regime of the ETE of a heterogeneous mixture, based on the ratio of the exothermic interaction times and the electric current variation during a thermal explosion. It is established that the dependence of the electric current variation rate on time has one peak when the sample is heated nonuniformly and two peaks when it is heated uniformly. An abnormally low effective flash point of the heterogeneous titanium - soot mixture is explained.

An experimental study of the effect of grinding on the thermal destruction of coal is carried out. Artificial neural networks are used to develop a model that allows predicting the degree of burnout of ground coals with high accuracy (an average relative error of 3% and a determination coefficient of 96%).

A method for calculating the optimal ratio of the fuel and solid propellant in the gas generator of a high-velocity flying vehicle is proposed. A thermodynamic approach is used to describe the processes in the gas generator. The ratio of the solid propellant and fuel in the gas generator is determined by their physical and chemical properties. An increase in the density and heat of combustion of the solid propellant and in the fuel porosity leads to reduction of the volume fraction of the solid propellant in the gas generator. An increase in the density, specific heat capacity, specific heat of sublimation, and temperature of sublimation of the fuel leads to an increase in the volume fraction of the solid propellant in the gas generator.

The objective of the present analysis is to investigate the effect of gaseous hydrocarbon fuels, such as Octane C₈H₁₈, Hexane C₆H₁₄, and Pentane C₅H₁₂ on the cyclic combustion process in an obstructed channel of the pulse detonation engine. Three-dimensional reactive Navier-Stokes equations are used to simulate the combustion mechanism of stoichiometric hydrocarbon fuels along with a one-step reaction model. The fuel is injected at atmospheric pressure and temperature and is ignited with pre-heated air. The investigation shows that initially a high-temperature combustion wave propagates with the local speed of sound; it creates turbulence after colliding with obstacles, resulting in an increase to supersonic flame speeds. Therefore, different values of the combustion flame propagation speed, combustion efficiency and impulse per unit area are obtained for these fuels. The detonation speed in the hexane-air mixture is about 5.8% lower than the detonation speed predicted by the NASA CEA400 code. However, it is observed that the octane fuel reduces the deflagration-to-detonation transition run-up distance as compared to other fuels.

The results of studying the explosive decomposition thresholds of PETN-aluminum composites depending on the concentration of inclusions at various sample densities are presented. An increase in the density of the samples reduces the minimum critical energy density of explosive decomposition and the concentration of inclusions at which explosions are observed.

It is a key problem to design and prepare metallized explosives of high energy and low sensitivity. In order to study the effect of the content of the boron/aluminum (B/Al) compound powder on the energy output property of metallized explosives in the air blast, three HMX-based explosives containing B/Al were designed and prepared. Air blast tests of cylindrical samples are performed, accompanied by numerical simulations by the finite element program LS-DYNA. The results show that the shock wave overpressures of explosives containing B/Al are higher than those of explosives containing Al under the same conditions. The deviations between the values calculated by the empirical equation and the measured values are smaller than 3.5 kPa, and the deviations between the values obtained in numerical simulations and the measured values are smaller than 4.9 kPa. Although the Al powder can enter the reaction with detonation products and air easier, the burning time of the boron powder is longer and the released energy is larger. Moreover, as the content of the B/Al compound powder increases, the burning time becomes longer, and the aftereffect work ability and the damage effect become stronger

Sound velocity variation behind a shock wave front is measured in pressed samples of micro- and nanodispersed mixtures of nickel and aluminum powders at pressures of 10, 30, and 60 GPa in order to verify the possibility of a reaction with the formation of nickel aluminide in a submicrosecond time range. It is shown that, in a pressure range of up to 60 GPa, the sound velocity in the samples from a nanodispersed mixture is higher than in the samples from a microdispersed mixture. Moreover, upon reaching 60 GPa, the sound velocities in both mixtures with account for errors are practically equalized, which is related to melting of the samples. Based on the data obtained, it is concluded that there is no noticeable progress of the Ni + Al reaction during less than 1 s.

This paper presents a computational comparison of the penetrability of two shaped charges - with conical and hemispherical (degressive thickness) liners. It is shown that close values of the penetrability of the charges are provided only in the ideal case of strictly axisymmetric motion of shaped-charge jets.

Shock-wave loading is an effective method of volumetric hardening of materials. This study of the structure and mechanical properties of 110G13L steel before and after explosive loading has shown that the mechanical characteristics reach maximum values at the surface of the explosively treated material: a hardness of 480 HV, a yield strength of 850 MPa, and a tensile strength of 1240 MPa. The thickness of the hardened layer is »40 mm. For steel without shock-wave loading, the listed parameters had values of 240 HV, 355 MPa, and 840 MPa, respectively. Metallographic studies revealed that the number of slip lines in grains decreases with distance from the explosive loading surface.

The breach of a steel column target (steel 45, 120 mm in diameter) by an inward-cutting circular shaped charge is considered. The jet penetration process is simulated by a 3D model run in the ANSYS/LS-DYNA program. The results are compared with actual tests, where photographs of the jet penetration process allowed observation of detonation forms, timing of the jets arising at the cross section of the detonation points, and detonation wave collision points. Different penetration effects are observed with 2-, 4-, or 8-point symmetrical synchronous initiation of detonation. With 2-point initiation, the circular-shaped charge can basically cut off the steel column target, but 4- and 8-point initiations are more effective. A greater number of detonation points provides more detonation wave collision points, higher jet velocity, earlier jet-target contact, greater penetration depth, and more rapid cutting of the target.