Home – Home – Jornals – Combustion, Explosion and Shock Waves 2017 number 3

This paper presents a numerical study of the combustion of a hydrogen-air mixture in a model ramjet combustor with separate hydrogen and air supply during activation of O₂ molecules with resonance laser radiation at a wavelength of 762.3 nm and 193.3 nm. The calculation is made using the parabolic Navier-Stokes equations taking into account chemical transformations, laser irradiation, and the unevenness of the air parameters at the combustor inlet due to the complex gas-dynamic structure of the flow in the air intake. It is shown that the combustion efficiency at the combustor outlet can be increased a factor of 2.8 by redistributing the hydrogen supply through the system of fuel tank pylons. Further increase in the combustion efficiency can be achieved by exposure of a narrow flow region to resonant laser radiation, more effectively at a wavelength of 193.3 nm. The combination of laser exposure together with the hydrogen supply redistribution increases the combustion efficiency by a factor of more than 4.7 compared to the base case. Furthermore, this provides a 95% increase in the axial force component in the inner portion of the engine flow path, which provides a positive contribution to the thrust. Estimation of the energy efficiency due to the use of laser radiation shows that the laser energy input required to achieve this effect is 40-80 times (depending on the fuel supply method) less than the increase in the chemical energy (compared to the case of no laser exposure) released due to fuel combustion.

The effect of a focused pulsed-periodic beam of a CO₂ laser on initiation and evolution of combustion in subsonic and supersonic flows of homogeneous fuel-air mixtures (H₂ + air and CH₄ + air) is experimentally studied. The beam generated by the CO₂ laser propagates across the flow and is focused by a lens at the jet axis. The flow structure is determined by a schlieren device with a slot and a plane sheet aligned in the streamwise direction. The image is recorded by a high-speed camera with an exposure time of 1.5 s and a frame frequency of 1000 s^-1. The structure of the combustion region is studied by an example of inherent luminescence of the flame at the wavelengths of OH and CH radicals. The distribution of the emission intensity of the mixture components in the optical discharge region is investigated in the present experiments by methods of emission spectroscopy.

The need to find reactive additives capable of reducing the combustibility of dimethyl ether is an important problem in connection with the widening use of ether as an environmentally friendly alternative motor fuel. In this study, the autoignition chemistry of mixtures of dimethyl ether with air in the presence of atomic iron was investigated by numerical methods. Atomic iron, which is an effective inhibitor of premixed laminar hydrocarbon flame was found to shorten the induction period. However, the additive affects only the first stage of the induction period. The mechanism promoting the low-temperature oxidation of dimethyl ether- air mixture by atomic iron involves the formation of hydroxyls in reactions involving iron compounds. Since the additive hardly changes the duration of the second stage of the induction period, it can be suggested that OH radicals play an insignificant role in the low-temperature oxidation of dimethyl ether at this stage.

The effects of silane temperature and ambient moisture on the ignition behavior are considered. The critical velocity for delayed ignition is determined for different silane temperatures and moisture contents in ambient air. The logarithm of the critical exit velocity is found to be inversely proportional to silane temperature. It is also observed that moisture in air has a strong inhibiting effect on silane autoignition in air. From a safety perspective, it is concluded that prompt ignition of silane is favored in a high-temperature and low-humidity environment.

This paper describes an experimental setup and method for determining the combustion completeness of the gas and condensed phases of gasification products of energetic boron-containing condensed formulations in high-enthalpy subsonic air flow. The results of investigation of the combustion of a two-phase fuel mixture in a channel of constant cross section at various temperatures, pressures, and component ratios are given. The combustion regularities of a boron-containing condensed phase in a high-enthalpy air flow are identified. The obtained data can be useful in computational and experimental studies of operation processes in power plants.

A performance analysis and experimental study of a hybrid gas generator to be used in a ducted rocket are presented. Such a system exhibits potential advantages with regard to safety, performance, costs, availability of the fuel components, storability, and thrust control. A combination of a paraffin wax fuel and oxygen in the gas generator ensures a high regression rate and reveals oxidizer-to-fuel ratios as low as 0.14 in the gas generator (compared to the stoichiometric ratio of 3.4). A fuel regression rate correlation versus the oxidizer mass flux is derived, presenting a major advantage for the fuel flow rate management in comparison to control of the solid propellant gas generator burning rate through the pressure exponent, which requires mechanical interference with the hot nozzle flow to ensure a change in the combustor pressure and a corresponding change in the burning rate. Evaluation of the ducted rocket (with different oxidizers) versus pure ramjet performance shows a higher specific thrust for the former, though the latter exhibits a higher specific impulse.

A cylindrical shock wave in a dusty gas under the action of monochromatic radiation into the stellar atmosphere with a constant intensity per unit area is discussed. The gas is assumed to be grey and opaque, and the shock is assumed to be transparent. The dusty gas is considered as a mixture of a non-ideal gas and small solid particles. To obtain some essential features of shock propagation, small solid particles are considered as a pseudo-fluid, and it is assumed that the equilibrium flow condition is maintained in the entire flowfield. The effects of the parameters of the gas non-idealness, the mass concentration of solid particles in the mixture, the ratio of the density of solid particles to the initial density of the gas, and the radiation parameter on flow variables are investigated. It is shown that an increase in the gas non-idealness and the radiation parameter has a decaying effect on the shock waves, whereas the shock strength increases with an increase in the ratio of the density of solid particles to the initial density of the gas. It is found that an increase in the gas non-idealness and the ratio of the density of solid particles to the initial density of the gas has the opposite effects on the fluid velocity, pressure, and shock strength. It is also shown that an increase in the radiation parameter has a trend to decrease the flow variables and the shock strength.

This paper presents the results of experiments to study the shock compression of samples of vanadium deuterides and hydrides of the following compositions: VX_0.51, VX_0.7-0.9 and VX _≥1.6, wher X is H or D. The experiments were carried out in a pressure range of 20-140 GPa. The technology of synthesizing samples using electrolytic vanadium containing not less than 99.7% of the basic substance. The shock adiabats of vanadium deuterides and hydrides were determined using the well-known reflection method. The samples were compressed using shock-wave generators based on the use of explosive charges of different power. The obtained experimental data are described by the equation of state developed using a model in which the specific heat and Gruneisen ratios of ions and electrons are functions of density and temperature. At low temperature, the specific heat changes in accordance with the Debye theory. The removal of the degeneracy of the electron gas at higher temperatures is considered. The influence of ionization processes on the thermodynamic functions is effectively taken into account.

This paper reports the synthesis, experimental and theoretical studies of a novel inorganic-organic cocrystal energetic material: methylamine triethylenediamine triperchlorate (MT). MT is synthesized by a rapid “one-pot” method. The performance test of MT shows that it is more powerful and has lower sensitivity in comparison to the benchmark energetic material, i.e., 2,4,6-trinitrotoluen (TNT). The molecular and crystal structures of MT are determined by means of x-ray diffraction (XRD). The compound crystallizes in a monoclinic system (space group Pn) with cell dimensions a = 8.975(18), b = 17.836(4), and c = 10.455(2) Å. The band structure and the density of states are calculated by an abbreviated form of the CASTEP code. The first principle tight-binding method within the general gradient approximation is used to study the electronic band structure, density of states, and Fermi energy. The results indicate that the main mechanism of cocrystallization originates from the Cl-O^…H hydrogen bonding between - ClO₄ and -NH₂.

Coal particles when subjected to shock waves can undergo rapid fragmentation, pyrolysis, and combustion, causing enhanced process intensity and efficiency. Particle fragmentation plays a crucial role in this process. Exposure of coal particles to a shock wave is modelled in the present work as combined convection and radiation at the surface and conduction in the interior. Local temperatures within a coal particle and the corresponding thermal stresses are computed to study particle failure. Particle fracture is modelled by a three-parameter Weibull probability to predict the failure location and time. Simulations indicate that pulverized coal of size up to 250 μm subjected to a shock wave for varying operational, thermal, and physical parameters can experience initial failure within 150 μs. Particles of size d ≥50 μm or higher wave strengths (with Mach numbers M ≥5) mostly trigger exfoliation, while interior fragmentation dominates at smaller sizes (d ≤ 25 μm). An initial fracture study reveals that pulverized coal with predominant sizes d ≤ 100 μm and the coal rank from lignite to bituminous coal is potentially suitable for detonation combustion in waves at Mach numbers M = 3-6. Coal particles under continuous exposure to post-shock conditions undergo recursive exfoliation until the core is 20-40 μm, after which an interior fragmentation phase is seen until the core is about 1-3 μm. Much finer coal particles, of the order of internal fragmenting cores, are hardly fractured due to low thermal stresses caused by rapid uniform heating. The fracture model approach for studying shock-induced combustion is validated by a reasonable match of the computed ignition delay with experiments. The fragmentation history indicates a substantial increase in the particle surface area and temperature under shock exposure, as against conventional combustion, leading to an increased order of the burning rates at the onset of ignition, which can sustain through the entire burning phase.

This paper presents the results of numerical analysis of a viscoplastic model of the formation of hot spots based on the solid-phase mechanism of hot-spot ignition of an energetic porous material under shock-wave loading. The highly viscous pore collapse is considered, which is of great interest in the theoretical study of shock-wave initiation of heterogeneous energetic materials. Interphase heat transfer was described under the assumption that the gas is ideal and hence the uniform pressure condition can be used. Parametric analysis was conducted, and the specific features of the effect of heat transfer and interphase heat transfer on the critical conditions of shock-wave initiation of chemical reaction in the energetic porous material were determined.

Reactive materials are a new class of energetic materials that extremely and efficiently release energy under the influence of high impact loading. An impact tester is used in the present study to explore the impact ignition characteristics of Al/PTFE reactive materials and the impact ignition pressure of Al/PTFE reaction materials under different conditions. The experimental result shows that the critical ignition pressure is approximately 1.44 GPa. Meanwhile, it also shows that the material compactness has a much less pronounced effect on the impact ignition pressure for this reactive material if the loading time scales are of the order of several milliseconds. Plastic work and viscous heat both play a significant role in impact ignition. Finally, it is shown that impact ignition ensures a higher energy release rate than surface ignition.

The spectral-kinetic characteristics of luminescence of PETN with iron nanoparticle inclusions are measured in real time under laser initiation of an explosion (wavelength is 1064 nm, and pulse duration is 14 ns). During the action of the laser radiation pulse, the luminescence of the samples is observed, and explosive decomposition occurs in a microsecond time interval. The spectral pyrometry method is used to establish the thermal nature of explosive luminescence. The explosion temperature is estimated to be 3400±100 K.

The detonation velocities close to ideal velocity relative to large charges of highly dispersed ammonium perchlorate (APC) and its mixtures with different explosive substances in thick-walled steel pipes are measured. The relationship of the detonation velocity of APC with its density and the relationship between the detonation velocity of mixtures with the correlation of components and oxygen coefficient of the mixtures are determined. The calculation of the detonation velocity of APC/explosive/Al three-component compositions is proposed for the first time.

This paper describes the results of the experimental study on the effect of structural parameters, such as diameter, height, and density, on the development of explosive transformation of an NCP energy-intensive metal complex during bridge initiation. The results are compared with those obtained in shock-wave initiation.

In this paper, the commercial finite element code LS-DYNA is employed to simulate the process of a certain kind of a linear shaped charge jet penetrating into a TC4 (Ti-6Al-4V) titanium alloy plate of moderate thickness. The fracture profiles agree well with experimental observations, which confirms the validity of the code and the Johnson-Cook material model applied to describe the TC4 plate. The fracture pattern of the plate is drawn based on this comparison.