Home – Home – Jornals – Combustion, Explosion and Shock Waves 2013 number 2

Thermodynamically coupled dehydrogenation of ethane in a membrane reactor is studied by mathematical modeling. The dehydrogenation of ethane in a membrane reactor with additional combustion of hydrogen is shown to have an advantage over dehydrogenation in a tubular reactor. Verification of the mathematical model of the process is performed.

The complete system of hydrodynamic equations describing the development of instability of the reaction front in the hydrodynamic approximation is reduced to a closed system of surface equations with the use of Lagrange variables, integrals of motion, and their analogs. In the adiabatic approximation, it is shown how to take into account the acoustic vibrations of the gas density caused by this motion.

The problem of flame propagation is studied as an example of unstable fronts that wrinkle on many scales. The analytic tool of pole expansion in the complex plane is employed to address the interaction of the unstable growth process with random initial conditions and perturbations. We argue that the effect of random noise is immense and that it can never be neglected in sufficiently large systems. We present simulations that lead to scaling laws for the velocity and acceleration of the front as a function of the system size and the level of noise and also analytic arguments that explain these results in terms of the noisy pole dynamics.

Problems of decreasing the flame speed resulting from pre-mixed gas explosions and attenuating explosion overpressures are discussed. A cylindrical test pipeline with an 89 × 4.5 mm cross section is used to study flame propagation characteristics of an acetylene–air mixture both in the empty pipeline and in the presence of aluminum silicate wool attached to the internal wall of the pipeline. Experimental results show that aluminum silicate wool, which is a kind of a fibroid porous material with a high specific surface area, decreases the increment of the outlet flame speed and attenuates drastically the explosion overpressure if the length of the porous insert exceeds the critical length.

An accumulated combustible dust layer on some hot process equipment, such as grinders, dryers, hot bearings, etc., can be ignited and lead to fires if the hot surface temperature is sufficiently high. Experimental tests are used to determine the minimum hot surface temperature for dust ignition, the ignition temperature of dust itself, and the ignition times in this study. Egyptian rice husk dust is sieved into different sizes (particle diameters) to be used in this investigation. The effects of the dust particle size and the sample size (depth of the dust layer) on ignition parameters are tested. The boundary between the ignition and non-ignition conditions is investigated precisely through a large number of tests. The results show that the minimum hot plate temperature for ignition of dusts decreases as the dust layer depth increases.

The so-called gradient relations, i.e., analytical formulas that define the relationship between partial spatial derivatives (gradients) of pressure, density, and gas particle velocity behind a plane detonation front and the acceleration of the front, are obtained within the framework of a model based on the assumption of isothermality of gas detonation products. It is shown that the relations can be used to simplify the description of overdriven detonation regimes in chemically reacting gas.

This paper reports on an investigation of detonation initiation with a transversal flame jet. Series of single-cycle experiments are performed, where stoichiometric propane–oxygen mixtures with nitrogen dilution at atmospheric initial pressure are used, both in the flame sub-chamber with varying configurations and in the test tube 70 mm in diameter and 1550 mm long. The experimental results show that the flame jet orifice diameter slightly affects the detonation initiation sensitivity, and the flame sub-chamber configurations exert a minor effect on the deflagration-to-detonation transition (DDT) distance, but the DDT time decreases as the flame sub-chamber length increases. Conventional spark ignition is investigated as a comparison experiment, and detonation initiation is not observed in mixtures with nitrogen dilution up to 65%.

Processes of suppression and quenching of detonation in a hydrogen–oxygen mixture by means of inserting inert particles into the flow field are numerically studied within the framework of a two-velocity two-temperature model of mechanics of heterogeneous media. The wave pattern in an inert cloud of particles, induced under the action of shock and detonation waves, is determined. The validity of using a one-velocity model for the description of detonation suppression and quenching by clouds of coarse particles is demonstrated. The effect of the volume fraction and diameter of moving particles on the detonation wave velocity is found. The limiting transition from a frozen detonation flow, which is formed in the case of large particle diameters, to an equilibrium flow formed in the case of small particle diameters is studied. Geometric limits of detonation are determined, and a comparison with similar results predicted by the one-velocity model is performed.

Processes of continuous spin detonation and pulsed detonation, as well as combustion of a hydrogen–air mixture in an annular combustor 306 mm in diameter in the regime of air ejection are studied experimentally. The specific flow rates of hydrogen are 0.6–9.8 kg/(s × m²). It is found that the greatest specific impulses of thrust generated by the combustor are reached in the case of continuous spin detonation. On the average, they are greater than the corresponding values by a factor of 1.5 in the case of burning the mixture in streamwise detonation waves, by a factor of 2 in the case of conventional combustion (by a factor of 3 at the maximum thrust impulse of 2200 m/s), and by a factor of 10 in the case of exhaustion of cold hydrogen. A change in the specific flow rate of hydrogen beginning from ≈1.2 kg/(s × m²) corresponding to the maximum thrust impulse decreases its value, and this decrease is more profound as the detonation limits in terms of the specific flow rate of hydrogen are approached. The maximum reactive thrust (83 N) is developed in the examined detonation chamber near the upper limit at the specific flow rate of hydrogen equal to 3 kg/(s × m²).

The effect of the initial temperature of a porous carbon particle on the characteristics of its combustion and spontaneous extinction is analyzed taking into account Stefan flow and heat loss by radiation. It is shown that in the case of forced ignition (increase in the initial temperature of the particle), the diameter and density of the particle after spontaneous extinction remain virtually unchanged. As a result, the extinguished particles have the same diameter but different density. It is shown that the dependence of particle diameter on stationary temperature can be used to determine the maximum combustion temperature and diameter of the particle during its spontaneous extinction. The effect of oxygen concentration on the region of ignition of a porous carbon particle, determined by the initial diameter and temperature of the particle, is analyzed. Analysis of the diffusion-kinetic relations shows that each of the two main heterogeneous reactions of carbon oxidation make similar contributions to the heat and mass transfer of the particle with the surrounding.

Studies on specific thermochemical processes contribute to the understanding of combustion processes and, meanwhile, to the calculus of the safety characteristics and the systems design parameters. In this paper, five different compositions have been studied through TGA and DTA. The reaction rate constants and the activation energies have been determined. Also, the solid combustion products obtained have been evaluated through SEM, EDX, and XPS. Nonisothermal kinetic analyses performed yield results that prove an important difference among the first fire compositions, where the activation energies are considerable, up to 400 kJ/mol, in comparison with the activation energies of the flare compositions, which are lower, 150–250 kJ/mol.

The thresholds of explosive decomposition of PETN (pentaerythrite tetranitrate) with the addition of ultrafine Al–C mechanocomposite particles were measured as a function of the concentration of the latter in the experimental samples exposed to laser pulses (1.064 nm, 12 ns). The sample density was 1.73 g/cm³, and the Al–C particle size at the distribution peak was 220 nm. The minimum threshold of explosive transformation corresponding to a 50% probability of explosion with an energy density of 4 J/cm² was reached at an optimum concentration of the mechanocomposite of 0.1–0.3%. Comparison with experimental data obtained for samples with aluminum nanoparticle additives was performed.

This paper presents the results of experimental determination of the electrical resistivity of an insulating polymer composition (Teflon film and high-vacuum leak sealant) under stepwise shock compression at pressures up to 150 GPa. The data obtained can be used in experiments to measure the electrical conductivity of materials in this range of shock pressures.

This paper considers the use of emulsion explosive compositions to join building bars and replace worn thread in the railway wheel axle. The compositions do not contain individual explosives and greatly increase the safety in explosive working of metals.

The damage characteristics of a typical semiconductor bridge and a micro-semiconductor bridge under the action of an electrostatic discharge are studied in experiments, which include semiconductor film shape measurement, resistance fluctuate evaluation, ignition characteristics, and invalidation threshold voltage measurement. The effect of multiple discharging on the bridge state is also discussed.

The behavior of two new complex salts containing cobalt and tungsten atoms was studied under different thermal conditions. It was found that when heated in air, the salt [Co(NH₃)₆](WO₄)NO₃ exploded at 260 ^oC, and further heating to 800 ^oC led to the formation of a mixture of oxide phases. Heating of [CoEn₃]₂(W₇O₂₄) × nH₂O was not accompanied by explosion and also led to the formation of cobalt and tungsten oxides. The complex salts were used to produce superhard (Vickers microhardness up to 35.8 GPa) coatings on titanium disks by means of shaped-charge explosion. High microhardness is associated with the formation of carbonitride crystalline phases on the target surfaces.

The present study aims at detecting the critical criteria and corresponding critical impact energy for initiation of strain localization during explosive cladding of the Inconel 625 superalloy as a cladding material and low-carbon steel as a substrate. The results do not reveal adiabatic shear bands, which are the main signs of strain localization, within the superalloy in all studied impact energies up to 205 kJ. At impact energies greater than 78–114 kJ, strain localization is observed in low-carbon steel, and microcracks develop within the adiabatic shear bands. The Johnson–Cook model is used to explains the results obtained and to study the thermomechanical behavior of materials.

The first experimental studies of the effect of the laser plasma generated by focused pulsed-periodic radiation of a CO₂ laser on the formation and evolution of combustion in a supersonic flow of homogeneous methane–air mixtures are performed. Results of multispectral investigations testify to a principal possibility of combustion initiation by an optical discharge. Conditions necessary for combustion stabilization are formulated.