Home – Home – Jornals – Combustion, Explosion and Shock Waves 2017 number 5

Interest in the combustion chemistry of multi-fuel mixtures is related to the need to study the combustion of natural gas, which is known to be a mixture of alkanes. It has been found by molecular beam mass spectrometry studies and numerical simulation that the width of the zones of consumption of hydrogen and methane in the H₂/CH₄/C₃H₈/O₂/Ar flame and the width of the zones of consumption of methane and propane in the CH₄/C₃H₈/C₄H₁₀/O₂/Ar flame differ significantly from each other. The causes of this phenomenon were determined by analyzing the simulation results. It has been found that in the presence of heavier compounds, lighter fuels, such as H₂ and CH₄, are formed which reduces the rate of their consumption and, hence expands the zone of their consumption in the flame. The influence of the presence of hydrogen in the fuel mixture on the concentration of C₂ hydrocarbons has also been studied. It has been established that the addition of hydrogen reduces the maximum concentration of ethane, ethylene, and acetylene in the flame, while the fraction of unsaturated C₂ hydrocarbons with respect to saturated ones also decreases.

An oxygen-diluted partially premixed/oxygen-enriched supplemental combustion (ODPP/OESC) counterflow flame is studied in this paper. Flame images are obtained through experiments and numerical simulations with the GRI-Mech 3.0 chemistry. The oxygen dilution effects are revealed by comparing the flame structures and emissions with those of a premixed flame and partially premixed flame (PPF) at the same equivalence ratio ( f _= 0.95 and f _f = 1.4). The results show that both PPF and ODPP/OESC flames have distinct double flame structures; however, the location of the premixed combustion zone and the distance between premixed/nonpremixed combustion zone are significantly different for these two cases. For the ODPP/OESC flame, the temperature in the premixed combustion zone is lower and the premixed zone itself is located farther downstream from the fuel nozzle, which leads to reduction of NO and CO emissions, as compared to those of the PPF. Therefore, by adjusting the distribution of the oxygen concentration in the premixed and nonpremixed combustion zones, the ODPP/OESC can effectively balance the chemical reaction rate in the entire combustion zone and, consequently, reduce emissions.

This paper describes the numerical modeling of gas flow in a plane vortex chamber by using the Navier-Stokes equations. The model is based on the laws of conservation of mass, momentum, and energy for nonstationary two-dimensional compressible gas flow in the case of axial symmetry with a tangential component of the gas velocity. The processes of viscosity, thermal conductivity, and turbulence are accounted for. It is shown that the transition of the kinetic energy of gas into thermal energy as a result of the transfer processes leads to the formation of hot spots in the boundary layers near the walls of the chamber. The gas temperature at these hot spots can exceed the gas ignition temperature, while the gas remains rather cold in the neighboring regions. This could be the reason for the cold gas self-ignition observed in the experiments. The turbulence of the flow and the processes of mixing and diffusion of the components make a significant contribution to the capacity of gas self-ignition.

A high-speed camera system is used to observe the diffusion flame of a Bunsen burner in linear motion. The resultant sequence of instantaneous motion pictures of the flame accelerating at 3.60 m/s² is processed and used to study the change in the flame area and specific floor area of the flame over different temperature ranges. The results indicate that the total flame area increases in the fuel control zone as the velocity increases over the range of experimental speeds employed (< 0.90 m/s); then the total area quickly decreases in the transition region and is stable in the cross-flow wind control zone. As the velocity of the fire source increases, the low-temperature and specific floor areas adopt more dominant positions in the low-speed fuel control zone. In the high-speed cross-flow wind control zone, the area of the high-temperature zone and specific floor area take the dominant positions. The transformation between the two situations occurs in the transition zone. The cross-flow wind increases the high-temperature specific floor area of the fire compared to that of a stationary fire; the consumption in the moving fire also becomes correspondingly more concentrated and fierce.

The effect of preliminary injection (pre-injection) of a gas (heated air, methane, or ethylene) ahead of the entrance of a three-dimensional inlet in a supersonic flow with Mach numbers M_¥ = 2-4, stagnation temperature T * = 300 K, and flow rates of the injected gas corresponding to 0-6% of the flow rate of air through the inlet is numerically studied. The gas is injected through orifices located behind the end faces of pylons mounted upstream of the inlet entrance. The computations are performed by the ESI-FASTRAN software package, which allows one to calculate three-dimensional viscous turbulent gas flows by a time-dependent method with the use of the Reynolds-averaged Navier-Stokes equations. Experimental investigations of pre-injection are performed with the use of small-scale and large-scale inlet models at M_¥ = 3-4 and 6. A positive effect of pre-injection on stable (without stalling) deceleration of the incident flow in the case of mass supply and on initiation of ignition and stable combustion in the combustion chamber is confirmed.

A new criterion for thermal explosion in exothermic reacting systems for the case of arbitrary one-step conversions. The method of calculation is based on the analysis of the equation of maximum temperatures obtained for closed systems with the consumption of reagents taken into account. The dependence of the maximum temperature on the Semenov and Todes parameters is bistable and the critical conditions are determined by the extremum conditions on the relevant parametric diagrams. The demarcation lines of ignition in the Todes criterion-Semenova criterion parametric plane are calculated for second-order exothermic reactions. Comparison of numerical and analytical calculations showed that they are in satisfactory agreement

The mathematical model of thermal explosion and synthesis of products in a mechanically activated 3Ni + Al mixture is simulated in a macroscopic approximation. It is shown that activation of original components significantly increases the formation rate of a Ni₃Al intermetallide. The experimental data are used to determine the thermophysical and kinetic constants of the process.

A model for the gasification of solid propellants containing a two-phase medium in an intermediate stage. The formation of the gas phase proceeds in two ways: chemical reactions result in gas products, which, in turn, initiate the formation of bubbles, in which vapor forms from the liquid phase of the fuel. Gas products play an important role only at the earliest stage of bubble development; the minimum size of gas-phase nuclei is determined from their minimum pressure. The he bubble volume grows primarily by evaporation of the liquid phase. A kinetic equation for the bubble concentration and the necessary boundary conditions are formulated. Arguments are given to suggest that a maximum temperature cannot be formed in the gasification zone and that natural turbulence can be generated by collapsing bubbles. The sound produced by burning solid rocket propellants is explained by the collapse of a huge number of microscopic bubbles. If the processes in the two-phase zone are neglected, the formulated system of equations goes over into the Belyaev-Zel'dovich model equations.

The combustion products of solid rocket fuels at a pressure of 40 atm have been studied by molecular beam mass spectrometry. A facility with a four-stage molecular beam formation system has been designed. The possibility of quantifying the composition of combustion products is shown using as an example a composite fuel based on ammonium dinitramide and polycaprolactone.

Five variants of calculating the burning rate of a solid propellant as a function of the pressure in a solid-propellant rocket motor are considered. Two variants of analytical expressions are proposed for approximating real dependences. In all variants, the pressure in the rocket motor can be presented by simple analytical expressions as a function of solid propellant parameters, charging conditions, and structural factors of the charge and motor.

The combustion and chemical transformation of highly exothermic MoO₃/NiO/Al/C and MoO₃/NiO/Al/B mixtures on the surface of a titanium base under the effect of centrifugal acceleration are studied. It is shown that chemical reactions occur not only in the combustion wave, but also on the surface of the titanium base, between the combustion products and the base material (titanium). The dynamics and mechanism of formation of layered (gradient) titanium-ceramics materials (Mo-Ni-C-Ti or Mo-Ni-B-Ti) is investigated.

Composite materials based on Nb with functional (Si, C, B) and alloying dopants (Hf, Ti, Al, etc.) are promising materials for aircraft engine applications. Our previous studies have shown that such composites can be synthesized in the autowave mode (combustion mode) using highly exothermic mixtures of Nb₂O₅ with Al, Si, Hf, and Ti. It has been found that in the combustion wave, Hf actively participates in reducing Nb₂O₅, which complicates its introduction into the composite material, and also leads to an excess of Al in the alloy. In the present study, we have investigated the possibility of replacing Hf in the starting mixture by less active HfAl₃ and have also determined the effect of dispersion of HfAl₃ on the Hf content in the composite material.

Propagation of a plane detonation wave in a stoichiometric mixture of a gas and aluminum particles in a plane channel with a linear expansion section is studied by methods of numerical simulation. The slope of the wall is varied from 15 to 60^o. The basic modes of detonation propagation are analyzed: supercritical (without failure), critical (with partial failure and re-initiation), and subcritical (with complete separation of the shock front and combustion front and with detonation failure). The detonation configuration formed in the expanding section can be a cellular structure with large differences in cell sizes at large angles of expansion or a close-to-uniform structure at the wall angle of 15^o.

This paper reports the experimental and theoretical studies of the synthesis and behavior of a cocrystal energetic material 2,4,6-trinitrotoluene/1,3,5-trinitrobenzene (TNT/TNB). The performance tests show that this material is more powerful and less sensitive than TNT and TNB. A molecular dynamic simulation is conducted for the cocrystal TNT/TNB by using a COMPASS force field with an NPT ensemble. The density function theory is applied to investigate the band structure and the density of states for various pressures and temperatures. The results show that the TNT/TNB crystal is sensitive to pressures in the interval of 35-50 GPa, and the melting temperature of the crystal is around »320 K, which agrees well with experimental results. The Hirshfeld analysis is carried out to ascertain weak interactions and associated two-dimensional fingerprint plots. The crystal packing is demonstrated to be ensured by H × O, C × O and O × O contacts.

This paper presents the results of numerical simulation of the Zaretskii's experiments on the loading of natural uranium in the temperature range 27-862^o in the phase-transition region. Simulation of these experiments is of interest because of the observed features of spall fracture of uranium in the phase-transition region. Spall fracture and compaction was simulated using the DRK-L model of the dynamics of growth and compacting in a liquid medium, which takes into account the effect of strength properties, pressure, surface tension, viscosity, and inertia forces on the growth and collapse of pores. The calculations are carried out according to the UP program - a Lagrangian technique for calculating deformation problems of continuum mechanics in a one-dimensional approximation.

This paper describes the experimental study of dielectric relaxation in energy condensed systems based on etherurate rubber plasticized by glycerin trinitrate and ammonium perchlorate, octogen, and aluminum powders in the frequency range of the electric field from 40 to 1.2 × 10⁹ Hz. Depending on the frequency of the field, it was possible to determine relaxation processes caused by dipole polarization, the bulk electrical conductivity of a polymer binder, and the influence of the surface of aluminum particles.