Vol 53, No 5 (2017)

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 (ϕ_Σ = 0.95 and ϕ_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.

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.

A new criterion for thermal explosion in exothermically reacting systems for the case of arbitrary one-step conversions is proposed. The computation method is based on the analysis of the equation of maximum temperatures obtained for closed systems taking into account reactant consumption. 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–Semenov criterion parametric plane are calculated for second-order exothermic reactions. Comparison of numerical and analytical calculations shows that they are in satisfactory agreement.

A model for solid propellant gasification is proposed which contains a two-phase medium in an intermediate stage. The formation of the gas phase proceeds in two ways: chemical reactions result in gaseous products, which, in turn, initiate the formation of bubbles in which vapor forms from the liquid phase of the propellant. Gaseous products play an important role only in the very early stage of bubble development; their critical pressure is used to determine the minimum size of gas-phase nuclei. The 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 suggesting that a temperature maximum cannot occur in the gasification zone and that natural turbulence can be generated by collapsing bubbles. The sound produced by solid propellant combustion 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 is transformed into the Belyaev–Zel’dovich model equations.

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.

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 the reduction of Nb₂O₅, which complicates its introduction into the composite material and leads to an excess of Al in the alloy. In the present study, we investigated the possibility of replacing Hf in the starting mixture by less active HfAl₃ and also determined the effect of the grain size of HfAl₃ on the Hf content in the composite material.

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 describes the experimental study of dielectric relaxation in energy condensed systems based on ether urethane rubber plasticized by glycerin trinitrate and ammonium perchlorate, HMX, and aluminum powders in the frequency range of the electric field from 40 to 1.2 · 10⁹ Hz. Depending on the field frequency, 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.