Effect of macroscopic heterogeneity of the medium on the solid-state combustion wave characteristics in thermally and chemically heterogeneous media

A macroscopically heterogeneous medium is modeled by a set of cylinders (rods) pressed from a mixture of solid reagents, which have coaxial cylindrical cores made from an inert material. A three-dimensional mathematical model describing propagation of combustion waves over one cylinder is studied by numerical methods. The influence of the distance between the centerlines of the neighboring cylinders and transverse sizes of the cylinders on characteristics of combustion waves propagating over a heterogeneous medium is considered. It is demonstrated that there exists an optimal distance between the inert rods, which ensures a much higher velocity of the combustion wave along the specimen than the theoretically predicted velocity of the classical combustion wave propagating over a solid specimen. New types of spinning waves are described, whose motion makes the high-temperature spot move inside the charge mixture. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 3, pp. 39–49, May–June, 2008.     

Combustion-wave propagation through an inert obstacle in real condensed systems

Displacement of combustion products behind the chemical-reaction front in a condensed system is shown to affect combustion-wave propagation through an inert obstacle. In the experiments, pressed specimens of a Ti+C+20%TiC mixture containing an obstacle in the form of a tantalum band 10 μm to 0.6 mm thick were used. Dependences of the delay of the combustion wave during penetration through the obstacle on the obstacle thickness were obtained for the cases of small (<1 mm) and large (≈4.5 mm) displacements of the material behind the chemical-reaction front. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 6, pp. 43–47, November–December 1998.     

Mechanism of catalysis in combustion waves of modern ballistite propellants

The effect of 12 catalysts that contain iron, lead, and nickel oxides, cobalt, potassium, copper, sulfur, and lead compounds, and soot additives on the structure and parameters of the combustion-wave zones for nitramine-containing ballistite propellant at pressures of 20, 50, and 100 atm is studied by a thermocouple technique. The many-parameter conversion of combustion waves by the catalysts is described in the form of matrixes of changes of the main combustion-zone parameters introduced by the catalyst. The zones in which these catalysts act are shown, and the magnitudes of the action are found. The resulting zone parameters make it possible to separate four regimes of catalytic action in the combustion wave: normal, blowing-off, compensation, and gas-phase. It is found that the significant increase in the combustion rate is generally caused by the increase in subsurface heat release in the c-phase and heat release on the burning surface. The reactivity series of these catalysts is given. It is shown that the combustion-wave parameters for catalyzed nitramine-containing ballistite propellants can obey unified laws. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 5, pp. 75–85, September–October 1999.     

Stabilized gas combustion wave in an inert porous medium

A stabilized gas combustion wave in an inert porous medium with intense internal interphase heat exchange (at low velocities) is considered to elucidate the mechanism underlying the increase in the burning velocity of the homogeneous gas mixture due to the porous medium. It is shown that the major factor of the increase in the burning velocity is conductive heat recuperation from the postflame zone to the region of the fresh mixture through the solid skeleton of the porous medium. Analytical dependences of the degree of increase in the burning velocity of the mixture in the stabilized wave on the determining parameters are obtained. The possibilities and restrictions of the use of the results to analyze the operation of porous burners are discussed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 8–15, September–October, 2008.     

Combustion of Ti+0.5C and Ti+C mixtures of bulk density in inert gas coflow

The combustion of Ti-C mixtures of bulk density under inert gas blowing conditions produced by evacuation of one end of the reaction cell was studied for the first time. The experiments showed that the tested mixtures in quartz cups were not ignited and did not burn without the inert gas (argon) flow. Increasing the rate of gas evacuation from the sample increased the rate of steady-state combustion of the mixture of titanium with carbon black, and for the mixture of titanium with graphite, stabilization of the flat combustion front was observed. It is shown that the presence of small pressure difference (up to 105 Pa) allows control of the combustion process and confirms the basic postulates of the convective-conductive theory of combustion for heterogeneous condensed systems. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 30–37, January–February, 2009.     

Filtration combustion of hydrogen-air, propane-air, and methane-air mixtures in inert porous media

The filtration combustion characteristics of hydrogen-air, propane-air, and methane-air mixtures in inert porous media have been studied experimentally. It is shown that the dependences of the combustion wave velocity on the fuel-air equivalence ratio are V-shaped. For hydrogen-air mixtures, the velocity minimum is shifted to the rich region, and for propane-air and methane-air mixtures, it is shifted to the lean region. For lean hydrogen-air and rich propane-air mixtures, the measured maximum temperatures in the combustion wave are found to be reduced relative to those calculated theoretically. For methane-air mixtures, a reduction in the measured temperatures is observed over the entire range of the mixture composition. The results are interpreted within the framework of the hypothesis of selective diffusion of gas mixture components. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 8–20, July–August, 2006.     

Propagation of spherical gas filtration combustion waves in inert porous media

The behavior of spherical combustion waves of gases in inert porous media is described in terms of one- and two-temperature models. Parametric dependences are obtained for the wave velocity and acceleration and for the temperature of the gas and porous medium in the wave. It is shown that in a diverging spherical gas flow, combustion waves initiated at different radii of the sphere converge to the standing wave coordinater ^*, and in a converging flow, on the other hand, they diverge from it. The experimentally observed propagation behavior of spherical combustion waves is well described by the proposed models. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 1, pp. 60–66, January–February 1999     

Heat fluxes to combustor walls during continuous spin detonation of fuel-air mixtures

Pioneering measurements of heat fluxes to the walls of flow-type combustors of different geometries were performed in regimes of continuous spin detonation of fuel-air mixtures under unsteady heating. These heat fluxes are compared with those observed in the regime of conventional turbulent combustion in the same combustor. Air is used as an oxidizer, and acetylene or hydrogen is used as a fuel. For identical flow rates of the fuel, the heat fluxes to the combustor walls in regimes of continuous spin detonation and conventional combustion are close to each other; their mean steady values are ≈1 MW/m² (≈0.5% of the enthalpy flux of the products over the channel cross section). In both detonation and combustion regimes, the maximum heat fluxes penetrate into the walls in the mixing region (where the heat release occurs). In the case of detonation, regenerative cooling of the combustor walls by the flow of the fresh mixture occurs in the heat-release region (region of propagation of the detonation-wave front). The regeneration becomes less effective in the downstream direction because of the shorter time of contact between the walls and the cold mixture and a longer time of contact between the walls and the hot products. More intense heating persists downstream of the front, where the regeneration ceases, but the temperature of the products is high. The character of heating of the wall in the region of rotation of the front of spin detonation waves depends on the number of these waves: the zone of the maximum heat release becomes narrower with increasing number of waves. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 80–88, January–February, 2009.     

Effect of convective motion on the flame structure in combustion waves propagating in heterogeneous systems under natural and artificial gravity conditions

Convective motion in combustion waves arises from natural or artificial gravity (centrifugal effect) for a wide range of heterogeneous systems, such as ammonium perchlorate containing solid additives, metal and nonmetal powders with a gas reagent, thermite mixtures, and hybrid layered structures. The paper summarizes the results obtained by the author and coworkers in studies of the effect of convective motion in combustion of heterogeneous systems for the period from the early 1970s to the present. It is shown that convective processes in combustion waves of heterogeneous systems may determine the structure and propagation mechanism of the combustion waves and the concomitant heat and mass transfer. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 86–92, July–August, 2009.     

Model of Vapor—Air Gasification of a Solid Fuel in a Filtration Mode

A two-temperature mathematical model of steady filtration combustion of a solid fuel in open systems is proposed. Air or a mixture of air with water vapors is considered as a gaseous oxidizer. The model takes into account that the heat capacities of the phases depend on temperature and composition and that the reactor length is finite and allows obtaining the composition of gaseous combustion products. Calculated results on gasification of a mixture of carbon with an inert component are presented. It is demonstrated that thermodynamic calculations are important for obtaining the upper estimate of gasification efficiency. Even a sufficiently long reactor becomes “short” in the regime of transient combustion waves, which results in more intense entrainment of heat by combustion products and, as a consequence, in lower efficiency of the process. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 1, pp. 65–72, January–February, 2006.     

Numerical study of combustion in a cylindrical porous burner

Hybrid combustion waves in a two-section apparatus with cylindrical symmetry were studied using a two-temperature mathematical model. A program was written and a large series of numerical calculations was performed using an algorithm developed earlier. For one-and two-section apparatuses with various flow rates, characteristics of the process such as the position of the steady-state point, the maximum gas temperature, the gas velocity at the steady-state point, the width of the combustion zone, etc., were determined. It is shown that, in the two-section apparatus, there is a stabilization of the position of the combustion front near the interface. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 3, pp. 17–21, May–June, 2008.     

Characteristics of HMX combustion waves at various pressures and initial temperatures

Temperature profiles and combustion-wave parameters are obtained experimentally for combustion of pressed HMX at room temperature and pressures of 1–500 atm and in the case of a change in the initial temperature of the specimens from −170 to +100°C at pressures of 1–75 (90) atm. The following combustion-zone parameters are determined: the heat effect in the c-phase, the heat transfer from the gas to the c-phase by thermal conduction and radiation, the rate of heat release in the gas near the surface, and the dimensions and temperature of the combustion zones. The authorsé previous conclusion that there is one process of decomposition and evaporation of HMX during its gasification in the condensed-phase reaction layer of the combustion wave is confirmed. Dependences of the fraction of decomposed HMX on the initial temperature of the specimens and the pressure are obtained. The differential characteristics of the combustion rate, surface temperature, and radiative heat transfer, required for the nonlinear theory of HMX combustion stability, are evaluated. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 2, pp. 59–66, March–April, 1998.     

Frontal transformation modes of structured energetic heterogeneous systems

The transformation of structured energetic heterogeneous systems is studied taking into account radiative heat transfer. It is shown that at the combustion front of porous heterogeneous media, radiative heat transfer has a significant effect on the structure of the front and the dynamics of its propagation. In the case of a nonlinear dependence of heat transfer on temperature, the reaction zone is concentrated in a narrow temperature range in the region of high temperatures. The dimensions of the reaction zone coincide with the width of the heating zone. In the case of dominant radiative heat transfer in multilayer systems, there may be quasihomogeneous or relay-race dynamics behavior of combustion-wave propagation, depending on the scale of heterogeneity. Oscillations in the displacement of the high-temperature zone are possible even for small scales of heterogeneity (microheterogeneous combustion mode).__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 2, pp. 51–61, March–April, 2005.     

Effect of radiative heat transfer on the combustion wave propagation in a model of a heterogeneous system

The subject of investigation was the behavior of integral curves of the problem of combustion-wave propagation in a model of a heterogeneous system. The influence of radiative heat transfer on steady combustion regimes was considered. It was shown that for sufficiently large gaps between the plates in the system (when a quasihomogeneous temperature distribution still exists) in the regime of weak retardation by the growing product layer, the radiation can result in a notable acceleration of the combustion wave. In the regime of strong retardation with the remaining conditions being the same, the radiation can be neglected. Translated fromFiz. Goreniya i Vzryva, Vol. 34, No. 3, pp. 69–76, May–June 1998.     

Steady states of the surface of a nonadiabatic flame near the limits

Within the framework of a weakly linear model, which describes a nonadiabatic flame near the limit of its propagation caused by heat losses, steady states of the combustion-wave from are studied. Three-dimensional structures of the wave front are formed because of diffusion-thermal instability of the planar flame. The limits of propagation of a curved flame front are shown to expand if the diffusion-thermal instability is taken into account: a cellular flame can exist at heat losses higher than the critical value for the two-dimensional flame. The stability of steady solutions, which describe the cellular flame near the limits of its propagation, is studied. For sufficiently high heat losses, steady solutions for a nonadiabatic flame with front discontinities are obtained. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 4, pp. 3–11, July–August 1999.     

Parameters of an accelerating spherical deflagration

A numerical analysis of an accelerating spherical deflagration is performed. It is shown that the deviations of the deflagration parameters from steady values are determined by the degree of increase in velocity in the law of motion for the flame front rather than by the acceleration value. Maximum deviations are observed at a front velocity equal to ≈0.1 of the sound velocity in the starting mixture. Analytical relations are obtained for the main parameters of the resulting flow for velocities up to a front velocity a factor of1.5 higher than the sound velocity in the starting mixture. Bauman Moscow State Technical University, Moscow 107005. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 2, pp. 11–22, March–April, 1997.     

Thermal Interaction of Two Flame Fronts Propagating in Channels with Opposing Gas Flows

A onedimensional nonstationary model is presented for propagation of two premixed flame fronts in narrow plane channels with regard for thermal interaction of flames through a dividing wall. The gas flows in channels considered are oppositely directed and equal in magnitude. It is shown that heat transfer through the heatconducting wall separating the plane channels leads to the appearance of a number of special features. The scheme of filtrational combustion of gases considered may be called the scheme of combustion with counter filtration; it is a new modification of systems with enthalpy excess. It is shown that the temperature at the combustionwave front may be greater than the adiabatic temperature of free plane flame with the same composition of the combustible mixture even if there are heat losses through external walls of the system. By solving the problem, dependences of the velocity of combustion waves on the distance between them are obtained, and the dynamic behavior of combustion waves is studied. The region of problem parameters, where selfstabilization of combustion waves is possible, is found.     

Combustion of a gasoline-hydrogen-air mixture in a reciprocating internal combustion engine cylinder and determining the optimum gasoline-hydrogen ratio

This paper reports results of an analysis of experimental data on the combustion of a gasoline-hydrogen-air mixture in a reciprocating internal combustion engine cylinder. The completeness of combustion of the mixture is shown to depend on the amount of hydrogen in the fuel mixture and the composition and physicochemical properties of the mixture. In particular, the conditions of addition of hydrogen to the gasoline-air mixture with active chemical action on the combustion process and the action of hydrogen as an additional fuel component are determined. A dimensionless universal relation is proposed that allows one to uniquely determine the initial composition of the fuel mixture (hydrogen to gasoline ratio) to accomplish combustion of the fuel mixture at the lean combustion limit. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 4, pp. 8–14, July–August, 2007.     

Combustion-Front Non-One-Dimensionality in Single- and Double-Base Propellants

Parameters of transverse waves propagating over the surface of specimens pressed from colloxylin and double-base propellants A and N are studied. By means of microvideofilming and thermocouple measurements, it is shown that the burning sites on the specimen surface are formed by a set of transverse waves. Under atmospheric pressure, the transverse-wave front has the form of a step 0.5–1.1 mm high and decreases with increasing pressure or initial temperature of the specimen. The front propagates with variable velocities in the horizontal and vertical directions. The mean velocity of the transverse wave is three to eight times higher than the normal burning rate of the specimen as a whole (with a wide spread of local values) and increases with increasing pressure. Behind the front, combustion can be interrupted till the arrival of the next transverse wave. As in the SHS process, the reason for the emergence of the transverse waves is combustion-wave spatial instability.