Characteristics of stationary spherical waves of gas combustion in inert porous media

Stationary spherical waves of gas combustion in porous media were studied theoretically and experimentally. It is shown that in spherically convergent and divergent gas flows, the waves differ in the type of stability: they are stable in a divergent flow and unstable in a convergent flow. A mathematical model for standing spherical waves is proposed that establishes the dependences of the coordinate of a standing wave on the gas-flow rate, mixture composition, and parameters of the porous medium and the gas and describes the experiment adequately. It is shown that allowance for heat losses leads to the appearance of limits of existence for standing spherical waves. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 2, pp. 9–19, March–April, 1998.     

Numerical study of the transfer of shock-wave loading to a screened flat wall through a layer of a powdered medium and a subsequent air gap

The two-velocity, two-temperature model with two stresses in a mixture of a gas and solid particles contacting each other is used to numerically study the dynamic effect of an air shock wave incoming onto a solid wall with a screening layer of a porous powdered medium at some distance from the wall. The process is described for the case of one-dimensional planar motion of the gaseous and disperse phases under the assumption of a viscoelastic behavior of the powder skeleton. The effect of stepwise shock waves onto the porous powdered screen is considered. The influence of parameters of the screening layer and the air gap on the dynamics of loading of the screened solid wall is analyzed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 1, pp. 132–142, January–February, 2007.     

Characteristics of RDX combustion zones at different pressures and initial temperatures

The RDX burning rates and temperature profiles in the combustion wave are measured with the pressure and initial temperature varied within wide limits. Parameters of combustion waves are found. The gas phase of RDX is demonstrated to have a single-zone structure. Two regimes of RDX combustion are found: a basic regime at p > 0.1 MPa and a special regime at p ≤ 0.1 MPa. The main characteristics of these regimes are obtained, and the wave regions responsible for the burning rate are determined. Both regimes are characterized by wide reaction zones in the gas phase. The processes of vaporization and decomposition of the condensed phase proceed simultaneously on the burning surface. The fraction of thermal expansion in the reaction layer of this phase is estimated. Laws of RDX gasiffication (pyrolysis) in the combustion wave are found, which turn out to be the same as those for HMX. Distributions of the heat-release rate in the combustion wave (in the reaction layer of the condensed phase and in the gas phase) are obtained. Sensitivity of the burning rate to the initial temperature is measured. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 5, pp. 32–45, September–October, 2006.     

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.     

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.     

Method of generating cold gases in solid-fuel gas generators

The principle of operation of a gas generator that is to burn a porous charge of a solid fuel in the filtration regime of gaseous combustion products toward the cold side, i.e., in the direction of displacement of the combustion front, is described. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 4, pp. 75–78, July–August 1999.     

Regimes of gas combustion in a porous body of a cylindrical heat generator

A stationary model of filtration combustion of a gas with allowance for conditions at the entrance to the porous body and conditions of heat exchange with the gas phase surrounding the burner and with the heat exchanger is proposed and numerically analyzed. Ranges of parameters where the regime of gas combustion with a narrow reaction zone near the outer surface of the porous body are determined. Mechanical stresses arising in the porous body owing to high temperature gradients and gas pressure in the pores are estimated. The rate of gas combustion and the radiative heat flux from the burner surface are plotted as functions of process-dependent parameters. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 18–29, January–February, 2009.     

Specific features of combustion in a variable-section narrow channel with a periodically changing gas flow

Results of numerical and theoretical research of flame behavior in a periodically changing gas flow during combustion in a variable-section narrow channel are presented. The influence of the mean mass flow of the gas and amplitude and frequency of periodic oscillations of the flow on the mean position and temperature of the flame front is studied. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 16–24, September–October, 2008.     

Dynamic combustion regimes of the Ti-(Ti+0.5C) layered system in a concurrent nitrogen flow

The filtration combustion of a layered porous fill consisting of alternating layers of a mixture of Ti + 0.5C a titanium powder with forced concurrent filtration of nitrogen was studied for the first time. The gas flow through the sample was provided by a vacuum pump attached to the lower end of the fill. The presence of the concurrent gas flow radically changes the character of propagation of the combustion front and the structure and composition of the products obtained. The layers consisting of carbonitride and titanium nitride make a single unit. The experiments provided scientific bases for the production of new laminated and composite ceramic materials by dynamic filtration combustion. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 44–51, November–December, 2008.     

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.     

Flame propagation in porous media wetted with fuel

Some specific features of flame propagation over a gas mixture with a very low value of enthalpy have been studied experimentally in an evaporative-diffusive regime in various porous media. The combustion wave is shown to propagate steadily in a high-porosity medium wetted withn-octane at velocities of3–10 cm/sec. We have also studied the effect of the volumetric heat capacity and thermal conductivity of the material of a porous medium on the velocity and characteristics of flame propagation both in a high-velocity regime for high-enthalpy gas mixtures and in a low-velocity regime for low-enthalpy ones. The existence conditions of an evaporative-diffusive regime have been considered. Institute of Chemical Kinetics and Combustion, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 3, pp. 76–85, May–June, 1997.     

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.     

Control of the deflagration-to-detonation transition in systems with resistance

This paper develops an approach to controlling gas combustion, including deflagration-to-detonation transition, based on using systems with resistance, such as porous media, periodic obstacles, rough tubes, etc. Gas combustion in these systems involves various physicochemical interactions: interfacial heat transfer, including combustion failure, flame quenching in fast pulsations (jets), transition to turbulence, generation of pressure waves in the flame zone, formation of hotspots, etc. These interactions result in a number of steady-state regimes with a uniform velocity of propagation of thermal waves — low-, high-, and sonic-velocity regimes, low-velocity detonation, and normal detonation with heat and momentum losses. Systems with porous media and periodic obstacles are considered as examples of systems with resistance. It is shown that with the effects of Lewis numbers taken into account, the steady-state velocities in the high-velocity regime for CH₄/Air, C₃H₈/air, and H₂/air systems over wide parameter ranges can be represented by a single relation Re = 6 · 10⁻⁴Pe³ in the coordinates (Re-Pe) for systems with porous media. Steady-state velocities in the sonic velocity regime for C₃H₈/air and H₂/air systems are described in the same coordinates by a single function Re = 120Pe^4/3 for systems with porous media and periodic obstacles. A condition for pressure generation in the flame zone at sonic velocities was obtained analytically. Problems involved in the implementation of the approach of controlling high-velocity combustion processes in systems with resistance are discussed.     

Characteristics of the boundary layer with hydrogen combustion with variations of thermal conditions on a permeable wall

The paper describes a numerical study of the influence of thermal and boundary conditions on the structure of laminar and turbulent diffusion flames in the cases with hydrogen injection through a porous surface and with hydrogen combustion in an air flow. Two types of boundary conditions are compared: with a given constant temperature T _w = const over the length of the porous surface for arbitrary intensities of fuel injection and with a constant temperature T′ = const of the fuel injected through the porous wall. The first case occurs during combustion of a liquid fuel whose burning surface temperature remains unchanged. Injection of gaseous fuel usually leads to the second case with T′ = const. Despite significant differences in velocity and temperature profiles, the skin friction coefficients in the laminar flow are close to each other in these two regimes. In the turbulent regime, the effect of the thermal boundary conditions on friction and heat transfer is more pronounced. Moreover, the heat flux to the wall as a function of fuel-injection intensity is characterized by a clearly expressed maximum. A principal difference of the effect of combustion on friction and heat transfer in the laminar and turbulent flow regimes is demonstrated. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 3, pp. 3–11, May–June, 2009.     

Flame spread over a liquid surface in a channel of finite section under oncoming air-flow conditions

The propagation of a combustion wave over a shallow hot liquid (n-butanol) blown over by an air flow was studied experimentally. The flame spread was accompanied by pulsations, whose amplitude depended on the oncoming gas velocity. Dependences of the average flame speed on temperature and oncoming gas velocity were obtained. The average speed was found to be independent of the liquid depth ahead of the flame within the experimental error. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 1, pp. 29–34, January–February, 2008.     

Hybrid autowaves in filtration combustion of gases in a catalytic fixed bed

A mathematical model of autowave processes in a heterogeneous medium with chemical reactions in the gas phase and on the catalyst is constructed. The coefficients of heat and mass transfer between the phases and the thermal conductivity of the solid phase are determined from the current values of system parameters. The model describes three types of autowaves: waves due to filtration combustion of gases in the low-velocity regime, waves due to a catalytic reaction, and hybrid waves. The behavior of phase trajectories of the dynamic system is studied by methods of qualitative and numerical analysis, and an effective technique is developed for searching for a physically grounded autowave solution of the problem. The laws of propagation of autowaves due to filtration combustion of gases and hybrid autowaves are numerically examined, and the effect of system parameters on the basic technological characteristics of autowave processes is studied. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 2, pp. 3–13, March–April, 2008.     

Filtration combustion of porous metallic samples in a gas diluted by an inert component

Filtration combustion of a porous layer with natural filtration of the oxidizer diluted by an inert component is studied. The problem of ignition of a porous layer by a heated surface permeable for the gas is solved, and the time of ignition is determined as a function of pressure in the reactor and initial porosity of the sample. Formation of an inert gas “plug” preventing chemical interaction is examined. It is shown that the mean depth of conversion of the condensed reagent in the porous layer depends on the sample length, porosity, pressure of the gas mixture, and concentration of the inert component. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 1, pp. 73–79, January–February, 2008.     

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.     

Emission phenomena in a SHS combustion wave

There are numerous experimental data indicating that the action of electromagnetic and magnetic fields on the combustion wave of self-propagating high-temperature synthesis (SHS) changes the process kinetics and the structure and properties of the reaction products. Emission phenomena in the combustion wave have received less attention. High rates of chemical energy dissipation in SHS (10¹² W/m³) are accompanied by physical phenomena such as the occurrence of a potential difference between the combustion front and reaction products, free electron emission from the combustion wave, and acoustic emission. Detailed studies of these nonequilibrium phenomena provide a deeper understanding of the reaction mechanism in solid flames to use nonthermal methods of combustion control. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 121–127, July–August, 2009.     

Filtration combustion of a gas in a semiinfinite porous medium

An asymptotic analysis of the combustion of a gas moving in a semiinfinite porous medium is performed for large values of the Zel’dovich parameter. The case of a highly porous medium with high gas permeability is considered. The basic terms of asymptotic expansions of the main parameters of the process for combustion and separation regimes are obtained by the method of joined asymptotic expansions. The effect of the rate of motion of the gas and heat transfer from the surface of the skeleton into the ambient medium on the combustion parameters is analyzed. The critical conditions of failure of stationary combustion at the external surface of the layer and the conditions of transition of the process to the separation and induction regimes are determined. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 4, pp. 3–14. July–August, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 98-01-03009).