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.     

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.     

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     

Formation of carbon clusters in deflagration and detonation waves in gas mixtures

Numerical and experimental results of studying the formation of carbon clusters due to propagation of deflagration and detonation waves in enriched acetylene-oxygen and acetylene-air mixtures are described. Experiments are performed in tubes of different diameters (including tubes filled by a porous medium) with wide-range variations of the initial pressure and the fuel-to-oxidizer ratio. A large variety of carbon clusters formed in different regimes of burning of the mixture is found. A typical size of condensed carbon particles is 15–100 nm. In the case of detonation in a porous medium, the size of carbon particles is 15–45 nm; in some tests, large individual fullerene-type particles 150, 400, and 950 nm in size are formed. The fraction of condensed carbon in the total amount of carbon in the initial mixture is found to depend on the wave type; detonation is characterized by the minimum “yield” of condensed carbon. The amount of condensed carbon increases with increasing acetylene concentration in the mixture and initial pressure. The size of carbon particles in the case of deflagration is greater than that in the case of detonation. Cooling of reaction products decelerates condensation and interrupts the growth of carbon particles. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 3, pp. 81–94, May–June, 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.     

Modeling of solid-state combustion in thermally and chemically heterogeneous media

A thermally and chemically heterogeneous medium is modeled by a set of cylinders pressed from a mixture of solid reagents with coaxially aligned cylindrical rods made from an inert material. The change in velocity of combustion waves propagating over one cylinder is studied by numerical methods under the assumption that there is no heat release from the cylinder surface. The mean velocity of the combustion front in the specimen is shown first to decrease and then to increase with increasing thermal conductivity of the inert rod. Spinning waves are obtained in the range of low velocities of the combustion front. The laws of variations of the maximum temperature in the combustion front are determined. It is shown that the inert rod may serve as a heat sink from the hot charge mixture, may be manifested as dilution by an inert component, enhance heat recuperation, and increase the combustion-wave velocity. Introduction of the inert rod may either destabilize combustion-wave propagation in the region of a stable plane front or stabilize the combustion wave in the range of parameters where the plane front is unstable. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 6, pp. 21–30, November–December, 2007.     

Numerical study of the effect of the parameters of a bulk layer and an incident shock wave on the pressure at a shielded flat wall

Results are presented from a numerical simulation to study the effect of the parameters of a porous, powdered layer and of an incident shock wave in air on the pulsed loading characteristics of a shielded flat wall. An analysis is made of the dependence of the total stress of the mixture and of the pressure of the pore gas at a rigid wall on the porosity of the medium, density of the material, particle diameter, depth of the shielding layer, and depth of the actuating shock wave. The depth of the screen is found to have the most significant effect on the amplitude of the total pressure of the powdered medium at the solid wall behind a reflected shock. A formula is obtained for estimating the number of maxima in the total pressure of the mixture at the wall during reflection of a shock wave from it. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 2, pp. 105–113, March–April 1999.     

Continuous spin detonation of hydrogen-oxygen mixtures. 1. Annular cylindrical combustors

A comprehensive numerical and experimental study of continuous spin detonation of a hydrogen-oxygen mixture in annular combustors with the components supplied through injectors is performed. In an annular combustor 4 cm in diameter, burning of a hydrogen-oxygen gas mixture in the regime of continuous spin detonation is obtained. The flow structure is considered for varied flow rates of the components of the mixture and the combustor length and shape. The dynamics of the transverse detonation wave is numerically studied in a two-dimensional unsteady statement of the problem with the geometric parameters of the combustors consistent with experimental ones. A comparison with experiments reveals reasonable agreement in terms of the detonation velocity and pressure in the combustor. The calculated size and shape of detonation fronts are substantially different from the experimental data. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 2, pp. 32–45, March–April, 2008.     

Gas detonation in a channel with transverse ribs

Results of experiments on detonation propagation in a rectangular horizontal channel with high ribs on the lower wall are presented. The experiments were performed with acetylene-oxygen mixtures. An interval of initial pressures is found, in which low-velocity detonation with a steady velocity of 0.38–0.55 of the Chapman-Jouguet velocity without losses exists. This detonation wave is a system consisting of a shock wave and a flame. Owing to gas outflow to the layer occupied by the ribs, the flame is maintained at a constant distance from the shock wave, which is approximately equal to the free transverse size of the channel. This distance weakly decreases with increasing initial pressure and is almost independent of the burning rate of the gas at standard temperature. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 5, pp. 82–86, September–October, 2007.     

Modeling of flame propagation in a mixture of combustible gases and particles

A physicomathematical model of flame propagation over a gas suspension composed of a mixture of gases (oxidizing, combustible, and inert components) and the particles of a condensed material that reacts heterogeneously with the oxidizing component is formulated. Numerical simulations are used to obtain a dependence of the flame velocity on the parameters related to the mass concentration of the particles, the particle size, the activation energy of a heterogeneous reaction on the particle surface, the heat of the heterogeneous reaction, and the mass exchange of the particles. Depending on the ratio of the dispersed-phase parameters, the flame velocity in this medium can increase severalfold in comparison with the flame velocity in a dust-free gas mixture or decrease. In the latter case, the effect of the particles is similar to the effect of the inert dispersed phase. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 2, pp. 3–9, March–April, 2000.     

Localization of combustion in a periodic velocity field

A direct numerical simulation of combustion-front propagation in a specified periodic velocity field of a medium was performed within the framework of a two-dimensional thermal diffusion formulation. The calculations showed that as the velocity amplitude is increased, the flame front separates into discrete burning segments, i.e., spatial localization of the combustion takes place. Only when heat losses are introduced into the model, flame quenching is observed at a certain amplitude of the medium’s velocity. The level of heat losses required to extinguish the combustion becomes lower with increase in the amplitude of velocity perturbations. On the whole, the obtained numerical results agree with results of an asymptotic analysis. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 3, pp. 19–28, May–June 1998.     

Motion of a shaped-charge jet in a porous medium

The problem of the motion of a shaped-charge jet in a porous medium is equivalent to the problem of a blunt cylinder in a hypersonic flow whose velocity at infinity is equal to the jet velocity in the porous medium. The flow pattern of the medium is the same as in the case of propagation of a blast wave generated by a point explosion of a cylindrical charge. The approximate theory of a strong explosion is used to obtain the basic relations for the shock wave and the expanding cavity in the hypersonic flow of a porous medium around the blunt cylinder. A comparison with experiments on the motion of a copper shaped-charge jet in porous aluminum is performed. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 6, pp. 119–124, November–December 1999.     

Selective Diffusion during Flame Propagation and Quenching in a Porous Medium

The propagation of propane-air flames in an inert high-porosity medium with nitrogen dilution and oxygen enrichment of the mixture was studied experimentally. It is shown that variation in the nitrogen or oxygen concentration (in the gas phase) leads to a more significant variation in the flame propagation velocity than in the laminar burning velocity; with the addition of nitrogen, the rate of increase in the flame velocity with the initial pressure becomes lower and the concentration range of flame propagation becomes narrower. At the flame propagation limit, the Peclet number obtained from the laminar burning velocity of the initial mixture is not constant but depends on the fuel-to-oxidizer ratio and the nitrogen content in the mixture. The results are interpreted from a physical point of view based on the hypothesis of selective diffusion. It is shown that accounting for the effects of the Lewis numbers of the fuel and oxidizer allows flame propagation in inert porous media to be described quantitatively over wide parameter ranges using a unified relation. At the flame propagation limit, the Peclet number constructed from the laminar burning velocity taking into account these effects is a constant.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 4, pp. 50–59, July–August, 2005.     

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.     

On the threshold nature of erosive burning

A physical explanation for the existence of the threshold of erosive burning is proposed. It is shown that this type of combustion occurs when the thickness of the laminar sublayer in the turbulent boundary layer becomes smaller than the thickness of the laminar combustion zone. In this case, turbulent flame in the gas phase is formed. Relations are obtained linking the critical (threshold) velocity of the blowing flow and the critical Vilyunov number to the properties of the propellant and the gas resulting from propellant decomposition. Simple exponential dependences on the blowing velocity are found for the burning rate. The simplest representation of the erosive burning rate is obtained using the Bulgakov-Lipanov number, whose threshold value is equal to unity. A new mechanism for the occurrence of negative erosion is proposed, according to which the burning rate decreases during blowing because the boundary layer is displaced, resulting in a decrease in the heat flux from the flame zone to the solid-phase decomposition surface. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 3, pp. 61–71, May–June, 2008.     

Modeling of unsteady filtration gas combustion

Unsteady filtration gas combustion with various gas flow parameters is studied by mathematical modeling. Transition processes due to a sudden change in the calorific value of the gas mixture, gas flow velocity, and flow direction are considered. Trends and mechanisms of change in the structure of the filtration gas combustion wave and its propagation velocity are analyzed for various types of transition processes. It is found that with a sudden change in gas flow parameters, the flame can abruptly move large distances in the porous medium. Subsequently, at the new flame localization, a wave of filtration gas combustion forms which corresponds to the changed parameters of the gas flow. If in the porous medium, the amount of heat is insufficient, the transition process ends with quenching. As the gas flow direction changes, the combustion wave continues to propagate in the former direction for some time, which can lead to the spread of the high-temperature zone in devices based on the reverse process with a homogeneous gas-phase reaction.     

Detonation waves in a reactive supersonic flow

A supersonic hydrogen—air flow is studied in detail, in particular, the fields of gas-dynamic parameters and chemical homogeneity of the mixture in various cross sections of the duct. The processes of excitation and propagation of a detonation wave in the downstream and upstream directions are considered. The detonation-wave velocity with respect to the mixture flow is found to differ from the nominal Chapman—Jouguet velocity for a quiescent mixture: the detonation-wave velocity is higher if the wave moves upstream and lower if the wave moves downstream. Some hypotheses on the reasons for these deviations of the experimentally measured velocity from the nominal value are given. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 5, pp. 85–100, September–October, 2006.     

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).     

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.     

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.