A. A. Korzhavin

Flame zone in gas combustion in an inert porous medium

The paper describes an experimental method for determining the propagation characteristics of a flame in a porous medium. The apparatus comprises a vertical tube filled with a medium of porosity 0.4. Photodiodes are provided for determination of the combustion zone boundaries, and photomultipliers are used to measure the extent of the combustion zone. The whole is filled with a stoichiometric methane air mixture and ignited with a single spark at the top end of the tube. Experiments are performed for mixtures at various initial pressures, and deductions are made concerning the simultaneous procession of the chemical transformations and combustion product cooling within the combustion zone. The flame propagation in a porous medium is determined to be turbulent in nature on the basis of observations concerning the relationship between the extent of the combustion zone and the thickness of a laminar flame front.

Diffusion combustion of a liquid fuel film on a metal substrate

Diffusion combustion of a film of a liquid fuel (n-undecane andn-butanol) deposited on the surface of a thin metal substrate is studied experimentally and theoretically. The experimental data obtained show that the mechanism determining the heating and evaporation of the combustible liquid is the heat transfer from the region of combustion products to the heating zone due to the high longitudinal thermal conductivity of the substrate. Prior to combustion, the combustible liquid may evaporate, not reaching the boiling point. A simple model that takes into account these features is proposed. The calculated velocities of flame propagation and temperature profiles are compared to experimental data. It is shown that the model gives a correct description of the dependence of the flame velocity on the substrate thickness, initial temperature, and properties of the substrate and liquid fuel.

Behavior of flames propagating over liquid films with metallic substrates

We have examined the propagation of a combustion wave over liquid fuel (n-butanol andn-undecane) films on substrates made form copper, aluminum molybdenum, and niobium under the conditions of a thermally thin layered system. It is shown that the flame edge is situated over the fluid surface, where the temperature of the layer system is sufficient to form a stoichiometric composition in the gas-phase mixture. The fuel film is evaporated at temperatures below the boiling point. The flame velocity does not depend (over a broad variation range) on the inclination angle of the substrate plane relative to the horizontal and is determined by the specific heat portion of the substrate in the substrate-fuel system. In the flame propagation mechanism, an important role is played by the heat transfer through the metallic substrate in the preignition zone. The heat recycling through the substrate results in an increase in the enthalpy in the gaseous phase due to the chemical component linked to the fuel vapor inflow from the film surface.

Gas combustion in a vessel with a highly porous inert medium

The authors investigate the basic properties of the gas combustion process in a porous medium differing significantly (in porosity and heat capacity) from the medium of packed spheres. The porous medium used was thermally reticualted polyurethane foam. The mixtures were ignited by a single spark in the upper foam-free region of the tube. An illustration shows a typical oscillogram of flame propagation in a tube filled with polyurethane. It was found that the basic principles of gas combustion in highly porous media remain the same as in low-porosity media. A unique feature is the possibility of significant heating of the medium due to its relatively low heat capacity. As a consequence, system pressure increases and the direction of motion of gas flows relative to the combustion wave changes.

Combustion of a liquid fuel in a rectangular channel

The possibility of flame spread over the surface of a liquid fuel (n-butanol) in two-phase flow with a gaseous oxidizer in a narrow rectangular channel is demonstrated. The results of a detailed experimental study of combustion in this system are given. Dependences of the flame propagation speed on the initial temperature and oxidizer and fuel flow rates are obtained.

Flame spread over liquid fuel films on metallic substrates

New experimental studies of parametric dependences of the flame spread velocity and limits for liquid fuel films on metallic substrates confirmed the main features of the physical model proposed previously. For thermally thin layered systems “fuel-substrate,” a steady-state regime of flame spread is possible. It is shown that the flame velocity depends on the effective thermal diffusivity of the layer system, and its value is determined mainly by the volumetric heat capacities of the components of the system and, to a lesser degree, by their thermal conductivities. The mechanism of flame spread includes a series of interrelated elementary processes: heat conduction over the substrate from the combustion zone to the preflame zone, heating and evaporation of the fuel by the substrate, formation of a combustible mixture, and heating of the metallic substrate by the combustion products. The flame edge is located at the liquid surface, where the temperature corresponds to the formation of a stoichiometric mixture under equilibrium conditions. The liquid fuel is completely evaporated from the substrate at temperatures below the boiling point.

Effect of initial temperature on the velocity of flame spread over a fuel film on a metal substrate

The influence of ambient temperature in the range of −42 to 25°C on the velocity of flame spread over films of liquid and solid fuels on a metal substrate in a thermally thin system is studied experimentally. It is shown that the previously proposed model for flame spread satisfactorily describes the dependence of the flame velocity on initial temperature. A strong influence of the phase transition of the fuel into the solid phase on the flame velocity was not observed. The limit of steady-state flame spread is due to the inability to provide a sufficient thickness of the evaporating film due to the Marangoni effect.

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.

Ignition of Filtration Gas Combustion Waves by the Flame of the Filtered Gas

Mathematical modeling of ignition of filtration gas combustion waves in a porous medium with external initiation of combustion by the filtered gas is performed. It is shown that the surface temperature of the porous medium at which the flame enters the latter is a function of system parameters. The existence of the lower and upper flammability limits in terms of the gas filtration rate is found. Dependences of the ignition time on parameters of the porous medium are obtained, and their interpretation is given.

Unsteady-state effects upon gas combustion in closed vessels with an inert porous medium

Two groups of unsteady-state effects have been studied. The first group includes the effects that are connected with initial and boundary conditions, i.e., combustion-wave formation and extnction near the walls of a vessel. The size of a combustion zone in a steady-state wave is the spatial scale of these processes. These effects are based on temperature nonequilibrium. between the gas and the porous medium. The second group includes the effects that are caused by both the dynamic change in the parameters of the reacting-gas state upon combustion and the corresponding loss of steady-flame stability which is manifested either in transitions of one combustion regime to another or in flame extinction.