Free-convective regime of flame spread over a fuel film on a substrate

Flame spread over a liquid fuel film on a thin metallic substrate under free convection was studied experimentally. Instantaneous flame velocities correlate with the flame length. The average flame velocity increases from 2 to 30–40 cm/sec with the slope angle of the substrate to the horizon varying in the range of 0–90°. For a substrate of specified width, the flame velocity is inversely proportional to the heat capacity of the unit area of the substrate-fuel system and to the differences between the temperature corresponding to the formation of a stoichiometric mixture of the saturated fuel vapor and air and the ambient temperature. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 5, pp. 21–30, September–October, 2007.     

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. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 3, pp. 25–30, May–June, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 98-03-32308) and the INTAS Fund (Grant No. 96-1173).     

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.     

Ignition and Combustion of Dust-Gas Suspensions

The ignition and combustion of dust-gas suspensions are considered. It is shown that the ability of these systems to accumulate heat is determined not only by their kinetic and thermal properties but also by the relation between their reaction surface and the heat-removal surface (f). Experimental information on flame temperatures, ignition delays, and flame propagation over gas suspensions is processed using the parameter f, and the postulate on the stimulating role of the developed reaction surface in activating these processes is validated. It is shown that during overall burning, diffusion combustion of particles occurs only for rather small values of f. The ambiguous effect of the parameter f on the ignition and combustion processes leads to the necessity of optimizing the fuel size distribution and concentration for the effective operation of the power devices. The role of the macroparameters of two-phase flames of refractory metals in the synthesis of combustion nanoproducts is analyzed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 6, pp. 3–14, November–December, 2005.     

Effects of octane number on autoignition and knocking phenomena in a stratified mixture

The effects of the fuel concentration gradient and the octane number on the autoignition and knocking phenomena in a stratified mixture were studied experimentally on a using a rapid compression machine using stratified mixtures of air and fuels n-heptane, iso-octane, n-hexane, and n-pentane with different octane numbers (0, 100, 25, and 62, respectively). In the chamber, the lower the vertical location, the richer the fuel concentration of the mixture. The mixture contains no gradient in the horizontal direction. The experimental results show that rapid spread of the flame is caused not by flame propagation but by sequential autoignition. Although ignition delays of a stratified mixture are not dependent on the fuel concentration gradient in the mixture, they are constant as long as mean equivalence ratio is the same, and they decrease with the decreasing mean equivalence ratio. In excess of certain gradient value, the knock intensity is smaller as the gradient becomes larger for all fuels tested regardless of their octane number. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 93–100, July–August, 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.     

Gas combustion in a narrow tube

A simplified model of flame propagation over a single capillary in the low-velocity regime is proposed. The model is based on the concept that the main features of flame propagation in the low-velocity regime are determined by the heat flux along the tube wall from the combustion products to the fresh mixture. Qualitative agreement with experimental results is obtained. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 2, pp. 22–26, March–April, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 99-03-32309).     

Flame propagation through an aluminum aerosuspension at reduced pressure

The rate of flame propagation through an aerosuspension of ASD-1 powdered aluminum is measured in the pressure range 0.1–0.05 MPa and at various component ratios corresponding to a fuel excess. Linear reduction in flame speed with reduction in pressure is observed. It is shown that the combustion of aluminum—air mixture is the most sensitive to pressure change. Spectrozonal cinerecording and optoelectronic image analysis are used to determine the temperature field in the flame front of an overenriched aerosuspension; the formation of eddy structures due to the hydrodynamic interaction of particle settling and the formation and propagation of a combustion surface is recorded. N. é. Nauman Moscow State Technical University. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 5, pp. 23–31, September–October, 1995.     

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.     

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.     

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.     

Characterization of passivity and pitting corrosion of 3003 aluminum alloy in ethylene glycol–water solutions

In this work, the passivity and pitting corrosion behavior of 3003 aluminum (Al) alloy in ethylene glycol–water solutions was investigated using various electrochemical measurements, Mott–Schottky analysis and surface analysis techniques. Results demonstrate that the passive film formed on Al alloy contains both Al oxide and Al alcohol, showing an n-type semiconductor in nature. There is an enhanced corrosion resistance of the Al-alcohol film, which is resistant to adsorption of chloride ions. The pitting corrosion of 3003 Al alloy occurs in the solutions containing a low concentration of ethylene glycol only, where the formed film is dominated by Al oxide. Chloride ions attack and replace the oxygen vacancies in the film, resulting in a local detachment of the film from the Al alloy. A galvanic effect exists between Al alloy substrate and the adjacent second phase particles. Pits form when Al alloy substrate is dissolved away and the second phase particles drop off from the substrate.     

Wildfire propagation: A two-dimensional multiphase approach

This paper describes a multiphase approach to determining the rate of propagation of a line fire through a randomly packed fuel bed of thermally thin cellulose particles and the induced hydrodynamics inside and above the litter. A set of time-dependent balance equations is solved for each phase (a gas phase andN solid phases) and the coupling between the gas phase and the solid phases is rendered through exchange terms of mass due to thermal degradation of the fuel material (heating, drying, pyrolysis, and char combustion), momentum, and energy. The radiative transfer equation for the fuel bed is deduced from the P1-approximation, and radiation from the flame to the fuel bed is accounted for using the empirical model of Markstein. The kinetics is incorporated to describe pyrolysis and combustion processes. Solution is performed numerically by a finite-volume method. The development of a line fire from the moment of initiation to quasisteady propagation is predicted and discussed. Results obtained by this multiphase model are compared to measurements made on laboratory fires using dead pine needles as fuel. The predicted rates of fire spread for some configurations, including slope effects, agree well with measured values. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 2, pp. 26–38, March–April, 1998.     

Propagation velocity of hydrocarbon-air flames containing organophosphorus compounds at atmospheric pressure

The propagation velocities of propane-air and methane-air flames of various compositions with and without the addition of 600 ppm trimethyl phosphate were measured and calculated. The flame propagation velocity was determined using the heat flux method, which allows a flame to be stabilized on a flat burner under nearly adiabatic conditions. The experimental results were compared with flame propagation velocities calculated using the PREMIX and CHEMKIN codes for the Westbrook mechanism for the destruction of trimethyl phosphate and the Curran mechanism for hydrocarbon oxidation. The methane-air flame was modeled using the GRI 3.0 mechanism. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 9–14, May–June, 2007.     

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 fuel films in opposed gas flow

The effect of the velocity of forced oxidizer flow on the pattern and velocity of flame spread over a fuel film was experimentally studied, and the limiting conditions of steady-state flame propagation were determined. New experimental evidence was obtained for the validity of the previously proposed model of flame propagation in a thermally thin system. It was found that, in a thermally thin system at a certain value of the gas flow velocity, laminar flame propagation is followed by spin flame propagation in a narrow range of gas flow velocities, and then by quenching. In the laminar layer-by-layer propagation regime, the flame velocity does not depend on the average velocity of the opposed gas flow. The proposed model for the laminar layerby-layer flame propagation agrees with experiment taking into account the fuel film flow under the action of the Marangoni effect due to the condensed-phase temperature gradient.     

Propagation and quenching of premixed flames in narrow channels

The shape and propagation of unsteady premixed flames in narrow channels with adiabatic and isothermal walls are numerically investigated in the present study. The flame chemistry is modelled by an one-step overall reaction, which simulates the reaction of a stoichiometric acetylene-air mixture. The numerical results show that both ignition methods and thermal boundary conditions affect flame formation and its shape. The flame keeps a single tulip shape in the whole process of propagating through the channel if plane ignition is used and a single mushroom shape if spark ignition is used. For isothermal cold walls, the flame cannot keep a single tulip shape or mushroom shape all the time. Under plane ignition, a transition from a tulip-shaped flame to a mushroom-shaped flame occurs as flames propagate from one end of channel to the other. Under spark ignition, the process is just the contrary. It is also shown that the heat loss at the cold walls has a dual effect on the formation of a tulip-shaped flame. Flame propagation and quenching in narrow channels of different heights are analyzed systematically, and a criterion is proposed to judge the flame states of partial extinction and total extinction. It is called the criterion of flame propagation and quenching in narrow channels. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 27–36, May–June, 2006.     

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.