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).     

Effect of modulations of opposed gas flow velocity on flame spread rate over a liquid surface

The effect of modulations of the velocity of the gas flow incident on the flame on the average flame velocity over a shallow liquid is studied. It is shown that the average flame velocity depends on the modulation frequency. If the modulation frequency is higher than the flame oscillation eigenfrequency, then, upon the imposition of the modulation, the flame velocity first increases and then gradually returns to the initial value. At frequencies close to the flame oscillation eigenfrequency, the average flame velocity is constant but is higher than the initial value. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 11–17, January–February, 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.     

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. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 3, pp. 15–18, May–June, 1998.     

Splitting flames in a narrow channel with a temperature gradient in the walls

A possibility of simultaneous formation of two chemical reaction fronts during nonstationary combustion of a gas in a microchannel with a temperature gradient in the walls is demonstrated. Combustion in a straight tube and in a gap between two disks with radial fuel injection is considered. In both cases, the characteristic transverse size of the channel is smaller than the critical diameter determined for the ambient temperature, and gas combustion occurs in the region where the wall temperature is higher than the ambient temperature. A numerical study of flame repetitive extinction/ignition (FREI) demonstrated a possibility of simultaneous formation of two chemical reaction fronts in the hot region of the channel. One front corresponds to conventional flame propagating upstream from the hot to the cold part of the channel, and the other front moves in the downstream direction and decays as the fuel burns out. Based on this study, a new mechanism of ignition and incomplete combustion of the combustible mixture in microsystems is proposed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 2, pp. 12–19, March–April, 2008.     

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.     

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.     

Operation of a gas generator controlled by supplying a gaseous oxidizer

The possibility of arranging a gas generator with the heat-release process being controlled by supplying a gaseous oxidizer is experimentally checked. Gaseous hydrogen, liquid gasoline, and solid hexamethylene tetramine (solidified alcohol) is used as a fuel. The gas generator with a proposed configuration is demonstrated to ensure stable operation during combustion of various fuels; the pressure in the gas-generator combustion chamber does not exceed the pressure of oxidizer supply and clearly correlates with variations of the oxidizer mass flow. Quasi-steady calculations allow determining all parameters of the process, including those that are not measured in the experiment. In particular, the temperature of combustion products is found to be 600–1900 K, and the gas generator forms a high-temperature mixture containing a non-reacted fuel (the air-to-fuel ratio is α = 0.55–2.30). __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 18–25, November–December, 2008.     

Experimental study of characteristics of a laminar boundary layer with hydrogen combustion

Results of an experimental study of a laminar boundary layer with combustion of a hydrogen-nitrogen fuel mixture uniformly injected through a porous wall into an air flow are presented. Data characterizing the ignition conditions are obtained. Based on the recorded temperature distributions, streamwise changes in the location and temperature of the flame front are analyzed as functions of the free-stream velocity (1–4 m/s), injection intensity, and fuel composition. It is demonstrated that heat transfer can be adequately described by a “standard” dependence for the boundary layer with boundary conditions of the second kind.     

Modeling combustion of lycopodium particles by considering the temperature difference between the gas and the particles

The effect of the temperature difference between the gas and the particles on propagation of premixed flames in a combustible mixture containing volatile fuel particles uniformly distributed in an oxidizing gas mixture is analyzed in this paper. It is presumed that the fuel particles vaporize first to yield a gaseous fuel, which is oxidized in the gas phase. The analysis is performed in the asymptotic limit, where the value of the characteristic Zel’dovich number is large, which implies that the reaction term in the preheating zone is negligible. Required relations between the gas and the particles are derived from equations for premixed flames of organic dust. Subsequently, the governing equations are solved by an analytical method. Finally, the variation of the dimensionless temperatures of the gas and the particles, the mass fraction of the particles, the equivalence ratio ϕ _g as a function of ϕ _u , the flame temperature, and the burning velocities of the gas and the particles are obtained. The analysis shows that the calculated value of ϕ _g is smaller than unity for certain cases, even though ϕ _u ⩾1. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 3, pp. 49–57, May–June, 2009.     

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.     

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 ignition of a solid-propellant plate by a low-temperature plasma jet

A problem of ignition of a semi-infinite solid-propellant plate by a low-temperature plasma jet formed in an igniter under a high-power electric discharge is considered. Particular attention is paid to formation and evolution of zones of forced gasification of the fuel under the action of a heat flux from the plasma to the propellant. The locations of these zones are determined by satisfying two conditions: the propellant surface reaches the gasification temperature and the heat flux exceeds a certain “threshold” value. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 1, pp. 23–28, January–February, 2007.     

Stabilization of an inverted propane-air flame on a string stretched along the flow

The stabilization conditions for an inverted flame on a long thin string stretched along the cylindrical burner axis were studied experimentally. Gas temperature distributions in the inverted flame are obtained. The boundaries of stable combustion that are simultaneously the condition of excitation of acoustic self-oscillations of the flame are found. Inverted flame blow-off velocities are measured, and the variation of its geometrical characteristics are studied. It is found that during overturning of the flame relative to the vector of the acceleration due to gravity, stabilization of the inverted flame in the open atmosphere is impossible. The physical mechanism involved in the formation of the inverted flame in flow parallel to the stabilizer surface is considered. The role of the hydrodynamic stretching of the flame in flame blow-off and extinction is determined. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 2, pp. 3–11, March–April, 2008.     

On the nature of the burning rate-controlling reaction of energetic materials for the gas-phase model

For model systems with known kinetics of elementary reactions (CH₃NO₂ and HN₃), temperature ranges are established in which the rate-controlling reactions are the initial endothermic decomposition of the starting material or the subsequent secondary reactions. Heat release in reactions of NO₂, NO, and N₂O with various fuels, such as CH₂O, CO, H₂, and HCN, is modeled to establish the kinetic parameters and nature of the rate-controlling reactions in gas flames of nitro compounds. It is shown that the activation energy of the heat-release reaction due to the interaction of NO₂ with a hydrocarbon fuel (which is characteristic of the first flame of nitro compounds) is in the range of 29–33 kcal/mole, depending on the type of fuel. According to the calculations performed, the activation energy of the rate-controlling heat-releasing process due to the deoxidation of NO and N₂O (which is typical of the second flame of nitro compounds) is 43–58 kcal/mole. In the range of high pressures, where the flames merge, the kinetic parameters of heat release are determined by the reactions of the most reactive nitrogen oxide NO₂. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 59–71, May–June, 2007.     

Experimental study of N<Subscript>2</Subscript> dilution on bluff-body stabilized LPG jet diffusion flame

The present paper reports the effects of N₂ addition and preheating of reactants on bluff-body stabilized coaxial LPG jet diffusion flame for two cases, namely, (I) preheated air and (II) preheated air and fuel. Experimental results confirm that N₂ addition to the fuel stream leads to an enhancement in flame length, which may be attributed to the reduction in flame temperature. The soot free length fraction (SFLF) also increases, which might be caused by the decrease in fuel concentration and flame temperature. The flame length and also the SFLF are observed to be reduced with increasing temperature of reactants and lip thickness of the bluff body. The NO _x emission level for all burner configurations are found to be attenuated with nitrogen addition, which can be attributed to the reduction of the residence time of the gas mixture in the flame. The emission index of NO _x (EINO _x ) also becomes enhanced with increasing lip thickness and reactant temperature due to an increased residence time and thermal effect, respectively. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 3–10, January–February, 2009.     

Flame stabilization in a narrow slot in an expanding gas flow

Conditions of flame stabilization in a narrow gap between two circular plates with gas injection through the center of one plate are considered. Lean hydrogen-air mixtures are used as a combustible gas. It is shown that the flame can become stabilized under certain conditions either at the tube exit, at the point of injection of the combustible mixture into the slot, or at a certain distance from the point of injection. Stable existence of such flames is observed even if the slot width is smaller than 0.1 mm. The burner walls are substantially heated in both cases. The basic trends of the flame behavior are found for the second case with variation of the flow rate of the combustible gas, composition of the mixture, and slot width. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 21–25, July–August, 2006.     

Thermal Decomposition and Combustion of Ammonium Dinitramide (Review)

A comprehensive review of thermal decomposition and combustion of ammonium dinitramide (ADN) has been conducted. The basic thermal properties, chemical pathways, and reaction products in both the condensed and gas phases are analyzed over a broad range of ambient conditions. Detailed combustion-wave structures and burning-rate characteristics are discussed. Prominent features of ADN combustion are identified and compared with other types of energetic materials. In particular, the influence of various condensed- and gas-phase processes in dictating the pressure and temperature sensitivities of the burning rate is examined. In the condensed phase, decomposition proceeds through the mechanisms ADN → NH₄NO₃ + N₂O and ADN → NH₃ + HNO₃ + N₂O, the former mechanism being the basic one. In the gas phase, the mechanisms ADN → NH₃ + HDN and ADN → NH₃ + HNO₃ + N₂O are prevalent. The gas-phase combustion-wave structure in the range of 5–20 atm consists of a near-surface primary flame followed by a dark-zone temperature plateau at 600–1000°C and a secondary flame followed by another dark-zone temperature plateau at 1000–1400°C. At higher pressures (60 atm and above), a final flame is observed at about 1800°C without the existence of any dark-zone temperature plateau. ADN combustion is stable in the range of 5–20 atm and the pressure sensitivity of the burning rate has the form r _b = 20.72p ^0.604 [mm/sec] (p = 0.5–2.0 MPa). The burning characteristics are controlled by exothermic decomposition in the condensed phase. Above 100 atm, the burning rate is well correlated with pressure as r _b = 8.50p ^0.608 [mm/sec] (p = 10–36 MPa). Combustion is stable, and intensive heat feedback from the gas phase dictates the burning rate. The pressure dependence of the burning rate, however, becomes irregular in the range of 20–100 atm. This phenomenon may be attributed to the competing influence of the condensed-phase and gas-phase exothermic reactions in determining the propellant surface conditions and the associated burning rate. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 6, pp. 54–79, November–December, 2005.