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.     

Low-temperature combustion of energetic materials in filled polymer systems

In systems filled by inert additives, combustion of cellulose nitrate (CN) proceeds in a flameless low-temperature regime with a low linear burning rate. At a standard temperature, the exponent in the low of CN combustion in ballasted mixtures with inert additives in the pressure range of 0.1 to 10 MPa is several times lower than that of pure cellulose nitrate and amounts to 0.23. The qualitative and quantitative composition of gaseous products of flameless CN conversion is found. It is noted that this composition approximately corresponds to data available in the literature for the products of thermal decomposition of cellulose nitrate at comparatively low temperatures. Based on this fact and on a weak dependence of the CN burning rate on pressure in ballasted systems, the process under these conditions is assumed to be controlled by conversion of the energetic component predominantly in the condensed phase. In the case of a composite consisting of cellulose nitrate, silicon carbide, and polymer binder, for samples 10–25 mm in diameter, armoring exerts practically no effect on combustion parameters. Combustion of the same mixture with smaller diameters of non-armored samples is unstable. The presence of a liner establishes a clearly expressed critical combustion diameter in the examined systems. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 98–102, May–June, 2007.     

Modeling of equilibrium combustion of kerosene by a mixture of gaseous hydrocarbons

It is demonstrated that the composition and parameters of state of the gas formed in the course of equilibrium combustion of aviation kerosene in air can be modeled by means of determining the same quantities for a certain mixture of hydrocarbon fuels. The latter can be a mixture of identical amounts of methane and acetylene. The equilibrium composition calculated for the model mixture exactly coincides with the composition of kerosene combustion products for identical values of pressure (or density), temperature, and oxidizer-to-fuel ratio. Combustion of kerosene and the model mixture in a constant-pressure (or constant-density) mode can ensure satisfactory agreement between the temperatures of the final products and their compositions if air with a lower-than-normal fraction of oxygen is used as an oxidizer. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 4, pp. 3–7, July–August, 2007.     

The effect of the structure of the starting mixtures on the combustion of gas-generating compositions based on sodium azide

It is established that the combustion of mixtures of sodium azide with aluminum oxide and potassium dichromate at the lower limit for the dichromate content occurs in a chaotic self-oscillatory regime. Modification of the structure of the starting mixture by increasing the dichromate particle sizes from 40–60 K to 250–500 μm lead to significant stabilization of its combustion parameters. The amount impurities and aerosols in generated nitrogen is considerably decreased by reducing the energy content of sodium azide based composition. Using the relay-race model of combustion of heterogeneous systems, it is shown that, under the given conditions, the burning rate of such compositions is determined by the time of heating of potassium dichromate particles to the ignition temperature by neighboring burning particles. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 72–76, November–December, 2008.     

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.     

Influence of the aerosol formed in the fast evaporation of superheated water on the combustion of methane-air mixtures in a closed vessel

The influence of the aerosol formed in the fast evaporation of superheated water at 150°C on the combustion of a methane-air mixture in a near-spherical closed vessel (volume 20 dm³) is investigated experimentally. The concentrational limits of flame propagation and the normal combustion rate are determined as a function of the mass concentration of the water aerosol. It is found that the flegmatizing ability of the aerosol is relatively low; it is not due to the formation of fairly large water drops but practically completely to the presence of saturated water vapor. Exponential reduction in normal combustion rate of the mixture with increase in concentration of the superheated-water aerosol is observed.All-Russian Scientific-Research Institute of Fire Prevention. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 2, pp. 17–22, March–April, 1995.     

Combustion of Ti+0.5C and Ti+C mixtures of bulk density in inert gas coflow

The combustion of Ti-C mixtures of bulk density under inert gas blowing conditions produced by evacuation of one end of the reaction cell was studied for the first time. The experiments showed that the tested mixtures in quartz cups were not ignited and did not burn without the inert gas (argon) flow. Increasing the rate of gas evacuation from the sample increased the rate of steady-state combustion of the mixture of titanium with carbon black, and for the mixture of titanium with graphite, stabilization of the flat combustion front was observed. It is shown that the presence of small pressure difference (up to 105 Pa) allows control of the combustion process and confirms the basic postulates of the convective-conductive theory of combustion for heterogeneous condensed systems. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 30–37, January–February, 2009.     

Combustion of the NiO−Al system under gas pressure

We have studied the special features and mechanism of combustion of the NiO−Al system under gas pressure. The pressure has been shown to suppress the spread of the initial mixture and to allow the final combustion products to be obtained in a cast form. The chemical composition, combustion limits, and combustion region of cast intermetallides have been found based on the ratio of reactants. Institute of Structural Macrokinetics, Russian Academy of Sciences, Chernogolovka 142432. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 5, pp. 20–24, September–October, 1997.     

Model of Vapor—Air Gasification of a Solid Fuel in a Filtration Mode

A two-temperature mathematical model of steady filtration combustion of a solid fuel in open systems is proposed. Air or a mixture of air with water vapors is considered as a gaseous oxidizer. The model takes into account that the heat capacities of the phases depend on temperature and composition and that the reactor length is finite and allows obtaining the composition of gaseous combustion products. Calculated results on gasification of a mixture of carbon with an inert component are presented. It is demonstrated that thermodynamic calculations are important for obtaining the upper estimate of gasification efficiency. Even a sufficiently long reactor becomes “short” in the regime of transient combustion waves, which results in more intense entrainment of heat by combustion products and, as a consequence, in lower efficiency of the process. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 1, pp. 65–72, January–February, 2006.     

Combustion of a gasoline-hydrogen-air mixture in a reciprocating internal combustion engine cylinder and determining the optimum gasoline-hydrogen ratio

This paper reports results of an analysis of experimental data on the combustion of a gasoline-hydrogen-air mixture in a reciprocating internal combustion engine cylinder. The completeness of combustion of the mixture is shown to depend on the amount of hydrogen in the fuel mixture and the composition and physicochemical properties of the mixture. In particular, the conditions of addition of hydrogen to the gasoline-air mixture with active chemical action on the combustion process and the action of hydrogen as an additional fuel component are determined. A dimensionless universal relation is proposed that allows one to uniquely determine the initial composition of the fuel mixture (hydrogen to gasoline ratio) to accomplish combustion of the fuel mixture at the lean combustion limit. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 4, pp. 8–14, July–August, 2007.     

Estimation of the pressure built up in a nonhermetic vessel upon combustion of a solid-propellant composition

We have developed an approximate mathematical model that allows one to calculate the pressure in a nonhermetic vessel upon combustion of a solid-propellant composition in it. Laboratory experiments showing the validity of the model were performed. It is shown that a maximum pressure is attained long before the combustion of the solid-propellant composition in a nonhermetic room. Institute of Fire Protection, Balashikha 143900. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 2, pp. 81–85, March–April, 1997.     

Modeling the process of methane-air plume extinguishing by a solid-propellant pulse aerosol generator

Operation of a pulse aerosol system of fire fighting designed for effective extinguishing of fires in gas wells is modeled by an example of quenching a methane-air subsonic plume escaping from a nozzle. The system consists of two separate parts: a charge of a unitary solid propellant (gas generator) and a container with fine-grain powder of a flame retardant. The combustion of the mixture is described by a one-step global reaction; the effect of the concentration of flame-retardant vapors on the combustion process is taken into account through reduction of the pre-exponent in the Arrhenius law and is described by an empirical dependence. A computational experiment shows that the use of the pulse aerosol system of fire fighting ensures effective transport of fine aerosol particles of the flame retardant and its vapors to the combustion zone in amounts sufficient to suppress the ignition spot. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 3, pp. 26–34, May–June, 2009.     

Effect of a heat-conducting element on the gasless combustion of cylindrical samples under nonadiabatic conditions

The effect of a heat-conducting element on the ignition and combustion of a cylindrical layer of a gasless mixture with a nonadiabatic lateral surface is studied. It is shown that the introduction of an element with high heat conductivity allows one to extend the region of ignition of the gasless composition by a heated surface and to increase burning rate under conditions of external heat release. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 1, pp. 66–71, January–February, 2007.     

Ignition and combustion of a hydrogen-air mixture in a rapid-compression combustor

In a rapid-compression combustor with a freely moving piston, the efficiency of thermomechanical conversion of energy was determined in the detonation combustion regime of a stoichiometric hydrogen-air mixture under conditions close to those observed during operation of a piston engine of internal combustion in the starting regime. It is shown that this regime of heat release is characterized not only by a dramatic pressure increase in the combustion chamber, but also by its subsequent rapid decrease caused by heat transfer to the cylinder walls and partial condensation of water vapors. The intensity of these components of the thermal process depends progressively on the pressure and temperature of the combustion products, which, in turn, depend on the parameters of the mixture immediately before its ignition. However, the relative increase in combustion pressure turns out to be minimum when the ignition is initiated near the top dead center. It is also shown that the coefficient of thermomechanical conversion of energy (an analog of the indicated efficiency of an internal combustion engine) reaches the maximum value of 31% if the mixture is ignited at a time of approximately 3/4 of a period of oscillations of the piston group after the beginning of compression of an air charge. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 6, pp. 3–14, November–December 1999.     

Combustion of titanium plates

The lower combustion limit is determined depending on the pressure of plates made of OT-4 titanium alloy in an O₂-Ar medium. The values of this limit decrease monotonically as the concentration of oxygen in O₂-Ar mixtures grows and increase with the sample thickness. The velocity of combustion propagation over the surface of the samples in a 75% O₂+25% Ar mixture at a pressure of ≦3.1 MPa is found. It is shown that, for titanium, the dependence of the combustion rate on the sample thickness is much weaker and, in contrast, it is much stronger on the pressure compared to similar dependences for cylindrical soft-steel samples known from the literature. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 4, pp. 60–65, July–August, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 96-03-32039a).     

Active control of combustion stability by means of an electrical discharge

A new method is proposed for controlling unstable combustion in combustion chambers by organizing a diffuse electrical discharge in the combustion zone with a special power supply that has an impedance that varies with the control task. The maximum shunt currents in the combustion zone of propane-air mixtures are determined as a function of the mixture composition. It is shown experimentally that when a constant current discharge is imposed on the combustion zone, unstable combustion is automatically suppressed at all harmonics, simultaneously, as a result of internal negative feedback. When a constant voltage discharge is applied, the opposite effect is seen, with the excitation and enhancement of unstable combustion. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 3, pp. 43–51, May–June 1999.     

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.     

Flame propagation in mixtures of monogermane and oxygen. II. Peculiarities of flame propagation and the structure of combustion waves

Different flame propagation regimes, including a two-wave regime, are studied in a closed pipe with single ignition of GeH₄−O₂ mixtures. It is shown that, depending on the initial conditions and, in particular, on the composition of the initial mixtures, spatially separated chemical reaction waves for oxidation and decomposition of monogermane are observed in certain parts of the reaction vessel which lead to the formation of a two-layer deposit of GeO/Ge. The temporal sequence of the separated combustion wave and wave from which layers of solid products are deposited, as well as the location of the deposition zones in the reaction vessel, are determined by two interacting chain processes—the oxidation and decomposition of monogermane. The thermal relaxation kinetics of the reactive mixture after passage of the combustion waves is determined by the rate of conductive heat transfer from heated solid particles (reaction products) to the gaseous phase. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 1, pp. 77–84, January–February 1999.     

Surface and layer-by-layer modes of niobium combustion in nitrogen gas: Experiment and theory

Infiltration-mediated combustion of porous Nb samples in nitrogen gas under pressures of 0.12–10 MPa was investigated by thermocouple method. Measuring the velocities of front propagation over the sample surface and along the sample axis, it turned out possible to determine the combustion front concavity inside of a sample body. Combustion with a concave front was found to occur within the pressure range 0.12–1.0 MPa while that with a planar front, in the range 1–10 MPa. Elucidated was the effect of sample melting on the phase/chemical composition of combustion products. Derived was an analytical expression for parameter SL defining a limit for deviation from the mode of planar frontal combustion: the criterion SL > 1 defines the domain of combustion with a concave front while SL < 1, the domain of planar-front combustion. Theoretical predictions were found to reasonably agree with experiment.