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.     

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.     

Noise and vibrations in a combustor with continuous spin detonation combustion of the fuel

Levels of vibrations of the wall of a flow-type detonation chamber of an annular cylindrical geometry in the region of detonation-wave rotation and the noise at a distance of 1 m are measured. In the case of continuous spin detonation of a hydrogen—air mixture, these values are found not to exceed the values inherent in conventional turbulent combustion of the fuel with the same flow rate in the same chamber. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 5, pp. 101–112, September–October, 2006.     

Filtration combustion of a carbon-inert material system in the regime with superadiabatic heating

Superadiabatic regimes of combustion of carbon mixed with an inert solid with filtration of the steam-air mixture are studied theoretically and experimentally. The temperature in the combustion wave and the composition of gaseous products are obtained as functions of the fraction of carbon in the fuel and the amount of steam in the gaseous oxidant. In the examined range of the control parameters, the maximum temperature in the combustion wave is shown to depend only slightly on the fraction of carbon in the mixture and the amount of steam in the oxidant gas. Simulations of filtration combustion of carbon with allowance for the kinetics of its oxidation are in good agreement with experimental results. The calculated combustion temperature coincides with that measured in experiments. In calculating the composition of the gaseous products, coincidence with experimental data is observed only for particular compositions with the mass content of carbon under 60%. As the fraction of the fuel exceeds 60%, the yield of CO and H₂ increases in experiments, though such a behavior is not predicted by the theoretical analysis. Hypotheses on the reasons for the disagreement in results are put forward and experimentally checked. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 3, pp. 30–38, May–June, 2008.     

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.     

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.     

Initiation of detonation in flows of fuel-air mixtures

Regimes of self-ignition of the fuel mixture obtained by controlled separate injection of hydrogen and air into a plane-radial vortex chamber with a rapid (0.2 msec) transition to detonation have been realized for the first time. Self-ignition occurs in the stoichiometric region with a slightly higher (up to 6–30%) content of hydrogen and, normally, in a subsonic flow. The energy of guaranteed detonation initiation is determined for combustors of different geometries and different ratios of fuel components by using a thermal pulse produced by blasting a wire by electric current. Detonation initiation is ensured by using energy of 0.1 J. It is found that the main contribution of energy into the flow of the mixture occurs at the stage of evaporation (ionization) of copper of the blasted wire. The continuous spin detonation regime is found to decay as the exit cross section of the combustor is reduced. In the regime of combustion, both detonation and conventional turbulent combustion, the pressure at the periphery of the plane-radial vortex chamber is lower and the pressure at the edge of the exit orifice is higher than that in the case of exhaustion of cold fuel components. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 110–120, May–June, 2007.     

Hydrogen combustion in an axisymmetric duct with a supersonic air flow

Results of an experimental study of hydrogen combustion in a supersonic high-enthalpy air flow in axisymmetric ducts with constant and variable cross sections are described. Conditions of flow reconstruction due to heat addition in ducts of different geometry are considered. It is demonstrated that a gas-dynamic action ensures stable self-ignition and it becomes possible to control combustion in expanding ducts with special profiles. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 3–8, 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.     

Effect of heat release conditions on the phase composition of the combustion products of a Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript>/Al/C thermite mixture

An X-ray method was used to study the phase composition of the combustion products of a Fe₂O₃/TiO₂/Al/C thermite composite mixture, in particular, during combustion of samples under cooling by water. The data of the experiments are compared with the results of thermodynamic calculations. A probable explanation of the reasons of the angular displacement of the line of iron in the X-ray spectrum of the combustion products and a mechanism for the crystallization of an eutectic TiC + α-Fe mixture from the melt of the Ti/Fe/C ternary system. The composition of the combustion products was found to be different from the equilibrium one, which is manifested in the presence of defects in the carbon sublattice of titanium carbide. A probable explanation of the deficiency of carbon is proposed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 4, pp. 39–43, July–August, 2008.     

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.     

Spatial forms of a combustion wave in energetic heterogeneous systems

The effect of the content of combustible particles on the spatial forms of the combustion wave in a gasless thermite-inert material model system is studied. It is shown that the lower flammability concentration limit for this mixture is related to the formation of a three-dimensional binding percolation cluster or to a percolation phase transition. Near the lower concentration limit, the combustion of model mixtures enters a smoldering regime in which separate combustion sources move randomly in the volume of the unburned material. If the initial composition contains more than 47% thermite particles, a motionless heated surface forms, along which separate incandescent combustion sources move. The motion of this surface in the unburned material occurs only locally as a result of motion of the combustion sources on it. The formation of this unified heated surface corresponds to the occurrence of a second percolation phase transition. When the volumetric content of thermite particles in the initial mixture is higher than 70%, separate combustion sources merge into a unified surface of layer-by-layer combustion due to the occurrence of a third percolation phase transition. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 4, pp. 60–65, July–August, 2008.     

基于氢气低温催化燃烧的燃料电池低温启动研究进展

相比于传统动力系统,基于燃料电池的动力系统具有很多优点,但在实际运用中仍有许多亟需解决的问题,其中包括燃料电池系统的低温启动问题。本文对比了各种燃料电池低温启动方案的工作机理及其优缺点,归纳并分析了氢气催化燃烧所用催化剂和催化燃烧反应过程以及燃料电池低温启动过程等方面的相关研究成果,研究了影响催化燃烧的主要因素,得出以下结论：基于氢气低温催化燃烧的燃料电池低温启动策略具有较高的可行性;在不同反应模型的情况下,氢气都可以在微尺度管道内实现稳定的燃烧;表面催化反应对空间气相反应有抑制作用;空间气相与表面催化的耦合反应能得到最高的温度;氢气/空气预混合气体入口流速、导热壁及导热壁材料、管径和当量比均对催化燃烧有着重要的影响。     

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.     

Dependence of the normal rate of combustion of methane-air and hydrogen-air mixtures on pressure and composition

Numerical methods were used to study the normal combustion rate of methane-air and hydrogen-air mixtures as a function of pressure in the range 0.005–2.0 MPa and composition. It was found that the closer the composition of the fuel mixture is to the limiting composition, the lower the pressure index of the normal combustion rate. It is shown that chemical reactions in the preflame zone have no appreciable effect on the normal combustion rate of a lean methane-air mixture. Balashikha. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 5, pp. 12–17, September–October, 1993.     

Heat fluxes to combustor walls during continuous spin detonation of fuel-air mixtures

Pioneering measurements of heat fluxes to the walls of flow-type combustors of different geometries were performed in regimes of continuous spin detonation of fuel-air mixtures under unsteady heating. These heat fluxes are compared with those observed in the regime of conventional turbulent combustion in the same combustor. Air is used as an oxidizer, and acetylene or hydrogen is used as a fuel. For identical flow rates of the fuel, the heat fluxes to the combustor walls in regimes of continuous spin detonation and conventional combustion are close to each other; their mean steady values are ≈1 MW/m² (≈0.5% of the enthalpy flux of the products over the channel cross section). In both detonation and combustion regimes, the maximum heat fluxes penetrate into the walls in the mixing region (where the heat release occurs). In the case of detonation, regenerative cooling of the combustor walls by the flow of the fresh mixture occurs in the heat-release region (region of propagation of the detonation-wave front). The regeneration becomes less effective in the downstream direction because of the shorter time of contact between the walls and the cold mixture and a longer time of contact between the walls and the hot products. More intense heating persists downstream of the front, where the regeneration ceases, but the temperature of the products is high. The character of heating of the wall in the region of rotation of the front of spin detonation waves depends on the number of these waves: the zone of the maximum heat release becomes narrower with increasing number of waves. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 80–88, January–February, 2009.     

Effect of Unmixedness in Large Eddies on Ignition and Combustion of Turbulent Jets of the Fuel in a Supersonic Flow

A method and results of computing the influence of unmixedness in large eddies generated by instability of a contact discontinuity on ignition and combustion of turbulent jets of a fuel in a supersonic flow are presented. The basic system of equations is the Reynolds-averaged Navier-Stokes equations. The influence of unmixedness on chemical reaction rates is reproduced by using a random number generator adjusted in accordance with the probability of origination of favorable conditions for proceeding of chemical reactions, which is computed in each node of the computational grid. The problem of combustion of a plane hydrogen jet injected into a supersonic flow along the plate surface is considered as an example. It is shown that allowance for unmixedness in the case where the flow and the jet have favorable initial parameters for self-ignition leads to a delay in heat release and to origination of pressure and temperature fluctuations. As the parameters approach the self-ignition limit, these fluctuations increase, and the combustion becomes intermittent. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 1, pp. 49–56, January–February, 2006.     

Initiation of combustion processes in a hydrogen-oxygen mixture under the action of a low-energy strong-current electron beam

The process of combustion initiation in a hydrogen-oxygen mixture by a beam of electrons with an energy of ≈10 keV and a mean cross-sectional size of ≈10 cm is considered theoretically and experimentally. The initial pressure in the mixture is 500 Pa. The reaction was visualized by recording the time evolution of fluorescence intensity in the lines of molecular (λ = 310 nm) and atomic hydrogen and sodium doublet (λ = 589 nm). The calculations are performed on the basis of a semi-empirical model developed in a three-temperature (T ₀, T _e , and T _v) approximation. The induction and burning times determined in experiments and calculations are in good agreement. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 2, pp. 10–17, March–April, 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.