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.     

Spontaneous change in porosity of a reacting pressed material and non-one-dimensional regimes of diffusion combustion

Essentially non-one-dimensional regimes of synthesis-wave propagation are considered within the framework of a model describing two-dimensional diffusion combustion and taking into account a change in porosity of a reacting composition attributed to a change in its physical and chemical properties. The surface and layer-by-layer burning of a pressed specimen in gas is described qualitatively as well as the interaction front propagation wherein unreacted material remains in skin layers of the specimen. The conditions of development of a gastight layer on the specimen surface and incomplete material conversion because of pore volume exhaustion have been determined. Dynamics of a number of unstable regimes of front propagation is studied. The role of diffusion supply of a gaseous reagent is analyzed under the conditions of changing porous space in the reacting material. The surface and layer-by-layer burning of pressed specimens, with the reagents interacting in an unstable regime, is considered.Institute of Structured Macrokinetics, Russian Academy of Sciences, Chernogolovka. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 2, pp. 47–58. March–April, 1995.     

Numerical Study of the Effect of Methane Combustion on Heat and Mass Transfer and Friction in the Boundary Layer

Calculation results are presented for laminar and turbulent boundary-layer flows on a porous plate with methane injection and combustion. The mathematical model is based on the boundary-layer approximation. Combustion was simulated by one global finite-rate reaction and the kinetic mechanism of hydrogen and carbon oxide afterburning. It is shown that injection and combustion in laminar and turbulent flows lead to more intense displacement of the flow away from the wall than in the case of injection into an isothermal flow, which decreases the friction drag and heat and diffusion fluxes. Combustion in a turbulent flow leads to flow laminarization and delay of the laminar–turbulent transition.     

The role of surface contact during burning of heterogeneous condensed systems

We examine the effects of surface processes and chemical transport reactions on the burning of a SHS system through variation of surface contact of the reactants. A model is proposed which can take into account variations in surface contact in the process of burn front propagation. We show that surface diffusion and chemical transport of one of the reactants to the surface of another are capable of increasing the burn velocity by a factor of 3–4. We also show that it is necessary for surface contact of the reactants to form in the heated zone of the combustion wave in order to attain maximum burn velocities.Chernogolovka. Translated from Fizika Goreniya i Vzryva, Vol. 27, No. 6, pp. 33–40, November–December, 1991.     

Diffusion Combustion of a System of Plane Supersonic Hydrogen Jets in a Supersonic Flow

Calculation results for diffusion combustion of a system of plane supersonic hydrogen jets in a supersonic flow are presented. The calculations are based on parabolized Navier–Stokes equations closed by the one-parameter (k – l) model of turbulence and a multistage mechanism of hydrogen oxidation. The influence of the air-flow composition and fuel-injection methods on the shape of the flame front and combustion efficiency of hydrogen is analyzed.     

Boundary layer model for char combustion and gasification

A non-steady boundary layer model is developed for numerical simulation of combustion and gasification of a single shrinking char particle. The model considers mass and energy conservation coupled with heterogeneous char reactions producing CO and homogeneous oxidation of CO to CO₂ in the boundary layer surrounding the char particle. Mass conservation includes accumulation, molecular diffusion, Stefan flow and generation by chemical reaction. Energy conservation includes radiation transfer at the particle surface and heat accumulation within the particle. Simulation results predict experimentally measured conversion and temperature profiles of a burning Spherocarb particle in a laminar flow reactor. Effects of bulk oxygen concentration and particle size on the combustion process are addressed. Predicted particle temperature is significantly affected by boundary layer combustion of CO to CO₂. With increasing particle size, char gasification to char combustion ratio increases, resulting in decreasing particle temperature and increasing peak boundary layer temperature.     

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.     

Effect of flow history on combustion in a laminar boundary layer

Results of numerical simulations of methane combustion in a laminar boundary layer on a porous plate with an impermeable initial section are presented. The analysis of results is based on comparisons of data with and without combustion, and also for different initial section lengths including the zero length. The flow history is demonstrated to exert a significant effect on heat transfer and friction in the boundary layer with injection without combustion, whereas the influence of the flow history in the case with combustion is smaller. The phenomenon experiencing the least effect of the flow history is heat transfer.     

Natural Buoyant Turbulent Diffusion Flame near a Vertical Surface

The structure and dynamics of a natural buoyant turbulent diffusion flame near a vertical surface with combustible gas exhaustion are numerically studied by using the FDS model and computer code. The flame is considered near the surface through which gaseous propylene is injected with a prescribed flow rate. Requirements are determined for the grid cell size in the near-wall region, which ensure sufficient spatial resolution of the boundary layer structure. It is shown that the predicted value of the total heat flux at the surface agrees with the measured results. Investigations of ignition and combustion of a vertical plate of non-charring thermoplastic (polymethylmetacrylate) with allowance for the material pyrolysis reaction show that the ignitor parameters determine the duration of the transient period, but weakly affect the growth of the heat release rate and the height of the pyrolysis region at the stage of developed burning. Significant effects of the ignitor shape, size, and temperature, as well as lateral entrainment of air on the velocity of the upward flame spread rate over the plate surface and on the shape of the pyrolysis front are revealed. The existence of critical parameters of the ignitor separating flame decay from developed burning is demonstrated. Three flame spread regimes with different pyrolysis front shapes are identified.     

Heat and mass transfer in a boundary layer with the evaporation and combustion of ethanol

Experimental data on heat and mass transfer in a boundary layer upon ethanol evaporation from a porous surface and its combustion in an air flow are reported. It has been established that variations in the flow velocity in the flow core weakly affect the temperature and concentration of substances on the reactor wall. The flame temperature and the distribution of mass flows over the wall depend essentially on the flow velocity. It has been observed that heat-and mass-transfer coefficients decrease in combustion. The representation of experimental data using overall enthalpies and generalized concentrations as transfer potentials suggests an analogy between the processes of heat and mass transfer in a reacting boundary layer.Novosibirsk. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 1, pp. 8–15, January–February, 1994.     

Aerosol-Generating Pyrotechnic Compositions with Components Interacting in the Combustion Wave

Observations were performed of the burning surface of the pyrotechnic aerosol-generating KNO₃/melamine/iditol system. Its effectiveness in suppressing a diffusion hydrocarbon flame and the quantity of the combustion residue were measured. An extremal dependence of the flame suppression effectiveness on the KNO₃/melamine ratio was obtained. The maximum effectiveness corresponds to the disappearance of the melt from the burning surface. This is due to the formation of nonmelting organometallic compounds in the combustion wave. The maximum flame-suppression effectiveness is achieved when the entire metal of the oxidizer is involved in the reaction. Possible versions of such reactions are discussed. Examples of calculation of optimal compositions of this type are discussed.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 3, pp. 86–89, May–June, 2005.     

Propagation of a burning front over a condensed material from a focus

Measured propagation speeds are given for the combustion front in a blind gap in a condensed substance, which are related to gap height and mean pressure. The front may propagate monotonically, which is characteristic of relatively high pressures and wide gaps, or as a result of additional burning foci arising at a certain distance from the main front. At low pressures and particularly with small gap heights, the front propagates in an unstable fashion over the surface.Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 28, No. 4, pp. 49–53, July–August, 1992.     

Comparison of different methods of modeling turbulent combustion in a boundary layer

Three methods of modeling a chemical reaction in the combustion of ethanol injected into the boundary layer through a porous plate are compared. In calculations of the average velocity and temperature and also heat-transfer coefficients, simple models of combustion are shown to be close in accuracy to a model that takes into complete account the kinetics of all reactions occurring during combustion. Simple models fail to determine the composition of reaction products, and it is necessary to take into account the formation rate of each substance entrained in combustion.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 4, pp. 37–42, July–August, 1996.     

Flame Propagation in a Variable–Section Channel with Gas Filtration

A one–dimensional unsteady model is proposed, which describes gas–flame propagation in a narrow variable–section channel with a gas counterflow and takes into account heat propagation over the channel walls. The case of the channel cross section changing slowly at a distance of the order of the thermal thickness of the combustion wave is considered. It is shown that various regimes of flame propagation are possible in such a system: a regime of flame propagation with a high velocity (of the order of the burning velocity of the flame), a regime of flame propagation with a low velocity as in the case of filtrational gas combustion in a porous medium, and an intermittent regime of combustion, where the flame has a high velocity in the wide section of the channel and a low velocity in the narrow section. A simple analytical model of flame oscillations in such a system is constructed. The possibility of these oscillations was predicted by numerical simulation results. The simple model considered is an attempt to take into account the large–scale inhomogeneity of the porous medium in simulation of filtrational combustion of gases.     

Reduction of metal ions in dilute solutions using a GBC-reactor Part II: Theoretical model for the hydrogen oxidation in a gas diffusion electrode at relatively low current densities

The GBC-reactor is based on the combination of a gas diffusion anode and a porous cathode. A theoretical model for gas diffusion electrode, valid at relatively low current densities, is derived. This is based on the pseudohomogeneous film model including an approximation of the Volmer–Tafel mechanism for the hydrogen oxidation kinetics. Results show a severe mass transfer limitation of the hydrogen oxidation reaction inside the active layer of the gas diffusion electrode, even at low current densities. Empirical formulae are given to estimate whether leakage of dissolved hydrogen gas into the bulk electrolyte occurs at specific process conditions. A simplified version of the model, the reactive plane approximation, is presented.     

Simulation of the Erosion Burning of a Granular Propellant

For combustion of axisymmetric propellant grains under blowing conditions, a mathematical model is proposed and numerical simulation is performed. The effects of incoming–flow parameters (velocity, pressure, and temperature) and surface dimensions and geometry on grain–burning rate are studied. Physical patterns of flow around burning propellant grains are presented.     

Unsteady combustion of a binary gasless mixture ignited by a hot wall

The effect of phase-transition parameters and chemical conversion kinetics on unsteady combustion of gasless systems with a melting reagent is studied. It is found that the change in the kinetic law and in the chemical reaction rate constant resulting from the emergence of the liquid phase in the system destabilizes the combustion. The mean front velocity in the self-oscillatory mode with an adiabatic burning temperature close to the melting point exceeds the front velocity in the steady-state mode. In this case, the front stability improves with an increase in the rate of heat withdrawal from the system and with a decrease in the burning temperature.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 2, pp. 45–50, March–April, 2005.     

Simulation of Convective Detonation Waves in a Porous Medium by the Lattice Gas Method

Gas—film convective detonation in a rigid porous medium is considered. The motion of the gas phase is described by a discrete stochastic lattice gas model taking into account the real laws of friction and heat exchange between the phases. The reaction kinetics was specified so that the characteristic time of combustion corresponded to experiment. The model simulates the main characteristics of the phenomenon: a nonflat (irregular) wave front, smooth increase in the pressure averaged over the charge cross section, friction–dominated mean flow, slow cooling of combustion products after completion of the reaction.     

The pressure drop in a porous material layer during combustion

During the combustion of a porous material layer, a manometer, which is attached to the cold end of the charge, records at the bottom of the layer a pressure reduction, which was discovered more than 20 years ago but which remains essentially unexplained up to the present. It is experimentally shown that this effect is similar to the pressure change in the cavities when a light gas (helium, hydrogen) diffuses from (or to) them under isothermal conditions and that it increases during the combustion mainly due to the accompanying Stefan type flow, and probably also as a result of the thermal diffusion. A pressure drop in the cavities is evidently made possible also by the pressure reduction in the flame which follows from the Hugoniot adiabatic theory.D. I. Mendeleev Mosk. Khim.-Tekhnol Inst. 125190, Moscow. Translated from Fizika Goreniya i Vzryva, No. 1, pp. 57–60, January–February, 1995.     

Numerical modeling of supersonic-flow deceleration in a two-dimensional duct in combustion of wall tangential hydrogen jets

Results are presented of a numerical modeling of the ignition and combustion of underexpanded turbulent hydrogen jets injected into supersonic air flow (M=2.63) along the walls of a two-dimensional duct. Calculations were performed by numerical integration of reduced Navier-Stokes equations using the method of global iterations. The kinetic mechanism of hydrogen combustion in air involved 13 reactions. In the calculations the duct height was varied. In a fairly narrow duct, the static pressure increased with flow deceleration to subsonic velocities due to ignition and combustion. The influence of combustion on the pressure distribution in the transverse direction is ambiguous. Initially, combustion increases the pressure nonuniformity (a new oblique shock wave occurs), while, downstream, the pressure profile is flattened out due to the appearance of a subsonic layer near the flame front.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 4, pp. 47–54, July–August, 1996.