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.     

Investigation of the structure of a diffusion hydrogen plume in a supersonic high-enthalpy air jet

The results of investigation of the plume structure in the case of diffusion combustion of hydrogen in a cocurrent supersonic high-enthalpy off-design air jet are presented. Based on the registration of radiation within the wavelength range of 260–350 nm across and along the flame, a three-dimensional tomographic reconstruction of the plume was obtained, which confirmed the interrelation between the gas-dynamic structure and combustion intensity. A possibility of existence of combustion regimes with periodically repeated cycles of complete extinction and subsequent ignition of hydrogen in accordance with the barrel-shaped structure of the off-design jet is established experimentally. The existence of local peripheral regions of combustion is noted, which can indicate the presence of vortex structures. In the three-dimensional representation (obtained under the assumption of axial symmetry of radiation), these vortex structures have the form of annular zones. In a real flow, helical structures are possibly formed at the plume periphery. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 5, pp. 3–5, September–October 1999.     

Determination of global parameters of gas-phase oxidation reactions from heat-evolution rates in a plug flow reactor

We have obtained curves of heat evolution in the process of gas-phase interaction of oxygen with 1,1-dimethylhydrazine, dimethyl formamide, kerosene (T-1 fuel), 1-methyl-1,2-dicyclopropylcyclopropane, n-butyl alcohol, and toluene in a plug flow reactor, with high dilution with nitrogen. The values of the kinetic constants for the processes have been determined using the data obtained. The adequacy of the kinetic models for chemical processes that cause self-ignition of combustible mixtures has been shown by comparing calculation results concerning self-ignition parameters with the available experimental data. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 2, pp. 20–25, March–April, 1998.     

Influence of geometric and regime parameters on flame stabilization of a swirl burner

The influence of geometric and regime parameters of a swirl burner with a central body and a diffuser on flow characteristics and flame stabilization in the combustion of a premixed gas mixture was investigated using Reynolds equations and thek-ε model of turbulence. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 5, pp. 21–26, September–October 1999.     

Induction period of a two-component aerosol of liquid oxidizer and propellant

The problem of self-ignition of a two-component aerosol consisting of uniformly mixed drops of an oxidizer and a propellant reacting in the gas phase is considered. The process of self-ignition of such an aerosol is divided into two stages: vaporization of aerosol components and subsequent chemical heating of the mixture. An approximate analytical formula is derived, for the period of self-ignition of a two-component aerosol. This formula takes into account the differences in thermophysical characteristics of aerosol liquids, mass concentrations, sizes of the drops of aerosol components, and the difference between the initial temperature of these drops and the initial temperature of the gas phase. The calculation results for the self-ignition period obtained using this analytical formula and the results of numerical solution of the problem in the zero-dimensional formulation coincide within 20% in a wide range of the governing parameters of the problem. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 6, pp. 15–21, November–December 1999.     

Self-ignition and special features of flow in a planar vortex chamber

Regular self-ignition of hydrogen and kerosene was registered upon their mixing with oxygen-enriched air in a straight-flow vortex, chamber of planar-radial geometry. These fuel components were injected into the chamber from tanks with room temperature and pressure 4–10 MPa. The velocity, pressure, and temperature fields in the chamber were studied using experimental methods and numerical modeling on the basis of full Navier-Stokes equations. The measurment and calculation showed that a region of elevated temperatures appears in an unsteady vortex flow. The calculation was performed for Reynolds number up to 5·10³ and revealed an increase in temperature with increase in Reynolds number. Nevertheless, both the calculated and measured temperatures were lower than the known self-ignition temperatures. The nature of the ignition, observed remains unclear. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 6, pp. 26–41, November–December 1999.     

Computational Analysis of Injection-Velocity Effects on Dynamic Parameters of Unconfined Fuel-Vapor Clouds

A computational investigation is performed to study the effects of injection velocity on the main dynamic parameters of the fuel cloud released into the open atmosphere. The volume, shape, and growth rate of the cloud, turbulence intensity, as well as the distribution of fuel concentration, temperature gradient, and self-ignition induction time are the most important parameters determining the mode of combustion that propagates through the cloud. A modified KIVA-based program is employed to fulfill the calculations. Systems of equations are solved by a finite-volume method. The k-ɛ model and discrete droplet model are applied for modeling gas-phase turbulence and liquid spray, respectively. The fuel-injection velocity is shown to have a major effect on turbulence intensity and uniformity of the cloud. With increasing injection velocity, the detonable part of the cloud rotates sooner and faster, and there is less time for ignition. A comparison with experimental results is performed for validation. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 5, pp. 29–40, September–October, 2005.     

Efficiency of hydrogen combustion in a high-temperature supersonic air flow for different methods of injection

The efficiency of hydrogen burnup in a supersonic air flow is studied for different methods of fuel injection. The combustion rate and combustion efficiency were determined by detecting the emission intensity of the OH radical over the length of the flare. The distinctive features and heat release characteristics of each of the delivery methods are established, so that a desired heat release behavior can be obtained by choosing suitable injectors. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 2, pp. 3–8, Mach–April 1999.     

Effect of heat transfer on the distributions of OH and CH radicals in the boundary layer with ethanol combustion

The reasons for formation of superequilibrium concentrations of radicals are studied by means of joint consideration of experimental data on the distributions of CH and OH molecules formed during diffusion combustion of ethanol and data on heat transfer in the chemical reaction region. The air flow velocity near the stagnation point in experiments with combustion is 0.7 m/sec, and the flow velocity along a flat plate is 10 m/sec (the turbulence levels are 1 and 18%). Mutual locations of specific features in the distributions of the heat-release rate and temperature are analyzed and compared with the distributions of OH and CH radicals. For all turbulence levels and flow velocities considered, the maximum concentration of radicals is reached on the boundaries of the heat-release region, whose locations are determined by molecular transport mechanisms. It is demonstrated that this conclusion is applicable to experimental data on diffusion combustion of a submerged hydrogen jet in air. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 3–11, November–December, 2008.     

Erosive burning of a solid propellant in a supersonic flow

We present results of an experimental study of burning of a ballistite solid propellant (gun powder H) in a supersonic flow. It is shown that the criterial dependence of the erosion coefficient on the Vilyunov parameter obtained on the basis of experimental results in a subsonic and a supersonic flow describes satisfactorily experimental data for a supersonic flow in the examined range of Mach numbers M=1–2.8 as well. A more correct approximation formula for the range of parameters considered is derived. The specific features of the flow along the surface of a solid propellant at M>1 is analyzed, and this analysis has revealed some problems in the interpretation of experimental data. Translated fromFizika goreniya i Vzryva, Vol. 34, No. 1, pp. 61–64, January–February, 1998.     

Recording local fluctuations of physicochemical parameters in a combustion wave in condensed systems

A method for recording fluctuations of physicochemical parameters in a combustion wave in condensed systems is proposed that makes it possible to estimate the frequency of these fluctuations experimentally. Experiments with a Zr+WO₃ pyrotechnic mixture using this method show that in the combustion wave there are fluctuation modes with frequencies ≈200 and ≈20 Hz corresponding to characteristic dimensions of heterogeneities of the system of order 0.1 and 1 mm, respectively. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 6, pp. 71–75, November–December 1999.     

Combustion of kerosene in a supersonic stream

Experiments on the organization of the combustion of kerosene in high-enthalpy supersonic air streams is analyzed. The use of promotor additives, as well as improvements in the atomization process, vaporization, and mixing, do not always facilitate efficient combustion development. The existence of a conversion process is found to have a significant effect on the ignition parameters. The burnup intensity can be ensured by adding hydrogen, and the relative position of the fuel injectors is important in that case. The fundamental role of wave structures in determining the length of the combustion zone in the channel is noted. The integral characteristics of combustion for hydrogen and kerosene are compared. Translated fromFizika Goreniya i Vzryva Vol. 35, No. 3, pp. 35–42, May–June 1999.     

Detonation waves in a reactive supersonic flow

A supersonic hydrogen—air flow is studied in detail, in particular, the fields of gas-dynamic parameters and chemical homogeneity of the mixture in various cross sections of the duct. The processes of excitation and propagation of a detonation wave in the downstream and upstream directions are considered. The detonation-wave velocity with respect to the mixture flow is found to differ from the nominal Chapman—Jouguet velocity for a quiescent mixture: the detonation-wave velocity is higher if the wave moves upstream and lower if the wave moves downstream. Some hypotheses on the reasons for these deviations of the experimentally measured velocity from the nominal value are given. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 5, pp. 85–100, September–October, 2006.     

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.     

Mathematical Model of Ignition of Condensed Systems by a High-Temperature Supersonic Underexpanded Jet

An approximate mathematical model is constructed and characteristics are calculated of ignition of a reactive plane infinite obstacle by a high-temperature nonstationary axisymmetric supersonic jet of combustion products escaping from the igniter. The approximate model data are compared with the results of numerical calculations using the system of equations of motion of an ideal gas, nonstationary equations of heat conduction and chemical kinetics, and conditions of conjugate heat exchange at the gas–condensed medium interface. The suggested approximate model adequately describes the ignition process and can be used for proximate evaluation of ignition time and temperature. Key words: ignition, jet, gas dynamics, supersonic flow, mathematical simulation.     

Self-ignition of a two-component gas suspension

The self-ignition of a motionless cloud of particles of two sorts (the two-component gas suspension) is studied. We obtain approximate analytical expressions for the induction period and critical conditions of self-ignition in the case where the heat exchange occurs between the cloud and the ambient medium and in the case where one of the components consists of the particles of a substance that is reactive in the endothermic chemical reaction. These formulas are compared with the numerical solution of the unsteady problem, and their domains of applicability are found. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 5, pp. 6–13, September–October 1999.     

Effect of the Wave Structure of the Flow in a Supersonic Combustor on Ignition and Flame Stabilization

Results of numerical and experimental investigations of a high-velocity flow in a plane channel with sudden expansion in the form of a backward-facing step, which is used for flame stabilization in a supersonic flow, are presented. The experiments are performed in the IT-302M high-enthalpy short-duration wind tunnel under the following test conditions: Mach number at the combustor entrance 2.8, Reynolds number 30 · 10⁶ m^?1, and total temperature T₀ = 2000 K, i.e., close to flight conditions at M = 6. The numerical simulations are performed by solving full unsteady Reynolds-averaged Navier–Stokes equations supplemented with the k–ω SST turbulence model and a system of chemical kinetics including 38 forward and backward reactions of combustion of a hydrogen–air mixture. Three configurations of the backward-facing step are considered: straight step without preliminary actions on the flow, with preliminary compression, and with preliminary expansion of the flow. It is demonstrated that the backward-facing step configuration exerts a significant effect on the separation region size, pressure distribution, and temperature in the channel behind the step, which are the parameters determining self-ignition of the mixture. The computed results show that preliminary compression of the flow creates conditions for effective ignition of the mixture. As a result, it is possible to obtain ignition of a premixed hydrogen–air mixture and its stable combustion over the entire channel height.     

Effect of flameout on Combustion of small particles in an acoustically oscillating flow

Combustion of magnesium and titanium particles in an acoustically oscillating flow is experimentally studied for the case where the particle dimensions are smaller than the amplitude of gas displacement in an acoustic wave. An increase in the combustion time of magnesium particles and a decrease in the combustion time of titanium particles upon application of acoustic vibrations is found. Characteristic features of fluctuations of the intensity of the luminous flux of a burning magnesium particle as a response to vapor-phase burning of a metal drop to an external acoustic action are revealed. An explanation of the shape of registered fluctuations is offered on the basis of an assumption about flame-front blowoff from the frontal point of the drop. Conditions necessary for flameout in an oscillating flow and effects that can be induced by flameout from the drop in the case of burning of sprayed fuels in intermittent burning devices are discussed. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 2, pp. 10–16, March–April, 2000.     

Basic self-ignition regimes and conditions for their realization in combustible gas mixtures

Self-ignition of hydrogen-air mixtures has been studied experimentally. This process was initiated behind a reflected shock wave at an initial pressure of up to0.5 MPa and with hydrogen volume contents ranging from9.5–20 and40–60%. Simultaneous recording of pressure and temperature profiles near the end of a shock tube made it possible to reveal the specific features of various self-ignition regimes and the regions of their existence. The peculiarities of the realization and evolution of mild and strong self-ignition processes are discussed, and a comparison with self-ignition of carbon-air mixtures is done. Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow 117977. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 2, pp. 3–10, March–April, 1997.     

Effect of the external electric field on the combustion of a suspension of aluminum particles in air

On the basis of experimental studies, it is shown that the rate of turbulent combustion of ASD-1 aluminum in air increases by a factor of 1.2–1.3 and the geometric parameters of the flame change under the action of a 0.9 kV electric field. These effects are explained. The positive result is obtained under the conditions of a developed turbulent flow and can serve as a basis for practical recommendations on the intensification and control of ignition and combustion processes under laboratory conditions (a laminar flow) and in power installations. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 6, pp. 23–28, November–December 1998