Relay-Race Combustion of Heterogeneous Systems

The structure of a combustion front in a multilayer system is analyzed taking into account radiative heat transfer in the gas interlayer. The effect of radiative heat loss on the structure and characteristics of the relay-race transformation regime is studied. The characteristics of the front and the transformation dynamics of the elements of the discrete system are determined. For the case of heat loss to the ambient medium, it is shown that the steady-state regimes are nonunique — high-temperature and low-temperature relay-race combustion fronts are detected. The absolute instability of the low-temperature regime and the dynamics of establishment of the high-temperature regime are studied in computational experiments under unsteady conditions. It is established that there is a critical heat loss above which there is flame extinction. The change in the thermokinetic characteristics during the process is analyzed, and it is shown that during combustion superadiabatic temperatures are reached. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 5, pp. 70–77, September–October, 2005.     

Diffusion-thermal stability of gas filtration combustion waves in an inert porous medium

A mathematical model for the filtration gas combustion taking into account thermal conductivity, diffusion, and intense interfacial heat transfer is presented. The temperature dependence of the chemical reaction rate is approximated by a δ-function and the thermal-expansion coefficient of gases behind the combustion front is taken into account. Unsteady combustion regimes are analyzed using the method of small perturbations. The boundaries of the longitudinal and spatial stability for steady regimes of the filtration combustion wave are obtained. The dependence of the Lewis number on the thermal-expansion coefficient of the gas mixture along the boundary of stability is derived, along with other relations.     

Formation of nitric oxide (NO) during propagation of a laminar flame over a homogeneous mathane—Air mixture

Formation of nitric oxide in the combustion of methane—air mixtures of various compositions was studied numerically. The system of equations takes into account 196 direct and reverse elementary chemical reactions for 32 substances. A procedure for unified numerical calculation of the indicated process at the flame front and at large distances from it is described. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 1, pp. 9–19, January–February, 1998.     

Stability of the combustion wave in a viscoelastic medium to small one-dimensional perturbations

The problem of stability of the conversion front in a viscoelastic medium is formulated. The stability study is performed by the method of small perturbations. Nonlinear equations are derived for decrements of decay and complex frequency. Several particular cases are analyzed. A significant effect of the time of relaxation of viscous stresses on the flammability limits for both high-velocity and low-velocity regimes is demonstrated. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 50–60, July–August, 2006.     

Aperiodic instability of the front of a crown forest fire

The stability of the three-zone structure of the front of a crown forest fire, which includes zones of vaporization, pyrolysis, and combustion gaseous products, is analyzed by the method of small perturbations. This model more accurately describes the physics of the actual process of propagation of upper forest fires. A characteristic equation is obtained and an analysis of its roots yields neutral curves for analysis of the monotonic instability of the fire front. It is shown that instability of fire-front propagation arises in a certain wavelength range. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 5, pp. 23–28, September–October 1998.     

Steady regimes of conversion in a viscoelastic medium

The problem of propagation of a steady conversion front in a viscoelastic medium is solved by the method of matched asymptotic expansions in the approximation of low strains. The heat flux is assumed to satisfy the Fourier law, and the components of the stress and strain tensors are related by the Maxwell relations including the shear coefficient of viscosity. The temperature of the products and the velocity of propagation of the steady reaction front are found. The solution of the problem is obtained for the limiting cases of the small and large times of relaxation of viscous stresses. It is demonstrated that the model contains different regimes of reaction-front propagation, like the coupled models of solid-phase combustion for a thermoelastic body, and viscous stresses insert additional features. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 5, pp. 63–73, September–October, 2006.     

Self-propagating high-temperature synthesis of porous Ti-Si-Al-C based materials

The pore formation process and the phase composition of the products of combustion of the Ti—Si—Al—C system were studied. The structure of Ti₅(Si,Al)₃C_0.6 single crystals formed in small amounts was determined. The dependences of the burning rate and elongation of the samples on their density and the gas pressure were determined. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 2, pp. 53–60, March–April, 2006.     

Chromium combustion in nitrogen

Results of investigation of chromium combustion in nitrogen are presented. It is shown that the combustion proceeds in the solid state. In the combustion wave, chromium reacts with nitrogen in steps. The first step—the formation of Cr₂N—proceeds in the separation regime. The maximum combustion temperature is limited by the temperature of dissociation of CrN. Afterburning is observed behind the combustion front. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 5, pp. 40–45, September–October 1999.     

Model for the propagation of a stationary reaction front in a viscoelastic medium

A coupled thermomechanical model for the propagation of a stationary chemical-reaction wave in a condensed medium is developed. Stresses and strains that arise during the reaction as a result of thermal and “concentration” expansion of the material are related by Maxwell’s equations for a viscoelastic medium. The expression for the heat flux is written as a generalized Fourier law with finite relaxation time for the heat flux. It is shown that deformation of the material in the reaction zone can lead to an apparent change in the activation energy, heat effect, and other characteristics of the system. This model allows for the existence of two different — subsonic and supersonic — regimes of propagation of the front, as well as the model in which the stress- and strain-tensor components are related by a generalized Hooke’s law. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 4. pp. 41–51, July–August, 2000.     

Properties of Solutions of Poly-(p-phenylenebenzobisthiazole) in Polyphosphoric Acids of Different Composition

The possibility of hydrolytic degradation of an oligomeric solvent—polyphosphoric acid—in finished spinning solution was established. The viscosity of the polymer solution can be decreased by decreasing the proportion of phosphoric anhydride in the solvent with a small change in the concentration of polymer in the solution; the properties of the fibre fabricated from such a solution do not worsen. When the degree of polymerization of the polyphosphoric acid decreases (up to its conversion to orthophosphoric acid), poly-(p-phenylenebenzobisthiazole) crystal solvates are crystallized into single crystals hundreds of micrometers in size. All-Russian Scientific-Research Institute of Polymer Fibres, Mytishchi. Translated fromKhimicheskie Volokna, No. 5, pp. 26–28, September–October, 2000     

Theoretical Analysis,Calculation, and Prediction of the Air Permeability of Textiles

Scientific principles were developed for predicting the air permeability of textiles based on current concepts concerning the air permeability of textiles and the assumptions of gas and hydrodynamics. Based on the physical concepts concerning flow of gases through porous systems, a theoretical model was elaborated to describe the dependence of the rate of passage of air through fabric on external ambient factors and the characteristics of their structure. The through porosity, thickness, and hydraulic diameter of the pores were determined with the most important characteristics of the structure of the fabrics in the complex index of their structure, with which their air permeability is closely correlated. The possibility of using the model for predicting the air permeability of fabrics was demonstrated. The differences in the results of determining the through porosity of fabrics obtained in using different methods were established during experimental testing of the theoretical model. Because of the very high reliability of the results, the estimation method was approved for use in research. A complex index for their structure which takes into account the effect of the most important structural characteristics — surface density, thickness, average density, filling of the material by the fibre — was proposed based on an analysis of the results of the study of the effect of the structural characteristics of nonwoven materials (NM) on their air permeability. The high degree of correlation of air permeability with this index allows using the dependences obtained with it for predicting the air permeability of NM. __________ Translated from Khimicheskie Volokna, No. 5, pp. 47–54, September–October, 2005.     

Basalt plastics — new materials for road construction

The effectiveness of using basalt fibres and substandard basalt wool — large-tonnage waste from chemical plants and nuclear generating stations — as a reinforcing component in road construction was demonstrated. Incorporation of basalt fibre or wool in asphalt concrete in an amount of up to 0.4 wt. % increased the strength characteristics of the asphalt concrete by 10–30%. The presence of a physicochemical reaction in the basalt fibre—polymer-asphalt binder system, which increased the strength characteristics, heat resistance, and lifetime of the polymer—asphalt—concrete, was demonstrated. IR spectroscopy showed ordering of the structure of the PAB due to formation of organosilicate compounds that strengthened the structure of the polymer asphalt concrete. Translated from Khimicheskie Volokna, No. 6, pp. 11–14, November–December, 2009.     

Modeling of low-speed combustion of a methane-air-coal dust suspension

A plane one-dimensional problem of the propagation of slow-speed combustion in methane-air-coal dust suspensions is solved. Methane contained originally in the gas is assumed to burn-up completely in the flame front. The front velocity relative to the gas is calculated from relations derived in analysis of the stationary problem using the theory of thermal explosion. The gas suspension is regarded as a two-temperature and two-velocity medium. Heterogeneous reactions of carbon (coke) combustion and carbon monoxide reduction, and the yield of volatile components and their gas-phase combustion together with carbon monoxide are taken into account. Based on numerical calculations, the coal dust is shown to affect unambiguously the flame velocity, with moderate content of the dust. In lean mixtures, with a moderate content of the dust the intensity of the combustion wave increases and decreases under conditions close to stoichiometric ones. Institute of Mechanics at Moscow State University, 119899 Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 4, pp. 46–54, July–August, 1997.     

Numerical study of heat waves in the oxidation of a magnesium wire

A mathematical model of ignition of magnesium samples is proposed based on the concept of the existence of thermal deceleration of a chemical reaction responsible for ignition. The model gives realistic values of temperatures after sample ignition and is in satisfactory agreement with experimental data on the dependence of the radius of a small particle on the limiting temperature of the ambient medium. It is shown that heat waves can be excited by heterogeneous oxidation of Mg wire located in the external flow. The range of parameters in which self-excited wave regimes are observed fits experimental data on oxidation of metal wires qualitatively and in order of magnitude. The problem of initiation of an ignition wave by the initial distributions of the sample temperature is solved numerically, and the stability of heat waves to small and finite perturbations is shown. Tranlated fromFizika Goreniya i Vzryva, Vol. 34, No. 6, pp. 29–38, November–December 1998.     

State of lithium cation in a solution of polyurethane in deviethylformamide

The viscosity characteristics of solutions of PU in DMF containing different amounts of lithium chloride and perchlorate wer investigated. The dependence of the relative viscosity of the ternary systems LiX—PU—DMF on the type and content of the lithium salt was established. A hypothesis was advanced concerning the coordination of lithium cations with polyurethane functional groups. __________ Translated from Khimicheskie Volokna, No. 6, pp. 5–6, November–December, 2006.     

Modeling the excitation of shock waves in tubes by gaseous detonation

The emergence and development of a viscous, thermally conducting, compressible boundary layer behind a shock front in a tube is investigated numerically. The reverse effect of the boundary layer on the inviscid flow in the center of the tube is studied. It is shown that, in order to account for the effect of the tube walls on the external flow, nonstationary boundary layers must be computed, since averaging the losses over the transverse cross section of the tube may be too crude an approximation. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 2, pp. 75–80, March–April 1999     

Parametric study of shock-wave formation in a spatially inhomogeneous self-igniting medium for arrhenius combustion kinetics

An approach to describing the formation and propagation of small gas-dynamic perturbations in a spatially inhomogeneous self-igniting medium for arbitrary chemical reaction kinetics was developed in a previous paper. In the present paper, the proposed approach is illustrated by studying the problem for a simple reaction with an Arrhenius reaction rate. A comparison of calculation results with the solution of the complete system of gas dynamics equations shows that the approach provides good accuracy of quantitative estimates. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 2, pp. 76–83, March–April, 2008.     

Formation of unburnt pockets in gaseous detonations

Formation of unburnt pockets in gaseous detonations is caused by two mechanisms: longitudinal and transverse mechanisms. The longitudinal mechanism is based on longitudinal instability of the detonation front. In the transverse mechanism, interactions between transverse waves lead to the formation of unburnt pockets. In this paper, the transverse mechanism is investigated via two-dimensional numerical simulations of propagation of gaseous detonation in a channel. A front structure that includes triple points, transverse waves, the incident shock, and the Mach stem is formed, as the detonation propagates in the channel. The origin of unburnt pockets is explained by the interaction between transverse waves or corresponding triple points that detach a portion of the reactant from the detonation front. It is observed that the size of the unburnt pockets and the depth of penetration to the products increase with increasing activation energy and that the shape of the pockets also changes with activation energy. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 2, pp. 84–92, March-April, 2009.     

Special characteristics of the filtration combustion of the system Ti?C?N in an audio-frequency sound field

The influence of sound fields on the filtration combustion of mixtures of titanium and carbon in nitrogen on the kinetics of self-propagating high-temperature synthesis and on the chemical and phase composition of the synthesis products is investigated. It is shown that filtration hindrances in a wide layer of the sample vanish if the charge material has an initial relative density of 0.2. The carbonitride component is formed most completely when the atomic fraction of carbon in the initial charge has values of 0.3 and 0.5. The optimum acoustic frequency stimulating the filtration of nitrogen deep within the briquette and yielding the most complete nitriding process is the resonance frequency of the acoustic system. Fast-framing photography exhibits the transient nature of the filtration combustion of titanium—carbon mixtures in nitrogen. Institute of Steel and Alloys, Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 5, pp. 44–51, September–October, 1995.