Mixing up to a molecular level and the development of a chemical reaction in a turbulent flow

Using the example of the process of ignition in flows of unmixed combustible components, an analysis is made of the effect of turbulence on the course of a chemical reaction. It is established that this process is conveniently described using the equation for the distribution of the probabilities of the temperature. This equation is derived and its solution is analyzed. In the case of mixing without chemical reactions, the intensity of the pulsations of the temperature is calculated without bringing in empirical constants. The ignition criterion is obtained.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 32–41, May–June, 1977.     

Effects of reagent consumption and heat loss in ignition in a vessel

Two approximate analytical methods are widely used in research on thermal ignition: the stationary theory [1] and the nonstationary one [2]. The first predicts the critical explosion conditions very closely. Direct numerical integration has been used [3] to obtain a solution for thermal ignition, which indicated that ignition near the heated walls can accompany ignition at the center. The difference between the critical conditions for ignition at the wall and self-ignition can be defined only from the interaction between the initial and boundary conditions. The extent of combustion is substantial in both cases [1], and it subsequently plays a substantial part in setting up the temperature conditions in the vessel. A study is made here of the thermal decomposition of methyl nitrate vapor, which incorporates the diffusion and finite reaction rate. Monte Carlo simulation is used with a planar electrically conducting medium [4].Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 192–196, January–February, 1975.     

Ignition of a gas in a boundary layer at a heated plate

The problem of the ignition of a moving homogeneous gaseous combustible mixture in a boundary layer along a heated flat semiinfinite plate is one of the main problems of the ignition of a combustible mixture in a flow (for example, [1]). The formulation of the problem includes the two-dimensional equations of motion and the equations of the transfer of heat and of the reacting substance, written taking a chemical reaction into consideration, as well as boundary conditions, and should lead to determination of the steady-state fields of the concentration and the temperature and, by the same token, of the position of the combustion zone. Different approximate numerical solutions of the problem were analyzed in [1–5]. One of the most important characteristics of the process is the length of the ignition, i.e., the distance from the edge of the plate to the point at which, thanks to the intrinsic chemical heat evolution in the gas, the heat flux from the plate to the gas becomes equal to zero. In the present work, for the case of large values of the activation energy of the chemical reaction and a sufficiently great temperature difference between the wall and the flow, an approximate expression is obtained for the length of the ignition.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 142–148, September–October, 1977.The authors thank V. M. Shevtsov for his aid in making the calculations.     

Theory of flame propagation in a gas and drop mixture

The equations of a reacting multiphase continuous medium [1] are used to investigate the problem of steady-state flame front propagation in a gas mixture with evaporating drops. A simple model for ignition of the liquid drops is proposed which is based on the application of the method of equally accessible surfaces [2] to the heat and mass exchange processes between the microflames surrounding the separate drops, the drops, and the carrying gas medium. The parameter distributions in the macroscopic flame front as well as the dependences of the flame propagation velocity in the gas suspension on a number of parameters governing the process under investigation are represented.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 4, pp. 101–108, July–August, 1973.     

Experimental investigation of the structure of coaxial nonisobaric reacting jets

This study, a continuation of an experimental program of research on isobaric coaxial jets [2], was primarily directed towards obtaining a more detailed picture of the flow structure, in particular by measuring the static pressure in the flow field of the jet with simultaneous visualization. It is shown that the nature of the axial static pressure distribution and the ignition lag are determined by the characteristics of the outer jet and are almost independent of the pressure ratio number (degree of underexpansion) of the inner nozzle and the rate of flow of fuel through it. The fuel forming the central jet self-ignites in a zone where not only the composition but also the temperature reach values corresponding to the ignition limits.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 184–186, January–February, 1987.     

Shock-initiated implosion of an elliptical cavity and detonation in a liquid layer

Many papers [1–9] have been devoted to the dynamical analysis of bubble implosion in a liquid layer. Experiments have shown that an initially circular cavity is displaced or transformed into an elliptical cavity during the implosion process due to instability, whereupon its further contraction produces cumulative jets. This problem is important in the study of surface wear in cavitation flow [7] and in the analysis of the impact sensitivity of liquid explosives [1–6]. The onset of accumulation is conveniently investigated by starting with an elliptical cavity or by displacing a circular cavity relative to the impact axis, thereby creating an asymmetrical pressure field about the center of the cavity. In the present article certain theoretical notions are advanced with regard to the onset of the cumulative jet in an elliptical or displaced cavity and its influence on the ignition of liquid explosives due to the formation of minute droplets [4] in the adiabatically heated gas inside the cavity. Experimental data on the jet formation time and the frequency of nitroglycerin detonations qualitatively support the theoretical predictions.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 78–85, September–October, 1971.     

Développement d''un modèle en vue de la prédiction du rallumage d''un foyer de turboréacteur à haute altitudeDevelopment of a model to predict the ignition performance of a turbojet engine at high altitude

Within the context of turbojet engine re-ignition after in-flight extinction, a thermo-diffusive model has been developed to describe the electrical ignition, at low pressure and low temperature, of a cluster of fuel droplets. The model involves the resolution of the conservation equations of mass, species and energy. It also takes into account the various physical and chemical phenomena occurring during the ignition process. This Note presents the ignition model and preliminary results of this model applied to an experimental configuration. To cite this article: V. Quintilla et al., C. R. Mecanique 330 (2002) 811–818.     

Analysis of a simple model of a bubble-liquid reactor

A mathematical model which describes the operation of a two-phase bubble-liquid reactor is studied. The model reduces to investigation of the Goursat problem for a hyperbolic system of two partial differential equations. A number of exact solutions to this problem are constructed that describe the operation of the reactor from the time of ignition until the onset of the stationary regime of operation. The problem is also investigated numerically.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 132–140, September–October, 1979.     

Ignition of solid propellants

At present, the study of solid-propellant ignition is of particular interest owing to the adoption of hybrid motors [1–3]. The status of experimental and theoretical research in this field can be evaluated on the basis of the rather extensive survey of American papers in [2]. It is noteworthy that a common deficiency in available references is the absence of exact ignition criteria; in most cases the propellant is assumed to have ignited when its surface temperature reaches a prescribed level (gasification temperature), or when the rate at which the temperature increases with time at the propellant surface is sufficiently high. Exact criteria for this rate, however, are not given. In this article, we present ignition criteria for solid propellants and these are based on a diffusion-burning model. It is shown that for a diffusion flame to exist above the propellant surface, two conditions must be satisfied simultaneously: 1) the propellant surface temperature must equal the gasification temperature for that propellant and 2) the temperature gradient at the surface must be smaller than some value which depends on the kinetics of the chemical reaction in the diffusion flame and on the rate of oxidizer input to the propellant surface during burning.Two ignition techniques are examined as examples: ignition by hot gases or radiant heat flow and ignition by means of an active film which reacts with a cold oxidizer; the film is applied to the propellant surface prior to ignition.     

Operating regimes of a chemical reactor with longitudinal and transverse mixing in the case of large Péclet numbers

A two-dimensional model of a chemical reactor with longitudinal and transverse mixing is investigated in the case of large Péclet numbers calculated from the effective thermal conductivity in the transverse direction. For this model the existence of at least one steady-state regime has been demonstrated [1], sufficient criteria of its uniqueness have been determined, an asymptotic expansion of the solution has been constructed in the case of small Péclet numbers, and the critical ignition and quenching parameters have been found. In this paper the other limiting case of the model, in which heat is propagated in the transverse direction much more slowly than it is transported by the flow along the reactor (large Péclet numbers), is analyzed in detail. An asymptotic expansion of the solution which closely coincides with the data of numerical calculations is constructed. The critical quenching and ignition conditions of the process are determined.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 120–127, January–February, 1987.     

Ignition of hydrogen in a turbulent boundary layer on a plate

The ignition of hydrogen blown into a turbulent supersonic boundary layer on a flat plate is investigated numerically. It is assumed that the mixture consists of six chemically active components H, O, OH, H₂O, O₂, H₂ and inert nitrogen N₂. The boundary layer is divided into outer and inner regions, for which different expressions for the coefficients of turbulent transport are used. The influence of pulsations on the rates of the chemical reactions, and also the back reaction of the chemical processes on the mechanism of turbulent transfer are not taken into account. The surface of the plate is assumed to be absolutely catalytic with respect to the recombination reactions of the H and O atoms. The influence of the blowing intensity, the Mach number in the outer flow, and the pressure on the ignition delay is analyzed. The possibility of effective porous cooling of the surface when there is combustion in the boundary layer is demonstrated.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 33–40, November–December, 1979.I thank V. G. Gromov and V. A. Levin for their interest in the work.     

Existence region for arcing conditions with negative anode potential drop

Conditions for ignition of a high-current arc with negative anode potential drop U_a is investigated, and the region within which these conditions exist and causes for a transition into conditions with positive U_a are studied. It is noted that a regime with negative U_a is most preferable for most plasma units.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 17–24, January–February, 1976.     

Evolution of a cloud of low-temperature light gas in an open atmosphere

The problem of the evolution of a hydrogen-air cloud formed as a result of the spillage and evaporation of a certain amount of liquid hydrogen is formulated. The effect of various factors (initial shape and temperature, condensation of atmospheric moisture) on the dynamics of the cloud and its dangerous (from the standpoint of ignition and explosion) propties is analyzed by means of numerical modeling and an approximate method based on a thermodynamic adiabatic mixing scheme. The results of the approximate theory, the numerical calculations and physical experiments are compared.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 92–101, May–June, 1991.The authors are grateful to I. A. Bolod'yan, V. I. Makeev, A. G. Frolov, and A. P. Chuguev for useful discussions and assistance.     

Experimental study of stable and unstable regimes of ethanol-to-acetaldehyde oxidation on a copper catalyst in a flow reactor

The oxidation of ethanol to acetaldehyde by atmospheric oxygen on a stationary copper catalyst in a flow reactor is investigated. It is shown that by operating in unstable rather than stable regimes the specific efficiency of conversion w of ethanol to the target product — acetaldehyde — can be increased by 13%. Operating in unstable regimes makes it possible, without reducing w (as compared with the most efficient stable regime) to increase the total productivity of the reactor by a factor of 2.89.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 11–17, May–June, 1989.     

Optimization of Dynamic Plastic Deformation of Plates with a Complex Contour

A method for studying dynamic deformation of ideal rigid–plastic plates with a complex contour on a viscoelastic foundation is proposed. The method allows one to optimize the process of pulsed forming. The optimization parameters are the amplitude of the pulsed load, viscoelastic damping coefficients of the foundation, the surface density of the plate material, and the shape and supporting conditions of the edges. Numerical examples of simply– and doubly–connected plates are given. It is shown that different combinations of the control parameters of the process can ensure the same final shape of the plate formed.     

Conjugated heat and mass transfer between a reactive solid and a gas in the presence of nonequilibrium chemical reactions

An investigation of a multicomponent boundary layer taking account of nonequilibrium chemical reactions has been made in a number of publications [1–3]; here, the temperature of the solid was assumed to be known or was determined from the condition of the conservation of energy at the interface between the gas and the solid, taking account of the solution of the equation of thermal conductivity in the solid phase. At the same time, heating of the material of a coating is an unavoidable step in any mechanism of thermokinetic decomposition and, in view of this, it is necessary to take account of the lag of the heat-transfer process inside the solid. Therefore, it is necessary to solve the equation of the energy balance in the solid phase simultaneously with the system of the equations of the boundary layer, i.e., the conjugate problem. The present article discusses the problem of flow around a solid in the vicinity of a frontal critical point, taking account of the dependence of the processes taking place in the solid body on the time, in the presence of two heterogeneous and one homogeneous reactions. The distributions of the velocity, the temperature, and the concentrations in the boundary layer are obtained, as well as the mass rate of entrainment of the material at different moments of time. The time of the change between kinetic and diffusion conditions of the course of the heterogeneous chemical reactions (the ignition time) is determined. It is established that, in the presence of a homogeneous chemical reaction, the mass rate of entrainment is less than with a frozen flow.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 121–128, March–April, 1974.     

Micromechanical study of formation and evolution of martensite for Cu–Al–Ni single crystal by moiré interferometry

The basic properties of pseudoelasticity of Cu–Al–Ni single crystal are studied to analyze the morphology associated with the formation and evolution of martensite and the shape memory recovery process at different temperatures. Use is made of a multifunctional macro–micro moiré interferometry measurement system. The β₁ to β₁^′ phase changes are identified with the stress-induced transformation of a Cu–13.7%Al–4.18%Ni (wt%) alloy. The invariant plane between the martensite and the parent phase is shown directly by fringe patterns. It is found that martensite appears in the shape of bands or thin plates on the surface of the specimen. The formation of martensite is a very rapid process and martensite ‘jumps' out until the specimen is completely transformed into a single variant. The results reveal the mechanism and process of stress-type and temperature-induced martensitic transformation.     

Solid propellant combustion in the presence of photoirradiation

Previous studies of the nonsteady processes associated with the irradiation of propellants with light have chiefly been devoted to the question of ignition [1–3]. It is also important to consider the effect of such an easily controlled influence as light on the propellant combustion process. We have attempted to estimate the dependence of the propellant burning rate on the intensity of the luminous radiation. Cases of steady-state combustion and combustion in the presence of a light flux varying harmonically with time are considered. It is assumed that the incident light flux is absorbed in the solid phase in accordance with the Bouguer-Lambert exponential law with constant transparency index. Steady-state combustion is considered within the framework of the Zel'dovich theory [4]. It is shown that in the steady state irradiation is equivalent to a certain increase in the initial temperature of the propellant. In the case of combustion with irradiation this makes it possible to use the data on steady-state combustion without irradiation. Nonsteady combustion in the presence of a periodically varying light flux is described with the aid of the Novozhilov model [5]. A correction to the mean burning rate (u °), proportional to the square of the light flux amplitude, is obtained. In the case of an exponential dependence of the burning rate on initial temperature the correction u ° is negative. The effect of irradiation on the stability of the steady-state propellant combustion mode is discussed.Translated from Zhumal Prikladnoi Mekhaniki i Tekhnicneskoi Fiziki, No. 5, pp. 70–77, September–October, 1971.In conclusion the authors thank O. I. Leipunskii and V. B. Librovich for their valuable advice.     

Effect of viscosity on the autoignition of a reacting moving mixture

Ya. B. Zel'dovich has established [1] that in a continuous-flow reactor two ignition regimes are possible: forced ignition and autoignition.It is important to consider the special properties of the autoignition regime associated with the hydromechanics of laminar flow and heat transfer through the pipe wall. In [2, 3] it was shown that the effect of heat of friction on heat transfer in long pipes is qualitative in character. Moreover, according to Schlichting [4], in certain cases the temperature gradient for such flows due to the heat of friction may reach 10–30°, which is comparable with the preexplosion temperature rise in the stationary theory of thermal explosion [5]. In this connection it is clear that under certain conditions the heat of friction may considerably reduce the explosion limit.This paper is devoted to a study of the effect of heat of friction on the explosion limit of a reacting fluid in a long cylindrical pipe. The dynamic autoignition regime due to heat of friction is examined. In particular, it is established that, other things being equal, by increasing the pressure drop it is possible to obtain explosion of the reacting system.     

Combustion of textile residues in a packed bed

Textile is one of the main components in the municipal waste which is to be diverted from landfill for material and energy recovery. As an initial investigation for energy recovery from textile residues, the combustion of cotton fabrics with a minor fraction of polyester was investigated in a packed bed combustor for air flow rates ranging from 117 to 1638 kg/m² h (0.027–0.371 m/s). Tests were also carried out in order to evaluate the co-combustion of textile residues with two segregated waste materials: waste wood and cardboard.

Textile residues showed different combustion characteristics when compared to typical waste materials at low air flow rates below 819 kg/m² h (0.186 m/s). The ignition front propagated fast along the air channels randomly formed between packed textile particles while leaving a large amount of unignited material above. This resulted in irregular behaviour of the temperature profile, ignition rate and the percentage of weight loss in the ignition propagation stage. A slow smouldering burn-out stage followed the ignition propagation stage. At air flow rates of 1200–1600 kg/m² h (0.272–0.363 m/s), the bed had a maximum burning rate of about 240 kg/m² h consuming most of the combustibles in the ignition propagation stage. More uniform combustion with an increased burning rate was achieved when textile residues were co-burned with cardboard that had a similar bulk density.