On the ignition of metal particles

The ignition and combustion of dust-gas suspensions are considered. It is shown that the ability of these systems to accumulate heat is determined not only by their kinetic and thermal properties but also by the relation between their reaction surface and the heat-removal surface (f ). Experimental information on flame temperatures, ignition delays, and flame propagation over gas suspensions is processed using the parameter f , and the postulate on the stimulating role of the developed reaction surface in activating these processes is validated. It is shown that during overall burning, diffusion combustion of particles occurs only for rather small values of f . The ambiguous effect of the parameter f on the ignition and combustion processes leads to the necessity of optimizing the fuel size distribution and concentration for the effective operation of the power devices. The role of the macroparameters of two-phase flames of refractory metals in the synthesis of combustion nanoproducts is analyzed.

Section: Self-ignition Of Particle-gas Suspensionsmentioning

confidence: 99%

Section: Self-ignition Of Particle-gas Suspensionsmentioning

confidence: 99%

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Ignition and Combustion of Dust-Gas Suspensions

Zolotko

Vovchuk

Шевчук

et al. 2005

“…Thermal explo- sion for various laws of oxidation of metals was analyzed in [9]. For a linear law, zero order of heat release takes place and (as for the case of explosive particles) the critical value of the Semenov criterion is Se cr = 1/e.…”

mentioning

confidence: 99%

Investigation of ignition of particle-gas flow based on the hot-spot ignition model

Егоров

Ivanin

Malinin³

2010

Electric-spark ignition of flow of an aluminum particle-air mixture in a channel with a sudden expansion was studied experimentally using the model of hot-spot ignition. The critical radius of the initial site of ignition was determined, the dynamics of the process was studied, and the effect of turbulence intensity on the development of the hot spot was established.The ignition of a particle-gas cloud by a spark discharge is described using the hot-spot ignition model [1][2][3]. The problem of hot-spot ignition was considered in [4][5][6][7].In the thermal theory of ignition, the problem of hot-spot explosion reduces to analyzing the evolution of the initial temperature profile in the reactive medium and to determining the critical value of the FrankKamenetskii parameter δ cr = R 2 0 /at a , where R 0 is the radius of the hot spot, a is the thermal diffusivity of the substance, and t a is the adiabatic induction period [7]. A single spherical metal particle in a heated gas is a very convenient object for describing thermal explosion due to two features: the absence of a temperature distribution in the particle and the very simple and physically accurate expression for the heat transfer coefficient α. For fine particles, despite the possible high temperatures of thermal explosion, the role of the radiative component α r is usually insignificant. Hence, the Biot criterion can be written as Bi = α c r 0 /λ p = λ g /λ p , where α c is the conductive component of the heat-transfer coefficient, r 0 is the metal particle radius, and λ g and λ p are the thermal conductivities of the gas and particle, respectively. Thus, the problem reduces to the Semenov-Todes formulation [8].The laws of thermal explosion for single metal particles and their clouds differ because in clouds there is thermal interaction between particles. Thermal explosion for various laws of oxidation of metals was analyzed in [9]. For a linear law, zero order of heat release takes place and (as for the case of explosive particles) the critical value of the Semenov criterion is Se cr = 1/e. For parabolic and exponential laws, strong kinetic (more precisely, diffusive) deceleration of the reaction due to the buildup of the film oxide takes place. Under real conditions of thermal explosion, it competes with Arrhenius thermal self-acceleration. As a result, the preexplosion picture differs markedly from the classical one [8].Because of the peculiarity of the kinetic law, the dimensionless analysis has some special features. Instead of the common Se and Td criteria, the process is defined by the parameters Ω = 3Se get Td and γ = r 0 3δ 0 Td, wherek get (T 0 ), r 0 is the initial particle radius, ρ is the particle density, δ 0 is the initial thickness of the oxide film, T 0 is the ambient temperature, Q is thermal effect of the reaction, and k get is the preexponent [8]. It is known [10] that ignition lasts from the beginning of spark discharge to the establishment of steadystate flame propagation. Here there are at least two problems. One of them is the form...

“…Expressions for the dimensionless quantities δ and τ can be found in [1,2]. System (1) is an autonomous dynamic system of the first order, which has no stationary solutions; this system describes a class of problems that arise in the heterogeneous-ignition theory, which includes ignition of an isolated metal particle during formation of the product oxide film [1], ignition of a dense aggregation of particles in the onetemperature approximation [2], and ignition of a particle during formation of an intermetallide layer with a narrow zone of intermetallic-compound homogeneity in the equilibrium phase diagram of components [3]. Specific features of individual problems are determined by a particular form of the dimensionless parameter δ, which does not need to be refined in statement (1).…”

mentioning

confidence: 99%

Criteria for thermal stability of parabolically oxidable heterogeneous systems

Filimonov

2006

A system of equations that interaction of an isolated particle or a dense ensemble of particles with an oxidizing agent in the case of parabolic growth of the product layer is analytically treated in the one-temperature approximation. It is shown that a jumplike transition from the lower reacting mode (regime with a temperature maximum) to the upper reacting mode (regime with a monotonically increasing temperature) occurs with a continuously changing ignition parameter. The critical ignition parameter obtained differs from its numerically predicted value by 7%.Key words: parabolic law, phase trajectory, critical conditions.In the combustion and thermal-explosion theory, in problems where heterogeneous chemical interaction of components has to be analyzed, one has often to deal with a parabolic law of product-layer growth, which describes the mechanism of diffusive self-retardation of the self-heating rate. In dimensionless variables, the system of equations has the form [1, 2](E is the activation energy, R is the universal gas constant, and T 0 is the temperature of the ambient medium to which the heat is removed), z is the dimensionless thickness of the oxide film, δ is the thermal-explosion criterion, and z 0 is the dimensionless initial thickness of the oxide film. Expressions for the dimensionless quantities δ and τ can be found in [1,2]. System (1) is an autonomous dynamic system of the first order, which has no stationary solutions; this system describes a class of problems that arise in the heterogeneous-ignition theory, which includes ignition of an isolated metal particle during formation of the product oxide film [1], ignition of a dense aggregation of particles in the onetemperature approximation [2], and ignition of a particle during formation of an intermetallide layer with a narrow zone of intermetallic-compound homogeneity in the equilibrium phase diagram of components [3]. Specific features of individual problems are determined by a particular form of the dimensionless parameter δ, which does not need to be refined in statement (1). System (1) was numerically analyzed for the first time by Khaikin et al. [1], who determined the critical value of δ from the breakdown of the curve Θ(τ ) and from the transition from the regime with a maximum to the regime with a monotonically increasing temperature. The physical mechanism of the temperaturecurve breakdown, however, remained unclear, allowing no mathematical formulation of the ignition criterion.Rozenband and Chernenko [2] gave an approximate analytical solution to problem (1) based on examining the dependence z(τ ) from the viewpoint of the lower ignition limit. The analysis was based on a criterion responsible for merging of inflection points in the curve z(τ ). Results obtained by this method agree well with simulated data, though this criterion is not a true ignition criterion. The true criterion, obviously, must be capable of defining the qualitative change in the behavior of Θ(τ ) and the transition from the regime with a temperature maximum to ...