Dynamic modes of Ti combustion in a coflow of N<Subscript>2</Subscript>-Ar mixture

Combustion of bulk density Ti powder (containing 20 wt % TiN as a diluent) in a coflow of N₂-Ar mixture was investigated upon variation in the nitrogen content of the gaseous mixture. The obtained data are believed to open up new horizons for fabrication of layered and composite ceramics by infiltration-mediated combustion.      

Combustion of bulk density powder mixtures in a coflow of nitrogen gas: 3. The Ti-(Ti + 0.5C) layered system

Explored is the filtration combustion in Ti-(Ti + 0.5C) layered powder systems in a coflow of nitrogen gas. The presence/absence of nitrogen coflow through the layered system was found to drastically affect the character of wave propagation and structure/properties of product. The obtained data can be regarded as a basis for fabrication of new layered and composite materials in the mode of dynamic filtration combustion.      

Passivation of Compacted Samples Made of Pyrophoric Iron Nanopowders during Their Interaction with Air

Combustion, Explosion, and Shock Waves - Various macrokinetic modes of interaction (self-ignition or combustion) of compact samples from nonpassivated (pyrophoric) and passivated iron nanopowders...     

Combustion of Ti+0.5C and Ti+C mixtures of bulk density in inert gas coflow

The combustion of Ti-C mixtures of bulk density under inert gas blowing conditions produced by evacuation of one end of the reaction cell was studied for the first time. The experiments showed that the tested mixtures in quartz cups were not ignited and did not burn without the inert gas (argon) flow. Increasing the rate of gas evacuation from the sample increased the rate of steady-state combustion of the mixture of titanium with carbon black, and for the mixture of titanium with graphite, stabilization of the flat combustion front was observed. It is shown that the presence of small pressure difference (up to 105 Pa) allows control of the combustion process and confirms the basic postulates of the convective-conductive theory of combustion for heterogeneous condensed systems. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 30–37, January–February, 2009.     

Approximate Analytical Method for Calculating the Time Characteristics of Ignition of a Gas Mixture by a Heated Body

Analytical expressions for calculating the time characteristics of ignition of gas mixtures by a heated body are obtained for the first time with the help of the wave theory of ignition. It is shown that ignition can occur in three different modes: kinetic (onetemperature), diffuse (twotemperature), and transitional, which combines the properties of both diffusion and kinetic modes. The parametric region of implementation of each possible ignition mode is determined. It is found that the transition from the kinetic to the diffusion mode occurs as the temperature of the heater changes only by one characteristic interval and is accompanied by a jumplike decrease both in the ignition delay and in the amount of energy necessary for combustion initiation. A relation is established between the laws of ignition and diffusion combustion of a single particle and parameters of ignition of the gas mixture of a given sort of particles. It is shown that there is a minimum in the dependences of the time of establishing of the zero gradient of ₀ on Semenov's criterion. The value of ₀ is found to be minimum if the heater temperature is higher than the temperature of ignition of a single particle by one characteristic interval. The numerical solution of the initial system of equation confirms the validity of the basic assumptions and results of the approximate analysis. The error in determining the time characteristics of ignition with the help of approximate formulas is lower than 50%. For the kinetic mode of ignition, a transformation of the time and space scales is found, such that the time characteristics of ignition in new dimensionless variables become independent of the mass concentration of particles.     

Ignition of porous bodies under conditions of counterflow nonstationary filtration of a gas

An analytical method for calculating the time characteristics of ignition of a porous body with outflow of an inert gas from the sample (counterflow nonstationary filtration) is developed employing the wave theory of ignition and an averaging technique using a certain weighting function. Numerical calculations confirmed the validity of the main assumptions of the theory on the stagewise nature of the ignition process and the wave mechanism of heating of the material. Complete qualitative and good quantitative agreement between conclusions of an approximate analysis and numerical results is shown. The error in determining the times of establishment of zero gradient and disturbance of temperature equilibrium does not exceed 50%. It is established that the equations of isothermal filtration are adequate for describing the process of gas escape from a porous body during ignition. It is shown that the gas mass flow can be calculated using a quasistationary approach. The region of applicability of the model of a semiinfinite body in ignition problems with counterflow nonstationary filtration of gases is determined. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 4, pp. 31–40, July–August, 2000.     

Ignition mechanisms in condensed systems using an incandescent surface for a parabolic interaction law

Using the ignition wave theory, the dependence of the ignition process stage periods for a condensed material and an incandescent surface, when a parabolic interaction law applies to the initial reagents, on the basic problem parameters — the dimensionless temperature gradient and the transformation depth in the intermediate combustion wave — has been determined for the first time. It is shown that the equation for a stage period can be represented in the form of a product of two functions: one being dependent on the temperature gradient, and the other — on the transformation depth. The dependence of the characteristic times on the transformation depth for a constant temperature gradient is found to be linear in nature. It is established that for a parabolic interaction law, there exists no limiting transition to the ignition mechanism that is characteristic of zero order reactions. The following critical conditions are formulated: an ignition does occur if a reaction zone is formed which is capable of spreading through the material in the combustion regime. Specialized numerical computations have confirmed the correctness of the assumptions of the stagelike character of the ignition process and of the wavelike nature of the heating of the material, on which the theory was based, and also validated the basic conclusions drawn from the approximate analysis.I.S.M., Russian Academy of Sciences, 142432, Chernogolovka. Translated from Fizika Goreniya, Vol. 30, No. 6, pp. 8–15, November–December, 1994.     

Ignition of condensed systems interacting through a layer of high-melting product

The functional dependence of the ignition delay time on the main parameters of the problem is determined for condensed systems interacting through a layer of high-melting product by a power law. A formula for determining the ignition temperature from the equality between the external heat flux and integral heat evolution in a chemical reaction in a stationary combustion wave with temperature equal to the ignition temperature is proposed and substantiated. It is shown that at a surface temperature below the ignition temperature the heating can be considered inert and the duration of this stage makes up the bulk of the delay time.Institute of Structural Macrokinetics, Russian Academy of Sciences, Chernogolovka 142432. Tranalated from Fizika Goreniya i Vzryva, Vol. 31, No. 4, pp. 3–9, July–August, 1995.