Combustion of gas mixtures with particles

Abstract

A multivelocity and multitemperature model, with heterogeneous chemical reactions, is used to describe the motion of such media. The article gives expressions characterizing the force, thermal, and mass interaction between the phases. A study is made of the problem of the normal propagation of the flame front inthe gas suspension for the case when the combustion of the particles takes place in the condensed phase. The values obtained for the rate of propagation of the flame front are given as a function of several parameters which determine the process under investigation (particle size, composition of the mixture, etc.). An analysis of the experimental data permits the isolation of three limiting sets of conditions for combustion of the particles. In the first place, purely heterogeneous conditions, when the chemical reaction takes place directly within or on the surface of the particles (in the condensed phase) and, by the same token, the heat of the chemical reaction is evolved directly in the condensed phase. For example, particles of graphite or carbon, which contain no volatile substances, burn in this way. In the second place, quasihomogeneous conditions, when combustion and heat evolution take place over the whole volume of the gas phase after vaporization and mixing of the fuel vapors with an oxidizer. Sufficiently fine particles of an easily vaporized fuel burn in this fashion. In the third place, vapor-phase conditions, when combustion and direct heat evolution take place in thin layers of flames surrounding the particles. These layers of flame have a considerably higher temperature than the surrounding gas and the particles. With regard to the combustion of a mixture with solid particles, the problem of the normal propagation of a flame front in a single-velocity and single-temperatu re approximation has been discussed by O. I. Leipununskii [1]. Papers [2, 3], for the case when the temperatures of the gas and the particles differ, discuss the combustion of a gas suspension under purely heterogeneous combustion conditions. Flame propagation in a gas suspension of an easily vaporizing fuel was studied experimentally by Burgoyne and Cohen [4]. They observed that, with an increase in the particle size, the quasi-homogeneou s conditions go over into vapor-phase conditions. In such mixtures, for quasi-homogeneou s conditions, the problem of the propagation of the flame front in its simplest form is investigated in the article of Williams [5].