Mathematical Modelling of the Combustion Process of Particles of Two-Component Metal Alloys in the Decomposition Products of Pyrotechnic Mixtures

Abstract

Purpose. The purpose of this work is to establish the mechanism and develop a mathematical model of the combustion process of AMC particles in gaseous decomposition products of nitrate-containing oxidants and organic additives to determine the dependence of their combustion time on technological parameters and external conditions. Methods. Modern methods of physical and chemical analysis: thermocouple methods of temperature measurement (tungsten-rhenium thermocouples were used); non-contact methods of recording the beginning and end of the burning of metal particles (temperature indicators, photosensitive elements, etc); film shooting methods (film camera “Konvas-auto” (shooting speed 30 frames/s)) and methods of micro-filming SKS-1M film camera (shooting speed 3000…5000 frames/s)) of the burning process of individual metal particles; methods of X-ray structural and micro-X-ray spectral analysis for conducting chemical analysis. Results. As a result of studies of the combustion process of aluminium-magnesium alloy (AMA) particles in products of thermal decomposition of pyrotechnic mixtures based on nitrate-containing oxidizers and organic additives, it was established that the process of combustion of alloy particles in gaseous decomposition products of mixtures that are formed (oxygen, oxygen + nitrogen or air in the presence of no more than 10 % CO + CO2) with an alloy content of no more than 80 % Al flows stably through the diffusion mechanism; with a content of Mg in the alloy of at least 20 %, the combustion process of the particles becomes unstable and becomes explosive as a result of their spontaneous fragmentation into smaller particles, the combustion of which becomes intermittent and accelerates sharply. Two points of view on the mechanism of diffusion combustion of alloy particles are also formulated: according to one of them, the particle combustion process takes place in two stages (first there is vapour phase combustion of Mg from the alloy, as the most volatile component, and then vapour phase combustion of the remaining molten drop of Al), and according to the second, simultaneous evaporation and combustion of both particles takes place. A mathematical model of diffusion combustion of particles of two-component metal alloys in gaseous products of thermal decomposition of pyrotechnic mixtures based on nitrate-containing oxidants and additives of organic substances has been developed. Using the example of AMA particles, it is shown that the developed combustion model allows, with an accuracy of 5…10 %, to determine the dependence of its combustion times on the technological parameters of the alloy powders (the content of the components in the alloy, the radius of the particle) and external conditions (the composition of the environment, the concentration of oxygen in it). As a result of calculations carried out on a PC in dialog and real-time mode, the following dependencies were established: an increase in the Mg content in the alloy and the concentration of O2 in the gaseous medium, as well as a decrease in the radius of the particle leads to a decrease in the burning time of the particle and a sharp increase in its burning rate, which contributes to destabilization and explosive development of the combustion process of pyrotechnic mixtures under external thermal conditions. Scientific novelty. As a result of studying of the process of combustion of AMC particles in products of thermal decomposition of pyrotechnic mixtures based on nitrate-containing oxidizers and additives of organic substances (in the amount of 5…10%), two points of view on the mechanism of diffusion combustion of alloy particles were formulated: according to one of them, the process of particle combustion proceeds in two stages (first there is a vapour-phase burning of Mg from the alloy, as the most volatile component, and then there is a vapour-phase burning of the remaining molten drop of Al), and according to the second, simultaneous evaporation and burning of both particles takes place; at the same time, spinel is the basis of combustion products in both cases MgAl2O4. Practical significance. The results of theoretical and experimental studies, which are obtained in the form of mathematical models and a database of experimental data on the influence of technological parameters of mixture charges on critical modes of explosive development of their combustion process under conditions of external thermal influences (elevated heating temperatures, external pressures, etc.), allow at the stage of manufacturing products by optimization of technological parameters (ratio of components, dispersion of metallic fuel and its nature) to increase their ignition temperature and reduce the probability of fire-explosive destruction of products in the conditions of their storage and transportation, taking into account the influence of external thermal actions; in the conditions of launch of products by optimising the speeds and angles at which they are fired during their application, to reduce the heating temperature of the metal shells of mixture charges and to reduce the number of premature explosive destruction of products during firing and flight.