Reactor model for ignition and burning of a gasifying solid

Abstract

Using ideas expounded in the book by Gray and Scott for the theoretical treatment of chemical reactors, a model is constructed of some of the events that take place during ignition of a solid energetic material that burns in the gas phase. Gray and Scott devote a chapter to a discussion of thermokinetic oscillations in a closed reactor. Although the mathematical character of their system is quite similar to the one described here, it is shown that the present system, in which some of the heat released is fed back to gasify the original solid material, cannot produce oscillations despite the use of the same (Sal'nikov) chemical-kinetic scheme for gas-phase combustion in the two situations. Instead, it is shown that when an initial temperature increase is sufficiently large, it stimulates a vigorous and accelerating rate of burning for as long as the supply of solid material is sustained. When the initial temperature rise is too small the initially rapid rate of reaction that it generates declines more and more; chemical activity in the gas phase goes asymptotically towards zero and, for all practical purposes, gas-phase chemistry becomes frozen. This decline into a chemically frozen state can, depending on the values of some of the system parameters, be replaced by a slow but continuing combustion of the energetic material all the way to complete consumption of reactants.

The present model forms a basis on which to build more detailed descriptions of the complex physical processes involved in two-phase combustion. It can also be used as a test problem for computer codes that aim to deal with very much more complicated scenarios than the present one.