Study of the mechanism of oscillatory solid-phase combustion by a non-linear chemical kinetic model

Abstract

Oscillatory solid-phase pyrotechnic combustion in the following system NH₄ClO₄+ Mg (powder)+ K₂Cr₂O₇ is studied, where NH₄ClO₄, Mg and K₂Cr₂O₇ are the oxidant, reducing agent and frequency-modulating species, respectively, in order to elucidate a more detailed description of the oscillatory mechanism. The simplicity of this pyrotechnic system serves our purposes very well.

It is shown that the combustion of the above system is highly oscillatory in nature. Once ignited, the mixture exhibits ‘pulsed burning’, the intensity of the combustion varying periodically between flash combustion (deflagration) and smokeless smouldering. A number of experiments were carried out which confirmed that the phenomenon is intrinsic and that the controlling process is chemical in nature.

According to the results of the experiments and the observed combustion phenomena, the following chemical kinetic mechanism for the oscillatory process is suggested 3Mg(s, l)+ O₂→ 2MgO + Mg(g)(1), m[Mg(g)+ Mg(s, l)]+ O₂→nMg(g)+ 2MgO (2), Mg(g)→ MgO (3). It is considered that the competiting reactions between the three phases of Mg with the oxidizable gases produced by the decomposition of NH₄ClO₄ control the oscillatory burning process. Based on this analysis, the following simplified qualitative chemical kinetical model is proposed. A + C → B + D; k₃, A +mB →nB; k₁(n > m), B → D; k₂ and kinetic equations are deduced. The results of the simulation of the autocatalytic model using non-linear numerical methods are reported and discussed. Steady-state solution and oscillatory behaviour is obtained, in good agreement with experimental phenomena.