On the controlling parameters of the thermal decomposition of inhibiting particles: A theoretical and numerical study

Alkali metal compounds, such as (NaHCO3)s and (Na2CO3)s, are widely used fire suppressants that can be spread as powders or aqueous solutions. Their efficiency depends strongly on the capacity of the alkali metal particles to decompose inside the flame front. The paper aims at providing a theoretical basis for: (1) manufacturing efficient powders; (2) selecting inhibitor candidates that could be efficient against a given reactive mixture. To do so, analytical expressions for the decomposition position of these inhibiting particles are sought to unveil the influence of flame and powder properties on the thermal decomposition problem. The latter is first solved analytically using a simple model for the thermal decomposition of alkali metals combined with a 2-step chain-branching mechanism for the flame chemistry. The analytical solutions are then confronted to numerical simulations using detailed chemistry for both CH4/air and H-2/air flames. The validity of the analytical derivations is discussed in terms of combustion regime, characterized in this paper by the ratio of chain-termination to chain-branching time scales. Based on these analytical derivations, a model is proposed for the critical particle size above which mild to no inhibition may be observed. This model accounts for the thermo-chemical properties of the flame/particle system, while being perfectly suited for fine-tuning using either numerical or experimental data. Finally, the paper opens a discussion on the capacity of alkali metal powders to efficiently mitigate highly reactive mixtures, e.g., C2H2/air and H-2/air flames, given the predicted particule sizes needed for fast decomposition in such cases. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.