Numerical modeling and experimental study of self-propagating flame fronts in Al/CuO thermite reactions

Nanothermites are promising energetic materials as their high-temperature reaction driven by the oxidation of a metallic fuel associated with the reduction of an oxidizer, can exhibit extremely fast burning rates, exceeding hundreds of m s-1. In addition, by modifying reactant size, stoichiometry and compaction conditions, reaction properties (temperature, intermediate reactions, by-products) and combustion rates can be tailored, making it possible to customize combustion properties for each application. Unfortunately, in spite of three decades of research in the field of thermites, there is no predictive physical models able to provide design guidelines to experimentalists. The reason of this is that the complex multiphasic physics governing thermite combustion, where combustion gases interact with burning particles, is still poorly understood and documented, while being the key step to depict the dynamics of the flame front. The purpose of this work is to propose a first one-dimensional (1D) model that describes the dynamics of the reaction front propagation in Al/CuO powdered thermite considering the reacting flow combined with heat transfer, chemistry and fluid flow. CuO was chosen as it is the widest used metallic oxidizer, that decomposes below the flame temperature, leading to a gas phase driven reaction. Separate mass, momentum and energy transport equations for the three phases, namely Al, CuO particles and gas mixture, are written in the frame of an Euler-Euler approach for multiphase reactive flows. These equations are coupled by modeled interphase transfer terms. The theoretical formulation and numerical methods are detailed. After validating the model with experimental case studies - specifically, the combustion of Al/CuO powder in open glass tubes - numerical experiments are performed to demonstrate the utility of the code in (i) analyzing the multiphase flow dynamics at the thermite flame front, and (ii) examining the critical powder characteristics that affect the burn rate.