On planar reaction front in condensed materials: Reduced model, propagation speed, reaction zone balance, and insights into battery thermal runaway

Abstract

In this work, the propagation and structure of planar reaction front in condensed materials is theoretically investigated and compared to the gas-phase cases. For condensed materials, the Lewis number (ratio of heat to mass diffusivities) approaches infinity due to negligible effect of mass diffusion, as such unique features are associated with a propagating reaction front, including distinct reduced models, discontinuity in wave propagation dynamics, and different controlling balance within the inner reaction zone. Large Ze number asymptotic analysis is performed to acquire the reaction front propagation speed and reveal the key controlling parameters, achieving very good agreement with numerical solutions. The results are further compared to those based on the large β asymptotic theory with different nondimensionalization strategies and controlling physical parameters. It is found that results in these two limits are in general different. Consistency can only be achieved when further assuming large thermal expansion ratio in the large β limit. In addition, by comparing to the finite Le number reaction front propagation in the large Ze number limit, it is shown that the reaction front in condensed phase propagates at least 2-3 times faster with the same thermochemistry. The dominant balance in the reaction zone is found to be fundamentally different when Le is taken infinity, further confirming the distinct nature of combustion. It is hence incorrect to tackle a combustion problem with infinite Le by extrapolating the results based on finite Le assumption. This study can provide useful guidance toward battery thermal runaway propagation and reaction front propagation in general condensed phase.