A diffusion-driven model for ignition of spherical and porous iron particles

The combustion modeling of spherical iron particles often employs Wagner's theory with planar geometric assumptions, which may not fully account for curvature effects, particularly for complex morphologies like porous particles. To address this, we develop an analytical model for iron particle combustion based on solid-phase Fe²⁺ diffusion within a spherical coordinate framework, thereby inherently incorporating curvature. This physics-based framework is extended to porous particles via a stochastic pore model. Calibrated against established oxidation kinetics, the model demonstrates superior agreement with experimental ignition temperatures compared to classical planar theory. It predicts a significantly lower ignition temperature for porous particles (722.0 K) than for non-porous ones (1127 K) and provides a criterion for transitions between diffusion-controlled regimes. The work demonstrates that a first-principles consideration of curvature within the solid-state diffusion framework can enhance model fidelity for spherical particles and offers a natural foundation for describing more complex geometries.