Aluminum combustion in strong convective flows

This paper addresses the behavior of aluminum combustion in high speed flows, typical of choked nozzles or behind detonation fronts. Numerical simulations are conducted on a single burning droplet with Reynolds numbers Red up to several hundreds. The combustion mass rate is found to increase with Red and is well described by the classical Ranz-Marshall correlation but only provided that Red is estimated at flame conditions. The exponent n, relating combustion time and droplet diameter ( tb∝dn ), is found to decrease from n =2 at quiescent conditions to n≈ 1 for the highest Red. This is indicative of a transition from diffusion-limited to kinetic-limited combustion regimes. A Damköhler number Da* —that depends on pressure, oxidizer content, droplet size, and Reynolds number—is proposed and allows to delineate this diffusion/kinetic regime through a unique master curve n=n(Da*). A correction on the burning time accounting for this change of combustion regime is also proposed and depends only on Da*. This work unequivocally confirms the existence of a flow-induced transitional regime for aluminum combustion that has significant implication in the burning time of aluminum droplets in strong convective flows.