Coupling of the micro and macro flame structures in aluminum–steam suspensions based on an Euler-Lagrangian modeling approach

Aluminum can serve as a carbon-free energy carrier suitable for transport and storage, releasing energy as heat and hydrogen during combustion in steam. This requires a detailed understanding of the macro- and microscopic flame structures that govern aluminum particle combustion and oxide nanoparticle formation and transport While advancements have been made in the fields of sophisticated boundary layer resolved single particle simulations and large scale flame simulations using simplified particle models, the coupling between the scales remains insufficiently understood. In this work, a combined simulation approach is presented for macroscopic flame propagation in aluminum–steam suspensions, with the Lagrangian particle model including an improved mechanistic two-diffusion-layer formulation of the structure and transport in the micro-flames which envelope each individually burning aluminum particle. In contrast to previous approaches, the model relaxes the unity Lewis assumption. This leads to an more accurate expression for the flame standoff ratio, informed by the mixture-averaged diffusion model. Validation relies on heuristic combustion time correlations and experimental data for the standoff ratio for particles between 20 μm and 500 μm diameter in steam, diluted with up to 50 % mole nitrogen or hydrogen. The flame simulations provide first estimates for the reaction front propagation speed in monodisperse ( 20 μm particle diameter) aluminum steam-suspensions for equivalence ratios between 0.5 and 1.3 . The simulation results show the link between the micro-flames around individual particles and macroscopic flame propagation along the particle suspension. By shifting the macroscopic conditions from lean to rich, qualitatively different flame structures are achieved, altering both single particle evolutions and macroscopic flame profiles in a coupled manner. These insights demonstrate the need to include and combine the processes from different scales in numerical simulations, especially in complex macroscopic configurations Novelty and significance statement The novelty of this work is the combined and coupled simulation of the micro-diffusion flame structure and macroscopic reaction front propagation in suspensions of aluminum particles in steam. The major methodological advancement which facilitates this simulation approach is the derivation of a new two-diffusion-layer formulation to represent the micro-diffusion-flame structure in a point-particle model. Compared to previous point-particle models for aluminum combustion, the model relaxes the common unity Lewis assumption, leading to enhanced model validity. The work is significant as it reveals a strong coupling between the micro flames and reaction front propagation using the example of generic lean and rich configurations. It hereby demonstrates the necessity to include sub-models for the microscopic flame structures into large scale burner simulations whenever the formation of nanometer sized oxide particles is a result of concern.