Effects of preferential concentration on the combustion of iron particles — A numerical study with homogeneous isotropic turbulence

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Iron particles, with their non-volatile combustion mode, remain in the dispersed phase throughout the combustion process, causing the flow in a typical iron powder combustor to be particle-laden and turbulent. Preferential concentration is a phenomenon prevalent in such turbulent flows that causes particle clustering and may be detrimental to the combustion process. To estimate the effects of clustering on the combustion process – the most significant of which is the extension of the combustion time – direct-numerical-simulations (DNS) are performed on a cubical domain with forced homogeneous isotropic turbulence. Three sets of simulations pertaining to Kolmogorov Stokes number St = 1 , 10 , 50 , turbulent Reynolds number Re λ = 5 , 10 , 20 , and global equivalence ratio (considering FeO as the oxidation product) ϕ = 0 . 25 , 0 . 5 , 0 . 75 are executed. The prevalence of clustering was found to be strongly sensitive to St , as reported in the literature. Increasing Re λ enhances the magnitude of clustering but retains the timescales of cluster formation. Increasing ϕ significantly extends the completion time of combustion owing to the depletion of O 2 in particle-rich regions. The particle combustion times are estimated for the combustion of a fully clustered distribution, a Poisson (random) distribution, and a particle-gas coupled 0D suspension model, and are compared. The Poisson distribution of particles burns faster with a higher peak mean temperature, possibly due to collective heating effects. The evolution of the mean temperature in the combustion of the clustered distribution is smooth and results in a smaller peak value. However, the total combustion time of a clustered distribution is significantly extended, by up to eight times at Re λ = 20 and ϕ = 0 . 75 . Analysis of the Voronoï volumes V norm at the start of combustion shows that particles in highly dense regions burn longer, as seen before in the literature. Furthermore, the combustion time exhibits a strong exponential dependence on V norm in the “cluster” regions, and an asymptotic behavior in the “void” regions. However, significant spread is observed in the correlation. Time-averaging V norm does not minimize this variation considerably. Analysis of the macroscale O 2 depletion zone indicates the importance of the macrostructure-proximity of multiple clusters-on the extension of the combustion time. Novelty and significance statement This study caters to the research and development of iron power technology, a novel carbon-free renewable energy storage alternative to fossil fuels. The focus of this article is the effects of particle clustering, through the phenomenon of preferential concentration, on the combustion process of iron particles. The statistical analysis methods developed in this work serve as a common framework for future computational investigations on turbulent iron-powder combustion. The in-depth quantitative analysis is novel and provides deep insight into the factors influencing the combustion of such distributions.