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

Abstract

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. To estimate the effects of clustering on the combustion process, direct-numerical-simulations are performed on a cubical domain with forced homogeneous isotropic turbulence. Simulations pertaining to Kolmogorov Stokes number $\mathrm{St}=1,10,50$, turbulent Reynolds number $\mathrm{Re_λ}= 5,10,20$, and global equivalence ratio (considering FeO as the oxidation product) $ϕ=0.25,0.5,0.75$ are executed. Increasing $ϕ$ significantly extends the combustion completion time. A Poisson distribution of particles burns faster with a higher peak mean temperature. 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 $\mathrm{Re_λ}=20$ and $ϕ=0.75$. Analysis of the Voronoi volumes $V_\mathrm{norm}$ 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 $V_\mathrm{norm}$ in the ``cluster'' regions, and an asymptotic behavior in the ``void'' regions. However, significant spread is observed in the correlation. Time-averaging $V_\mathrm{norm}$ does not minimize this variation considerably. Analysis of the macroscale $\mathrm{O_2}$ depletion zone indicates the importance of the macrostructure -- proximity of multiple clusters -- on the extension of the combustion time.