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

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 $\mathrm{S}\mathrm{t}=1,10,50$, turbulent Reynolds number $\mathrm{R}{\mathrm{e}}_{\lambda }=5,10,20$, and global equivalence ratio (considering FeO as the oxidation product) $\phi =0.25,0.5,0.75$ are executed. Increasing $\phi$ 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 $\mathrm{R}{\mathrm{e}}_{\lambda }=20$ and $\phi =0.75$. Analysis of the Voronoi volumes $V_{\mathrm{n}\mathrm{o}\mathrm{r}\mathrm{m}}$ 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 $V_{\mathrm{n}\mathrm{o}\mathrm{r}\mathrm{m}}$ in the cluster'' regions, and an asymptotic behavior in the void’’ regions. However, significant spread is observed in the correlation. Time-averaging $V_{\mathrm{n}\mathrm{o}\mathrm{r}\mathrm{m}}$ 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.