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

TL;DR

This numerical study investigates how particle clustering due to preferential concentration affects iron particle combustion in turbulent flows, revealing extended combustion times and the significance of macrostructure and local density.

Contribution

It provides detailed insights into the impact of particle clustering on combustion duration and temperature evolution using direct numerical simulations in homogeneous isotropic turbulence.

Findings

Increasing equivalence ratio extends combustion time.

Clustered particle distributions burn slower with lower peak temperatures.

Combustion time exponentially depends on local particle density in clusters.

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 $\mathrm{Re_\lambda}= 5,10,20$, 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 $\mathrm{Re_\lambda}=20$ and $\phi=0.75$. 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.