A Quantitative Analysis of the Ignition Characteristics of Fine Iron Particles

TL;DR

This study quantitatively analyzes the ignition characteristics of fine iron particles, focusing on how factors like oxide layer thickness and particle size influence ignition temperature and delay time, with implications for combustion processes.

Contribution

It provides a comprehensive kinetic and thermal analysis of iron particle ignition, highlighting the role of oxide layer thickness and particle size, and introduces a $d^2$-law scaling for ignition delay time.

Findings

Ignition temperature (~1080 K) is largely independent of particle size above 5 microns.

Initial oxide layer thickness significantly affects ignition temperature.

A $d^2$-law scaling relates ignition delay time to particle size.

Abstract

Ignition of iron particles in an oxidizing environment marks the onset of a self-sustained combustion. The objective of the current study is to quantitatively examine the ignition characteristics of fine iron particles governed by the kinetics of solid-phase iron oxidation. The oxidation rates are inversely proportional to the thickness of the oxide layer and calibrated using the experimentally measured growth of iron-oxide layers over time. Steady-state and unsteady analysis have been performed to probe the dependence of the critical gas temperature required to trigger a thermal runaway (namely, the ignition temperature $T_\mathrm{ign}$) on particle size, initial thickness of oxide layer, inert gas species, radiative heat loss, and the collective heating effect in a suspension of particles. Both analyses indicate that $T_\mathrm{ign}$ depends on $\delta_0$, i.e., the ratio between the initial oxide layer thickness and particle size, regardless of the absolute size of the particle. The unsteady analysis predicts that, for $\delta_0 \lesssim 0.003$, $T_\mathrm{ign}$ becomes independent of $\delta_0$. Under standard conditions in air, $T_\mathrm{ign}$ is approximately 1080 K for any particle size greater than 5 microns. Radiative heat loss has a minor effect on $T_\mathrm{ign}$. The collective effect of a suspension of iron particles in reducing $T_\mathrm{ign}$ is demonstrated. The transition behavior between kinetic-controlled and external-diffusion-controlled combustion regimes of an ignited iron particle is systematically examined. The influences of initial oxide-layer thickness and particle temperature on the ignition delay time, $\tau_\mathrm{ign}$, of iron particles are parametrically probed. A $d^2$-law scaling between $\tau_\mathrm{ign}$ and particle size is identified.