Physics

Geology

Ignition system

Pure mathematics

Mathematics

Diffusion

Suspension (topology)

Analytical Chemistry (journal)

Optics

Combustion

Oceanography

Physical chemistry

Particle size

Chemistry

Metallurgy

Materials science

Iron oxide

Particle (ecology)

Oxide

Chromatography

Homotopy

Radiative transfer

Thermodynamics

Combustion and Flame

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.

A quantitative analysis of the ignition characteristics of fine iron particles