  Three-dimensional CP-DNS of reacting iron particle dust clouds in a turbulent
mixing layer are conducted. The simulation approach considers the Eulerian
transport equations for the reacting gas phase and resolves all scales of
turbulence, whereas the particle boundary layers are modelled employing the
Lagrangian point-particle framework for the dispersed phase. The CP-DNS employs
an existing sub-model for iron particle combustion that considers the oxidation
of iron to FeO and that accounts for both diffusion- and kinetically-limited
combustion. At first, the particle sub-model is validated against experimental
results for single iron particle combustion considering various particle
diameters and ambient oxygen concentrations. Subsequently, the CP-DNS approach
is employed to predict iron particle cloud ignition and combustion in a
turbulent mixing layer. The upper stream of the mixing layer is initialised
with cold particles in air, while the lower stream consists of hot air flowing
in the opposite direction. Simulation results show that turbulent mixing
induces heating, ignition and combustion of the iron particles. Significant
increases in gas temperature and oxygen consumption occur mainly in regions
where clusters of iron particles are formed. Over the course of the oxidation,
the particles are subjected to different rate-limiting processes. While
initially particle oxidation is kinetically-limited it becomes
diffusion-limited for higher particle temperatures and peak particle
temperatures are observed near the fully-oxidised particle state. Comparing the
present non-volatile iron dust flames to general trends in volatile-containing
solid fuel flames, non-vanishing particles at late simulation times and a
stronger limiting effect of the local oxygen concentration on particle
conversion is found for the present iron dust flames in shear-driven
turbulence.
