Effect of oxidizer composition on flame propagation modes in iron particle clusters

Iron dust flames can propagate either in a discrete mode, without simultaneous heat release among neighboring particles, or in a continuous mode, which is characterized by multiple neighboring particles burning at the same time. These characteristic flame propagation modes have been analyzed in previous numerical studies varying particle sizes and loadings. The results indicated that, in addition to the heat transfer, the depletion and replenishment of oxygen in the gas phase can also be a decisive factor for the propagation mode. Therefore, the oxidizer composition is an important parameter, which has, however, received less attention in the literature so far in the context of transition between the continuous and the discrete propagation modes. Thus, in this study, we explore how varying the oxidizer composition impacts the transition between the two propagation modes using boundary-layer resolved simulations with increasing complexity. We observe that the transition consistently occurs near the particle spacing/loading associated with stoichiometric conditions. This holds true for various oxygen concentrations and two different particle diameters that were investigated with oxygen–nitrogen as the oxidizer (i.e. with a mixture Lewis number close to unity). In contrast, for thermally thick oxygen–xenon mixtures (with Lewis numbers smaller than one), which have been used previously in microgravity experiments to investigate the different propagation modes, we find that the transition occurs near the particle spacing predicted by the ratio of the particle combustion time and the heat transfer timescale. Ultimately, our analysis, supported by numerically estimated boundaries, reveals that the transitional particle spacing—indicating the shift between different propagation modes—depends on the stoichiometric particle spacing when the Lewis number equals unity. For Lewis numbers less than unity, this transition is governed by the heat transfer time scale. • Propagation of iron dust flames analyzed using boundary layer resolved simulations. • Transition between continuous and discrete mode evaluated for varying compositions. • Transition to the discrete mode occurs close to stoichiometry for O 2 -N 2 mixtures. • For thermally thick mixtures (O 2 -Xe), transition occurs at rich conditions. • Scaling of transition particle spacings depends on the oxygen Lewis number.