Evaluating regime diagrams for closed volume hybrid explosions

Hybrid mixtures of combustible dust and flammable gas represent an enhanced industry hazard due to increased explosion severity over that for the constituent fuels at their given concentrations. The current investigation extends the understanding of hybrid explosion dynamics by identifying and evaluating new explosion regimes on the dust/gas concentration plane. This work builds on previous studies that identified five regimes: gas-driven explosion, dual-fuel explosion, dust-driven explosion, synergic/synergistic explosion, and no explosion. For low ignition energy (e.g., spark ignition) the gas-driven and dual-fuel regimes are extended to include: gas-only explosion, two-stage explosion, single-stage explosion, and dust-only explosion. For high ignition energy (e.g., 10-kJ ignitors) the hybrid behaviour in the dust-driven regime depends on the dust reaction mechanism. For heterogeneous combustion, addition of flammable gas has a minor impact on explosion parameters. For homogeneously reacting dust, two new regimes are proposed: isolated particle combustion close to the dust flammability limit and group combustion further away. It is hypothesized that flammable gas addition has a larger impact in the isolated regime, as the gas acts to bridge individual diffusion flames during flame propagation and explosion. Ongoing research is investigating this hypothesis and reviewing the use of Computational Fluid Dynamics to close the gaps in understanding for hybrid explosion systems.