Comparative study of explosion processes controlled by homogeneous and heterogeneous combustion mechanisms

The dust explosion behaviors induced by two different combustion mechanisms (homogeneous and heterogeneous mechanisms) were comparatively investigated, based on the experiments under different dust concentrations, particle sizes and initial pressures in Siwek 20-L chamber. Based on the thermo-gravimetric analysis (TGA), sweet potato dust and magnesium dust were selected as the representative dusts with homogeneous and heterogeneous combustion mechanisms, respectively. Experiments find that these two dusts have different behaviors in the explosion kinetics due to different combustion mechanisms. For sweet potato dust, the explosion pressure p_max, the pressure rise rate (dp/dt)_max and the combustion fraction η exhibit similar variation trends as dust concentration increases and they all reach to the maximum values at the worst-case concentration; while for magnesium dust, the variation of (dp/dt)_max is somewhat different from that of p_max, that is, the (dp/dt)_max will achieve the maximum at the concentration higher than the worst-case and keep stabilized with further increase of dust concentration. As the particle size decreases, the (dp/dt)_max for sweet potato dust will increasingly rise and gradually approach to a stabilized value, but for magnesium dust, the increase of (dp/dt)_max becomes pronounced only in the range of smaller particle sizes. To account the effect of initial pressure on p_max under different combustion mechanisms, a dimensionless pressure P_R was introduced to denote the relative intensity of explosion. It is found that, for sweet potato dust, the increased initial pressure will promote the explosion process (or with high P_R) for the dust cloud with high concentration due to the augmented oxygen concentration, but for the dust cloud with low concentration, the increased initial pressure will suppress the explosion process due to the increased resistance in devolatilization. For magnesium dust, the rise of initial pressure will generally promote the explosion process even for the dust cloud with low concentration; however, in the case of small particle size, the promotion of increased initial pressure to the explosion process is not so pronounced.