PresentationThe 20L sphere is one of the standard devices accepted as an international normativity used for dust explosivity characterization. One concern about the effectiveness and reliability of this test is related to the particle size variation due to particles agglomeration and de-agglomeration. These phenomena are determined by the turbulent regime of the dust cloud during the dispersion. This variable must be considered since it determines the uncertainty level of the ignitability and severity parameters of dust combustion. In this context, this study describes the influence of the cloud turbulence on the dust segregation and fragmentation through an experimental and computational study. The behavior of the gas-solid mixture evidenced with the standard rebound nozzle was compared with that observed with six new nozzle geometries. Thereafter, the variations of the Particle Size Distribution (PSD) that occur during the dispersion within the 20L sphere were analyzed for two different powders: carbon-black and micrometric wheat starch. This description is performed with the implementation of two complementary approaches. On the one hand, an experimental approach characterizes the turbulence levels with Particle Image Velocimetry (PIV) tests that are complemented by the description of the PSD variations with granulometric analyses. On the other hand, a computational approach described the dispersion process with CFD-DEM simulations developed in STAR-CCM+ v11.04.010. The simulation results established that the homogeneity assumption is not satisfied with the nozzles compared in this study. Nonetheless, the particles segregation levels can be reduced using nozzles that generate a better dust distribution in the gas-solid injections. Subsequently, an additional first-approach CFD model was established to study the behavior of the combustion step when a starch/air mixture. This model considers the gas- phase reactions of the combustible gases that are produced from the devolatilization of Wheat starch (CO, CH4, C2H4, C2H6, C2H2 and H2) and allowed to establish the approximate fraction of the particle mass that devolatilizes, as well as to confirm that the modelling of the pyrolysis stage is essential for the correct prediction of the maximum rate of pressure rise
