CFD-DPM and experimental study of the dynamics of wheat starch powder/pyrolysis gases hybrid mixtures in the 20-L sphere

Abstract This study comprises an assessment of the behaviour of wheat starch/pyrolysis gases hybrid mixtures under different operating conditions of Ignition Delay Time (tv) and nozzle geometry in the context of the 20-L Sphere Test. The behaviour of the system was determined through the study of the interaction between the chemical reactions of the phases and the turbulence levels produced by the different operating configurations. For these purposes, CFD simulations based on a coupled Eulerian-Lagrangian formulation were developed considering the two main steps of the test: i) Dispersion of the powder, and ii) Pyrolysis/gas-oxidation reactions. The CFD model included a few important improvements to other computational methodologies proposed in previous studies, such as the incorporation of more rigorous kinetic mechanisms for the devolatilization reactions of the starch particles and the combustion reactions of the pyrolysis gases. In parallel, experimental tests were performed in order to validate the CFD model and characterize the powder samples. In particular, the composition of the pyrolysis gases emitted by the particles was determined by a modified setup of the Godbert-Greenwald furnace, and the Particle Size Distribution (PSD) was measured through laser diffraction techniques. The performance of the CFD model was satisfactory in terms of predicting parameters related to the hydrodynamic or reaction-kinetic behaviour of the system, such as the pressure evolution during dispersion and the (dP/dt)m (average deviations from experimental values of ~4.0% and 11.9%, respectively). Furthermore, the results suggest that the combined effect of the dust-phase and gas-phase reactions have a distinctive dissipative effect on the turbulence levels and that higher turbulence decay levels during the explosion step of the test are related to higher explosion severities.