Comparative study by PEPT and DEM for flow and mixing in a ploughshare mixer

Abstract

Investigation of the granular flow induced by a single plough mixer was performed using positron emission particle tracking (PEPT) and Discrete Element Modelling (DEM). Both approaches showed the development of two loops of circulation, one on either side of the plough. Particles are pushed forward through the bed by the plough and then picked up and thrown through the space above the bed, before falling to the surface and then slowly flowing back into the trench left behind the plough blade. The flow patterns obtained experimentally were compared to those obtained using DEM for a speed of 2.25Hz and showed reasonable qualitative agreement. However, the angle of repose of the trench down which the avalanching back flow occurs was found to be too shallow in the DEM predictions. This is due to the rice grains being represented as spheres in the model — a demonstration of the modelling penalty in over-idealising particles and ignoring their shape. In contrast, the PEPT process used was unable to resolve the dilute cloud of ballistic material thrown by the plough and was also unable to identify dead regions of the bed. Uneven sampling of the phase space in PEPT can be a consequence of density and size differences between the tracer particle used and the bulk material. Mixing rates for different blade speeds were characterised for both PEPT and DEM. Good quantitative correlation was found and a consistent picture of increasing mixing efficiency with increasing plough speed was observed. Finally, observations on the averaging of PEPT and DEM data and on the comparison results from these two methods are made.