Analysis of Tribo-Electric Charging of Spherical Beads Using Distinct Element Method

Tribo-electrification is an important phenomenon in powder handling, as it could cause hazards and nuisance, such as dust explosion, adhesion to walls and blockage of pipelines leading to powder losses and difficulties in flow control. At present, there are only a few methods available to predict the dynamic charging of bulk powders, and they are not suitable for testing a small quantity of powders or handling highly active powders, for which full isolation would be needed. In this paper, the operation of a simple test device is analysed. Tribo-electrification of a single spherical bead inside a sealed capsule is induced through a horizontal motion using a shaking device at various frequencies. The single particle contact charge obtained from experiments is incorporated in numerical simulations based on Distinct Element Method (DEM) and is used to predict the transient charge accumulation of an assemblage of beads. In the model, the tribo-charging of a single bead is related to the contact area of the bead on impact with the capsule wall. The effects of long range forces, space charge and image charge at boundary condition are investigated for bulk charging. It is shown that the inclusion of space charge effects and boundary condition into the DEM model leads to a better prediction of charge build-up when compared with the experimental results.