Influence of weak electrostatic charges and secondary flows on pneumatic powder transport

Abstract

The dynamics of the transported powder determines the functionality and safety of pneumatic conveying systems. The relation between the carrier gas flow, induced by the flown-through geometry, and the powder flow pattern is not clear yet for electrostatically charged particles. This paper highlights the influence of relatively minor cross-sectional secondary flows and electrostatic forces on the concentration and dynamics of the particles. To this end, direct numerical simulations (DNS) capture the interaction of the continuous and dispersed phases using a four-way coupled Eulerian-Lagrangian strategy. The transport of weakly charged particles in channel flows, where turbopheresis defines the particle concentration, is compared to duct flows, where additional cross-sectional vortices form. For both geometries, the Stokes number (St ¼ 8, 32) and the electrical Stokes number (St el ¼ 0, 1, 2, 4 ½ Â10 À3 ) are varied, and the turbulent carrier flow was fixed to Re τ ¼ 360. The presented simulations demonstrate that secondary flows, for the same Re τ , St, and St el , dampen the effect of particle charge. In a duct flow, vortical secondary flows enhance the cross-sectional particle mobility against the direction of electrostatic forces.Compared to a duct flow, in a channel, the wall-normal aerodynamic forces are weaker. Thus, electrical forces dominate their transport; the local particle concentration at the walls increases. Further, electrostatic charges cause a stronger correlation between the gas and particle velocities. In conclusion, despite being weak compared to the primary flow forces, secondary flow and electrostatic forces drive particle dynamics during pneumatic transport.

Influence of the Reynolds number from Reτ = 150 to 210 on size-dependent bipolar charging

“…Triboelectric charging causes a build-up of charge in particulate systems. In industrial environments, the excessive build-up causes unwanted agglomeration and wall-sheeting in the fluidized beds [1][2][3][4][5], pneumatic conveyors [6][7][8][9][10], or during powder handling in general [11]. This is an especially severe problem in the pharmaceutical industry [12][13][14][15] because the formulation of powders is strictly given and often cannot be changed to reduce the charging.…”

Issue Highlights

“…On the contrary, in a channel, the wall‐normal aerodynamic forces are weaker, electrical forces dominate the transport, and the local particle concentration at the walls increases. [ 1 ]…”

Introduction to the Special Series on Process Safety

“…The first manuscript to be published in the special series comes from Germany, with Holger Grosshans contributing the paper titled Influence of Weak Electrostatic Charges and Secondary Flows on Pneumatic Powder Transport . [ 4 ] As noted by the author, pneumatic powder conveyance can lead to rapid particle electrification and a subsequent dust explosion initiated by an electrostatic spark discharge.…”