Gas-solid flows with tribocharging

Granular materials develop static charges when they come into contact with each other or other solid surfaces such as bounding walls, which is known as triboelectrification or tribocharging. Tribocharging may lead to safety hazards in extreme cases [1]; in other instances, it is believed to lead to undesirable outcomes such as sheeting in polymerization reactors [2]. The interplay between gas-particle flows and charge accumulation on particles is a topic of much interest. In this study, we probe the coupling between tribocharging in gas-solid flows via modeling and computations, aided by supplementary experiments.
We first study this coupling through CFD (Computational Fluid Dynamics) – DEM (Discrete Element Method) simulations of gas-particle flows, augmented by a finite-volume based particle-particle and particle-mesh type Poisson solver for the electric field [3] and Laurentie et al. model [4] for tribocharging. In the Laurentie et al. model, the charging tendency of particles is captured by effective work function difference between the contacting surfaces and the electrical field at contact. The effective work function is a lumped parameter that can be determined through experimental studies [4] or quantum mechanical calculations using the density functional theory [5]. In a recent study [6], we performed vibrated bed experiments and measured average charge on polyethylene (PE) particles at different controlled humidity conditions; DEM simulations of the same system were then performed to estimate the effective work function differences between the PE particles and the glass container at different humidity levels. CFD-DEM simulations of fluidized beds with these work function values captures the qualitatively different fluidization behavior seen at low and high humidity levels.

As CFD-DEM simulations are limited to relatively small systems and there is a need to examine the interplay of flow and tribocharging in large fluidized beds, we have also formulated and analyzed a kinetic-theory based Euler-Euler model for monodisperse particles with tribocharging. Towards this end, we have derived (i) the mean charge transport equation from the Boltzmann equation, assuming Maxwellian distributions for particle velocities and charges and allowing for conduction of mean charge through collisions in the presence of electric field; and, (ii) boundary condition capturing tribocharging at the wall. These models have been implemented in an open-source continuum physics software. Model predictions are then assessed through comparison with (i) hard-sphere Euler-Lagrange simulations and (ii) experimental studies.