Coupling of hydrodynamics and triboelectric charging in fluidized beds

It is well known that materials can develop static charges upon mechanical contact with other materials [1]. This phenomenon, known as triboelectric charging or tribocharging, is at play, for example, when one rubs a balloon against wool cloth and observes the balloon sticking to the cloth. It is also known that granular materials in transport lines and fluidized beds tend to become charged due to contacts with walls and other particles [2,3,4]. In the present study we probe the interplay between tribocharging and fluidized bed hydrodynamics through simulations.

We have simulated gas-solid flow in a small 3D fluidized bed with a conducting wall using the Computional Fluid Dynamics-Discrete Element Method. All the particles are assumed to have the same size and same work function, which is different from that of the wall. The work function based model of Laurentie et al. [5] is employed to evolve the extent of tribocharging. This model has been found be succesful in capturing the extent of particle charging in hopper-chute flows [6,7].

The electrostatic forces on the particles are determined using the hybrid approach of Kolehmainen et al. [8], which finds the long-range contribution to the electric field by solving the Poisson equation, estimates the short-range contribution through a truncated pairwise sum, and adds a correction to avoid double counting.

Simulations reveal that the triboelectric charging behavior can be strongly coupled to particle dynamics. With small levels of charge, the behavior of the bed remains similar to the case without triboelectric charging, and the charge evolution could be predicted based on the work function difference alone. When the electrostatic forces become comparable to the gravity, the system behavior is altered due to particle-particle and particle-wall electrostatic interactions. In this case, the particles gain charges that are larger than the maximum charge anticipated by the work function difference between the particles and the wall alone. This is attributed to bed-induced electric field. Most of the hydrodynamic changes is due to wall-particle interactions that cause highly charged particles to adhere on the walls and also induce a core-annular flow.