3 - Momentum Transfer and Charge Transfer

Summary

Introduction

In gas–solid flows, flow patterns of both phases depend not only on the initial conditions and physical boundaries of the system but also on the mechanisms of momentum transfer or the interacting forces between the phases. The forces controlling the motions of particles may be classified into three groups: (1) forces through the interface between fluid and particles, (2) forces due to the interactions between particles, and (3) forces imposed by external fields. Although interparticle forces and field forces do not directly change the course of the fluid motion, they may indirectly influence the motion via particle–fluid interactions.

Among the significant characteristics in a gas–solid flow are the charge transfer and charge accumulation among particles due to the interparticle collisions and/or particle–wall collisions. The charges carried by the particles may directly affect the particle motion and particle concentration distribution in the flow by the electrostatic forces, especially when an external electric field is applied. Even in the absence of an external electric field, the accumulation of particle charges may yield adverse effects such as arc discharge and dust explosion.

In this chapter, various forces affecting the motion of a gas–solid flow along with the momentum equations governing the motions of particles are delineated. The mechanisms of charge generation and charge transfer by collisions of particles are discussed.