COMPARISON OF CONVECTIVE HEAT TRANSFER IN PACKED BEDS AND GRANULAR FLOWS

Abstract

Stationary and contact-dominated moving-particle beds are two examples of high-particle-density gas−solid flows. In packed beds, only the gaseous phase moves so that the flow depends on the bed structure and the associated fluid properties. In granular flows, both phases move, but the particle collisions govern the transport so that the fluid effects are negligible. Though the phenomena involved in the momentum transport of the two flows are quite distinct, the convective heat transport depends on similar mechanisms. Both problems are generally modeled as a one-phase continuum with a uniform velocity and effective thermal properties. If the gas-solid medium flows adjacent to a heated impermeable surface, the near-wall particle distribution alters the uniform velocity and modifies the near-wall thermal conductivity. For packed beds, the distribution depends on the particle size and uniformity, and for granular flows, it depends on the flow rate, surface conditions, and type of particles. Though both flows appear macroscopically one-dimensional, the local fluid and particle motions exhibit lateral variations. Both the near-wall effects and the local velocity variations contribute to deviations from the one-phase continuum model. This review begins by presenting the continuum model and by comparing to the model experimental results for both flows. The noncontinuum effects are described for the core and near-wall regions of the flows. The review also discusses current efforts in advancing the modeling of these flows and concludes by suggesting issues that deserve continuing attention.