Thermophysical Properties and Heat Transfer Coefficients for Flowing Packed Particle Beds

Accurate determination of heat transfer coefficients for flowing packed particle beds is essential to the design of particle heat exchangers and other thermal and thermochemical processes. The discrete nature of granular materials alters the thermal transport processes in both the near-wall and bulk regions of flowing particle beds from their stationary counterparts. Heat transfer correlations based on the stationary particle bed thermal conductivity could be inadequate for flowing particles in a heat exchanger. Thermophysical properties of the continuum were often obtained from measurements on stationary beds owing to the difficulty of flowing bed measurements. In this work, we found that the properties of a stationary bed are highly sensitive to the method of particle packing and there is a decrease in the particle bed thermal conductivity and increase in the near-wall thermal resistance, as measured by an air gap thickness, on the onset of particle flow. These variations in the thermophysical properties of stationary vs. flowing particle beds can lead to large errors in heat transfer coefficient calculations. Therefore, we modelled heat transfer coefficients for granular flows using the experimentally determined flowing particle bed thermal conductivity and near-wall air gap for ceramic particles - CARBO CP 40/100 (mean diameter=275 um), HSP 40/70 (404 um) and HSP 16/30 (956 um); at velocities of 5-15 mms-1; and temperatures of 300-650 oC. These calculations, which show good agreement with experiments in literature, can be used as a guide for heat exchanger designs.