Heat Transfer Coefficients of Moving Particle Beds from Flow-Dependent Particle Bed Thermal Conductivity and Near-Wall Resistance

Determination of heat transfer coefficients for flowing packed particle beds is essential to the design of particle heat exchangers, and other thermal processes. While such dense granular flows fall into the well-known plug-flow regime, 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. As a result, heat transfer correlations based on the stationary particle bed thermal conductivity could be inadequate for flowing particles in a heat exchanger. Earlier works have achieved reasonable agreement with experiments by treating granular media as a plug-flow continuum with a near-wall thermal resistance in series. However, the properties of the continuum were often obtained from measurements on stationary beds owing to the difficulty of flowing bed measurements. In this work, it was 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, measured as an effective air gap thickness, on the onset of particle flow. These variations in the thermophysical properties of stationary and flowing particle beds can lead to errors in heat transfer coefficient calculations. Therefore, the heat transfer coefficients for granular flows were calculated using experimentally determined flowing particle bed thermal conductivity and near-wall air gap for ceramic particles -CARBOCP40/100(275 um), HSP40/70(404um) and HSP16/30(956um); at velocities of 5-15mms-1; and temperatures of 300-650C. The thermal conductivity and air gap values for CP40/100 and HSP40/70 were further used to calculate heat transfer coefficients across different particle bed temperatures and velocities for different parallel-plate heat exchanger dimensions.