Heat Transfer to Small Cylinders Within a Packed Bed

Christopher Penny

doi:10.32920/ryerson.14665854

Abstract

Heat transfer to small cylinders within a porous media has been experimentally and analytically studied extensively over a varying degree of sample and particle sizes and fluid flow regimes. In general, the observations, trends and empirical correlations developed for these systems do not accurately extrapolate down to small cylinders operating under the packed bed condition. The objective of this research is to develop an empirical correlation that expresses the Nusselt number of small cylinders immersed horizontally within a packed bed subject to forced convection heat transfer, in terms of the pertinent test parameters and material properties. Heat transfer to small cylinders within a porous media has been experimentally and analytically studied extensively over a varying degree of sample and particle sizes and fluid flow regimes. In general, the observations, trends and empirical correlations developed for these systems do not accurately extrapolate down to small cylinders operating under the packed bed condition. The objective of this research is to develop an empirical correlation that expresses the Nusselt number of small cylinders immersed horizontally within a packed bed subject to forced convection heat transfer, in terms of the pertinent test parameters and material properties.A set of seven small cylinders ranging in size from 1.27 to 9.53mm were resistively heated within a 311mm diameter lab-scale packed bed. The porous medium in which the samples were immersed was fine alumina oxide sand, with mean particle sizes ranging from 145 to 33μm. Four separate Type K thermocouples were used to measure temperatures at pertinent locations within the apparatus: bed temperature, inner sample temperature, left and right sample temperatures. The apparatus was operated under flow rates up until incipient fluidization. The trends observed in this research compared well with published data, though the correlations developed from other research consistently under-predicted the heat transfer capacity within the packed bed. The correlation that was developed for calculating the mean Nusselt number was accurate to within ±15% for the entire range of tested and published data.

Highlights

Heat transfer derived from flow through a porous medium has several engineering applications in areas such as industrial and geophysical contexts, thermal insulation techniques, geothermal processes, chemical and nuclear engineering and hydrology applications
Under the packed bed condition the flow rates through the porous medium do not surpass the minimum fluidization velocity
This study extends the range of flow speeds developed within [3], as the physical flow and heat transfer rate characteristics developed for fluidized bed regimes do not accurately extrapolate into the packed bed condition

Summary

Introduction

1.1 INTRODUCTIONThe heat transfer between immersed objects suspended within a porous material has been extensively studied since the nineteenth century due to the efficient and effective heat transfer capacity such a system provides. Under the packed bed condition the flow rates through the porous medium do not surpass the minimum fluidization velocity Under this condition the heat transfer within the bed is much less than that of the same system operating under the fluidized state. The velocity at which incipient fluidization is apparent is likewise defined as the minimum fluidization velocity, Umt· This value is established based on physical properties associated with both the gas and the contained particles This parameter defines the maximum operational velocity to be implemented in this research, as velocities rates below this represent the packed bed condition. The expression used to relate the actual volumetric flow within the system to the displayed standard flow rate was the following, based on material provided by the flow meter manufacturer (TSI):

Objectives

Methods

Conclusion