Heat transfer phenomena in flow through packed beds

Abstract

In order to simulate the thermal-fluid performance of a pebble bed reactor such as the PBMR, heat transfer phenomena in packed beds must be characterized. In the pseudo-heterogeneous simulation approach that is often employed, the bed is not modeled as a single lumped entity but rather is discretized into control volumes, each with a given homogeneous porosity. Therefore, the Nusselt number characteristics for pebble-to-fluid heat transfer must be investigated for homogeneous porosity packed beds. The purpose of this study is to measure the heat transfer coefficient (Nusselt number) for pebble-to-fluid convection heat transfer for a given set of discrete homogeneous porosities and then compare it with existing correlations. A literature study was conducted and it was found that several heat transfer phenomena exist in a packed bed and that in order to obtain useful results it is necessary to isolate connective heat transfer from conduction and radiation heat transfer. Convective heat transfer for packed beds can be divided into the following two divisions, namely: • Pebble-to-fluid heat transfer; • Wall-to-fluid heat transfer; In this study the heat transfer coefficient (Nusselt number) for pebble-to-fluid convection heat transfer is measured for three discrete homogeneous porosity test sections ( 0.36; 0.39; 0.45) that form part of the PBMR High Pressure Test Unit (HPTU). As part of the experimental procedure the standard uncertainty due to the instrument inaccuracies were determined. Data from the physical tests was systematically processed to obtain results of Nusselt number as a function of Reynolds number. From the processed data the relevant non-dimensional parameters could be plotted along with the standard uncertainty in each data point. Repeatability of the data as well as the comparison of the data with correlations from the literature survey is also done and graphically illustrated. From the results it can be concluded that the HPTU test facility provides good quality results with high repeatability and relatively low uncertainty. The maximum standard uncertainty of 10.88% implies that the data measured on the HPTU is reliable. However, significant differences were found in the values measured for the homogeneous porosity test sections versus that of randomly packed beds that were employed in studies by other authors.