Numerical and experimental study of stagnant effective thermal conductivity of a graphite pebble bed with high solid to fluid thermal conductivity ratios

Abstract

In this work, a numerical method combining conduction and radiation is used to study the stagnant effective thermal conductivity of a simple cubic packed bed with high solid to fluid thermal conductivity ratios. The experiment is designed to verify the numerical model and the numerical results agree well with the experimental results. It is found that the effective thermal conductivity accounting for conduction and radiation declines first until 690 K as the temperature rises and then increases. This trend is a result of the interactions between different heat transfer components. Compared with the numerical values and experimental values, the ZBS correlation with a proper value for the empirical parameter φ selected is capable of predicting the stagnant effective thermal conductivity in the bulk region. Lastly the different heat transfer components caused by the solid/fluid conduction, contact conduction and radiation are extracted from the total effective thermal conductivity and their individual contributions are analyzed. The contact conduction makes a major contribution to the effective thermal conductivity but gets weaker as temperature rises. The radiation overtakes contact conduction after 920 K. The solid/fluid conduction has little influence on the effective thermal conductivity.