Investigation of heat transfer enhancement in mixed-sized pebble beds with a variable number of small spheres

Inserting small spheres into a pebble bed can enhance heat transfer, lower the surface temperature of fuel elements, and reduce the possibility of forming local hot spots. However, the reasons for the heat transfer enhancement are still not clearly explained. In response, this study explored the mechanism by numerically investigating the effects of different number of inserted spheres (from one to eight) on the heat transfer performance in a face-centered cubic (FCC) pebble bed. It was found that (1) the average heat transfer coefficient (HTC) value peaks when the number of small spheres is five, showing 7.7 % enhancement compared to the HTC of the pebble bed without small spheres; (2) a maximum of 2.6 % of the total power generation is transferred through small spheres to the coolant, and increases as the sphere number increases. The small spheres vary the flow field in the gap and enlarge the convective surface areas of adjacent pebbles, demonstrating a combined impact on the heat transfer and prompting case 5 (5 small spheres with a diameter of 2.210 cm) to perform best. These findings can help establish the heat transfer characteristics of high-temperature reactor cores and serve as a reference for understanding the mechanism and future pebble-bed core designs.