Effect of pebble diameters on the heat transfer characteristics of a structured pebble bed in an HTGR

Investigation on the positions of hot spots appearing in an operating pebble-bed of a high temperature gas-cooled reactor (HTGR) and seeking ways to reduce the possibility of their appearance have attracted scientists' attention. Improving the convective heat transfer coefficient (HTC) of the bed could reinforce heat transferring and thus lower the temperature of the pebbles. In our previous studies, heat transfer characteristics of a face-centered-cubic (FCC) structured pebble-bed (pebble diameter of 12 cm) were discussed. In this study, 3 FCC beds were packed with pebbles of 3 different diameters (10 cm, 12 cm, and 14 cm) and the impact of pebble diameter on the heat transfer coefficient was firstly investigated; then, a small sphere was placed in the pebble-bed packed with 10 cm-diameter pebbles and how the sphere size affecting the heat transfer characteristics was studied. It was found that (1) reducing the pebble diameter improved the heat transfer performances, specifically, the bed with a pebble diameter of 10 cm demonstrated the best heat transfer ability and an enhancement ratio of 10.4% was obtained compared to the bed with 12 cm-pebbles; (2) the average HTC of the bed increased with the inserted sphere size, particularly, comparing to the pebble-bed without a small sphere, 27% enhancement was achieved for the bed packed with 10 cm-pebbles and a 4.14 cm-sphere; (3) A generic correlation of the Nusselt number was proposed as Nu 1/4 0.1941Re(0.8)Pr(0.4)-0.3226(L/D-1.027)(RePr0.4)-Re-2-Pr-0.8. Such findings provide references for reactor designers and will help to develop a safer pebble-bed core. (C) 2020 Elsevier Ltd. All rights reserved.