CFD Investigations of Subcooled Flow Boiling in a Random Pebble Bed

Abstract

A new concept of nuclear reactors, Pebble Bed Water Reactor (PBWR), has attracted wide attention due to the advantages in the inherent safety performance, high conversion efficiency and low power density design. In this paper, the RPI wall boiling model was applied to solve the local flow and heat transfer of subcooled flow boiling in a random pebble bed with the tube diameter to particle diameter ratio D/d of 4. The effects of inlet temperature and particle diameter on the subcooled flow boiling characteristics were analysed. It was found that the hot spots and bubbles began to appear at the downstream of the contact points between neighbour pebbles or between pebbles and tube wall. With a higher inlet temperature, the onset of boiling occurred earlier and the void fraction near walls increases obviously. With a smaller pebble diameter, the onset of boiling was delayed and the bubbles occupied a smaller fraction of pebble surfaces. This study can provide useful information to the design of the PBWR.