Study on heat transfer behavior in rod bundles with spacer grid

Experiments have been carried out to investigate the heat transfer downstream of spacer grids in a vertical 5 × 5 rod bundle cooled by single phase water. Wide ranges of mass flow rate (Re from 2000 to 30,000) and heating power (25-150 kW/m2) are examined. Buoyancy force as a prominent factor to influence the development of heat transfer downstream of spacers is observed for the first time. For small buoyancy affected conditions, the heat transfer downstream of spacers decay exponentially to the fully developed values, while for highly buoyancy effect conditions, the heat transfer development downstream of spacers shows complicate behaviors. An explanation of the effect of buoyancy force on heat transfer downstream of spacers is addressed. Existing correlations perform unfavorable predictions with current experiments because of the ignorance of buoyancy effect. Based on the current experimental data, a new correlation accounts for the buoyancy effect has been proposed and shows good predictions.