Experimental investigation of reflood heat transfer in the wake of grid spacers

A transient thermal-hydraulic experiment over realistic ranges of system parameters in a simulated fuel rod bundle is reported in the hope of gaining an insight into the most important heat transfer mechanisms which are responsible for the development of the cladding temperature transient. The enhanced mist cooling downstream of grid spacers has been identified as one of such mechanisms on the assumption of the entrainment of small droplets generated from large droplets in the flow which are intercepted by the grid spacers located at distributed intervals along the whole length of the bundle. This concept is supported by reported results of a recent LDA velocity and size measurement of droplet size-population distributions across a grid spacer plate. A suggested form of correlation for enhanced droplet heat transfer can lead to results readily useful for incorporation in the existing analytical codes for the improved prediction of thermal-hydraulic behavior of LOCA reflood of pressurized water reactors