Experimental Study of Differences in CHF between Upflow and Downflow in Vertical Rectangular Channels.Effect of Subcooling.

A detailed understanding of critical heat flux (CHF) for vertical rectangular channels is required for the thermohydraulic design and safety analysis of research nuclear reactors in which flat-plate-type fuel is employed. In this study differences in CHF between upflow and downflow were investigated, focussing especially on channel outlet subcooling, because differences in CHF between upflow and downflow under the subcooled condition at the channel exit had not been fully clarified systematically. With the investigation of the existing CHF data, it was elucidated that there was a systematic tendency in CHF between upflow and downflow under the subcooled condition at the channel exit with demensionless subcooling and coolant mass flux, and a new CHF correlation was proposed for the vertical rectangular channels based on investigation results. The condition that CHF differs between downflow and upflow was also made clear within the range investigated in this study with pressure of less than 4MPa and dimensionless mass flux of less than 2200.