EFFECTS OF THE SPACER AND MIXING VANES ON CRITICAL HEAT-FLUX FOR LOW-PRESSURE WATER AT LOW-VELOCITIES

The effects of spacer grids and mixing vanes on critical heat flux (CH F) have been experimentally investigated for low-pressure water at low , using two different types of test sections: an annulus (19-mm I.D., 33.5-mm O.D., 0.7-m long) and a round tube (10.8-mm I.D. and 0.7-m lon g). An obstruction-type spacer and a split-vane-type mixing vane were used for the annulus and round tube test sections, respectively. The s pacer or mixing vane generally increase the CHF for flow rates higher than a threshold; however, they do not affect or even decrease the CHF under lower flow rates. The major contributing mechanisms would be (a ) directing entrained liquid droplets to the heated wall (positive eff ect) and (b) break-up of the liquid film in annular flow (negative eff ect).