Effects of the spacer and mixing vanes on critical heat flux for low-pressure water at low-velocities

Abstract

The effects of spacer grids and mixing vanes on critical heat flux (CHF) 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 long). An obstruction-type spacer and a split-vane-type mixing vane were used for the annulus and round tube test sections, respectively. The spacer 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 effect) and (b) break-up of the liquid film in annular flow (negative effect).