(An) experimental study of the effects of the mixing vane on the CHF in the low-pressure, low-flow conditions

Abstract

The effects of the spacer grid and mixing vane on the critical heat flux (CHF) have been experimentally investigated under low-pressure, low-flow and high-quality conditions using two types of test sections: an annulus and a round tube. In the annulus test section (0.7m long, 19 mm I.D., 33.5 mm O.D.,) the ring type spacer grid was used to know the variation of the CHF and the first location of the CHF according to the axial location of the spacer grid. The transparent outer tube was used to observe the mechanism of the CHF and the flow patterns when the CHF occurred. In the tube test section (0.7 m long, 10.8 mm I.D.), the split vane type mixing vane was invented and installed in the tube. The test conditions were atmospheric pressure, low mass flux and constant inlet subcooling. Experimental results shows that (1) the spacer grid usually increases the CHF although gives no effect on, or decreases the CHF in some cases, depending on the mass velocity and its axial location, (2) especially in the lower mass flux ($G < 100 \sim 150 kg/m^2s$), the spacer grid or the mixing vane can not increase the CHF, on the contrary it decreases the CHF, (3) the location of the first CHF is usually just upstream of the spacer grid, but it is same to the bare test section``s when the flow is low or the spacer grid is located far from the downstream end of the test section. The major mechanisms of the effects of the mixing vane on the CHF are discussed. They may be (1) directing the entrainments to the heated wall to increase droplet deposition at the wall (an increasing effect), and (2) breakup of the liquid film in annular flow at the wall (a decreasing effect).