Experimental and numerical study on the chf performance of mixing vane

Abstract

In this study, CHF enhancement by various mixing vanes was evaluated and the flow characteristics were investigated through the CHF experiments and CFD analysis. CHF tests were performed using 2x2 and 2x3 rod bundles and R-134a as a working fluid. Test section geometry was the same as that of commercial PWR fuel assembly except the heated length (1.125m) and number of fuel rods. From the CHF tests, it is founded that the CHF enhancement by mixing vanes under higher mass flux $(1400 kg/m^2sec)$ and lower pressure (15 bar) conditions is bigger than the CHF enhancement under other conditions. Among the mixing vanes used in this study, the swirl vane showed the best performance under relatively low pressure (15 bar) and mass flux $(300~1000 kg/m^2sec)$ conditions and the hybrid vane was the best near the PWR operating conditions. It is also found that there exists an optimum angle of mixing vane to maximize the CHF enhancement ratio. Along with the experiments, detailed flow characteristics were investigated by CFD analysis using the same conditions as the CHF tests. For calculation of subcooled flow boiling, the wall partitioning model was applied to the wall boundary and various two-phase parameters were also considered. The reliability of CFD analysis in boiling analysis was checked by comparison of average void fractions of analysis and experiments and they were well agreed. From the CFD analysis, void fraction flattening by the lateral velocity induced by mixing vane was observed. By the lateral motion of liquid, void fraction in near wall was decreased and that of core region was increased: the void fraction flattening. The decrease of void fraction in near wall region promotes liquid supply to the wall and consequently the CHF can be increased. For the quantification of the void flatness, an index was developed and applicability of the index in CHF assessment was confirmed. On the contrary, the critical void fraction achieved by CFD analysis was not...