In the development of spacer grid with mixing vanes, a useful guideline for significant CHF enhancement is greatly valuable to minimize efforts and expenses. A systematic approach was made to draw the information through a series of experiments and CFD analysis. Conclusively, a turbulent flow structure with large swirl motion in a subchannel center would be much desirable rather than that with strong cross flows in the rod gap regions. It was about 26% relatively in terms of CHF enhancement near PWR operating conditions. On the other hand, it turned out that CFD analysis for single phase heat transfer has a large uncertainty to infer the CHF performance due to a neglect of bubble behavior by the strong swirl motion, which affects the CHF deeply.
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