The Impact of Mixing Vane Arrangement on Mixing Coefficient β for Subchannel Analysis

Mixing vanes attached to a space grid play an important role in heat transfer enhancement, thus increasing critical heat flux. Subchannel analysis and computational fluid dynamics (CFD) are usually applied to simulate the coolant flow behavior in a fuel assembly. In subchannel analysis, the mixing effect, mainly turbulent mixing, produced by mixing vane grids (MVGs) is represented by a coefficient β without considering flow direction and mixing vane arrangement. However, in CFD computation, the mixing effect can be simulated more closely. The objective of this paper is to evaluate the mixing coefficient β used in subchannel analysis by a CFD code. Then, the effects of the three MVGs are compared qualitatively and quantitatively.

Through the analysis, an effective mixing coefficient adopted in the subchannal codes should be related to the vane arrangement. Improvements for β are needed to better reflect the true mixing function from the spacer grid relevant to its mixing vane arrangement. Besides the lateral velocity distribution, secondary flow intensity, temperature distribution, and thermal nonuniformity are different for different vane arrangement patterns.