Abstract
A major function of Mixing Vane Grids (MVGs) in Light Water Reactors (LWRs) is mixing. Strong mixing results in more uniform coolant temperature distribution, which often leads to improvement in Critical Heat Flux (CHF). In subchannel analysis, a typical mixing coefficient β is often used to represent MVG mixing performance. Despite of this, due to the periodical mixing caused by MVGs with complex geometry under a wide range of operating conditions, a simple constant mixing coefficient cannot accurately reflect the true characteristics of MVG. In order to explore the fundamental mixing characteristics introduced by mixing vanes, a 2×1 subchannel system was selected to study the mixing mechanism. Due to the wall effect, different boundary settings in the 2×1 subchannel gaps were analyzed and compared with 5×5 simulation results to get the most similar simulation results. To evaluate the effect of grids on mixing, the temperature distributions and thermal non-uniformity (Et) between bare rod bundles and grid-enclosed rod bundles were compared. A new concept of local mixing and subchannel mixing was proposed, by comparing the local Et (defined based on mesh size used in CFD calculation), the subchannel Et (defined based on the subchannel area) and temperature contours. In subchannel analysis, lumped parameters could not accurately represent the true mixing caused by mixing vanes. They overestimated the mixing effect and neglected the important near wall mixing phenomenon.
Published Version
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