IMPROVEMENT OF MIXING VANE CROSSFLOW MODEL IN SUBCHANNEL ANALYSIS

摘要

Subchannel analysis is widely used for development of CHF correlations and prediction of CHF values and locations; however, the current subchannel codes usually neither reflect the contribution of spacer grid mixing vanes on local crossflow, nor do they present the difference between various types of mixing vanes and their influence on flow field. The lack of crossflow modeling leads to major challenge on predicting CHF values and CHF locations in an axially non-uniform heating system, such as the reactor core. In a high axial power peaking system, not only the prediction of CHF value, but also the prediction of CHF location are critical in properly providing safety margin. Therefore, it is necessary to improve the mixing vane crossflow model in subchannel analysis. This paper presents the improvement of mixing vane crossflow model by applying the Distributed Resistance Method (DRM). The term of mixing vane resistance is expressed in momentum equations of subchannel analysis, and the new equations are solved to obtain the influence of mixing vanes on local pressure drop and crossflow. The improved (with DRM) model is applied to subchannel code COBRA-Ⅳ to analyze thermal hydraulic performance in a 5×5 rod bundle with one spacer grid having classical split vanes. The results are compared with those of numerical analysis available in literature. The results show that mixing vanes not only cause increase of local pressure drop in subchannels, but also cause increase of local crossflow in gaps among fuel rods, which reduces modeling uncertainties in lumped parameter codes. The analysis indicates that the improved mixing vane crossflow model performs satisfactorily predicting the most important qualitative trends for flows in rod bundle with spacer grid having mixing vanes, therefore it could be used as an approach to optimize the parametric studies for grid design.