We improved the computational grid and schemes in the VOF (volume of fluid) method with the standard turbulent model in our previous study to evaluate CCFL (countercurrent flow limitation) characteristics in a full-scale PWR hot leg (750？mm diameter), and the calculated CCFL characteristics agreed well with the UPTF data at 1.5？MPa. In this paper, therefore, to evaluate applicability of the VOF method to different fluid properties and a different scale, we did numerical simulations for full-scale air-water conditions and the 1/15-scale air-water tests (50？mm diameter), respectively. The results calculated for full-scale conditions agreed well with CCFL data and showed that CCFL characteristics in the Wallis diagram were mitigated under 1.5？MPa steam-water conditions comparing with air-water flows. However, the results calculated for the 1/15-scale air-water tests greatly underestimated the falling water flow rates in calculations with the standard turbulent model, but agreed well with the CCFL data in calculations with a laminar flow model. This indicated that suitable calculation models and conditions should be selected to get good agreement with data for each scale. 1. Introduction Reflux condensation by steam generators (SGs) is considered as one of the possible core cooling methods under hypothetical accident conditions in pressurized water reactors (PWRs). In the reflux condensation, the steam generated in the core and the water condensed in the SG form a countercurrent flow in a hot leg, which consists of a horizontal pipe, an elbow and an inclined pipe. As reviewed by Al Issa and Macian [1], many experiments have been conducted to investigate the countercurrent flow limitation (CCFL) in the hot leg, and empirical correlations were proposed using Wallis parameters [2]. The review showed that many differences between CCFL data were simply due to geometrical effects. To compare CCFL characteristics in hot leg models, Vallée et al. [3] selected three geometrical factors, which were the horizontal pipe length to diameter ratio ( ), the inclined pipe length to diameter ratio ( ), and the elbow angle . They showed that even for similar geometrical factors there was clear deviation between CCFL characteristics due to scale effects. Moreover, effects of fluid properties on CCFL characteristics in a hot leg have not been clearly discussed. Therefore, in order to evaluate effects of scale and fluid properties better, numerical simulation using CFD (computational fluid dynamics) software is expected to be useful. In order to investigate effects of scale and
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