Numerical simulations for effects of pipe size on countercurrent flow limitation in slightly inclined pipes

Under postulated accident conditions in a pressurized water reactor (PWR) such as loss-of-RHR (residual heat removal systems) during mid-loop operation, steam and condensate water form countercurrent flows in a hot leg and a pressurizer surge line, so that countercurrent flow limitation (CCFL) may occur. For CCFL in the hot leg, we measured CCFL characteristics in a 1/15-scale model using air and water [1], carried out numerical simulations for a full-scale hot leg using a volume of fluid method (VOF), and derived a CCFL correlation [2] using Wallis parameters [3]. For CCFL in the surge line (consisting of a vertical pipe, a vertical elbow, and a slightly inclined pipe with elbows), we measured CCFL characteristics in a 1/10-scale model using air and water [4]. However, the layout of the surge line is different in each PWR plant and a generalized method to predict CCFL characteristics in the inclined pipe with elbows is necessary. Therefore, we did one-dimensional (1D) computations [5] and three-dimensional (3D) numerical simulations [6] for the 1/10-scale air-water experiments [4] to validate the 1D computation and 3D simulation. In this study, we did 1D computations and 3D simulations for the 1/10-scale and full scale models to confirm effects of the pipe size on CCFL characteristics. Working fluids in the computation were air and water at room pressure and temperature and these conditions allowed us to evaluate pure effects of the pipe size.