Development of a simplified boiling model applied for large-scale detailed two-phase flow simulations based on the VOF method

Abstract

The Japan Atomic Energy Agency (JAEA) is developing an evaluation method for a two-phase flow in the reactor core using simulation codes based on the volume of fluid (VOF) method. However, it is impossible to simulate boiling on the heating surface in large-scale domains, such as fuel assemblies, using this simulation method because simulating boiling based on the VOF method requires fine meshes to resolve the initiation of boiling. Therefore, the JAEA started developing a simplified boiling model (SBM) for the two-phase flow simulation in fuel assemblies. In the SBM, the motion and growth equations of a bubble are solved to obtain their diameter and time length at the detachment, the size scale of which is within/around the calculation grid size of the numerical simulation. This information is given as the vapor injection flow rate for the boundary condition on the heating surface. JUPITER calculates the bubble behavior with a scale of more than several millimeters. Using the developed SBM, this study simulates convection boiling on a vertical heating surface. A comparison between the simulation and experimental results showed good reproducibility of the heat flux and velocity dependency on the passage period of the bubble.