Verification and validation of CFD and its application in PWR fuel assembly

摘要

Fuel assembly is one of the most important components in Pressurized Water Reactor (PWR). Mixing Vane Spacer Grid (MVG) provides support for the fuel bundle. However, the most important role for MVG is to mix the coolant and decrease the temperature gradient, ultimately enhance the heat transfer and improve Critical Heat Flux (CHF). Hence the performance of MVG are directly related to the safety and economics of PWR. Due to the complexity of MVG and the broad range of thermal hydraulic conditions involved, full scale, time consuming and costly full length rod bundle experiments (usually 5 × 5 or 4 × 4 or 6 × 6) were performed to study the complicated thermal hydraulic phenomenon. Recently, with the development of computer technology, Computational Fluid Dynamics (CFD) has become one of the most popular tools to simulate the flow field of the fuel assembly. In this paper, the method based on CFD to study the MVG performance is reviewed. The main process for CFD in simulating flow field of MVG is divided into two parts. The first part is the verification and validation of CFD codes under single phase-and two-phase flow. The second is the application of CFD for exploring the MVG performance. For the verification of single phase flow CFD simulation, mesh sensitivity, turbulent model effects were examined, the validation including axial pressure drop, lateral temperature distribution, and lateral velocity distribution. For two phase flow verification and validation, the sensitivities of different drag and non-drag forces on void fraction distribution were tested. The experimental data with a tube and an annual flow fields under two phase boiling were chosen to validate the two phase flow simulation and to cover different aspects of internal and external wall boiling effects. The verified and validated CFD codes were then used to study the performance of MVG. Considering the complexity of MVG, a separated effect study approach is presented to evaluate the performance of MVG. Another systematic approach from simple geometry to 5 x 5 MVG model to study the effect of different components, including vane, dimple, and spring were proposed for the simulation of two phase flow. Some indexes were reviewed and used to evaluate the performance of MVG.