Parameter Sensitivity of CFD Model of Subcooled Flow Boiling in Tube

The critical heat flux is essential to the operational safety of nuclear power plants, and it plays a vital role in the thermal-hydraulic and structural design of a fuel assembly. To improve the heat transfer efficiency of the reactor fuel assembly, it is necessary to accurately calculate the twophase flow boiling characteristics and the critical heat flux in the fuel assembly. Accurate simulation of subcooled flow boiling in simple geometry (such as round tubes, annular tubes, and rectangular channels) can lay a foundation for the research of boiling crisis phenomenon in complex structures (such as fuel rod bundle). Compared with singlephase flow, mass, momentum, and energy transfer process occur at the interface of twophase flow. In addition to the conventional turbulence model and grid model, the interactions between liquid and vapor phases need to be characterized by various interphase forces. Furthermore, the wall boiling model is needed to describe the distribution of heat flux at the heating surface, which includes several empirical auxiliary models. Accurate simulation of flow boiling phenomenon requires the further study of many sub-models in numerical calculations. In this study, the commercial computational fluid dynamics package STARCCM+ based on the Eulerian twofluid model combined the wall boiling model was used to simulate the subcooled flow boiling in a vertical tube, and obtain the distribution of wall temperature, mainstream temperature and void fraction. Based on the experimental results, the sensitivity analysis of the parameter settings of the grid models, turbulence models, boiling models and interphase force models was carried out. The results demonstrate that for Eulerian twofluid model, with the increase of the number of grids, the results are not more accurate. The height of the first layer of grid on the heating surface has a significant effect on the calculations. By setting different first layer grid heights for comparison, it is found that when the grid height is low enough to make the minimum y+<40, the local parameters in flow boiling calculation would change dramatically (especially the void fraction near the wall), which would affect the stability of numerical calculation. The calculation results under different turbulence models and drag force models have relatively small differences while the turbulent dissipation force and lift force in the nondrag force have a greater impact on the results. The combination of Li Quan or HibikiIshii nucleation site density model and Kocamustafaogullari bubble departure diameter model can accurately calculate the wall temperature and void fraction. This study can provide more references for the numerical simulation of subcooled flow boiling in the fuel assembly.