Development and validation of advanced CFD models for detailed predictions of void distribution in a BWR bundle

In recent years, a commonly adopted approach is to use Computational Fluid Dynamics (CFD) codes as computational tools for simulation of different aspects of the nuclear reactor thermal-hydraulic performance where high-resolution and high-fidelity modeling is needed.;Within the framework of this PhD work, the CFD code STAR-CD [1] is used for investigations of two phase flow in air-water systems as well as boiling phenomena in simple pipe geometry and in a Boiling Water Reactor (BWR) fuel assembly.;Based on the two-fluid Eulerian solver, improvements of the STAR-CD code in the treatment of the drag, lift and wall lubrication forces in a dispersed two phase flow at high vapor (gas) phase fractions are investigated and introduced. These improvements constitute a new two phase modeling framework for STAR-CD, which has been shown to be superior as compared to the default models in STAR-CD.;The conservation equations are discretized using the finite-volume method and solved using a solution procedure is based on Pressure Implicit with Splitting of Operators (PISO) algorithm, adapted to the solution of the two-fluid model.;The improvements in the drag force modeling include investigation and integration of models with dependence on both void fraction and bubble diameter. The set of the models incorporated into STAR-CD is selected based on an extensive literature review focused on two phase systems with high vapor fractions.;The research related to the modeling of wall lubrication force is focused on the validation of the already existing model in STAR-CD.;The major contribution of this research is the development and implementation of an improved correlation for the lift coefficient used in the lift force formula. While a variety of correlations for the lift coefficient can be found in the open literature, most of those were derived from experiments conducted at low vapor (gas) phase fractions and are not applicable to the flow conditions existing in the BWRs. Therefore, it is important to extend validity of the current correlations for the lift coefficients to higher void (gas) phase fractions. After investigating the underlying physics and analyzing a large amount of experimental data, an improved model for lift force at different void fraction levels, including large bubbles and slug flow regime, is proposed. The model is implemented in STAR-CD and validated.;The validation of the models is performed against five different experiments, characterized by different geometries at different boundary conditions. Comparison with the already existing models in STAR-CD code is performed and it is found that the newly integrated force models for drag and lift forces leads to more accurate void distribution predictions.