Investigation of coarse-grid CFD approach for nuclear engineering application

In this thesis, an innovative coarse grid CFD approach is developed that aims to exploit the capabilities of sub-channel codes and CFD methods while overcoming their limitations. In the approach, a very coarse mesh is implemented in the CFD software OpenFOAM and a new wall treatment, based on the traditional concept of the wall function, is applied to the wall boundary conditions of the domain to take into account the low resolution of the grid which does not allow to effectively capture the effect of the solid walls on the thermo-hydraulics of the flow. To investigate the performance of the new approach, the method is implemented first in three simple test cases for which the sub-channel codes are the state-of-the-art thermo-hydraulic analysis since they are single-phase flow problems in which there are no prevailing 3D flow conditions. An additional test case representing a 2x2 fuel bundle with three full-length rods and one half-length rod is investigated to verify the behavior of the new approach in cases where secondary flows are present. The results for the pressure fields are compared with the analytical pressure profiles for the four test cases that well represent the ones that would be obtained with sub-channel code analysis, while the results for the wall shear stresses obtained in the four test cases are compared with the ones obtained with a more refined mesh in which the traditional wall function approach is implemented since they should be the best estimation of the actual wall shear stresses at the wall domain. For the first two cases, the developed approach produces reasonable results with a good agreement to the analytical pressure profiles while the other two test cases show that the methodology has a limited applicability and, before proceeding with the extension of the new approach to single-phase problems with 3D prevailing phenomena and two-phase problems, it is necessary to solve the issues that emerge for some types of cases.