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.