In the framework of CEA RetD program to develop the Advanced Sodium Technological Reactor for Industrial Demonstration (ASTRID), a high efficiency compact heat exchanger technology is developed, to provide solid technological basis for the use of Brayton power conversion system. Multi-channel compact heat exchangers are necessary for the present application because of the low heat transfer capacity of gas foreseen, i.e. nitrogen at 180 bar. On the gas side, the severe constrain of a maximum pressure drop of 1 bar leads to increase the number of channels to feed. On the other hand, a minimum required size of Na channels section due to plugging risk induces very low pressure losses. Considering an additional inlet flow condition, a real risk of bad flow distribution remains. As a result, the thermal performance and thermal loading of the heat exchanger degrades due to it.The present work concerns the program dedicated to the optimization of Na headers in order to have an allowable flow distribution. Turbulent structures in the entrance header, as well as flow distribution in ASTRID large channel bundle are characterized by computational fluid dynamics (CFD) simulations and PIV experiments under similar conditions. Different RANS turbulence model approaches are used to analyze velocity and vorticity fields in a specific header configuration. The comparison with experimental data suggests models taking into account anisotropic effects are more suitable to investigate the complex flow patterns.