The aim of this work is to investigate experimentally the increase of mixing phenomenon in a coolant flow in order to improve the heat transfer, the economical operation and the structural integrity of Light Water Reactors-Pressurized Water Reactors (LWRs-PWRs). Thus the parameters related to the heat transfer process in the system will be investigated. Data from a set of experiments, obtained by using high precision measurement techniques, Particle Image Velocimetry and Planar Laser-Induced Fluorescence (PIV and PLIF, respectively) are to improve the basic understanding of turbulent mixing phenomenon and to provide data for CFD code validation. The coolant mixing phenomenon in the head part of a fuel assembly which includes spacer grids has been investigated (the fuel simulator has half-length of a VVER 440 reactor fuel). The two-dimensional velocity vector and temperature fields in the area of interest are obtained by PIV and PLIF technique, respectively. The measurements of the turbulent flow in the regular tube channel around the thermocouple proved that there is rotation and asymmetry in the coolant flow caused by the mixing grid and the geometrical asymmetry of the fuel bundle. Both PIV and PLIF results showed that at the level of the core exit thermocouple the coolant is homogeneous. The discrepancies that could exist between the outlet average temperature of the coolant and the temperature at in-core thermocouple were clarified. Results of the applied techniques showed that both of them can be used as good provider for data base and to validate CFD results.

A coolant mixing investigation in a head of a half-size model of VVER-440 fuel assembly (simulator) has been performed at KFKI. The PIV and PLIF measurements have been done under a selected list of power distribution options, flow rates and powers. The experiments were focused on obtaining a data for investigating the trends in temperature difference between the value registered by a thermocouple and that obtained using PLIF technique. The coolant temperature distribution has been measured in many positions along the coolant trajectory and where coolant flow leaves the rod bundle and in the cross section location of thermocouple, thus the dynamics of effect of mixing process is also declared. PIV and LPIF results show their ability to verify the primary results of CFD calculations.