Study of natural convection around a vertical heated rod using PIV/LIF technique

The Nuclear Training Reactor of the Institute of Nuclear Techniques (Budapest University of Technology and Economics, Hungary) is a pool-type reactor, with light water moderator. The maximum thermal power is 100 kW. The reactor core consists of 369 fuel elements (10 mm of diameter, 590 mm of length), which are arranged in a square lattice. The fuel rods have an active length of 500 mm and an inactive length both in the upper and the bottom part of the fuel rods. The fuel elements are cooled by natural convection of the water. In certain operation states the cooling system pumps colder water below the core, but even in these cases natural convection ensures the refrigeration of the fuel rods. The reactor pool contains 8.5 m3 of water, which is responsible for the cooling of the fuel rods and the biological shielding. Investigation of reactor excursion scenarios showed that the process of the excursion and the maximum energy release are determined basically by the efficiency of the natural convection around the rods, the velocity of the cooling water flow and the heat transfer coefficient. These processes were investigated using an electrically heated rod, which models the geometry of the fuel rods in the training reactor. The active length of the model is the same as the active length of the real fuel rods. The electric power of the model rod can be variable between 0-500 W. The rod is placed in a glass tank which has the shape of a square-based prism. The height of the tank is 1 m, and the dimension of the square is 0.15 m x 0.15 m. PIV (Particle Image Velocimetry) and LIF (Laser Induced Fluorescence) techniques were used to study the velocity and temperature field of the natural convection around the electrically heated vertical rod. Several experiments were made with different amount of heating and different positions of the detected area. The PIV measurements gave us detailed, 2D images about the velocity field, as the equipment was installed in such way that the middle plane of the rod could be detected. The LIF measurements provided - after a complicated calibration and data processing - the temperature field of the water on the same plane. A flow near the rod was observed and fully analyzed. The results allowed us to investigate the laminar and turbulent boundary layers. The velocity of the flow next to the rod had a maximal value around 0.02 m/s, when the electric power of the rod was set to 100 pc. The flow had a turbulent regime in this case, its pattern showed a significant, vertical stream with vortexes near the rod. The natural convection was less dynamic with low settings of the electric power. CFD modelling of the measurements were also carried out using ANSYS CFX. The aim of the study was to give an experimental support for the safety analyses of the Training Reactor and a basis for the validation of the CFD models investigating natural convection. (authors)