Hydrodynamic characterization of a single-phase turbulent flow in a mixing tee junction

Mixing tee junctions are parts of the residual heat removal system (RHRS) of a nuclear pressurized water reactor. When mixing hot and cold flows in such a junction, temperature and velocity fluctuations downstream of the tee are responsible for initiation and propagation of cracks in pipe walls: it is the thermal fatigue phenomenon. To better understand its mechanisms, hydrodynamic and thermal loadings can be investigated separately. At CEA-Grenoble, the HYPI facility is aimed at studying local hydrodynamic characteristics of the single-phase turbulent flow in a test-section which geometry is representative of a RHRS tee junction. The choice of a 1/3 scale led to use 52.5 mm in diameter pipes. The main branch of the tee is horizontal, while the side arm is vertical, oriented upwards. Operated with water in isothermal conditions (30 deg. C), a representative outlet Reynolds number of 6.2 105 is investigated, with 20% of the total flowrate coming from the side branch. The test-section is built in plexiglass, allowing flow visualization and laser-based measuring techniques such as LDV and PIV. Upstream, inside and downstream of the junction, average and fluctuating components of the velocity are measured, providing 3D maps of the flow field. A spectral analysis of the velocity fluctuations exhibits dominant frequencies characterizing the hydrodynamic load on the pipe. The objective of this detailed experimental database is now to perform comparisons with LES-based computational code. (authors)