Mini-channel flow experiments and CFD validation analyses with the IFMIF Thermo- Hydraulic Experimental facility (ITHEX)

The design of the IFMIF High Flux Test Module (HFTM) is based on the predictions for the heat transfer in narrow channels conducting helium flow of 50 oC inlet temperature at 0.3 MPa. The emerging helium flow conditions are in the transition regime of laminar to turbulent flow. The rectangular cooling channels are too short for the full development of the coolant flow. Relaminarization along the cooling passage is expected. At the shorter sides of the channels secondary flow occurs, which may have an impact on the temperature field inside the irradiation specimen's stack. As those conditions are not covered by available experimental data, the dedicated gas loop ITHEX has been constructed to operate up to a pressure of 0.42 MPa and temperatures of 200 oC. It's objective is to conduct experiments for the validation of the STAR-CD CFD code used for the design of the HFTM. As a first stage, two annular test-sections with hydraulic diameter of 1.2 mm have been used, where the experiments have been varied with respect to gas species (N2, He), inlet pressure, dimensionless heating span and Reynolds number encompassing the range of operational parameters of the HFTM. Local friction factors and Nusselt numbers have been obtained giving evidence that the transition regime will extend to Reynolds 10,000. For heating rates comparable to the HFTM filled with RAFM steels, local heat transfer coefficients are in consistence with the measured friction data. To validate local velocity profiles the ITHEX facility was further equipped with a flat rectangular test-section and a Laser Doppler Anemometry (LDA) system. An appropriate optical system has been developed and tested for the tiny observation volume of 40 μm diameter. Velocity profiles as induced by the transition of a wide inlet plenum to the flat mini-channels have been measured. Whereas the CFD models were able to reproduce the patterns far away from the nozzle, they show some disagreement for the conditions at the nozzle. Several possible options to cope with this deficiency will be presented. A systematic analysis using STAR-CD between 8 different low Reynolds number turbulence models showed that the k-ε low Reynolds model is most appropriate in describing the transition region. On the basis of these experimental results and analyses obtained so far with ITHEX it can be concluded that STAR-CD and it's k-ε low Reynolds model is validated for the thermohydraulic design of the HFTM. (author)