Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.
Within the framework of radioactive waste management, the VALIDA program started to provide reliable data for the validation of numerical tools used to model the cooling of spent nuclear fuel containers in dry storage facilities. The design of such facilities implies thermal-hydraulic calculations in order to predict containers and wall temperatures. One has to make sure that these temperatures never exceed critical values. The understanding of mixed-convection flow in realistic conditions and more particularly the interaction between a global cross-flow circulation and local natural convection effects is a key point of these design studies. VALIDA experiments were carried out in this way at CEA on a multi canister configuration (7 rows heated tube bundle mounted vertically) in a special wind tunnel (length:12m, height: 3m width: 2m13) and cooled by a cross-flow air circulation. During the experiments, the air flow rate, the velocity profile and the heating power are controlled and have been adjusted to simulate various thermal-hydraulic conditions. A staggered tubes of seven rows of heated tubes (diameter 0.64m and height 2m) are placed in the wind tunnel, the 18 canisters arrangement use a triangular pitch (P/D = 1.66). Instrumentation includes thermocouples in the air flow, on the cylinders, and on the walls; the wind tunnel is rigged with two air-velocity measurement systems: LDV (Laser Doppler Velocimetry) and PIV (Particle Image Velocimetry). One presents the main experimental results reached with different values of the parameters: air velocity (0.25 to 1 m/s) and power density (300 to 600W/m²). From the downstream air measurements, a visualization of the temperature plume is obtained at different location behind the last tube. Measurements of air velocity are also performed with LDV laser in the air gap above the canisters. All the results show that the flow pattern of air strongly depends on the ratio of the buoyancy to the inertia forces. Convective transfers areas involving predominately forced or natural convection are distinguished thanks to established heat transfer correlations. A dimensionless buoyancy number Bo* is defined to characterize the experimental flow regimes obtained.