Investigation of natural convection heat transfer to the cooled top boundary of a heated pool

摘要

Experiments have been performed in the framework of the BALl program for the investigation of turbulent heat transport to the upper cooled top boundary of a volumetrically heated pool. A specific LIF (laser induced fluorescence) technique has been used to visualise the plumes detaching from the top boundary. Results concerning flow configurations, detachment time intervals. size of plumes and characteristic distances between plumes will be presented as a function of Rayleigh and Prandtl numbers. A model has been developed for the description of these plumes and the resulting heat transfer. A physical understanding of the effect of Rayleigh and Prandtl numbers on heat transfer has been gained. The so-called soft turbulence regime is associated with negligible interaction between plumes detaching from the boundary and the main flow. In this regime, the Nusselt number is proportional to Ra~(1/3). The so-called hard turbulence regime is associated with an interaction between the plumes and the main flow. The plumes are locally and intermittently destroyed by a laminar boundary layer developing from large eddies issued from the main flow and impacting the boundary. The development of these laminar layers explains the dependency of the Nusselt number as approximately Ra~(2/7) in this regime. For elevated Rayleigh numbers, the increase of the Reynolds number in the main flow explains the development of fully turbulent boundary layers. In the so-called asymptotic regime, experiments by Chavanne (1997) have shown that Nusselt is approximately proportional to Ra~0.4. Effects of the aspect ratio have been reported.