Heat thermal structure in the interfacial boundary layer measured in an open tank of water in turbulent free convection

The thermal structure in the boundary layer and its relation to the heat flux from the cooling and evaporating surface of a deep tank of water are investigated. When a deep layer of water in contact with still air above loses heat to the air, the cooled water in a region just under the surface converges along lines and then plunges down in sheets. These sheets of falling water dissipate as they move into the body of the water, which is in turbulent motion. The vertical profiles of the horizontally averaged temperature and its standard deviation agree fairly closely with theoretical profiles based on time averages of the solution to the heat diffusion equation. The differences between observed and thus predicted profile shapes are consistent with the expected effects of the falling cold thermals and the warm return flow, which are neglected in the theories. The profiles of the standard deviation have large values up to the interface and lie between predictions based on boundary conditions of constant surface temperature and constant heat flux, in keeping with the experimental conditions. The relation between the net heat flux and the temperature difference across the boundary layer is given in non-dimensional form by N = 0sdot 156 R 0sdot 33 , which is in good agreement with the asymptotic similarity prediction N vprop R 1/3 but lower than theoretical calculations of the upper bound of N vs. R .