HEAT-TRANSFER IN TURBULENT FALLING FILMS

In condensers with high condensation rates or in falling film evaporators the falling film at the vertical tube wall shows a turbulent flow pattern. For optimal dimensioning of these apparatuses the knowledge and the description of the heat transfer in this turbulent falling film is necessary. Especially in fluids with a low thermal conductivity in liquid phase (i.e. hydrocarbons) the main heat transfer resistance is in the falling film, also if there are several components in gas phase. For calculating the heat transfer usually dimensionless Nusselt equations were used. These equations mainly base on experimental results which were found with water vapour as test fluid. It is shown that these equations fail for fluids with low thermal conductivity resp. high Prandtl numbers. Based on a Nusselt equation for turbulent tube flow a new equation for turbulent film flow is derived. A comparison between calculated and measured data for Prandtl numbers up to 50 shows good agreement.