A numerical and experimental study of breakdown of falling film on horizontal circular tubes

Abstract

Abstract Falling film horizontal tube heat exchangers are used in several industrial applications such as sea water desalination, multi effect distillation, chemical industries, ocean thermal energy conversion systems and refrigeration. Different flow patterns that occur during falling film evaporation on an array of horizontal tubes are droplet, droplet-columnar, columnar, columnar-sheet and sheet modes. From the heat transfer point of view, a sheet mode (continuous falling film spanning the intertube space) is the most desirable falling-film mode. Falling film breakdown leads to the occurrence of dry patches on tubes which cause heat transfer deterioration. In the present study, falling film breakdown on horizontal tubes under adiabatic condition has been investigated both numerically and experimentally. The study focuses on the influence of tube pitch to diameter ratio on the threshold film Reynolds number required to maintain continuous falling film. Results indicate that the threshold film Reynolds number increases with the increase in the pitch to diameter ratio for a given diameter of the tube. The study also shows that as the number of tubes in the column increases, the threshold film Reynolds number required to maintain continuous film over all the tubes increases. The study highlights the need for correlations for the column-sheet mode transition that take into account the effect of number of horizontal tubes in a vertical array.