Heat Transfer Performance for a Falling-Film on Horizontal Flat Tubes

Local and average heat transfer behavior for a falling film on horizontal flat tubes is explored through an experimental approach. Experiments are conducted using water, ethylene glycol, and their mixture (50% by volume) under different heat fluxes and tube spacing, with a range of flow rates that covers all flow modes. It is found that the local heat transfer coefficient decreases with distance from the top of the tube. The distribution of the heat transfer coefficient along the axial direction depends on the flow mode: it is constant for the sheet mode, shows small variations for the jet mode, and has variations as large as 20% for the droplet mode. Heat flux has almost no effect on the average Nusselt number within the experimental range. The average Nusselt number for the flat tube is close to that for round tubes in the droplet flow mode, however, in the jet and sheet modes the flat-tube Nusselt number is much larger than the round-tube Nusselt number. Boundary-layer theory is used to explain the local heat transfer coefficient distribution and the experimental data show good agreement with the boundary-layer theory for most cases. New curve fits for the average heat transfer coefficient for three flow modes at different tube spacing are provided and the maximum deviation of the data from the fit is less than 14%.