FLOW AND WETTING BEHAVIOUR OF FALLING LIQUID FILMS INSIDE INCLINED AND INTERNAL STRUCTURALLY-FINNED TUBES
The objective of the research project is to gain fundamental knowledge of the flow and wetting behaviour
of a falling liquid film on structured surfaces during evaporation. In order to investigate the distribution
and wetting behaviour of liquid films a special test rig was designed and built. This test rig allows to research
the influence of inclined (0° - 20° from the vertical), internally structured tubes and the fluid properties on the stability of the liquid film. Another major research aspect is the investigation of the film stability, the film behaviour during partly film evaporation and the determination of mean heat transfer coefficients. A major advantage of the test rig is the possibility of visual observations of the falling liquid film inside of the tube without disturbing the falling film and the vapour flow. The use of a special camera enables the investigation of the liquid film along the whole length of 2 m. Propane and water are planned as
working fluids covering a wide range of properties (9.55×109 < Ka < 1.47×1012; 1 < Refilm < 1000; 3 < Pr < 7). Details of the test plant and first results will be presented. Previous experiments have shown a break up of the film in vertical and slightly inclined smooth tubes at a wide range of wetting rates. Experiments with structured surfaces showed that a film break up can be avoided even at low wetting rates and 20° inclination by using a suitable structure.