An experimental study of falling liquid films in countercurrent annular flow in a vertical tube

The hydrodynamic characteristics of a counter-current annular flow with a falling liquid film inside a vertical pipe (50.8 mm ID, 244 cm length) have been studied experimentally over the film Reynolds number (ReL= 4Gamma/mu) range of 1358 to 7845 using kerosene and air as the working fluids. A laser induced photochromic dye tracer technique was used to measure the instantaneous velocity profiles in freely falling liquid films (no gas flow case) and falling films at the onset of flooding with significant interfacial shear. A laser displacement sensor and high speed video photography were also utilized to measure the film thickness fluctuations at two different positions along the test section.;Time-averaged film thickness data were in good agreement with Nusselt's theory in laminar region and other experimental data from the literature in turbulent region. However, the flow visualization work indicated significant differences between the measured velocity profiles and Nusselt's predictions in wavy turbulent films. The wave-induced turbulence produced flat velocity profiles within the large disturbance waves and also affected the flow even in the substrate film depending on the wave amplitude. The large disturbance waves were seen not to transport large fractions of the liquid mass as previously believed.;Measurements at the onset of flooding indicated neither the interfacial wave growth nor reversal in the direction of wave propagation could be responsible for the initiation of flooding. Furthermore, the measured velocity profiles showed only a slight reduction in liquid velocity within a narrow region close to the gas-liquid interface, and no flow reversal in the liquid film was observed at the measurement location for all flooding conditions studied. Thus, flooding in counter-current annular flow is believed to be caused by the gas-liquid interactions and resulting instabilities in the liquid film flow in the liquid inlet or outlet sections, and not in the vertical tube itself.;Flow visualization experiments also revealed the occurrence of circulatory motions with significant velocities normal to the tube wall under the large waves, which may be the dominant mechanism for enhanced wall-to-liquid and interfacial transport phenomena in wavy films.