Characteristics of developing free falling films at intermediate Reynolds and high Kapitza numbers

Experiments on developing free falling films have been carried out in a vertical rectangular channel, using smooth inlet conditions, with three liquids characterized by high Kapitza numbers, Ka (i.e. water, 1.5% butanol and 2.5% butanol solutions with surface tension 75, 50 and 40 mN/m, respectively) at intermediate Reynolds numbers (ReL<400). Film thickness measurements supported by visual observations suggest that reduced surface tension significantly affects the inception of waves at the entrance region of the film. However, no significant qualitative difference is observed concerning developing wave patterns in the longitudinal direction, as large “tear-drop” type waves appear downstream in all cases, with the smaller Ka liquids being more unstable as expected. The new data show that above ReL∼200, the RMS values of film thickness fluctuations, beyond the wave inception region, tend to be nearly independent of ReL. Results concerning other typical falling film characteristics also show that above ReL∼200 (for the ReL range of the present experiments) the wave structure is weakly dependent on ReL beyond the wave inception region. The dominant disturbance frequencies on the wavy film surface as well as the wave celerity are in good agreement with earlier findings. Finally, results from previous linear stability analyses are compared with the new data showing in general satisfactory agreement especially with measurements corresponding to developing nearly two-dimensional waves.