화학공학소재연구정보센터
International Journal of Multiphase Flow, Vol.30, No.7-8, 853-876, 2004
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 (Re-L < 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 Re-L similar to 200, the RMS values of film thickness fluctuations, beyond the wave inception region, tend to be nearly independent of Re-L. Results concerning other typical falling film characteristics also show that above Re-L similar to 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. (C) 2004 Elsevier Ltd. All rights reserved.