Formation of confined free film in the window on a vertical perforated plate

Liquid film is a common contact way between gas/liquid and can be divided into wall-bounded film and free film according to the film formation process and the number of free surfaces. When a plate is perforated, both kinds of films exist. This paper describes some experimental observations of free-surface flows arising when a thin liquid film flows through the window on a vertical perforated plate. Dozens of rectangular windows (9~1152mm²) and six fluids (Ka from 52 to 3000) were used to investigate the flow mechanisms. Several typical flow patterns including pass-around flow, pass-through flow, bias flow and back-side flow are well defined under such a flow condition. Here a special focus is given to the window region, where various free-surface flow patterns composed of droplets, columns, sheets and their combinations were observed with increasing flow rate. Meantime, free film in the window is surface tension dominated and susceptible to disturbance, resulting in coalescence or break of liquid columns and films. At a critical flow rate, liquid film is able to full fill the window, forming a stable complete confined free film. Mutual influence between confined free film in the window region and wall-bounded film around shows special wavy trains, which is also called "twin liquid film". Based on experimental data and scaling analysis, an empirical equation which relates Reynolds number Re, Kapitza number Ka and a dimensionless length Nx is proposed to characterize the film formation conditions. It is found that critical film formation Reynolds number increases with Kapitz number and window size. And hysteresis phenomenon is manifested by obviously different flow transition Re for confined free film formation and breaking. The results can help window geometry optimization industrial processes to improve local heat and mass transfer. It can also enrich the traditional film flow investigations.