Flow pattern transition of twin-liquid films with countercurrent gas shear

The flow patterns of twin-liquid film, which combine wall-bounded film supported by a solid wall and confined-free film through the opening window, have been investigated by high-speed cameras. The effects of window length, fluid viscosity, and surface tension on flow patterns have been investigated with the variation of liquid Reynolds number (ReL) and gas velocity (U). The experiments revealed a spectrum of confined-free film flow patterns, including droplets, columns, films, and bubbles. It has been established that gas shear stress exerts negligible influence on twin-liquid films at gas velocities below 3 m/s. With increasing gas velocity, the confined-free film exhibits periodic oscillation due to the gas shear and results in a periodic flow rate distribution on the wall beneath the window, which induces the periodic large waves of the wall-bounded film and bubbles formation downstream of the window. Ligament breakup at lower ReL and bag breakup at higher ReL (with a gas velocity exceeding 7 m/s) are identified as two primary breakup modes of the twin-liquid film. Based on the diverse film flow patterns, two correlations correlating the flow pattern transitions of the twin-liquid film with gas velocity, liquid properties, operating conditions, and window geometry have been established.