Behavior of a falling liquid film of highly viscous fluid in the counter-current flow condition was examined. In experiments, water and silicon oils of 500, 1000 and 3000 cSt were used as the liquid phase and air was adopted as the gas phase. A test section vertically oriented was a circular pipe of 30 mm in inner diameter and 5.4 m in length. Flooding velocities of the air-water system were well correlated with traditional correlations such as the Wallis correlation and the Kamei correlation. However, the flooding velocities of silicon films were greatly lower than the expected. When the effect of the viscosity was incorporated into the Wallis correlation, it predicted the experimental results well. The flooding in the air-silicon system was initiated by sudden growth of a wave on the film as in the air-water system although the film Reynolds number of the falling silicon film was considerably low; 0.02 ∼ 4. A considerable amount of droplets were detected a long time before the initiation of flooding in the air–silicon oil experiments as well as in the air–water experiments. The correlations tested for the onset condition of entrainment gave much higher gas velocities than the measured. Predicted velocities were rather close to the flooding velocities. The falling film thickness was predicted well by applying the universal velocity profile to the film flow over a wide range of a film Reynolds number; ranging from a water film to a 3000 cSt silicon oil film.

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