The intertube falling film: Part 1—flow characteristics, mode transitions, and hysteresis

When a liquid film falls from one horizontal tube to another below it, the flow may take the form of discrete droplets, jets, or a continuous sheet; the mode plays an important role in the wetting and heat tranafer characteristics of the film. Experiments are reported that explore viscous, surface tension, inertial, and gravitational effects on the falling-film mode transitions. New flow classifications, a novel flow regime map, and unambiguous transition criteria for each of the mode transitions are provided. This research is part of an overall study of horizontal-tube, falling-film flow and heat transfer, and the results may have important implications on the design and operation of falling-film heat exchangers.