Flow visualization with high-speed video of evaporating water films falling over flat horizontal tubes, representative of the external surfaces of microchannel tubes, is presented. Experiments were conducted with 1.4 mm thick and 27 mm tall tubes over a film Reynolds number range of 23 \u3c Re \u3c 126. In addition to a qualitative description of the flow mechanisms, this work quantifies key droplet and wave characteristics using image analysis techniques. A semi-autonomous edge-detection technique is used to develop a mathematical description of the droplets and waves, allowing the surface area, volume, velocity, and frequency of the droplets, as well as the width, surface area, and velocity of the waves, to be measured. The results are useful for developing accurate, phenomena-based models for falling-film evaporation over flat horizontal tube banks
