Dominant flow mechanisms in falling-film and droplet-mode evaporation over horizontal rectangular tube banks

•Evaporating water films falling over rectangular horizontal tubes are examined.•Flow visualization data collected for Reynolds number range of 23 < Re < 126.•Droplet and wave characteristics quantified using semi-autonomous image analysis.•Results are useful for developing phenomena-based falling-film evaporation models.

Visualization of evaporating water films falling over flat horizontal tubes, representative of the external surfaces of rectangular microchannel tubes, is presented using high-speed video. Experiments were conducted with a bank of three tubes at a saturation temperature of 17 °C. In addition to a qualitative description of the flow mechanisms, this work quantifies key droplet and wave characteristics using a semi-autonomous image analysis technique that develops a mathematical description of the droplets and waves. This allows the surface area, volume, velocity, and impact frequency of the droplets, as well as the width, surface area, and velocity of the waves to be measured. It was observed that droplet diameters, surface areas, and volumes are smaller than those measured in flow over round tubes, but were not influenced significantly by Reynolds number. The observed roll waves demonstrated similar surface area growth rates throughout their development, with stretched profiles relative to those described in flow over round tubes.