Falling films on internally cooled horizontal tube banks are widely used in the absorbers of absorption heat pumps. Recent literature suggests that the droplets that form on the underside of the tubes play a large part in the absorption process. Flow visualization of aqueous lithium-bromide solutions falling over 15.9 mm OD tubes using high-speed video is presented here. The results illustrate the characteristic droplet evolution pattern, including the axial elongation along the tube, the formation of a primary droplet, trailing liquid thread, and satellite droplets, and the formation of saddle waves due to the spreading lamellae of the primary droplet impacts. In addition, a new image analysis method is developed to quantify the surface area and volume of the droplets during their formation, detachment, fall and impact. Using a semi-automated edge-detection process, a mathematical description of the interface of the droplets is generated for each frame of a sequence. The results show that the surface area and volume of a droplet between the tubes increase until the primary droplet impacts the tube beneath. The surface area and volume both drop sharply after impact as the main liquid inventory of the droplet joins the film surrounding the tube. As the trailing liquid thread breaks into satellite droplets, the surface-area-to-volume ratio reaches a maximum. These results are useful for developing more realistic models of heat and mass transfer in droplets pendant from horizontal tubes.
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