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ISSN Online: 2377-424X

ISBN Print: 978-1-56700-421-2

International Heat Transfer Conference 15
August, 10-15, 2014, Kyoto, Japan

Heat Transfer and Interaction of Suspended Droplets and Locally Heated Liquid Layer

Get access (open in a dialog) DOI: 10.1615/IHTC15.evp.009523
pages 2375-2386

要約

The heat and mass transfer processes near the liquid-gas interface were studied. A horizontal water layer with the thickness of 200-400 ????m was heated locally by the heater with the diameter of 1mm, located on the layer bottom in the center of a cuvette with the diameter of 50 mm. Liquid evaporates intensively from the region of liquid above the heater. The flow of the vapor-gas mixture moves upward, it is cooled in the ambient air, vapor condenses, and the formed condensate droplets move in the vapor-gas flow. Some droplets, which come into the zone above the heater, can levitate stably above the liquid surface in the ascending flow of the vapor-gas mixture. Formation of hexagonally ordered structure, consisting of numerous condensate microdroplets, known as the “Droplet Cluster” [Fedorets 2004], takes place. The typical diameter of droplets in the cluster is several dozens of microns, the distance from the lower droplet surface to the liquid layer is comparable with the droplet diameter. The condensation growth of droplets occurs with time, and when some critical mass is achieved, the droplets coalesce with the liquid layer, causing cluster collapse. In experiments the process of droplets coalescence with the liquid surface was recorded by a video camera with the speed of up to 20000 f/s and by an IR camera with the speed of up to 1500 f/s for surface temperature measurements. To get the video image, the Schlieren system with reflection was used. Propagation of capillary waves formed on the liquid surface at coalescence of cluster droplets was registered. When a microdroplet merges with the liquid layer, surface energy of droplet ????????D2 converts to the energy of capillary waves. The wavelengths measured in experiments are in the range of 200-300 ????m, velocity of their propagation is 1-2 m/s, and this correlates well with the theory of capillary waves. While propagating, the waves generated by one droplet touch the adjacent droplets, causing their coalescence. This is the domino effect, when coalescence of one droplet causes the snowballing collapse of the whole cluster in a few thousandths of a second. We can also observe interference of capillary waves generated by adjacent droplets. The surface temperature of the cluster droplets was several degrees lower than the temperature of liquid surface above the heater. According to the estimate, the temperature gradient in the vapor-gas mixture above the liquid surface is up to 40 K/mm.