Evaporation of pure liquids with increased viscosity in a falling film evaporator |
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Authors: | Felix Weise and Stephan Scholl |
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Institution: | (1) Institute for Chemical and Thermal Process Engineering (ICTV), Technical University of Braunschweig, Langer Kamp 7, 38106 Braunschweig, Germany |
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Abstract: | The present study investigated fluid dynamics and heat transfer of viscous pure liquids in a falling film evaporator. This
is of special benefit as it avoids mass transfer effects on the evaporation behaviour. Experiments at a single-tube glass
falling film evaporator were conducted. It allowed a full-length optical film observation with a high-speed camera. Additionally
the evaporator was equipped with a slotted weir distribution device. Test fluids provided viscosities ranging from μ = 0.3
to 41 mPa s. The Reynolds number was between 0.7 and 1,930. Surface evaporation and the transition to nucleate boiling were
studied to gain information about the film stability at maximum wall superheat. A reliable database for laminar and laminar-wavy
viscous single component films was created. The experimental results show a significant enhancement in the wave development
due to the film distribution. A wavy flow with different wave velocities was superposed to the film in each liquid load configuration
without causing a film breakdown or dry spots on the evaporator tube. It was found that nucleate boiling can be allowed without
causing film instabilities over a significant range of wall superheat. |
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