Characterization of Thin Film Over Vertical Fluted Tube: An Experimental Approach

Abstract

Enhancement of heat transfer coefficient in evaporation processes using axially fluted surfaces compared to smooth surfaces is prominently displayed over several decades. Experimental research shows that with the help of a vertical tube evaporator, high heat transfer coefficient in the range from 10,000 to 20,000 W/m2K can be achieved. Heat transfer coefficient enhancement is closely related to the formation of thin film over the vertical tubes. However, the lower thickness of thin film over vertical tubes causes dry patches during the evaporation process leading to significant salt depositions, thus decreasing the evaporator's overall performance. Limited experimental studies are made to characterize the external evaporating thin-film formation for variation in feed rate, temperature, and tube length. In this paper, an experimental setup is designed to study thin-film formation on a fluted aluminum pipe of 24 mm outer diameter with 21 flutes. A detailed experimental investigation is performed to study the thin-film characteristics with the use of digital imaging. The minimum flow rate to avoid trough flow with no external heat source is recognized as 1 l/m for feed temperature ranging from 22 to 70°C up to 350 mm tube length. This finding is vital for applying the fluted tube in a vertical tube evaporator for water distillation to avoid dry patches contributing to salt depositions.KeywordsVertical fluted tubeExternalEvaporatingThin film