Only a wet tube is a good tube: understanding capillary-assisted thin-film evaporation of water for adsorption chillers. (25th January 2019)

• Record Type: Journal Article
• Authors: Seiler, Jan; Lanzerath, Franz; Jansen, Christoph; Bardow, André
• Is Part Of: Applied thermal engineering. Volume 147(2019)
• Page Start: 571
• Page End: 578
• Publication Date: 2019-01-25
• Subjects: Capillary action -- Copper coating -- Overall heat transfer coefficient -- Tube -- Adsorption heat pump; Heat engineering -- Periodicals; Heating -- Equipment and supplies -- Periodicals; Periodicals; 621.40205
• DOI: 10.1016/j.applthermaleng.2018.08.024
• Languages: English
• ISSNs: 1359-4311

Abstract

Highlights: Novel experiment to study the impact of driving force with continuously varying filling levels. Correlation of surface properties to thin-film evaporation performance. Key variables identified for heat transfer by thin-film evaporation. Dry-out leads to drop in heat transfer at high driving forces. 10-fold increase in heat transfer from uncoated to coated tubes. Abstract: Efficient evaporation of water at low temperatures is challenging due to its low saturation pressure. As a consequence, the preferred evaporation by nucleate boiling can only be achieved at the cost of high superheats. However, low superheats can still lead to efficient evaporation by thin-film evaporation. In this work, we experimentally characterize the heat transfer for thin-film evaporation on coated copper tubes, which use capillary action to create a thin film on their surface. The overall heat transfer through the tubes is determined at all filling levels for evaporator inlet temperatures of 10, 15 and 20 °C with varied driving force. Our experiments reveal that poor coatings suffer from dry-out at high driving forces whereas tubes with good coatings remain fully wetted even at high driving force. Furthermore, we show the impact of surface properties on thin-film evaporation: high porosity, surface extension and roughness promote the creation of a thin film on the tube. Thereby, the heat transfer UA -value is increased up to a factor of 10.