Capillary-assisted evaporation of water from finned tubes: Impacts of dynamics and experimental setups

Capillary-assisted thin-film evaporation has attracted increased attention recently to efficiently evaporate the natural refrigerant water at low pressures. Although capillary-assisted evaporators have been studied in many publications, it remained unclear if published results can be compared. Respective measurements are often conducted under different conditions in terms of setup, procedure and evaluation, and potentially important factors are not specified or disregarded. In this work, we compare experimental setups and procedures. Experiments with finned copper tubes are conducted at RWTH Aachen and Fraunhofer ISE. A set of requirements is identified to ensure good agreement of the overall heat transfer coefficients. Major requirements are the specification of the heat exchanger’s surface properties, analysis of measurement uncertainty, well-specified input conditions, and control of non-condensable gases (NCG). Furthermore, dynamic experiments with continuously decreasing filling levels prove to be well-suited to quickly assess the heat transfer at all filling levels in a single experiment. Thus, this work identifies approaches for fast, reproducible and comparable characterization of capillary-assisted evaporation