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

: Seiler, J.; Volmer, R.; Krakau, D.; Pöhls, J.; Ossenkopp, F.; Schnabel, L.; Bardow, A.


Applied thermal engineering 165 (2020), Art. 114620
ISSN: 1359-4311
Journal Article
Fraunhofer ISE ()

Capillary-assisted thin-film evaporation is a promising approach to overcome the heat transfer challenges associated with sub-atmospheric evaporation of water in refrigeration applications. Consequently, thin-film evaporation is currently studied in several labs. However, so far it has been unclear whether results from different labs can be compared. In this work, we therefore investigate – for the first time – the impact of experimental setups and procedures on sub-atmospheric capillary-assisted thin-film evaporation of the refrigerant water and deduce methodological recommendations for improving reliability and comparability of measurement results. We present results of evaporation from finned copper tubes with decreasing filling levels from two different experimental setups:
At higher driving force, the resulting dynamics of logarithmic mean temperature differences, heat flows and overall heat transfer coefficients determined in both setups are generally in good agreement but absolute values show deviations of up to 24%. For lower driving forces, the results are identical within uncertainty of measurement. We conclude that the impact of experimental setup is important when comparing absolute values from different setups, but obtaining comparable results is generally possible.
Furthermore, we compare experiments with continuously decreasing versus constant filling levels and show that both procedures yield the same results within measurement uncertainty. With decreasing filling level, however, overall heat transfer coefficients are systematically 5–10% higher. Thus, experiments with continuously decreasing filling level are well-suited for a fast analysis of all filling levels in a single experiment.