Capillary-assisted thin-film evaporation of water–ethanol mixtures

Abstract

Adsorption chillers often employ the refrigerant water despite its two challenges: (1) The temperature limitation due to its freezing point and (2) the need to evaporate at low pressure for typical cool temperatures in cooling applications. These challenges can be addressed individually: Anti-freezing agents help to overcome challenge (1), while capillary-assisted thin-film evaporation can mitigate the static pressure issue associated with challenge (2). However, the combination of both measures remains to be explored. Hence, this study investigates the heat transfer during evaporation of water as primary refrigerant, ethanol as anti-freezing agent, and their mixtures on finned tubes exploiting capillary action. The results show that capillary-assisted thin-film evaporation can also be exploited for ethanol and water–ethanol mixtures. Ethanol can achieve overall heat transfer coefficient $U$ comparable to water, while $U$-values of water–ethanol mixtures decreased up to 51 %. The most likely reason for the heat transfer deterioration are mass transfer resistances due to increased viscosity of water–ethanol mixtures. These results provide insights into the heat transfer of alternative refrigerants to expand the cooling temperature of adsorption chillers.