Interfacial temperature measurements in evaporating cooling for different tube orientation in a gravitational field

In this work we report on the Infra-Red temperature measurements of an ethanol evaporating meniscus interface formed at the mouth of a capillary tube of 1 mm internal diameter with changing axis orientation respect to the gravity vector. Five tube orientations have been analyzed, namely: 0degree (vertical tube with meniscus interface facing upwards); 45degree; 90degree (horizontal tube); 135degree; and 180degree (vertical tube with meniscus interface facing downwards). What we found is that when the tube is vertical (with the meniscus facing upwards or downwards), the temperature profiles along vertical diametrical sections of the tube are broadly symmetrical with respect to the tube axis. Instead with the other tube orientations, the symmetry of the temperature profiles found for the vertical tube is lost and the upper parts of the temperature profiles are locally at higher temperature than the lower part. This clearly shows that the physical presence of the tube in the gravitational field distorts the buoyancy driven cold plume in the external airflow that is ultimately responsible for the asymmetry found in the temperature and velocity fields in the internal liquid. We adapted a simplified dimensional analysis for the inclined tube, which was originally developed for isothermal tubes, that also takes into account the changing external tube wall temperature and the predicted results are broadly in line with the experimental trends. The results reported in this experimental study on evaporation are very relevant to engineering applications such as evaporative coolers, vapor compression refrigeration systems, drying of drops, combustion, heat pipes, and porous media.

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