ISBN 9783843944328

978-3-8439-4432-8, Reihe Thermodynamik

Stefano Ruberto
Experimental Investigation of the Phase Change of Freely Suspended Supercooled Water Droplets

147 Seiten, Dissertation Universität Stuttgart (2020), Softcover, A5

Zusammenfassung / Abstract

The objective of the present thesis is the experimental study of evaporation and sublimation of supercooled and frozen water droplets. To this end, single, freely suspended water droplets were optically levitated in an observation chamber at controlled temperature and relative humidity. Two measurement techniques were used to determine the droplet size. In the case of supercooled droplets, the regular pattern of scattered light was evaluated. Since droplets deform during freezing, the scattered light no longer has a regular pattern. Therefore, they were observed by means of shadowgraphy. However, an investigation of the chaotic pattern of scattered light showed that it still contains plausible size information.

Evaporation rates were measured at three ambient temperatures below zero degree and varying relative humidity. Hereby, it could be shown that the D²-law also applies to supercooled water droplets, although they are in a metastable state. The results show a linear dependence of the evaporation rate on the relative humidity at constant ambient temperature. A comparison of the experiments with a simplified model based on the D²-law and with direct numerical simulations shows good agreement. Sublimating frozen droplets were observed at one temperature and three different relative humidities. At this temperature freezing was initiated by homogeneous nucleation. The shadow images show a deformation of the droplets, which consists in the formation of spikes or bulges. For this reason, only the largest detectable circle was evaluated. Despite the fact that spherical droplets were considered in the numerical simulations, a good agreement between the simulation results and experiments can be stated. Relative humidity was identified as the most influencing parameter. This is reasonable as the phase changes are mainly diffusion driven.