Experimental Investigation of the Internal Flow in Evaporating and Freezing Water Droplets
Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
This report presents a fundamental study on the flow inside water droplets as they evaporate or freeze. The internal flow is visualized with the particle image velocimetry (PIV) method, and the results are analyzed from the viewpoint that thermal gradients in one way or another induces the flow. Light refraction at the droplet surface causes a distorted flow field, which is corrected by using an algorithm based on the ray tracing method.Room temperatured 10 μL droplets (base diameter of about 3-4 mm) are evaporated on two different surfaces; sapphire and polycarbonate. The surface temperature is varied between 40, 50 and 60 ºC. The results show that the internal velocities are greater when the droplet lies on the sapphire plate, which has higher thermal conductivity than polycarbonate. The velocities also increase when the plate temperature is increased. The flow is mainly directed upwards through the droplet symmetry axis and downwards along the surface. This implies that natural convection is the dominating mechanism of the internal flow in evaporating droplets.The same droplet volume is examined in the freezing experiments. The surface material is sapphire and the plate temperature is varied between -10.0 and -12.8 ºC. Two different types of freezing are observed. Either the droplet starts to freeze the moment it touches the cold surface, and the ice climbs from the droplet bottom to the top, or the droplet stays in the liquid phase for some time even after it is placed on the surface. Once the droplet starts to freeze, the droplet surface freezes first, in the wink of an eye, followed by ice climbing from the bottom to the top inside the droplet. Since the droplet surface freezes first, no internal flow can be seen. The internal flow in the first mentioned type of freezing is directed upwards along the centerline and downwards along the surface, which indicates thermo-capillary convection.The main differences between the internal flow in freezing and evaporating water droplets are the velocity magnitudes and the locations of the velocity maxima. Up to five times higher velocities are found inside freezing water droplets. The highest velocities are found close to the surface in freezing droplets, while in evaporating droplets, the velocity maximum is located on the symmetry axis.
Place, publisher, year, edition, pages
2016. , 45 p.
IdentifiersURN: urn:nbn:se:ltu:diva-52237Local ID: 95eadc1f-9df4-4db6-ae14-d97fe5c06cdcOAI: oai:DiVA.org:ltu-52237DiVA: diva2:1025607
Subject / course
Student thesis, at least 30 credits
Engineering Physics and Electrical Engineering, master's level
Validerat; 20160624 (global_studentproject_submitter)2016-10-042016-10-04Bibliographically approved