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Experimental study of the internal flow in freezing water droplets on a cold surface
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0001-5310-9761
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0001-8235-9639
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0001-8355-2414
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2019 (English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 60, no 12, article id 182Article in journal (Refereed) Published
Abstract [en]

The study of a freezing droplet is interesting in areas, where the understanding of build up of ice is important, for example, on wind turbines, airplane wings and roads. In this work, the main focus is to study the internal motion inside freezing water droplets using particle image velocimetry and to reveal if mechanisms such as natural convection and Marangoni convection have a noticeable influence on the flow within the droplet. The flow has successfully been visualized and measured for the first 25% of the total freezing time of the droplet when the velocity in the water is the highest and when the characteristic vortices can be seen. After this initial time period, the high amount of ice in the droplet scatters the PIV light sheet too much and the images retrieved are not suitable for analysis. Initially, it can be seen that the Marangoni effects have a large impact on the internal flow, but after about 15% of the total freezing time, the flow turns indicating increased effects of natural convection on the flow. Shortly after this time, almost no internal flow can be seen.

Place, publisher, year, edition, pages
Springer, 2019. Vol. 60, no 12, article id 182
National Category
Fluid Mechanics and Acoustics Applied Mechanics
Research subject
Fluid Mechanics; Experimental Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-76844DOI: 10.1007/s00348-019-2823-1ISI: 000497768000004Scopus ID: 2-s2.0-85075101925OAI: oai:DiVA.org:ltu-76844DiVA, id: diva2:1372670
Note

Validerad;2019;Nivå 2;2019-11-25 (johcin)

Available from: 2019-11-25 Created: 2019-11-25 Last updated: 2019-12-10Bibliographically approved
In thesis
1. The Internal Flow in Freezing Water Droplets
Open this publication in new window or tab >>The Internal Flow in Freezing Water Droplets
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Det inre flödet i frysande vattendroppar
Abstract [en]

The aim of this work has been to study the internal flow in freezing water droplets on a cold surface and to investigate the different heat transfer mechanisms involved. This is an interesting topic with a great number of applications, specifically in areas where the prevention of unwanted icing is important, e.g. in the case of airplane wings and propellers, wind turbine rotor blades, and roads surfaces.

Combining experimental and numerical methods, this study uses Computational Fluid Dynamics (CFD) to build a model of the freezing process and Particle Image Velocimetry (PIV) to aid for a better understanding of the freezing process. For the numerical part of the study, a model of a droplet with a rigid boundary was created where only the interior was of interest and different boundary conditions on the droplet surfaces were used to induce a flow inside the droplet. The heat transfer mechanisms studied was conduction, natural convection and Marangoni convection. For comparison, an experimental method was developed to visualize the movement of the water and to estimate the velocities inside the droplet. In order to compensate for the refraction at the droplet surface a velocity correction method was applied. The internal flow in freezing droplets was also compared to the internal flow in evaporating droplets. 

The results show that the freezing time is not affected considerably between experiments and the numerical model when including different heat transfer mechanisms, instead the size and contact angle to the surface as well as the substrate temperature are the largest contributors. The direction of the flow and the velocity of the water are highly dependent on the heat transfer mechanisms and these are more difficult to mimic in the numerical model. In the experimental work it was found that the flow is controlled by Marangoni convection for a short time period in the beginning of the freezing process. After this, natural convection instead dominates the flow. When including only conduction and natural convection in the numerical model it can be seen that the gravity effects are most pronounced around the density maximum for water (at T = 4°C). When introducing Marangoni convection in the model the highest velocities are seen in the beginning of freezing. It was found that neither only natural convection nor only Marangoni convection could in itself describe the flow seen in the experimental work. In previous research it has been shown that Marangoni convection is reduced approximately 100 times in the real water droplets compared to theory. This condition yields the best correspondence between numerical results to the experimental results, although there are still differences that have to be investigated further. For evaporating droplets, the Marangoni convection seems to have a little or no effect on the flow.

The main conclusion is that it is possible to work with a simplified CFD model and still capture the main flow features and freezing characteristics in freezing water droplets. Furthermore, an experimental method for studying the freezing droplets and for comparison of the numerical work has also been constructed with good results. For the future it would be interesting to further develop the CFD model for even better correspondence with the experimental work and to unravel the differences between theory and real droplets.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2020
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-77126 (URN)978-91-7790-515-8 (ISBN)978-91-7790-516-5 (ISBN)
Public defence
2020-02-27, E231, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2019-12-12 Created: 2019-12-10 Last updated: 2019-12-12Bibliographically approved

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Karlsson, LinnLycksam, HenrikLjung, Anna-LenaGren, PerLundström, Staffan

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