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  • 1.
    Karlsson, Linn
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    The Internal Flow in Freezing Water Droplets2020Doctoral thesis, comprehensive summary (Other academic)
    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.

  • 2.
    Karlsson, Linn
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lycksam, Henrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Ljung, Anna-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Experimental study of the internal flow in freezing water droplets on a cold surface2019In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 60, no 12, article id 182Article in journal (Refereed)
    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.

  • 3.
    Karlsson, Linn
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Ljung, Anna-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Modelling the dynamics of the flow within freezing water droplets2018In: Heat and Mass Transfer, ISSN 0947-7411, E-ISSN 1432-1181, Vol. 54, no 12, p. 3761-3769Article in journal (Refereed)
    Abstract [en]

    The flow within freezing water droplets is here numerically modelled assuming fixed shape throughout freezing. Three droplets are studied with equal volume but different contact angles and two cases are considered, one including internal natural convection and one where it is excluded, i.e. a case where the effects of density differences is not considered. The shape of the freezing front is similar to experimental observations in the literature and the freezing time is well predicted for colder substrate temperatures. The latter is found to be clearly dependent on the plate temperature and contact angle. Including density differences has only a minor influence on the freezing time, but it has a considerable effect on the dynamics of the internal flow. To exemplify, in the vicinity of the density maximum for water (4 C) the velocities are about 100 times higher when internal natural convection is considered for as compared to when it is not.

  • 4.
    Karlsson, Linn
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    A Numerical and Experimental Investigation of the Internal Flow of a Freezing Water Droplet2015Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The overarching aim of this work is to study the freezing process of a single water droplet freezing on a cold surface, which is an interesting and important phenomenon with possible applications in many areas. Understanding the freezing process of a single water droplet is for example an important step when preventing unwanted icing, e.g. in the case of airplane wings and propellers, wind turbine rotor blades, and road surfaces.As a step in understanding the freezing process, the study specifically focuses on the internal flow in the droplet during the freezing process. To do this, the study combines the use of Computational Fluid Dynamics (CFD) to build a model of the freezing process and experimental methods, i.e. Particle Image Velocimetry (PIV) to validate the numerical results. Focus is to start with the heat- and mass transfer inside the droplet using simple geometries with a rigid boundary, not modelling the outside environment as the air and the cooling plate. These components will be incorporated in the model further on.Three papers will be included in the study. In Paper A the CFD model is created and tested on a simple 2D-geometry of a droplet. The numerical result is partially compared to experimental work found in literature. In Paper B the numerical model is developed even further and a more realistic geometry of a real droplet, although with rigid boundaries, is used. The numerical results are as for Paper A validated with experimental results found in literature. In Paper C the internal flow inside the droplet has been investigated experimentally to estimate the velocities in the water, so that in the future the results can be used to validate the numerical work.The results show that is possible to work with a very simple CFD model and still capture the main flow features and freezing characteristics in a freezing water droplet. In line with previous research, this study confirms that the natural convection induced by gravity is significant for the internal flow, as compared to conduction and effects of ice creation. If studying the freezing time the internal flow has little effect. However, when estimating the velocities in the water it is crucial. It can be seen that the gravity effects are most pronounced around the density maximum for water (at T = 4◦C). The experiments show that the method used to study the flow inside the droplet is a working method, and the velocities in the water has been estimated. The next step is to further develop the CFD model and validate the numerical work with the experimental results. An interesting next step is to incorporate a moving interface to capture the volume expansion during the phase change.

  • 5.
    Karlsson, Linn
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Ljung, Anna-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    The impact of a fixed contact angle on a freezing water droplet2015Conference paper (Refereed)
  • 6.
    Karlsson, Linn
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Ljung, Anna-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    The influence of a fixed contact angle on a freezing water droplet2015Conference paper (Refereed)
  • 7.
    Karlsson, Linn
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Ljung, Anna-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Influence of internal natural convection on water droplets freezing on cold surfaces2014Conference paper (Refereed)
  • 8.
    Karlsson, Linn
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Ljung, Anna-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, T. Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Influence of internal natural convection on water droplets freezing on cold surfaces2014Conference paper (Refereed)
1 - 8 of 8
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