Experimental study of the internal flow in freezing water droplets on a cold surface

Abstract

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.

Numerical simulation on the freezing of deposited or impinged water droplets on a cold surface

“…This flow is a result of the circulation flow near the contact line and the descent of cool water near the cap of the droplet. This second circulation flow is similar to the internal flow field of freezing water droplets measured by Karlsson et al (2019). The droplets' volumes and the temperature in the present study are lower than the corresponding ones for Karlsson et al's measurement.…”

Influence of Substrate Material on Flow in Freezing Water Droplets—An Experimental Study

“…The experimental setup can be viewed in Figure 2. The set up is similar to the experimental setup presented by Karlsson et al [30,31] with the modification that the laser is now directed from the side instead of below. This was done so that the plates were not limited to transparent materials.…”

“…In the numerical analysis, the Marangoni effect was not included, but they showed that the buoyancy force and the internal natural convection were important for the internal flow. Furthermore, Karlsson et al [30] experimentally showed that there is a turn in the internal flow of a freezing water droplet. A probable cause for this is that the Marangoni effect generates a flow along the surface and up the middle of the droplet in the beginning of the freezing process.…”

“…In Karlsson et al [31], internal flow in both freezing and evaporating droplets was compared and a shift in direction could be observed for both freezing and evaporating droplets. The setup in Karlsson et al [30,31] required a transparent substrate for the laser as well as a thin layer of frost to initiate freezing. In particular, the need to generate a new layer of frost for each case induces an increased possibility of randomness regarding e.g., contact radius and height.…”

“…This paper investigates three areas experimentally; the first step is to reduce uncertainties and randomness in the freezing process by exchanging the frost layer with a mechanically constructed groove on the substrate to control the contact radius and to make the freezing process start immediately after impact; secondly, to modify the set up in Karlsson et al [30,31], to enable opaque substrate materials; and, thirdly, to expand the knowledge of what affects the internal flow by studying how materials of substrates such as copper, aluminium and steel affect the internal flow in a freezing droplet.…”