In the atmosphere, water droplets which may stay liquid although cooled down to temperatures of approx. -20°C are present. Due to the temperature range within which they exist, aircraft pass clouds of these droplets especially during their starting and landing. The impingement of the droplets onto the cold aircraft parts triggers the freezing process of the supercooled water, which results in the build-up of ice layers on top of the aircraft’s surface, as shown in Figure 1 for an ice accretion at the leading edge of an airfoil and in Figure 2 for an Iiced aircraft nose.
By changing the aerodynamics of the aircraft and increasing the aircraft’s weight, the ice accretions lead to an increased drag and fuel consumption as well as a decreased lift. Icing may also occur on non-heated measuring probes, leading to a wrong estimation of the flight conditions. Hence, icing affects the safe and economic operation of an airplane, which may in the worst case lead to an aircraft crash.
To improve existing tools for icing simulations, the problem is investigated experimentally and numerically. The experimental investigations lead to a better understanding of the dominating physical mechanisms during the process and furthermore serve as the basis of subsequently derived mathematical models. Two aspects of a droplet impact onto a cold surface accompanied by the solidification of the liquid are investigated separately. On the one hand, the interplay between hydro- and thermodynamics is investigated with experiments of impacting water droplets and on the other hand, the influence of the substrate material on the solidification process is examined by looking on the freezing process of supercooled sessile droplets.
During a droplet impact, hydro- and thermodynamic processes take place in parallel potentially influencing each other, as shown in Figure 3 and in this video. It shows the freezing process of a receding water droplet, initially at +20°C after an impact onto an inclined surface cooled down to -17°C. When the solidification of the droplet starts, the actual shape of the spreaded liquid is fixed, leading to an amount of ice on top of the impact surface. Due to the fact that the liquid is still moving before it solidifies, the moment in which freezing starts has a strong influence on the size of the area that is covered by ice.
A supercooled liquid is in a metastable state and its solidification is an irreversible process. It takes place in two subsequent phases. During the first phase, the liquid solidifies in form of a dendritic structure accompanied by the release of latent heat. According to the release of latent heat, only an amount of the supercooled liquid freezes, i.e. just enough to warm up the mixture of solid and liquid to the freezing temperature which represents the thermodynamically stable and thereby preferred state. While the duration of the first phase is in the order of milliseconds, that of the second phase is in the order of seconds. After the first phase, the initially supercooled liquid is a mixture of liquid and solid, where the solid is immersed more or less homogeneously in form of a porous dendritic structure through the whole volume. If further heat is conducted away from the mixture, the remaining liquid freezes isothermally at the melting temperature. Figure 4 shows the first phase of solidification of a supercooled sessile water droplet, observed in two dimensions. The freezing starts at a single nucleation site at the substrate which is followed by the parallel propagation of the enveloping front of dendrites. After this phase, the mixture is at the melting temperature of water and due to the cold substrate on which the droplet is placed, the second phase of solidification starts at the droplet’s bottom, as observable in the last frames of Figure 4 as a bright stripe next to the substrate.
M.Sc. Markus Schremb (SLA)
- M. Schremb, I. V. Roisman, C. Tropea; Different Outcomes after Inclined Impacts of Water Drops on a Cooled Surface; ICLASS 2015 –International Conference on Liquid Atomization and Spray Systems; Tainan, Taiwan; (2015)
- M. Schremb, I. V. Roisman, S. Jakirlic, C. Tropea; Spreading and Freezing of a Droplet Impacting onto an Inclined Cooled Surface; SAE 2015 – International Conference on Icing of Aircraft, Engines, and Structures; Prague, Czech Republic; (2015)