Abstract

Droplet Impact and Solidification on Solid Surfaces in the Presence of Stagnation Air Flow. Morteza Mohammadi, Ph.D.
Concordia University, 2016

Understanding the fundamentals of ice accretion on surfaces can help in proposing solutions to reduce or prevent ice accumulation on aircraft components and power lines. The main way in which ice forms on a surface is the solidification of supercooled droplets upon impacting on the surface. On an aircraft wing, ice accumulation can easily change the flow pattern, which could result in an increase in drag force. This research investigates the use of superhydrophobic coatings (surfaces with contact angles larger than 150) to counteract icing (anti-icing) as a result of their extremely low surface energy. The main goal of this study is to assess the performance of superhydrophobic surfaces in the presence of stagnation flow to mimic flight conditions (e.g. droplet impinging on the leading edge of an aircraft’s wing). A wide range of droplet impact velocities and stagnation flows in splashing and non-splashing regimes (at high and low Weber numbers) were carried out on surfaces with various wettabilities. The results were analyzed in order to highlight the advantages of using superhydrophobic coatings. Free stream air velocity were varied from 0 to 10 m/s with a temperature which was controlled from room temperature at 20 oC down to -30 oC.
It was observed that while the presence of stagnation flow on hydrophilic (i.e. aluminum substrate) results in thin film formation for droplets with Weber numbers more than 220 upon impact in room temperature condition, instantaneous freezing at the maximum spreading diameter was observed in low temperature condition where air and substrate temperature was below the -20 oC. Same phenomenon was observed for hydrophobic substrate at aforementioned temperature. On the other hand, striking phenomenon was observed for superhydrophobic surface when stagnation air flow is present. Although it was expected droplet contact time to be increased by imposing stagnation air flow on an impacting droplet it was reduced as a function of droplet Weber number. This was referred to the presence of full slip condition rather than partial one where the spreading droplet moves on thin layer of air. Consequently, it promotes droplet ligament detachment through Kelvin-Helmholtz instability mechanism. While in low temperature condition above temperature of heterogeneous ice nucleation (i.e. -24 oC)1 supercooled water droplet contact time is reduced up to 30% to that of still air cases, droplet solidified diameter was increased up to 2 folds for air velocity up to 10 m/s compare to the still air condition at temperatures as low as -30 oC. These results were compared with a new predictive model of droplet impact behavior on the superhydrophobic substrate.
New universal predictive model of droplet impact dynamics on the superhydrophobic surface was developed based on the concept of mass-spring model2 which was validated against experimental results. In the new model, viscosity effect was considered through adding a dashpot term in mass-spring model. In addition, the effect of stagnation flow was also integrated to the model through classical Homann flow approach.3 For non-isothermal condition, the effect of phase change (i.e. solidification) on droplet wetting dynamics was coupled to the model through classical nucleation theory. The universal model was compared against experimental results in room and low temperature conditions (i.e. supercooled condition) for model’s validation.