Investigation of transient phase change phenomena of water droplets for marine icing applications

Sultana, Kaniz Ronak (2019) Investigation of transient phase change phenomena of water droplets for marine icing applications. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Icing reduces the stability, reliability, productivity and safe operation of offshore exploration vessels, icebreakers and marine structures. A better understanding of the fundamentals of ice accumulation on variable surfaces will be helpful for developing solutions to reduce or prevent ice accretion on offshore vessels and marine structures. The main way that ice accretes on vessels and offshore structures is the solidification of wave-generated saltwater spray. Additionally, in cold climate conditions, vessels and offshore structures can also experience atmospheric or fresh water icing. This research investigates the primary cause of both ice accretion processes. Results were analyzed in terms of pre-impact and post-impact processes. The pre-impact results of a single water droplet suggest the droplet is not in thermal equilibrium with ambient air. Additionally, the nucleation process can occur at higher temperatures than its equilibrium freezing point. Furthermore, it predicts that the nucleation temperature is controlled by the droplet’s volume and the atmospheric temperature. Larger sized droplets have a higher nucleation temperature than smaller sized droplets. Moreover, internal circulation can enhance heat transfer from the droplet to the surrounding air and can accelerate nucleation in cold environmental conditions, as well as influence the fragmentation process. It was observed that the drag force is not only a function of droplet size, but also of ambient temperature and internal circulation. The post-impact results show that for a droplet impacting on a semi-infinite medium, the thermal penetrated depth is minimal. Therefore, during the post-impact study, the substrate can be considered as an isothermal surface. Furthermore, the droplet solidification process is primarily affected by droplet size, spreading area, surface temperature and pre-impact velocity. The lower the object temperature, the faster the cooling rate. Additionally; the larger the droplet size, the more time it takes to solidify. Experimental studies on droplet impact behaviour on both bare and coated substrates were analyzed. The experiments suggest that a lower spreading and longer freezing time occur more frequently on coated substrates than on uncoated substrates. The main reasons for this are surface roughness, contact angle hysteresis, surface energy, surface properties, the thickness of the coatings and droplet-substrate surface tension. The new experimental data and numerical predictions presented in this thesis will help to develop superior and more accurate ice prediction and prevention models.

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