Droplet growth kinetics on superhydrophobic surfaces and future work on combating cryogenic surface contamination

Condensation;Hydrophobicity;Thermal Radiation;Spacecraft Cryocoolers

Carpenter, James S. ; Miljkovic ; Nenad

关键词: Condensation;    Hydrophobicity;    Thermal Radiation;    Spacecraft Cryocoolers;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/101378/CARPENTER-THESIS-2018.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

In the first part of this thesis, I discuss my work on droplet growth kinetics during dropwise condensation. Accurate heat transfer predictions during dropwise condensation depend on the growth rates of the condensing droplets. This includes both the heat transfer rate during droplet growth as well as following droplet sweeping—where large droplets sweep away other droplets on an inclined surface. Droplet growth rates on a given surface primarily depend on background vapor pressure and the extent of wall subcooling. Here, I report that larger droplets affect the growth rates of nearby smaller droplets during dropwise condensation on superhydrophobic aluminum surfaces. In large systems, neglecting this influence is less of a concern as the error will largely be contained in the uncertainty of the heat transfer rate prediction. However, in smaller phase-change heat transfer systems, like those currently being explored to cool MEMS/NEMS devices, accurate predictions of droplet growth rates are crucial. In this work, using high-angle environmental scanning electron microscopy (ESEM), I report visualizations of the effect that larger droplets have on the growth rates of nearby smaller droplets.Furthermore, I also hypothesize the underlying mechanism behind this phenomenon and discuss my ongoing work to validate this hypothesis.In the second part of this thesis, I propose a project to use functionalized surfaces to combat contamination of low-emissivity cryogenic surfaces. Future crewed space missions require high capacity cryocoolers for zero boil-off (ZBO) systems. Due to parasitic heat loads, cryocoolers have low efficiency, making approaches to limiting these loads highly desired. Water cryodepositing onto low-emissivity cryogenic surfaces results in high parasitic radiative loads. In this project, I propose a method for combating this load increase. Specifically, I propose to use a functionalized (biphilic) surface to getter water onto a small percentage (< 5 %) of the cryogenic surface, thus maintaining an overall low-emissivity. I aim to determine whether (1) the spatial control of water nucleation that the biphilic surface exhibits in non-cryogenic conditions extend to cryogenic conditions, (2) the biphilic surface maintains the low-emissivity of the cryogenic surface, and (3) the biphilic surface is durable in space-cryocooler applications. I discuss the preliminary considerations and objectives for this work, including the proposed fabrication and characterization process for the biphilic surfaces, as well as the proposed experimental apparatus to be used in conducting experiments.