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An Exploration of Nanostructured Surfaces for Wicking and Vaporization Enhancement

  • Author(s): Wemp, Claire Kunkle;
  • Advisor(s): Carey, Van P;
  • et al.

This dissertation summarizes the results from experiments and models related to spreading and vaporization of water droplets on zinc oxide, nanostructured, hydrophilic surfaces. The motivation of this research is water reduction in large scale power plants, where water usage for spray cooled condensors can run in the millions of gallons per day. This dissertation will examine the fundamental framework necessary to build a scientific argument for applying these surfaces to condensers as a way to increase the cooling effects of droplet evaporation.

The nanostructured surfaces for this dissertation research were fabricated using two different zinc oxide (ZnO) seeding nanostructures. One with 6μm diameter and the other of 20μm diameter. A hydrothermal synthesis technique was used to grow a ZnO crystalline nanostructure on a clean metal substrate. This novel approach to metal substrate deposition, only previously done on copper, was demonstrated to be successful for both copper and aluminum substrates. The morphology of the surface, which was examined and processed using SEM imaging was the basis for developing spreading models.

Experimental tests in spreading were run on both copper and aluminum substrate surfaces and data from these experiments shows that the more wetting these surfaces are, the more likely they are to exhibited a two-stage spreading process. These two steps, defined in this thesis as synchronous spreading and hemi-spreading have significant relevance in the spreading and evaporation processes. Synchronous spreading happens very rapidly and is the primary spreading stage of concern for evaporation applications. The model based knowledge of what surface characteristics affect the synchronous spreading speed, and thus evaporation performance, proved to be very useful in determining the cause of rapid evaporation. In addition, the hemi-spreading region proved very useful in model correlation as a tool to apply a morphology prediction technique that could relate experimentally documented peformance with the actual physical morphology of the surface, such as nanoscale structure spacing, and surface thickness.

The models presented in this dissertation explore (1) the spreading of a liquid droplet on a nanoporous nanostructured surface and (2) the vaporization and heat transfer on a nanoporous nanostructured surface. These models shown that there is a strong correlation between surface morphology, liquid spread rate, and heat transfer performance. The characteristics built into the models point to possible ways to optimize surfaces for maximum heat flux performance. This dissertation will also explore how these models are remarkably accurate at predicting not only spreading and vaporization performance, but also the scale and morphology of the nanostructure itself.

In addition, these models allow for a deeper understanding of the evaporative mechanisms taking place when liquid droplets are evaporated on nanostructured surfaces.

This full examination of the experiments and models of spreading and evaporating of liquid droplets on nanostructured surfaces reveals the first steps forward to optimizing surfaces for application outside of the lab. Applying this technology to spray cooling applications is now closer than ever.

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