Abstract

Liquid jet impingement is a technique ubiquitously used to rapidly remove large amounts of heat from a surface. Several different regions of heat transfer spanning from forced convection to nucleate, transition, and film boiling can occur very near to one other both temporally and spatially in quenching or high wall heat flux scenarios. Heat transfer involving jet impingement has previously shown dependency both on jet characteristics such as flow rate and temperature as well as surface material properties. Water droplets are known to bead up upon contact with superhydrophobic (SH) surfaces. This is due to reduced surface attraction caused by micro- or nanostructures that, combined with a natively hydrophobic surface chemistry, reduce liquid-solid contact area and attraction, promoting droplet mobility. This remarkable capability possessed by SH surfaces has been studied in depth due to its potential for self-cleaning and shear reduction, but previous research regarding heat transfer on such surfaces shows that it has varying effects on thermal transport. This thesis investigates the effect that quenching initially hot SH surfaces by water jet impingement has on heat transfer, particularly regarding phase change. Two comparative studies are presented. The first examines differences in transient heat transfer from hydrophilic, hydrophobic, and SH surfaces over a range of initial surface temperatures and with jets of varying Reynolds number (ReD), modified by adjusting flow rate. Comparisons of instantaneous local heat flux from the surfaces are made by performing an energy balance over differential control volumes across the surfaces. General trends show increased heat flux, jet spreading velocity and maximum jet spread radius when ReD is increased. An increase in inital surface temperature resulted in increased heat flux across all surfaces, but slowed jet spreading. The local heat flux, average heat rate, and total thermal energy transfer from the surface all confirmed that SH surfaces allow significantly less heat to transfer to the jet compared to hydrophilic surfaces, due to the enhanced Leidenfrost condition and reduced liquid-solid contact on SH surfaces which augments thermal resistance. The second study compares jet impingement heat transfer from SH surfaces of varying microstructures. Similar thermal effects due to modified jet ReD and initial surface temperature were observed. Modifying geometric pattern from microposts to microholes, altering cavity fraction, and changing feature pitch and width had little impact on heat transfer. However, reducing feature height on the post surfaces facilitated water penetration within the microstructure, slightly enhancing thermal transport.

Degree

College and Department

Ira A. Fulton College of Engineering and Technology; Mechanical Engineering

Rights

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