Abstract

As the demand for more powerful and smaller electronics rise, the need for creative cooling solutions to prevent burnout becomes increasingly paramount. In response to recent cooling needs, new cooling techniques, such as jet impingement cooling, spray cooling, and heat pipes, have risen in popularity for their simple design and efficiency in thermal transport. This interest has risen in both industry and academia, where research has been conducted to optimize the heat transfer performance for these systems and how these systems can be implemented in new technologies. One method that has risen in interest is affecting the surface, through physical and chemical treatments, and how these different treatments can enhance the heat removal rate for different systems. This dissertation presents two main regions of research that, when combined, can enhance understanding of cooling rates for different surface treatments. The first region is utilizing time domain thermoreflectance (TDTR) to measure the heat transfer coefficients in a microchannel experiencing jet impingement cooling. This current study presents the findings of experiments that measure the heat transfer coefficients on surfaces exposed to hot-spot heating and cooled using water jet impingement at Reynolds numbers up to 6432. The heat transfer coefficients were found using TDTR with a water jet on a fused silica (FS) glass substrate coated with a thin-film Hafnium-alloy (Hf). The heat transfer coefficient data are based on a local, micron-sized hot-spot region (generated by the TDTR pump laser) that is translated at different locations relative to the stagnation point. The study shows that at different microchannel regions (relative to the stagnation point) and for different Reynolds numbers for the jet that the TDTR method can detect changes in the heat transfer coefficient. Along with a novel method to measure the heat transfer coefficients using TDTR, several studies on different surface conditions are presented in the dissertation. Physical changes in wetting performance is analyzed through soft wetting materials and the impact the stiffness has on the hemwicking performance. Through a novel, in house stamping apparatus, polydimethylsiloxane (PDMS) samples were created of varing stiffness of 0.338 MPa to 1.98 MPa. Through analyzing the hemiwicking velocity, hemiwicking diffusion, and initial hemiwicking wicking velocity of ethanol, isopropyl alcohol, and isooctane, it was observed that the stiffness of soft materials can play a significant impact on the overall wicking performance. Furthermore, a deformation model is presented based on pillar deformations observed with PDMS/ExoFlex hybrid samples as the working fluids evaporated from the wicking arrays. The chemical impact on the overall wetting performance is also analyzed and presented in this dissertation. Two main methods were implemented to track the changes in wetting through surface chemistry; through the application of a polyvinyl alcohol (PVA) self-assembled monolayer (SAM) and applying a spiropyran (SPCOOH) to a microstructured gold surface. The changes in hemiwicking velocity and meniscus extension with the PVA SAM displayed an important aspect of chemical interactions with respect to hemiwicking performance, which affects the heat transfer performance of a microstructured surface. The SPCOOH studies revealed a change in wetting behavior which further emphasized the importance of intermolecular interaction on wetting performance, but also revealed a controlling aspect the PVA SAM experiments did not exhibit. Along with these studies, a preliminary study of controlling the intermolecular interactions of metamaterials through strain to change the surface wetting is presented in this dissertation. Through the use of a simple, uni-axial strain instrument, different metamaterials composed of hafnium dioxide, titanium nitride, and tungsten deposited on Kapton tape were subjected to strains up to 8%. While under strain, significant changes in the advancing, receding, and equilibrium contact angle were observed for both polar and non polar fluids on the surface. These changes are attributed through changes in the intermolecular forces and verified through changes in the reflectivity while strain is applied to the metamaterials.

Notes

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Graduation Date

2020

Semester

Fall

Advisor

Putnam, Shawn

Degree

Doctor of Philosophy (Ph.D.)

College

College of Engineering and Computer Science

Department

Mechanical and Aerospace Engineering

Degree Program

Mechanical Engineering

Format

application/pdf

Identifier

CFE0008771;DP0025502

URL

https://purls.library.ucf.edu/go/DP0025502

Language

English

Release Date

6-15-2021

Length of Campus-only Access

None

Access Status

Doctoral Dissertation (Open Access)

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