Abstract

Drop impact on a surface has an effect on nearly every industry and this impact may have adverse effects if not controlled. Superhydrophobic (SH) surfaces have been created with the extreme ability to repel water. These surfaces exist in nature but may also be fabricated using modern techniques. This thesis explores heat transfer from these SH surfaces to drops impacting them. This thesis is devoted to increasing the breadth of knowledge of thermal atomization during drop impingement on superheated SH surfaces. When a water drop impinges vertically on a horizontal superheated surface, intense atomization can occur. The atomization is caused by rapid vapor generation at the surface and the corresponding formation and collapse of vapor bubble cavities. This thesis is divided into two main works, experimental quantification of thermal atomization and analytical prediction of vapor generation. An experimental exploration, comprising chapter 3 contains experimental work done on drop impingement on nanostructured surfaces. of this thesis, presents results of experiments meant to quantify the amount of thermal atomization during drop impingement on superheated superhydrophobic surfaces. Effects of time, surface temperature, and surface geometry are investigated. Superhydrophobic surface geometries explored in this work included post, rib, and carbon nanotube (CNT) structures. Each surface is characterized by its temperature jump length. It is shown that, in general, atomization intensity decreases with increasing temperature jump length. It is also shown that atomization is completely suppressed on surfaces with nanoscale surface features and high cavity fraction (e.g. CNT structures). This work also relates the effect of temperature jump length on the maximum atomization temperature and the maximum atomization time. Both quantities show a systematic relationship with temperature jump length. The analytical portion, comprising chapter 4 of this thesis, presents an analytical model used to predict the amount vapor generated during drop impingement on superheated SH surfaces. This vapor generation is then correlated to experimental values of atomization. Atomization is caused by vapor generation so their magnitudes are thought to be proportional. Two existing analytical models for drop contact area of impinging drops are combined to predict drop spread for all impact scenarios. An analytical model for heat flux is used to find heat transfer to impinging drops and mass flow rate of vapor generated from boiling.

Degree

College and Department

Ira A. Fulton College of Engineering; Mechanical Engineering

Rights

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