Surfaces with controlled morphology and wettability for advanced phase change heat transfer applications.

摘要

Surface morphology and wettability have been identified as the key factors affecting phase change heat transfer. Controlling the surface characteristics would in turn provide new opportunity in phase change heat transfer applications. In this study, we explore various functional surfaces for advanced phase change heat transfer applications including advanced evaporators for micro heat pipes and new boiling heat transfer surfaces.;We first investigate the use of Cu2O and CuO nanostructures to alter the wetting characteristics of copper surfaces. We perform detailed characterization of the morphology and the wettability of copper oxide films formed by different oxidation schemes. Based on our comparative study, we demonstrate the ability to tune the wettability of copper surfaces from superhydrophilic to superhydrophobic.;We then demonstrate controlled bubble nucleation, growth, and departure by altering the wetting characteristics of surfaces for boiling heat transfer and microfluidic applications. Compared with a hydrophilic silicon surface, the bubble departure diameter is observed to be ∼2.5 times smaller on superhydrophilic CuO film and ∼3 times larger on hydrophobic Teflon surface. The experimental findings well corresponds to our numerical simulations. Based on the experimental and numerical findings, a new bubble departure model is suggested for superhydrophilic surfaces. By observing steady-period bubble nucleation on precisely defined hydrophobic islands, we also provide quantitative physical evidence consistent with the existent of nanobubbles of the order of 1 microm and contact angle with water greater than ∼170°.;Lastly, we introduce new evaporators for micro heat pipes. The new evaporator wicks composed of superhydrophilic copper micropost arrays are fabricated using electrochemical deposition combined with controlled chemical oxidation scheme. We perform capillary rate of rise experiments in conjunction with numerical simulations to characterize the capillary performance of micropost wicks. For the heat transfer performance test, the superhydrophilic wicks are self-saturated by a finite capillary pressure gradient and placed in 90 deg orientation to prevent flooding. The experiment is conducted at approximately 0.1 atm in a vacuum chamber. Integrated CuO nanostructures significantly enhance both capillary and heat transfer performance. As a solid fraction increases, the heat transfer rate increases due to the increases in thin film area. The critical heat flux, however, decreases since the capillary performance decreases with increasing solid fraction.