| Summary: | 博士 === 國立交通大學 === 機械工程系所 === 105 === Phase change is a commonly seen process in a wide range of systems, including desalination, power generation, water-harvesting, and electronics cooling. Micro/nanostructured surfaces have been recognized to have a huge potential in promoting the efficiency of phases interaction in phase change processes. This thesis aims to improve the heat and mass transfer in condensation by using micro/nanostructured surfaces, and to promote the anti-frosting and de-frosting abilities by using micro/nanostructured surfaces.
Condensation is a common phenomenon and is widely exploited in power generation and refrigeration systems. We reported a new concept to enhance condensation by controlling heterogeneous nucleation on superhydrophobic (SHB) surfaces. Condensation on plain silicon surface (plain Si), silicon nanowire coated (SiNW) surface and microgroove with silicon nanowire coated (MG/SiNW) surfaces have been examined. Heterogeneous nucleation on the MG/SiNW surface could be spatially controlled by manipulating the free energy barrier to nucleation through parameterizing regional roughness scale. Moreover, the nucleation site density (NSD) can also be manipulated by tailoring the density of the microgroove on the surface. Our experimental results show that the MG/SiNW surfaces can effectively promote condensation by utilizing the spatial control of nucleation. This suggests that potentially high heat and mass transfer rates can be achieved on the MG/SiNW surfaces. It is worth noting that utilizing micro/nanostructured surface is not necessarily advantageous because the apparent Cassie droplets are usually in fact partial Wenzel in condensation. The Wenzel droplets would result in an increase in droplet departure diameter and thereby deteriorating the condensation performance on the micro/nanostructured surfaces. To attain the efficient shedding of Cassie droplets in condensation on a SHB surface, a Bond number (a dimensionless number for appraising dropwise condensation) and a solid−liquid fraction smaller than 0.1 and 0.3, respectively, were suggested.
Ice formation is a catastrophic problem affecting our daily life in a number of ways. At present, de-icing methods are costly, inefficient, and environmentally unfriendly. Ice can be formed on a solid surface either by condensation-freezing process or by frosting process. Although SHB surfaces can potentially be an ice-phobic surface in the condensation-freezing process, frosting is expected at a very cold temperature. Thus, indiscriminate frost formation is found everywhere on the solid surfaces through the frosting process, eliminating the ice-phobic function on the SHB surfaces.
Frosting on plain Si surface, SiNW surface, v-shaped microgroove (VMG) surfaces and trapezoid microgroove (TMG) surface have been systematically investigated. It was found that ice embryos could preferentially nucleate at the microgroove on the VMG surfaces and TMG surface. Ice NSD could also be manipulated by tailoring the number of microgrooves on the surfaces. Besides, ice crystals grew and stacked along the direction of grooves on VMG surfaces. The spatial control of frost formation and the confinement of ice growing kinetics on VMG surfaces could effectively improve the anti-frosting and de-frosting performances. The VMG surface possessed the longest ice-covering time (the time required for ice to cover the whole surface area in the frosting experiments) and the shortest dwell time (the time period during which ice covered the whole surface area after switching off the Peltier cooler in the de-frosting experiments) among various kinds of surfaces. This implied that v-shaped microgroove surface exhibited the best anti-frosting and de-frosting performances among the studied surfaces.
This thesis has demonstrated the concepts of designing micro/nanostructured surfaces, which can improve condensation performance and anti-frosting/de-frosting abilities. We anticipate that the concept could be adopted in the other phase change process such as boiling or evaporation process.
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