| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系 === 104 === Filmwise/dropwise hybrid wettability surfaces have well condensation heat transfer performance than a fully superhydrophobic surface, because of enhanced water transportation. This study mainly investigate the CHT performance of various hybrid copper surfaces designed according to CHT theory. Currently, the issue is very important because of high erergy use effiency and energy-saving are concerned. Also the issue is applicable to kinds of phase change thermal control devices, heat exchangers, energy-saving systems so that energy consumption and waste heat is minimized. The demand for environment protection and the development of thermal science are able to be realized.
Hybrid wettability surfaces were fabricated by screen printing comprising hydrogen peroxide (H2O2) oxidation and Teflon coating. Superhydrophilic and superhydrophobic regions were present, and water contact angles of both the regions were approximately 7° and 146°, respectively. Thus, the wettability difference with considerable surface tension force inequilibrium induced droplets seating on the surface move fast.
Hybrid wettability surfaces were designed according to tree-like hybrid structures, so most of them consisted of water galleries and water paths for water transportation. Optimization of theoretical heat flux of the designed hybrid surface was conducted by current condensation heat transfer theory and geometrical features. The main parameters in the theory are superhydrophilic/superhydrophobic area ratios and the theoretically maximum droplet radius on the surface. In comparison, theorecial heat flux of the designed hybrid surface is as 1.15 ~ 1.52 times as that of the surface with rectangular strip water gallery utilized in much related research.
Effective thickness as planar layer of coating was proposed to replace the general problem of thickness deviation in measurement. The calculation of effective thickness was according to experimentally measured heat flux, maximum droplet radius and comparison among theoretical heat flux values. The effective thickness was appropriate to calculate reasonable thickness of coatings with non-planar morphology and microstructures. In this study, the effective thickness of Teflon coating as a planar layer was about 250 nm, which was close to the experimental result compared with the experimentally measured 20 μm. Thus, the effective thickness eliminated experimental error in measurement.
In experiment, most designed hybrid surfaces performed condensation heat transfer well and the enhancement relative to the superhydrophobic Teflon-coated surface is higher. For example, the heat flux of hybrid surfaces with triangular gradient pattern array and apex angle 8° is about 1.21 times that of the superhydrophobic Teflon-coated surface within vapor ― surface temperature difference range 4 K to 10 K; the heat flux of hybrid surfaces with horizontal triangular water gallery is about 1.03 ~ 1.303 times within range 3 K to 10 K; the heat flux of hybrid surfaces with reverse V shaped water gallery is about 1.07 ~ 1.12 times within range 6 K to 10 K; the heat flux of hybrid surfaces with triangular gradient gallery is about 1.19 ~ 1.92 times within range 2 K to 10 K; not only was experimental heat flux of most designed hybrid surfaces high, but also was obviously higher than that of he surface with rectangular strip water gallery used as a reference for comparison. The reason why the designed hybrid surface performed well was found due to significant condensate water droplet transportation on the surface, resulting into overwhelming condensation heat transfer. Therefore, the feasibility of the designed hybrid surface in applications is evident without any doubt.
This study met the optimization of condensation heat transfer performance on superhydrophilic/superhydrophobic hybrid surfaces proposed by theoretical analysis and experimental investigation. It contributed an up-to-date technical idea to current phase change thermal control devices, and revealed a developable field in condensation heat transfer study.
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