| Summary: | 碩士 === 國立中央大學 === 能源工程研究所 === 105 === Condensation, a two-phase heat transfer processes, is commonly utilized in industrial systems. Water vapor condensation has been studied by several researchers and different technique has been proposed to enhance condensation heat transfer. Functionalized surfaces can enhance condensation heat transfer by means of transferring heat with dropwise condensation due to hydrophobic coatings. Many studies have investigated outside water condensation heat transfer of functionalized surface.
However, there are several applications which use refrigerant with low boiling temperature instead of water such as waste heat recovery technologies with low thermal efficiency and high capital cost. Enhancing condensation heat transfer of refrigerant can considerably increase and decrease the thermal efficiency and capital cost of these systems respectively.
Due to the lack of study on outside refrigerants condensation of heat transfer with functionalized surface, this study focuses on the effect of graphene-coated copper tube on the performance of outside condensation heat transfer. The graphene was in-situ grown cooper tube surface by chemical vapor deposition (CVD) method. In this research two different refrigerant R245fa and R141b are used. Moreover, the heat transfer efficiency of outside condensation for graphene coated and uncoated copper tubes are investigated and compared.
The contact angle of deionized water (DI-water) and R141b droplet on pristine and graphene-coated copper plate are measured. The droplet contact angle on graphene-coated and uncoated copper plate are measured to be 88.3o (DI-water)/ 23.6 o(R141b) and 79.8o(DI-water)/ 15.7 o (R141b) ,respectively.
The experimental observation and results show no drop-wise condensation and heat transfer enhancement for R245fa and R141b on graphene-coated copper tube when compare to that of uncoated tube. The results show that the experimental heat transfer coefficient at low subcooled wall temperature has no significance difference (5 %) with Nusselt correlation but at high subcooled wall temperature the difference increases to 23%. The difference of experimental data with Nusselt correlation at high wall subcooled temperature is because of the effect of vapor shear which is not considered in Nusslet correlation.
R245fa is a symmetric polarized molecular, thus lower the degree of dipole. Based on the principle of like-dissolves-like, the graphene is a non-dipole molecular, leading to hydrophilic surface when it was subjected to the contact from R245fa molecular. From the contact angle measurement by real-time macroscopy, it was found out the R141b exhibit hydrophilic type droplets on graphene surface, which was consistent with the observed phenomenon of R141b testing in chamber.
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