산화 그래핀에 의한 임계열유속 증진 연구

Abstract

Critical heat flux (CHF) is the operation limit of thermal systems using nucleate boiling heat transfer phenomenon, including nuclear power plant. Accordingly, the CHF performance determines overall efficiency and safety margin of the system. So, various methods to modify the heating surface were studied, whereby a good wetting surface was reported as efficient way to enhance CHF performance. The graphene coated surface, however, showed a dramatic enhancement boiling performance without wetting characteristics. Both of CHF and boiling heat transfer coefficient were enhanced by 80%, and a rapid temperature jump was eliminated after CHF trigger, which could dramatically increase the safety margin of the systems. This result could not be explained by the conventional model focusing on surface wettability, which indicates that another parameter should be considered in order to understand the CHF enhancement on graphene surface. The graphene is evaluated as a promising material for modification of boiling heat transfer surface with following advantages; i) 2D graphene flake is adaptable to apply a heat transfer ‘surface’ in real application rather than 1D materials, i.e. nano particles or 1D carbon nanotube, ii) graphene is chemically stable so that it has advantage to be used for long periods of time, and iii) graphene has extraordinary mechanical endurance against external disturbance. In this reason, mechanism of CHF on graphene and its optimization should be investigated. Even though the reduced graphene oxide (RGO) surface has been investigated intensively as a heating surface, the graphene oxide (GO) surface is still unexplored in application for heat transfer system. GO has almost same properties with RGO except oxidation level, so a comparative study of deposition feature of GO flakes and its boiling characteristics with those of RGO flakes would help to understand nucleate boiling characteristics on graphene material. In this study, it was report an experimental investigation into the boiling heat transfer characteristics of graphene oxide (GO) colloidal suspensions on a Nichrome wire and a silicon dioxide plate surface. Pool boiling experiments were conducted with a GO colloidal suspension at three concentrations: 1, 5, and 10 mg/L. The characteristics of the GO colloidal suspension were compared to those of reduced GO (RGO) colloidal suspensions examined in our previous studies, in which we investigated the critical heat flux (CHF) enhancement mechanism in graphene colloids. Even though GO and RGO share almost identical geometrical and chemical characteristics, the GO and RGO flakes formed a two-dimensional (2D) laminate film and a three-dimensional (3D) porous network on a 2D film by nucleate boiling, respectively. Both species showed dramatic CHF enhancement, but not surface wettability enhancement. The 2D structure formed by nucleate boiling exhibited a delayed CHF phenomenon, and we found that the GO flakes were well-aligned in the 2D film. Boiling hydrodynamics was examined under high-speed camera visualization. For analysis of CHF enhancement on the GO laminate surface, the discussion focused on geometrical characteristics, since it is well known that a GO laminate membrane shows an unimpeded penetration feature via water molecules. GO laminates consists of numerous nano-channels, which can transport confined water molecules with ultra-low friction flow, so called nanocapillary, whereby growth and formation of dry spots can be prevented. This study demonstrates that dramatic CHF enhancement is the consequence of the nanocapillary flow of nano-confined water molecules via GO laminate film based on the measured water penetration flux. Finally, the experimental estimation of nanocapillary speed into nano-channels was developed and verified to predict the CHF gain through the experiments.