그라핀 필름 코팅 히터에서의 풀 비등 임계열유속 증진에 대한 실험적 연구

Abstract

Graphene is composed of single layer of carbon atoms in a hexagonal lattice and it has novel properties such as superior thermal conductivity, electronic conductivity, high transparency and mechanical properties. Among them, higher thermal conductivity has gained much attention in the heat management application. The thermal conductivity of graphene is two times higher than diamond which has the highest thermal conductivity in the existing material. Among these properties, superior thermal conductivity has much paid attention for thermal application. Boiling heat transfer is one of the efficient cooling methods in thermal system running at high temperature such as nuclear power plant since it used latent heat of vaporization during the phase change from liquid to vapor. However, there is an inherent limitation

the critical heat flux. CHF is an important parameter which affects the safety and economic limitations. Therefore, there are many researches to enhance the CHF, and many recent studies have revealed that the significant CHF enhancement due to surface modification. The surface modification influenced various surface characteristics such as wettability and liquid spreading ability. Among them, it is well known that the wettability is a key parameter for enhancing CHF. However, in the recent study, it is turned out that the increasing wettability is not the necessary condition to enhance CHF. (Park et al., 2010) CHF is influenced by wettability, but also influenced by thermal activity which is independent with surface characteristics. In this study, to enhance CHF, graphene was coated on the heater surface with various thicknesses. The investigation of surface characteristic and thermophysical properties of heater was conducted. Experiments were conducted in pool boiling condition at atmospheric pressure and high-speed visualization was also followed. First, the experimental results on graphene coated surface showed the significant enhancement of CHF compared with bare surface. The enhancement ratio was increased as increasing of the thickness of graphene film up to asymptotic. Second, to explain CHF enhancement, surface characteristic (surface wettability and roughness) and thermophysical properties (thermal activity) were investigated. To check the surface wettability, contact angle was measured. The contact angle on graphene coated surface was increased compared with bare surface. In general, the CHF is strongly influenced by surface wettability. However, in this study, CHF was enhanced independently the surface wettability. From these results, it was confirmed that the surface characteristic doesn’t affects to CHF. Then, to explain the CHF enhancement, the relationship between thermal activity and CHF was investigated. The result showed that CHF was increased with increasing thermal activity up to the certain point. Lastly, the boiling and CHF phenomena was investigated by high-speed visualization. Through the visualization, it was turned out that the CHF enhancement on the graphene coated surface was due to the vigorous heat spreading.