MWCNT/Fe3O4 하이브리드 나노유체에서 열전도 및 광-열 변환 성능에 미치는 자기장 효과

Abstract

Due to the unregulated consumption of fossil resources, environmental pollution and catastrophic disasters were frequently occurred in the last decades. As a result, humankind has to harvest clean energy which does not emit greenhouse gases and toxic pollutions. Among the clean energy technologies, the solar collector is considered as the most efficient way to harvest thermal energy. Recently, many researchers have developed new working fluid in the solar collector, and it is called nanofluid that dispersed nano-sized materials into the base fluid such as water, ethylene glycol, thermal oil. Nanofluid can enhance the heat transfer efficiency of the solar collector, therefore it can reduce the size of solar collector. In this study, photo-thermal conversion performance and thermal characteristics of MWCNT/Fe3O4 hybrid nanofluid suspended in water-ethylene glycol (20wt%), were experimentally investigated in the absence and presence of an external magnetic field. MWCNT/Fe3O4 hybrid nanofluid was prepared by various weight concentrations (0.005, 0.05, 0.2wt%) using a two-step method. The effect of magnetic field on the photo-thermal conversion performance and thermal properties of MWCNT/Fe3O4 hybrid nanofluid was investigated under various magnetic intensities such as 250, 500, and 750 Gauss. The experimental result shows that adding the MWCNT nanoparticles into the Fe3O4 nanofluid can achieve higher thermal properties of Fe3O4 nanofluid. The thermal conductivity of Fe3O4, MWCNT, and MWCNT/Fe3O4 hybrid nanofluid increased with the increase of temperature and weight concentration. The maximum thermal conductivity of Fe3O4, MWCNT, and MWCNT/Fe3O4 hybrid nanofluid were 0.562, 0.58, and 0.569 W/mK, respectively, at concentration and temperature of 0.2wt% and 50oC without an external magnetic field. In addition, the thermal conductivity of Fe3O4 and MWCNT/Fe3O4 hybrid nanofluid increased with the increase of magnetic field intensity. The maximum thermal conductivity of Fe3O4 and MWCNT/Fe3O4 hybrid nanofluid was 0.583 and 0.59 W/mK with the magnetic intensity of 750 Gauss, respectively. These were 4.69% and 4.73% enhanced, compared to those of without the external magnetic field. Optical test results revealed that solar weighted absorption fraction of Fe3O4, MWCNT, and MWCNT/Fe3O4 hybrid nanofluid increased with the increase of weight concentration and penetration distance. MWCNT/Fe3O4 hybrid nanofluid increased significantly the solar weighted absorption fraction compared to that of Fe3O4 nanofluid. Moreover, photo-thermal conversion of Fe3O4, MWCNT, and MWCNT/Fe3O4 hybrid nanofluid increased with the weight concentration and operating time. In the presence of external magnetic field, 0.2wt% MWCNT/Fe3O4 hybrid nanofluid exhibited significant photo-thermal conversion enhancement, which was about 33% compared to that of 0.2wt% MWCNT/Fe3O4 hybrid nanofluid without the external magnetic field.