Experimental Study on Pool Boiling Heat Transfer Characteristics of Graphite Oxide Nano Coating on Copper Surface

Abstract

Graphene materials have great application prospects in the field of advanced heat transfer equipment for nuclear reactors due to their excellent thermal conductivity and chemical inertia. In order to explore the enhanced heat transfer performance of graphene materials, boiling deposition method to prepare graphene oxide nanocoatings was used in this paper. The working fluid concentrations of graphene oxide nanofluids are 0.001, 0.003 and 0.005 mg/mL, and the heating time is controlled at 1 hours and 2 hours. Six types of graphene oxide coatings were prepared on exposed copper surfaces through boiling deposition. The coating surface was observed through optical photography and scanning electron microscopy (SEM). By comparing and analyzing the influence of graphite oxide coating on surface morphology characteristics, it is found through optical photos that the deposition of graphene oxide on the copper surface shows an uneven spot like distribution. With the increase of nanofluid concentration and preparation time, the coverage gradually increases. When the preparation time is 2 hours, the graphene oxide coating completely covers the substrate copper surface. The deposition of graphene oxide on the copper surface exhibits an uneven spotted distribution. Through scanning electron microscopy analysis, it is found that the uneven spots are the vaporization core during boiling. By using X-ray energy dispersive spectroscopy (EDS) to scan the element content distribution in the spot area along a straight line, it is found that the carbon element content in the vaporized core is the lowest, and the carbon element content around it first increases and then decreases. These indicate that the deposition of graphene oxide nanofluid occurs through the phase transition at the bottom of the bubble, which is the reason for the non-uniform distribution of the deposition layer spots. Using the above graphene oxide coating for pool boiling experiments, it is found that when the coverage rate of graphene oxide coating is low, its impact on pool boiling heat transfer characteristics is minimal. When the coverage rate of the nano coating is high, it can significantly increase the critical heat flux, but the subsequent thickening of the coating has a smaller effect on improving the critical heat flux. Within the experimental range, the critical heat flux increases by 40% to 55% compared to the bare surface. Mechanism analysis finds that the sedimentary layer improves wettability and thermal conductivity, both of which jointly suppress the irreversible expansion of the evaporation drying area at the bottom of the bubble under high heat flux conditions, significantly increasing the critical heat flux density. Due to the similarity between the surface morphology of the coating and the exposed copper surface, the effect on the density of the vaporized core during pool boiling is relatively small, so the effect on the pool boiling heat transfer coefficient is relatively small.

Keywords

• graphite oxide
• surface characteristics
• pool boiling
• heat transfer characteristics
• critical heat flux