Comparison between pool boiling system of graphene quantum dots and nitrogen-doped graphene quantum dots suspended in binary base fluids

Using nanofluids instead of conventional heat transfer fluids as a passive method is a well-established and widely used technique by researchers to increase the rate and thermal performance of engineering equipment. In this study, the hydrothermal method with a bottom-up approach was used for the synthesis of graphene quantum dots and nitrogen-doped graphene quantum dots. Then, nanofluid samples were prepared in a two-step process, by adding nanoparticles to binary base fluids of deionized water and ethylene glycol in volume concentrations of (50:50) and (60:40), in four concentrations of 100, 200, 500, and 1000 ppm. In order to better understand the boiling heat transfer mechanism and measure its characteristics such as critical heat flux and heat transfer coefficient, an experimental system was designed and built. Nanofluids based on graphene quantum dots have unique features such as compatibility with the environment, economic efficiency, high stability and suitable heat transfer capability. For this reason, their selection in the pool boiling heat transfer process, in addition to saving energy, is introduced as one of the most effective options for improving CHF and HTC. The tests were performed under saturated conditions, atmospheric pressure, and on a vertical flat and polished copper thermal plate. Prepared nanofluids GQDs and N: GQDs based on DI-water maintained their apparent stability for two months. For GQDs nanofluids at an optimal concentration of 500 ppm with a volume ratio of (60:40) DI-water and EG, compared to DI-water, the greatest increase in CHF and HTC is 90.69, 85.011 % and for N: GQDs at a concentration of 500 with a volume ratio (50:50) DI-water and EG, 75.37 and 78.17 % compared to DI-water.