Effect of aluminum oxide and reduced graphene oxide mixtures on critical heat flux enhancement

Pool boiling experiments were carried out to investigate critical heat flux (CHF) characteristics in mixtures of aluminum oxide (Al2O3) nanoparticles and reduced graphene oxide (RGO) colloids under saturated temperature and atmospheric pressure conditions. Nichrome (Ni-Cr) wire (diameter: 0.2 mm; length: 85 mm) was fixed between electrodes horizontally and heated using Joule heating. The working fluids were prepared using different concentrations of Al2O3 suspension (0.0001-0.01 vol%) and RGO colloids (0.00005-0.005 vol%), in concentration ratios varying from 0.25 to 10. Deionized water was used as a control. In the Al2O3 suspensions, the CHF increased with concentration, showing a saturated enhancement of 54% above 0.001 vol% concentration. In the RGO colloids, CHF enhancement varied between 10 and 37% as the concentration increased. A dramatic CHF enhancement of 473% was observed when the Al2O3/RGO mixture was equally mixed at 0.0005 vol% of each (sample: A0005R0005). Upon completion of the pool boiling experiments, a coating layer was observed on each specimen; the coating layer and specimen surface were characterized using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. When Al2O3 and RGO were used as individual working fluids, the morphologic characteristics of the coating layer were similar to typical shapes seen in previous reports. Upon altering the concentration ratio, different clusters of Al2O3 and RGO flakes appeared in the coating layer, providing coating layers of different appearances. A porous superstructure formed in A0005R0005 and the macrostructure consisted of Al2O3 and RGO agglomerations. The enhancement mechanisms could not be explained based solely on the hydrophilic contact angle. Therefore, an additional experiment to quantify capillary flow rate through the porous media was performed with consideration of the marginal heat flux gain.