Convection heat transfer enhancement of a vertically heated surface via rising bubbles in graphene oxide (GO) nanofluid

• Bubbles rising in GO-water nanofluid capable of enhancing heat transfer. • 0.1 g/L GO-water nanofluid has overall better heat transfer performance. • Heat transfer enhanced with increased air injection rate from 6 to 36 mL/min. • Optimum GO concentration exists for better heat transfer performance. • Maximum enhancement of 145 % was achieved by 0.2 g/L GO-water nanofluid at 36 mL/min. Nanofluids enhance heat transfer due to the addition of highly thermally conductive nanoparticles into the base fluid, which has low thermal conductivity. On the other hand, rising gas bubbles in a liquid medium can enhance heat transfer over a heated surface due to mixing effects induced by the vortices caused by the bubbles rising motion. This study aims to integrate the effects of graphene oxide (GO) nanofluids and rising air bubbles to enhance heat transfer over a vertically heated surface. Air bubbles were injected into deionised water-based GO nanofluids, and heat transfer performance was quantified by the heat transfer coefficient ( h ¯ ). With bubbles rising in a GO-water nanofluid of concentration 0.1 g/L and an air injection rate of 6 mL/min, h ¯ increased by 22.3 % compared to deionised water without bubbles. However, when nanofluids were used without bubble injection, the heat transfer performance deteriorated, proving the effectiveness of rising bubbles in enhancing heat transfer in GO-water nanofluids. The improvement was higher for rising bubbles in GO-water nanofluids than bubbles in deionised water alone. Moreover, the heat transfer coefficient increased with increasing air injection rates. Among all the liquid mediums tested, the 0.1 g/L GO-water nanofluid outperformed deionised water, 0.05 g/L GO and 0.2 g/L GO, achieving the highest h ¯ value of 1044 W/(m 2 ·K) at an air injection rate of 36 mL/min. Therefore, with optimum concentration, rising air bubbles in GO-water nanofluid are capable of dissipating heat and enhancing heat transfer on a heated surface.