The present work deals with the preparation of reduced graphene oxide-Fe 3 O 4 (rGO-Fe 3 O 4 ) nanocomposite and its nanofluid by an ultrasound assisted method for convective heat transfer study. Formation of the rGO-Fe 3 O 4 nanocomposite with a uniform distribution of smaller sized (10−20 nm) Fe 3 O 4 nanoparticles on graphene nanosheets was confirmed from UV/vis, transmission electron microscopy, Raman, X-ray diffraction, and X-ray photoelectron spectroscopy analysis. Thermal conductivity of prepared rGO-Fe 3 O 4 nanocomposite based nanofluids with the aid of ultrasound showed an 83.44% enhancement for 0.2 vol % concentration of the rGO-Fe 3 O 4 nanocomposite at 40 °C. Rheological study revealed non-Newtonian behavior of the nanofluids. Various viscosity models were used to predict the behavior of rGO-Fe 3 O 4 nanocomposite based nanofluids. The estimated heat transfer coefficient with the use of 0.02 vol % rGO-Fe 3 O 4 nanofluid at the exit of the test section was 4289.5 W/m 2 •K for the Reynolds number equal to 7510 ± 5. A new correlation for the estimation of Nusselt number has been proposed for the rGO-Fe 3 O 4 nanofluid which fits the experimental data very well.

The fact that some solid materials with suitable phases possess higher thermal conductivity than liquids gave rise to their utilization in heat transfer processes. Fluids that contain nanoparticles of any conducting solid material that enhance the thermal conductivity of the conventional heat transfer fluids are known as nanofluids. Ever since graphene has been known for its high thermal conductivity, it has found applications in nanofluids. There are many investigations on enhancement of thermal conductivity using graphene derivatives-based nanofluids. Graphene can form composites with metal, metal oxides, polymers, and other carbon forms. Nanohybrids of graphene derivatives and other conducting materials can enhance the heat transfer properties of fluids. This review presents an in-depth dicussion of the studies done on graphene derivatives (like graphene oxide)-based and graphene nanocomposite-based nanofluids and their preparation methods, properties, and applications. Although there have been many studies on the thermal properties of graphene oxide-, graphene-, and graphene nanocomposite-based nanofluids, there still remains a wide scope for further discoveries.

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