Massive improvements in the thermophysical properties of nanofluids over conventional fluids have led to the rapid evolution of using multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) in the field of heat transfer. In this study, the heat transfer and entropy generation abilities of MWCNTs/GNPs hybrid nanofluids were explored. Experiments on forced convective flow through a brass microtube with 300 μm inner diameter and 0.27 m in length were performed under uniform heat flux. MWCNTs/GNPs hybrid nanofluids were developed by adding 0.035 wt.% GNPs to MWCNTs water-based nanofluids with mass fractions of 0.075-0.125 wt.%. The range of the Reynolds number in this experiment was maintained at Re = 200-500. Results showed that the conventional approach for predicting the heat transfer coeffcient was applicable for microtubes. The heat transfer coeffcient increased markedly with the use of MWCNTs and MWCNTs/GNPs nanofluids, with increased pressure dropping by 12.4%. Results further showed a reduction by 37.5% in the total entropy generation rate in microtubes for hybrid nanofluids. Overall, MWCNTs/GNPs hybrid nanofluids can be used as alternative fluids in cooling systems for thermal applications.

The development of new classes of fluids with enhanced heat transfer capabilities has been the subject of significant contemporary research. One area of interest in this field involves the use of nanomaterials to improve the properties of heat-transfer fluids. This research experimentally investigates promising hybrid nanofluids that contain graphene nanoplatelets (GNPs) and multi-walled carbon nanotubes (MWCNTs). The present article reports thermophysical properties, heat transfer coefficient, and pressure drop for MWCNTs/GNPs water based hybrid nanofluids that flow through a circular tube (D_in = 1.1 mm). The flow was assumed as a fully laminar flow (Re = 200–500), and a uniform heat flux was applied to the tube surface. Different weight concentrations of MWCNTs/water nanofluids (0.075, 0.125, and 0.25 wt%) were used and mixed with (0.035 wt%) GNPs to prepare hybrid nanofluids. Heat transfer coefficient was significantly enhanced using MWCNTs and MWCNTs/GNPs hybrid nanofluids. The enhancement of heat transfer coefficient is found to be proportionally dependent on the nanoparticle concentrations and inversely related with the Reynolds number. The positive effect of adding GNPs to different concentrations of MWCNTs enhanced the heat transfer coefficient. The maximum enhancement was recorded for 0.25 MWCNTs/0.035 GNPs hybrid at Re = 200 for a 43.4% increase with an 11% rise in pressure drop.