Thermophysical Profile and Synergistic Effects of Graphene-Thermal Oil Nanofluids with Hybrid Additives of MWCNTs and CNFs

Graphene nanoplatelets (GNP) are emerging as promising nanomaterials in nanofluid technology due to their exceptional intrinsic thermal properties. The hybrid combination of GNP with Multi-walled carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) can demonstrate a synergistic effect, impact stability, and effective thermal behavior, which is yet to be investigated. Therefore, this research explores the thermal profile, i.e., thermal conductivity, specific heat capacity, and density of GNP, GNP + MWCNTs, and GNP + CNF-based hybrid Nanofluids. A two-step method is employed to formulate three sets of nanofluid mass concentrations, ranging from 0% to 2.0%, with an optimized concentration of non-ionic Span85 surfactant. The dynamic stability is analyzed using viscosity profiles over time at three different temperatures, exhibiting excellent stability at high temperatures. Experimental thermal conductivity analysis of nanofluids reveals a direct relationship with increasing temperature and nanofluid concentration, with maximum enhancements of 182.19%, 175.34%, and 169.86% for GNP, GNP + MWCNTS, and GNP + CNF nanofluids, respectively, at a 2.0% weight concentration. Specific heat capacity (SHC) increases with temperature but decreases with higher concentrations, with decrements of 37.06%, 29.3%, and 22.1% observed at 2.0% concentration for GNP, GNP + MWCNT, and GNP + CNF nanofluids, respectively. While density increases with mass concentration, the enhancement remains negligible. The synergistic effects in thermal conductivity favor GNP nanofluids over hybrid systems, yet hybrid nanofluids exhibit superior SHC and density. Multivariable correlations are developed from experimental data, demonstrating an excellent prediction of thermal properties. The findings highlight the potential of GNP and its hybrid nanofluids for improving energy efficiency in thermal management systems.