Thermophysical Properties And Heat Transfer Performance Of N-Doped Carbon-Metal Oxide Nanocomposite Nanofluids

Abstract

Nanofluids, made by dispersing solid nanoparticles in a base fluid such as water or ethylene glycol, raise the thermal conductivity of ordinary coolants but come with two familiar penalties: the viscosity rises with loading, and the particles tend to settle or clump over time. Nitrogen-doped carbon combined with a transition metal oxide is one of the more promising particle chemistries for getting round these penalties. The carbon phase (graphene, graphene oxide or nanoplatelets) carries a high intrinsic thermal conductivity and, once nitrogen is doped into its lattice, improved wettability and dispersibility in polar base fluids; the metal oxide (Fe3O4, Al2O3, TiO2, CuO and related oxides) adds mass, magnetic or optical function, and anchoring sites that keep the carbon sheets from restacking. This review gathers what is known about these hybrid nanofluids. We first cover how they are prepared, the two-step and one-step routes, the choice of base fluid, and the surfactant or functionalization strategies that decide stability. We then treat the thermophysical properties one at a time: thermal conductivity and the mechanisms proposed for its enhancement (Brownian motion, the interfacial nanolayer and particle aggregation), dynamic viscosity, specific heat capacity, density and electrical conductivity, with reported enhancement figures drawn from the experimental literature. The heat-transfer section covers convective performance in laminar and turbulent flow, the Nusselt-number and heat-transfer-coefficient gains reported for nitrogen-doped graphene and carbon-metal oxide hybrids, entropy generation, and device-level results in heat exchangers and solar collectors. We close with the problems that still limit use: the trade-off between conductivity gain and pumping penalty, long-term stability, the gap between property measurements and full-system performance, and the shortage of data specific to nitrogen-doped carbon-metal oxide hybrids as opposed to their single-component parents.