The low thermal conductivity of conventional heat transfer fluids is the major limitation in effective removal of the heat generated in heat exchangers, radiators, electrical motors and generators. Nanofluids, called as next generation heat transfer fluids are a suspension of nanoparticles in base fluids such as DI water and ethylene glycol. Nanofluids exhibit high thermal conductivity and heat transfer properties and hence they can improve the efficiency of cooling systems.
Carbon nanotubes (CNT) and Graphene show very high thermal conductivity of about 5000 Wm-1K-1. Hybrid nanocomposite of CNT and graphene can be advantageous for enhancing the heat transfer properties due to the intrinsic high thermal conductivity of individual counterparts. We have prepared nanofluids by dispersing nitrogen doped hybrid nanocomposite of CNT and graphene. Nitrogen doped hybrid nanocomposite is synthesised by pyrolysis of polypyrrole wrapped hybrid structure consisting of CNT and graphene. Nanofluids are prepared by dispersing specific amount of nanocomposites in DI water and ethylene glycol (EG). Nanofluids show very good stability and uniform dispersion of nanoparticles. A significant enhancement in thermal conductivity of 17.7 % is achieved with 0.02 % volume fraction in DI water and 15.1 % with 0.03 % volume fraction with EG.
Addition of nanomaterials as additives in base lubricant oil is a rapidly progressing field of research, since nanomaterials are different from traditional bulk materials due to their extremely small size and high specific surface area. Researchers have tried a variety of nanomaterials (carbon nanotubes, fullerenes and graphite nanosheets) dispersed base oils to improve the antiwear and friction reduction. We have employed solar exfoliated graphene (sG) based nanolubricants for lubrication purpose. At lesser concentrations of sG synthesized by solar exfoliation, the tribological properties of sG-engine oil nanolubricants improve enormously. For 0.025 mg/ml of sG in engine oil, frictional coefficient (FC) and wear scar diameter (WSD) are reduced by 80% and 33% respectively. This demonstrates that the maximum reduction in frictional coefficient can be achieved with sG based engine oil without modifying the surface of solar exfoliated graphene.
It is also observed that FC and WSD increase with increase in concentration of sG beyond optimal concentration, which can be attributed to the coalescence and segregation of particles. Formation of nanobearing between the balls plays a major role in reducing the friction and wear. The excellent performance of graphene on tribological properties of these oil-based nanofluids is attributed to the ultimate mechanical strength and topological structure of sG.