Heat transfer performance and transport properties of ZnO–ethylene glycol and ZnO–ethylene glycol–water nanofluid coolants

•High thermal conductivity and low-viscous ZnO–ethylene glycol nanofluids prepared.•ZnO–ethylene glycol–water nanofluids prepared by hierarchical method.•Liquid layering and Brownian motion contribute to thermal conductivity enhancement.•Improvement in nanofluid cooling performance inline with thermal conductivity rise. Experiments were carried out on preparation and characterization of ZnO–ethylene glycol (EG) and ZnO–ethylene glycol–water nanofluids and analysis of their performance as coolants. Favorable interactions between ZnO nanoparticles and ethylene glycol molecules ensured superior transport properties of ZnO–EG nanofluids. These interactions were utilized during formulation of ZnO–EG–water nanofluids with preservation of ethylene glycol molecules over ZnO nanoparticles’ surface rendering them with better transport properties. ZnO–EG nanofluids containing 4vol.% nanoparticles showed thermal conductivity enhancement of 33.4% and viscosity reduction of 39.2% at 27°C. Similarly, 2vol.% ZnO–EG–water nanofluids showed thermal conductivity enhancement of 17.26% and viscosity reduction of 17.34% at 27°C. Disturbance of hydrogen bonding network of ethylene glycol by ZnO nanoparticles resulted in reduced dispersion viscosity. Empirical models were developed to predict the thermal conductivity enhancement and viscosity reduction of the nanofluids apart from elucidating mechanisms for the same. Transient heat transfer experiments showed that ZnO–EG and ZnO–EG–water nanofluids had better heat absorption characteristics compared to their respective base fluids. The enhancements in heat transfer were proportional to thermal conductivity enhancements, which showed that superior thermal conductivity of nanofluids could be harnessed for cooling applications.