Thermophysical and pool boiling characteristics of ZnO-ethylene glycol nanofluids

Abstract

Thermal conductivity and viscosity of surfactant free and stable ethylene glycol (EG) based ZnO-nanofluids, prepared using long duration sonication, have been investigated both as a function of ZnO nanoparticle volume fraction and temperature (10–70 °C). Thermal conductivity enhancement of > 40% have been observed (ZnO volume fraction: 0.0375), which is substantially higher than that reported earlier on EG based oxide nanofluids. The observed enhancement of the studied ZnO-EG nanofluids is comparable to that of graphene oxide (0.04 volume fraction)–EG nanofluids. Viscosity of ZnO-EG nanofluids displays transition from Newtonian behavior at lower ZnO concentration to non-Newtonian characteristics at higher ZnO content and lower temperatures. Most importantly, viscosity of ZnO-EG nanofluid is found to be nearly independent of ZnO nanoparticle loading. The large thermal conductivity enhancement and marginal viscosity penalty of ZnO-EG nanofluids is believed to be the consequence of prolonged ultrasonication (∼60 h), which results in superior fragmentation and uniform distribution of ZnO nanoparticle clusters in the base fluid (EG). Results on the pool boiling characteristics of ZnO-EG nanofluids are reported first time and an enhancement of ∼22% in the boiling heat transfer coefficient is achieved with 0.016 volume fraction of ZnO nanoparticle loading. Finally, surfactant free ZnO-EG based nanofluids, prepared using long duration sonication, possesses all the features (viz, high thermal conductivity enhancement, nearly no viscosity penalty and enhanced heat transfer coefficient) required for an energy efficient coolant.