Improved transient heat transfer performance of ZnO–propylene glycol nanofluids for energy management

Highlights: • Transient heat transfer characteristics of ZnO–propylene glycol studied. • Linear increase in heat removal rate with increasing ϕ under constant heat flux conditions. • ∼26% increase in 'h' for 2 vol.% ZnO–PG under constant temperature condition. • ZnO–PG acts as true fluids with enhanced thermal properties. - Abstract: Experiments were performed to study the transient heat transfer characteristics of probe-ultrasonicated, surfactant-free, ZnO–propylene glycol (ZnO–PG) nanofluids under constant heat flux and constant bath temperature conditions. Under constant heat flux conditions, ZnO–PG nanofluids exhibited linear increase in heat removal rates with increasing nanoparticle concentration. An enhancement of 4.24% in heat transfer rate was observed with 2 vol.% ZnO–PG nanofluid. The overall heat transfer coefficient and nanofluid-side heat transfer coefficient under constant bath temperature condition were found to increase with increasing nanoparticle volume concentration. About ∼26% increase in coolant side heat transfer coefficient was obtained with 2 vol.% ZnO–PG nanofluid under constant bath temperature condition which could be attributed to thermal conductivity enhancement as well as improved natural convection due to viscosity reduction. Our results indicate that the surfactant-free, ZnO–PG nanofluids behave like liquid coolants but with improved thermal conductivity and specific heat and hence suitable for energy management