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 similar to 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. (C) 2015 Elsevier Ltd. All rights reserved.