Significant enhancement of thermal conductivity in toluene-based nanofluids employing highly dispersed and concentrated ZrO2 nanodots

This study deals with the influence of high concentration and high dispersion nanodots in toluene-based fluids on the improvement of the thermal conductivity of ZrO2/toluene nanofluids by experimental measurement and theoretical analysis. For this purpose, the high concentration (13.21 vol%) ZrO2/toluene nanofluids were diluted to 2.20%, 4.41%, 6.61%, 8.81% and 11.01%, respectively. And their thermal conductivities were measured using the transient planar heat source method (TPHS). Subsequently, the predicted values of four classical theoretical models were compared and discussed with experimental values in detail to verify the applicability of theoretical models. The results indicate that almost all of the thermal conductivities of ZrO2/toluene nanofluids are higher than that of the pure toluene fluid, and the trend is rising with the increase in temperature. Moreover, the thermal conductivity enhancement of ZrO2/toluene nanofluid with 13.21 vol% has achieved 38.7% and 59.9% at room temperature and 62.8 degrees C, respectively. The visualized behaviors of nanodots, including migration, collision, energy exchange between the nanodots reveal the physical mechanism that contributes to the thermal conductivity of nanofluids. And the thermal conductivity enhancement of nanofluid per unit concentration quantifies the mechanism. The Brownian motion of nanodots may play a crucial role, which influences the move, collision, and micro-convection. The predicted values of four models were compared and discussed with experimental values to verify the applicability of theoretical models. The analysis reveals that all predicted values are closer to experimental results at low temperatures. Compared with other models, the thermal conductivity value predicted by the Shukla model is closer to the experimental value at middle and high temperatures. In this work, the thermal conductivity of ZrO2/toluene nanofluids has been greatly improved, which has shown its promising applicability in thermal management in practical engineering applications.