Experimental And Simulation Study Of Natural Convection Heat Transfer Of TiO₂/Ethylene Glycol Nanofluids

Natural convection heat transfer is important for engineering and industry areas.Scientists and engineers have made great effort to enhance the heat transfer efficiency by dispersing nanoparticles in base liquid.As a common working media of natural convection heat transfer,the study of its heat transfer property is inadequacy.Therefore,in this article we will study the natural convection heat transfer property of nanofluid by using numerical simulation method and experimental method.This work conducts heat transfer experiments with natural convection in a80×80×180 mm vertical square enclosure?width×height×length?using variable volume fractions from 0.26%to 0.78%of TiO2/ethylene glycol?EG?nanofluids prepared using a two-step method.There was no sedimentation after the preparation of nanofluid even after one month.Nusselt numbers under different Rayleigh numbers are also obtained from experimental data,and for Rayleigh numbers between 10⁷?Ra?10⁸ the heat transfer of Ti O₂/EG nanofluids is worse than pure EG.The increased viscosity due to the addition of nanoparticles has a greater negative effect than the positive effect of the increased thermal conductivity.We found that the heat transfer efficiency decrease as volume fraction of the nanofluid increase and increase as the Rayleigh number increase.To further understand the heat transfer law of nanofluid,a single phase heat transfer model was established and carried on numerical simulation of natural convection heat transfer by the FLUENT software.Measurement of the thermal transport properties of the nanofluids shows thatthermal conductivity increase as the volume fraction and the temperature of nanofluid increase;thermal conductivity ratio increase as the volume fraction increase but decrease as temperature increase.Furthermore,the viscosity of the nanofluids increase as volume fraction increase and decrease as temperature increase;the viscosity ratio increase as the volume fraction increase but almost the same regardless of temperature at a fixed temperature.In contrast to pure EG,there is a shear thinning non-Newtonian behavior in the low shear rate region of?<10s^-1,although nanofluids are still treated as a Newtonian fluid for high shear rates of?>10s^-1.The results showed that the heat transfer efficiency increase as the Rayleigh number increase.The heat transfer efficiency is best while the volume fraction is low and temperature difference is high.The nanofluid flows faster as the Rayleigh number increase,this promotes the heat transfer.