Mathematical Model And Medium-High Temperature Experimental Research Of Nanofluid Effective Thermal Conductivity

Nanofluids that are produced by dispersed nanoparticles into traditional heat transfer fluids, show greater thermal conductivities than regular fluids. Nanofluid provides theoretical challenges because the measured effective thermal conductivity containing a few loadings of nanoparticle showed greater enhancement than traditional models predicted. The effective thermal conductivity of nanofluids as a function of nanoparticle volume fraction has been widely investigated. However, the temperature dependence has still not been fully understood, with data only available up to a maximum temperature of 138oC. This study theoretically and experimentally investigates the effective thermal conductivity of nanofluids.A mathematical model for the interfacial nanolayer with a nonlinear thermal conductivity distribution is presented, which the interfacial thermal conductivity profile and the slope are contiguous at the interface with the nanoparticle and the base fluid. An expression is given for the effective thermal conductivity of the nanofluid which includes the effect of the nanolayer. The results obtained with this model compare well with available experimental data as well as with other earlier models. The model is then used to evaluate the impact of the nanoparticle radius, nanolayer thickness, nanoparticle volume fraction, and thermal conductivity ratio of the nanoparticle to the base fluid. Thermal conductivity of nanoparticle has much less effect on the enhancement effective thermal conductivity of nanofluid than other factors, like nanoparticle dimension, interfacial nanolayer thickness, etc.A spatial averaging-based model, which considering the nanoparticle dimension and shape, thickness and nonlinear thermal conductivity distribution of the interfacial nanolayer, ratio of thermal conducitivity of the particle to the base fluid, shows good agreement with available experimental data and provides better predictions for the effective thermal conductivity compared to other models. The effective thermal conducitivity increases with decreasing nanoparticle outer diameter and increasing nanolayer thickness. The present model predicts the nanoparticle size and shape and interfacial nanolayer thermophysical properties as the important factors influencing the effective thermal conductivity of nanofluid.The nanofluid effective thermal conductivity enhancement by including aggregation volume fraction considering the aggregate state of nanoparticles is analyzed. An aggregate-based model is proposed to predict the enhanced thermal conductivity of CNT-based nanofluid. The present model gives closer predictions with majority of the previous experimental data than the results predicted by the Wiener bounds, which sheds light on the thermal conductivity mechanisms in nanofluids with respect to nanoparticle aggregate state in base fluids.The nanofluid stability is experimentally evaluated over time based on the thermal conductivity measurement at high temperatures closer to 200oC. A transient hot-wire system was used to measure the nanofluid effective thermal conductivity at medium-high temperatures. Experimental data is presented for the effective thermal conductivity of nanofluids as a function of nanoparticle volume fraction and temperature. The thermal conductivity enhancement increases much smaller at higher temperatures than at lower temperatures, probably due to nanoparticle Brownian motion induced microconvection and interfacial thermal resistance at higher temperatures for nanofluids including spherical nanoparticles. For CNT-based nanofluids, the thermal conductivity enhancement decreases with increasing temperature due to various aggregate state of the CNTs at high temperatures.