Experimental And Numerical Study On Heat Transfer Enhancement Of Nanofluids

Our research focuses on convective heat transfer characteristics of nanofluid. The main purpose is to explore the factors impact on nanofluid convective heat transfer characteristics with naofluids thermal conductivity and viscosity of experimental and theoretical analysis. Based on lattice Boltzmann method, we can reveal the reason that heat transfer enhancement of nanofluid.The conductivities, viscosities and heat transfer characteristics of nanofluids with CuO, SiO2, Al2O3 were measured respectively. Under different particle size, material and volume fraction conditions, the experimental results show that the nanofluids conductivities increase with the volume fraction of particles but decrease with the particle size. The nanofluid thermal conductivity is proportional to the particle thermal conductivity. Through investigation on the mechanism of nanofluids thermal conductivity, we find the two important factors:one is the micro-convection of nanoparticles, the other is the effect of solid-liquid interface.Through the experimental analysis of viscosity, we can see that the low concentration of nanofluid is a kind of ideal Newtonian fluid. The viscosities increase with volume fraction linearly and the size effect of nanoparticle is very obvious. The viscosity increases with decreasing particle size. In this paper, for the 7nm SiO2-water nanofluid at a volume fraction of 2%, there is an increase about 120% of the viscosity in contrast to water viscosity. pH value of nanofluids plays a very important role in nanofluid viscosity. The study on pH value shows that pH value increases with addition of nanoparicle. For the nanofluid with smaller than 20nm particle size, the pH value effect is more significant than the nanofluid with bigger particle size. And the viscosity of different particle size will be response to the different pH value. With the increasing particle size, the peak value of viscosity will be moved to the bigger pH value.In the convective heat transfer experiments, the sample nanofluids used in this study show that the heat transfer performance of nanofluid is better than the base fluid in terms of Nu with Re, and the convective heat transfer coefficient of the dispersion increases with the volume fraction of nanoparticles. From the sample nanofluids with different particle size we can see that the convective heat transfer coefficient of SiO2-water and A12O3-water nanofluids increase with the decreasing particle size. But the convective heat transfer coefficient of CuO-water nanofluid increase with the particle size in the turbulent region. Analysis the reason for this Phenomenon maybe the viscosities of SiO2-water and A12O3-water nanofluids are much higher than CuO-water nanofluid, so in turbulent region, the turbulence effect will be surpass the conductivity increase caused by Brownian motion. The bigger particle will be likely appeared in the boundary layer, this Equivalent to decrease the thermal boundary layer. So, from a macro perspective, the convective heat transfer coefficient of CuO-water nanofluid increases with the particle size in the turbulent region.To understand the heat transfer enhancement mechanism of the nanofluid flow from the particle level, the Lattice Boltzmann method is used because of its mesoscopic feature and its many numerical advantages. The buoyancy force, gravitational force, Brownian force, drag force and interaction potential were introduced into the two-component LBM model. From the simulation results, it can be seen that the temperature distribution of the nanofluid seems to become irregular compared to that of pure water because of the Brownian motion of the suspended nanoparticles under the action of various forces. The convective heat transfer coefficient of CuO-water nanofluid increases with volume fraction but decreases with the particle size. For example, the enhancement of the heat transfer coefficient with 0.3% volume fraction is about 5%, the enhancement of the heat transfer coefficient with 0.5% and 0.8% are about 11% and 21% respectively. At the same time, we have compared the simulation results with the experiment results under Re=800,1000, 1200,1400 conditions. It can be seen that the simulation results coincide with the experimental results. The results also proved that the feasibility and effectiveness of the lattice Boltzmann model in successful simulation of nanofluid flow and heat transfer characteristics.