Natural Convection Heat Transfer Of Nanofluids In A Square Enclosure

Nanofluids as a new kind of working media for heat transfer have graduallybecome a research focus in recent years. It’s well known that the thermalconductivity of nanofluids is bigger than that of water at static state, butresearches about the effect of thermal conductivity of nanofluids in practicalapplication, for example natural convection, are still relatively little.Therefore, Al₂O₃-water nanofluids, TiO₂-water nanofluids andnanodiamond-water nanofluids are prepared and their thermal conductivity aswell as rheological property are tested. Test results show that the thermalconductivity of Al₂O₃-water nanofluids is the highest and that of nanodiamond-water nanofluids is the lowest in condition of the same temperature and weightpercentage. There exists deviation between predicted value and test value aboutthermal conductivity at different temperatures for the same nanofluids. Thereason is the Maxwell model doesn’t fully consider the effect of temperature.Nanofluids at low volume fraction can be treated as Newtonian fluid. In therange of low shear rate, nanofluids present the shear-thinning; in the range ofhigh shear rate, nanofluids appear shear-thickening.Natural convection experimental setup is built in this work. Research aboutnatural convection heat transfer of nanofluids in a square enclosure is conductedusing water and TiO₂-water nanofluids（vol=0.94%） as working mediarespectively. Experimental results indicate that natural convection heat transferof TiO₂-water nanofluids isn’t better than water, even when Ra is low, thereexists a relatively bigger difference between the experimental results andtheoretical results of water. The possible explanation is that the negative effectof enhanced viscosity is stronger than the positive effect of enhanced thermalconductivity on the natural convection in this work.According to the coupled lattice BGK model, a Lattice Boltzmannsimulation program for natural convection heat transfer of nanofluids in a squareenclosure is made. The simulation of natural convection heat transfer experimentof water and TiO₂-water nanofluids（vol=0.94%） shows that the numerical resultsincrease linearly, which is as same as the theoretical values. Numerical results are a little smaller than experimental results, but they agree with each otherbasically. The numerical results of natural convection heat transfer of differentvolume fractions Al2O3-water nanofluids are a little bigger than the experimentalresults, but little difference exists when the volume fraction increases.