Study On Mechanism Of Thermal Conductivity Enhancement Of Nanofluids

Heat transfer of nanofluids has been studied for about 20 years as an emergent interdiscipline. Many researchers found that most nanofluids are of attractive character of great thermal conductivity (TC) enhancement, and many models had been developed to explain the TC at nanoscale. However, there is no theory that can explain the enormous TC enhancement of gold nanofluids compared to its extremely low volume fraction.Enlightened by those theories proposed by many researchers, we proposed that the TC of gold nanofluids was mainly affected by clusters and Brownian motion of gold nanoparticles(NPs) other than the basic TC of solid-liquid suspension which is close to nothing. Here we presented a new model in which fractal theory was adapted to simulate the NPs’distribution to explain the clusters’effect on TCE, fractal dimension Df was obtained by calculating the TEM images through box-counting method, a set of ramdon numbers was used to calculate clusters’size and micro-convection heat transfer model was induced to explain the dynamic term of TC. A factor f(Φ,t) was used to modify this model against the experimental data.It has fully taken clusters, distribution, micro-convection induced by Brownian motion, temperature and diffusion of particles into consideration, which is able to predict the TC enhancement of gold nanofluids with a deviation of less than 1.5% from experimental data. We also have discovered that, for this kind of nanofluid with extremely low volume fraction, the effect of temperature affect the TC was exponential and much greater than the effect of either the volume fraction or particle size. Therefore, this model provides not only a new direction of explaining TCE of nanofluids, but also theoretical foundation of application on heat transfer enhancement of nanofluids.