Effect Of Nanofluid Microstructure On Its Mass Transfer And Heat Transfer

The concept of nanofluids was first proposed by Choi et al.of Argonne National Laboratory in 1995.It is a kind of stable suspension formed by adding a small amount of nanoparticles(usually from several nanometers to several hundred nanometers in size,usually made of metal,metal oxides,non-metallic organic compounds or inorganic compounds)into the liquid medium(base liquid)..It has received extensive attention from scholars because it greatly enhances the heat transfer capability of the base fluid.Nanoparticles suspended in nanofluids can enhance their thermal conductivity,but the mechanism by which nanoparticles enhance heat transfer is still unclear.At present,some studies have shown that the aggregation of nanoparticles is one of the important mechanisms to enhance the thermal conductivity of nanofluids.The fractal dimension of the nanoparticle aggregate morphology has a significant impact on the thermal conductivity of the nanofluid.In this paper,the Green-Kubo formula is used to calculate the thermal conductivity under equilibrium molecular dynamics.At the same time,the fractal dimension under different aggregate patterns is calculated by Schmidt-Ott relation.Comparing the thermal conductivity with the fractal dimension at the same volume fraction,it can be seen that there is a higher thermal conductivity at lower fractal dimensions.In addition,the difference in tight and loose aggregation of nanoparticles can be seen by the radial distribution function.The base liquid molecules reach a dynamic equilibrium in the nano-thin layer near the nanoparticles.This helps us understand the effect of nanoparticle aggregate morphology on thermal conductivity.These results are of great significance for guiding nanofluids with different microstructures in different applications.In addition,with the development of nano-fuels,the influence of nanoparticle behavior on the evaporation characteristics of nano-fuels has attracted the attention of scholars.The evaporation behavior of nanofluid droplets containing nanoparticles plays an important role in the combustion of nanofuels.In the final stage of nano fuel droplet combustion,the average evaporation rate(AER)of nano fuel droplets will drop sharply because the nanoparticles aggregate on the surface of the droplets to form a hollow shell.In order to illustrate the microscopic mechanism of the nano-fluid surface tension and the average evaporation rate caused by the nano-particle motion behavior,the numerical simulation method based on Brownian dynamics is used to investigate the surface tension and average evaporation rate of nanoparticle motion behavior.By simulating the liquid evaporation under different volume fractions and different nanoparticle distribution patterns,it was finally found that the distribution of nanoparticles and the volume fraction of nanoparticles would greatly affect the average evaporation rate of nano fuel droplets.And Dh-law can be derived,that is,the h-th power of the equivalent diameter D of the nanofluid droplets is proportional to the evaporation time t,and the size of h is related to the volume fraction of the nanoparticles in the nanofluid.The conclusion of this study has an important guiding role for how much proportion of nano-particles can be added to nano-fuels to improve their combustion performance most effectively.