| With the increasing power performance of internal combustion engines,the thermal load of equipment is also increasing dramatically.The cooling capacity of traditional coolants has its upper limit,and the use of cooling media with better heat transfer performance can break the cooling limit of traditional coolants.Nanofluids have better heat transfer performance than base fluids,and this characteristic provides ideas for the development of new engine coolants.In this study,the conventional alcohol-based coolants have been used as the base fluids and nanoparticles have been added to them to form the nanofluid coolants.The thermal conductivity of several nanofluid systems has been simulated and the microscopic mechanism of the enhanced thermal conductivity of nanofluids has been analyzed.The results of the study may provide some reference for the application of high thermal conductivity nanofluids in engineering.Based on the above concept,this study was carried out step by step.The thermal conductivity of Au-Ar nanofluid systems were firstly calculated by molecular dynamics method.The influence law of volume fraction and particle size on the thermal conductivity of nanofluids was obtained by changing the volume fraction and particle size of nanoparticles,and the inner mechanism of thermal conductivity enhancement of nanofluids was also obtained by studying the system structure change and the nature of adsorption layer on the surface of nanoparticles.Then the conventional ethylene glycol and propylene glycol coolants were used as the research objects,nanoparticles were added to them,and the thermal conductivity of the nanofluid coolants were calculated and compared with traditional coolants.The structural changes of the nanofluid systems and the related properties of the adsorption layer on the surface of the nanoparticles were explored.The results show that the thermal conductivity of the Au-Ar nanofluid system is positively correlated with the volume fraction of nanoparticles and negatively correlated with the particle size.The thermal conductivity calculated by equilibrium molecular dynamics and nonequilibrium molecular dynamics is not much different.The calculation results using the LJ potential function between Au atoms are slightly higher than the EAM potential,and the maximum error between the two potential functions is 5.4%.There is a non-escape adsorption layer of Ar atoms on the surface of nanoparticles,and the thickness of adsorption layer on the nanoparticle surface is about 0.35 nm in the system with the particle radius of 0.8 nm.The thickness of the adsorption layer is negatively correlated with the particle size,that is,the smaller the particle size,the thicker the adsorption layer and the higher the thermal conductivity of the nanofluids.The arrangement of the base liquid atoms in the adsorption layer is similar to the characteristics of the orderly arrangement of solids,which makes the nanofluids exhibit the microstructural characteristics similar to that of solids.The presence of the adsorption layer is the intrinsic dominant factor for the improvement of the thermal conductivity of nanofluids.It is also found that the thermal conductivity of ethylene glycol-water coolants is positively correlated with the water content of the systems,and the addition of Au nanoparticles increases the thermal conductivity of the coolants at all concentrations,with the maximum increase up to 17.2%.The addition of the nanoparticles has no significant effect on the conformation of ethylene glycol molecular chain,and slightly enhances the van der Waals interaction energy in the system,but has no significant effect on the Coulomb interaction energy.Through the analysis of the radial distribution function and density distribution of the nanofluid systems,it is found that the base solution molecules form a high-density aggregation region around the nanoparticles.The high-density interval of ethylene glycol molecules is(0.65,1.25)nm due to the different strength of the particles on different types of base solution molecules.However,water molecules mostly gather in the form of clusters in the(1.15,1.65)nm interval,where the maximum density layer of water molecules appears near 1.5 nm away from the center of the nanoparticles,that is,there is stratification of the base liquid molecules in the adsorption layer on the surface of nanoparticles.At the same time,it is also found that the thermal conductivity of propylene glycolwater coolants is also positively correlated with the water content of the systems.Under the premise of the same type,particle size and volume fraction of the added nanoparticles,the maximum increase in thermal conductivity of propylene glycol nanofluid coolants is 15.1%,which is slightly lower than that of ethylene glycol nanofluid coolants.The existence and stratification of the adsorption layer on the surface of the nanoparticles are the common characteristics in the alcohol nanofluid systems.However,in the propylene glycol nanofluid coolants,the maximum density layer of water molecules is located near 1.8 nm away from the center of the nanoparticles.This indicates that the closer the position of the layer with the highest density of water molecules,the less obvious the stratification of the adsorption layer,the higher the mixing degree of alcohol molecules and water molecules in the adsorption layer,and the greater the thermal conductivity of alcohol nanofluid system.In the anhydrous propylene glycol nanofluid coolants,the increase of thermal conductivity is related to the type of nanoparticles added.The specific performance is Cu>Au>Fe,that is,the higher the thermal conductivity of the nanomaterial added in the base fluid,the greater the thermal conductivity of the nanofluid formed.This is because the higher the thermal conductivity of the nanomaterial,the greater the number density of the base fluid molecules in the adsorption layer,the better the heat transfer performance of the adsorption layer,and the greater the macroscopic thermal conductivity of the nanofluid. |
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