Molecular Dynamics Simulation Of Thermal Conductivity Of Nanofluids

Nanofluids formed by adding a small volume fraction of solid nanoparticles to conventional fluids can greatly enhance the heat transfer performance of conventional fluids.Up the internal combustion engine heat load,the internal combustion engine cooling oil cavity need to take the increase of heat,existing coolant heat conduction ability is not enough to transfer the heat load of internal combustion engine cooling oil cavity of heat discharge of internal combustion engine,and nanofluid with a conventional fluid with high thermal conductivity,can consider to use it to solve the above problems.This research mainly focuses on the mechanism of improving the thermal conductivity of nanoparticles and the application of nanofluids in coolant.In this paper,molecular dynamics method is used for simulation calculation,and the results are analyzed and compared with the corresponding literature values,experimental values and several commonly used prediction models.Firstly,the system was modeled.The models established in this paper included Cu-Ar nanofluid model,Cu-water nanofluid model,and three kinds of alcohol(Ethanol / Ethylene glycol / 1,2 Propylene glycol)mixed with water and Cu nanofluid model.Next,the force field was applied to the system,and lj-12-6 and EAM potential functions were used for Cu nanoparticles,the water molecules using SPC/E model,alcohol molecules by PCFF force field parameters,and adopt the equilibrium molecular dynamics and non-equilibrium molecular dynamics for calculation,thermal conductivity on the volume fraction,particle size and particle surface adsorption layer on the influence of thermal conductivity calculation,alcohol concentration also considered in calculation process of the influence of thermal conductivity.The results show that the thermal conductivity of the nanofluid is positively correlated with the change of the volume fraction of nanoparticles and negatively correlated with the change of the size of nanoparticles.The thermal conductivity of several nanofluids calculated in this paper can be increased by up to 32.1% by particle volume fraction,and35% by particle size.There is an adsorption layer on the surface of nanoparticles,and the adsorption layer is the main reason for the increase of the thermal conductivity of nanofluids.It is also found that the thickness of adsorption layer decreases with the increase of particle size and is positively correlated with the thermal conductivity.The effect of the volume fraction of Cu-Ar nanoparticles on the thermal conductivity was calculated and analyzed.It was found that the thermal conductivity calculated by EMD was slightly higher than that calculated by NEMD.The NEMD result is larger than the EMD result in particle size calculation.The Ar component of the base liquid contributed the most to the thermal conductivity of the system,while the Cu particle itself and the Cu-Ar cross part contributed less to the thermal conductivity of the nanofluid.For thermal conductivity based on Green-Kubo formula according to the function decomposition,found that use LJ potential function with the collision between Cu atoms(V)related items(PV,KV,VV)plays a main role to the contribution of the thermal conductivity,some paragraphs enthalpy(h)and PP,KK contribution to thermal conductivity is very small,and the thermal conductivity and decreases as the particle size plays an obstacle,and KP cardinal constant,but in the use of the EAM potential function of the system,the contribution of KP and PP is relatively high.It is also found that the accuracy of EAM potential function is higher than that of LJ potential function between Cu atoms.The thermal conductivity of Cu-water nanofluids is similar to that of Cu-Ar nanofluids through the analysis of the calculation results of Cu-water and several alcohols.In Cu-water nanofluid,the arrangement of H(Water)and O(Water)atoms in the adsorption layer region on the particle surface is different,with O atoms closer to the particle surface and H atoms farther away.Next,the thermal conductivity of different concentrations of Ethanol / EG /PG-aqueous solution and the nanofluids formed by using it as the base solution were calculated and analyzed,the results showed that the alcohol solution of thermal conductivity increases with the increase of the proportion of water,this is mainly due to the increased with the increase of the proportion of water,gradually reduce the system temperature gradient,cause the fluid in the process of heat conduction thermal resistance is reduced,less energy loss,caused by heat conduction ability stronger.The analysis of alcohol-Cu nanofluid showed that the interaction between alcohol molecules and water molecules was not related to the content of the two molecules.The adsorption layer on the surface of the particles is stratified,with alcohol molecules in the inner layer and water molecules in the outer layer.The stratification phenomenon is mainly reflected in the maximum water molecular density layer,in which the adsorption layer of Ethanol nanofluid is obviously stratified,and the maximum water molecular density layer is at r=2 nm.Secondly,NP stratification in PG nanofluid,the highest water molecular density layer is at 1.8 nm.Finally,EG nanofluid,the maximum density layer of water molecules is about 1.5 nm,which is relatively earlier than the first two.It is also found that the closer the maximum water molecular density layer is to the surface of the particles,the higher the thermal conductivity of alcohol nanofluids is.The existence of adsorption layer is an important reason for improving the thermal conductivity of alcohol nanofluids.