Heat Transfer Enhancement And Flow Characteristics Of Nano Fluids By Molecular Dynamics Simulation

Nanofluid is a new functional fluid that has been developed in recent decades.It is very important to study the thermal properties and micro-mechanism of nanofluids,which is of great significance to solve the problem of heat dissipation and flow in microsystem.The scale of nanoparticles is close to the scale of atomic or molecular diameter,and molecular dynamics simulation is the main method to study nanometer scale problems.In this paper,water-based nanofluid is the main research object,and the heat transfer mechanism and the flow characteristics of nanofluid was analyzed.The main conclusions for this study are as fllows:In this paper,the molecular dynamics software was used to simulate the thermal conductivity of the basic liquid water.The accuracy of the model and the calculation method was verified by comparing it with the existing experimental data.Then,different types of nano-fluid models were established to study the microscopic mechanism of thermal enhancement of nanofluid.The simulation results show that the aggregation and interaction between the nanoparticles become stronger and the thermal conductivity of the nanofluid increases with the increase of the volume fraction of the nanoparticles.Box shape and spherical nanoparticles are more likely to collide with each other,thus strengthening the thermal conductivity of nanofluids.The nanoparticle property is also one of the main factors that affect the thermal conductivity of nanofluids.The ratio of atoms with high potential energy is higher,so that the thermal conductivity of nanofluid increases significantly.With the increase of temperature,the strength and frequency of collision between the internal particles of nanofluids increase which leading to the increasing thermal conductivity of the nanofluids.The simulation results show that the microstructural changes of the basic liquid and the micro-motion behavior of the nanoparticles all affect the energy transfer process within the nanofluids,which is the main micro-mechanism leading to the thermal enhancement of the nanofluids.The viscosity of nanofluids is closely related to the shear rate,temperature,volume fraction and the shape of nanoparticles.In the process of confined shear flow,the random motion behavior of the nanoparticles and the shear behavior of the plates affect the flow characteristics of the nanofluids.The collision,aggregation and separation of nanparticles happened in the process of confined shear flow,and there is a thin layer of liquid adsorption on the surface of the nanoparticles.In addition to the random movement of nanoparticles in x direction,there also have random movements in y and z direction.Various movement behavior of nanoparticles occurred,such as translation,rotation and Brownian motion,and the random motion behavior of the nanoparticles is mainly due to the small scale.The shear flow has a certain effect on the translational velocity of the nanoparticles,and the translational velocity components in x direction are slightly larger than those in y and z direction,but they are still in the same order of magnitude.The collision,aggregation and separation behavior of nanoparticles make the atom density of nanofluids appear unevenly distributed,which is beneficial to strengthen the mass transfer process of nanofluids.The microstructure of the nanofluids changes obviously in the electrostatic field,and water molecules of nanofluids have a perfect structure,which is similar to ice structure.With the increase of electric field intensity,the intramolecular energy of nanofluids decreases and the diffusion coefficient of water molecules in nanofluids decreases drastically.The thermal conductivity of nanofluids in x and y directions are same,which is smaller than that of nanofluid in z direction.The thermal conductivity of the nanofluids exhibits anisotropic characteristics under the action of the electric field,and the anisotropy of the thermal conductivity becomes more pronounced as the electric field strength(0-3.5V/?)increases.The electrostatic field changes the microstructure of the nanofluids,and the interaction motion between the nanoparticles and the basic liquid water molecules make a difference.The variation of the microstructure and the particle motion behavior enhances the energy transfer process and the conversion efficiency of nanofluids to a certain extent,and strengthens the thermal conductivity of the nanofluids.