Molecular Dynamics Analysis Of The Influence Of Wettability On The Heat Transfer Performance Of Vapor Condensation Process

Surface condensation is one of the most important heat transfer methods.The study of condensation on the macro has been perfected relatively,but the formation process and mechanism of nanoscale droplets and liquid film in the early stage of surface condensation has not been fully understood.In the field of molecular dynamics simulation,the vapor temperature selected for simulation of water vapor condensation is mainly above 450 K,and the influence of non-condensable gases on the phase transition process is rarely studied,while the temperature of some working fluids such as water vapor in the condenser used by the power plant is below 450 K.In view of the above-mentioned situations,the molecular dynamics simulation technique was used to study the surface condensation process of relatively low temperature and density vapor in this paper.The influence of vapor temperature and non-condensable gases on condensation progress at different wettability surfaces was researched.It provides theoretical supports for further improving the heat transfer efficiency of condensation process.In order to provide better reference for condensing process in condenser,copper is chosen as the surface to carry out simulation study.The software LAMMPS was used to simulate.The main work of this paper is as follows:The TIP4 P model was used to describe the water molecules,and the EAM model was used to describe the copper atoms.The TIP4 P water model was verified by density and thermal conductivity,and it was found that the error between the simulated value and the experimental value was small,which could meet the simulation requirements.The contact angle is the key parameter to characterize the wettability.A model of a water droplet moving on the copper surface was established,which was used to simulate the change of the potential well of the interaction potential of copper and water with the contact angle of water droplets.It was found that the potential well from 0.01 ev to 0.03 ev could characterize the wettability of the copper surface from hydrophobicity to hydrophilicity.A model which can supply the water vapor to the copper surface continuously has been established,which has been improved to adapt to the low density of water vapor.The condensation process of water vapor on hydrophilic surface and hydrophobic surface was simulated by this model.The results show that the heat and mass transfer performance of the vapor of different temperature condensing on the hydrophilic surface is the same: as the water vapor contacts the surface,and the heat transfer efficiency is basically unchanged.However,when the vapor condenses on the hydrophobic surface,the heat and mass transfer performance is affected by the vapor temperature: when the temperature is 450 K,the vapor condenses quickly after contacting with the surface,and the heat transfer efficiency changes little;while the temperature is between 350 K and 400 K,it takes a long time for the vapor contacting with the surface to start the condensation.The heat transfer efficiency first increases and finally becomes constant.hence,according to the change of heat transfer efficiency,the process of condensation of vapor below 450 K was divided into three stages: non condensation,accelerated condensation and stable condensation.It shows that the stronger surface wettability and higher vapor temperature can improve the heat transfer performance.The lower the vapor temperature,the greater the influence of wettability on the heat transfer efficiency,and the less vapor condenses on the hydrophobic surface.This indicates that for the vapor condensation process at lower temperature,the excessive wettability may play a counterpart role in the condensation process.In addition,the non condensable gas was added to the above condensation model to simulate the vapor condensing process containing non condensable gas.The simulation results show that the non condensable gas has a inhibitory effect on condensation,and the effect of the non condensable gas on hydrophobic surface is larger than that on the hydrophilic surface.Therefore,when selecting hydrophobic materials for condensation,more attention should be paid to prevent leakage of non condensable gases in equipment.