| Coal-fired and nuclear power are the main way of generating electricity.Under actual high temperature and high pressure operating conditions of boilers and nuclear reactors,surface boiling occurs on metal walls.However,the surface wettability will affect the efficiency of heat energy conversion and utilization and the safety of equipment.At present,most of the researches on water wetting on metal surfaces under high temperature and pressure conditions are based on the measurement of apparent phenomena such as contact angle,and the research on the microscopic mechanism of surface wetting is still very scarce.In this paper,molecular dynamics simulation is used to study the wettability of water droplets on the surface of metal(nickel)under high temperature and high pressure,which is a study of wetting behavior at the molecular levelFirstly,the molecular dynamics simulation of high temperature and high pressure pure water at different temperatures was carried out.The number,average energy and spatial distribution of hydrogen bonds in high temperature and pressure water over a large density range(0.4ρ_c~2ρ_c)were obtained.The results show that the number and average energy of hydrogen bonds in high temperature and high pressure water increases with the increase of its density,but they increase more slowly whenρ>1.4ρ_c.At higher temperature,the number and average energy of hydrogen bonds are less.The bonding space uniformity of hydrogen bond in high temperature and high pressure water increases with the increase of density and temperature.Secondly,we simulated the infiltration process of spherical nano-droplets on a smooth nickel plate in the temperature range of 298 K~538 K,pressure of 7 MPa,15 MPa and 20 MPa.The law of static contact angle changing with temperature is obtained and compared with the experimental value.The results show that the contact angle of nano-droplets decreases with the increase of temperature in the range of 298 K to 538 K.The decline curve is divided into two straight lines with different slopes:the first drops slowly and the second drops rapidly.The segmented temperature is 458 K,which is similar to the experimental contact angle-temperature curves.Under the condition of constant temperature,the contact angle of nano-droplets increases slightly with the increase of pressure.Finally,the change of interfacial tension with temperature was studied,and the Young’s equation is used to verify the law of contact angle changing with temperature.The relationship between the number,average energy of nano-droplet hydrogen bonds and temperature,pressure was studied which was used to explain the changes of contact angle.The results show that the interfacial tension of solid-liquid and vapor-liquid fluctuates and decreases with the increase of temperature.However,the interfacial tension of solid-vapor increases with the increase of temperature.Therefore,the trends of interface tension and contact angle are consistent with Young’s equation.Under certain pressure,with the increase of temperature,the number of hydrogen bonds in nano-droplets decreases gradually.In the range of 298 K to 538 K,the total number of hydrogen bonds decreased by 63%,but the pressure had little effect on the number of hydrogen bonds. |
