Effect of pressure on slip length of supercritical water flow in graphene nanochannels

Abstract

Flow transport of supercritical fluids (SFs) in nanopores widely occurs in shale and deep geothermal systems. Pressure is an important parameter for describing SFs, but the effect of pressure on the flow and slip behaviors of SFs in nanochannels is unknown. In this study, molecular dynamics simulations of the Poiseuille flow of supercritical water (SCW) in graphene nanochannels are performed by applying a driving force to water molecules between two walls. Fluid pressure is realized by varying water density. The effect of pressure on the slip length under different surface wettability is investigated. The results show that the slip length nonmonotonically varies with the pressure, which is divided into three regions according to the three-regime-model of SFs. For the gas phase and gas-like SCW, the slip length gradually decreases with increasing pressure. For two-phase-like SCW, the slip length increases with pressure. For liquid-like (LL) SCW, the slip length decreases with increasing pressure. The slip length nonmonotonically varies due to the nonlinear variation of the fluid viscosity and interfacial friction with pressure. For LL SCW, the slip length is linearly related to the inverse of the density ratio, and the effect of pressure and wettability on the slip length is evaluated using the density ratio and energy barrier. Furthermore, the relationship between the slip length and pressure is verified for application to a wide range of wettability, different channel heights, and different fluid types and solid wall types. This study deepens the understanding of the microscale flow theory under supercritical conditions.