Couette flow at high Knudsen number between wall and liquid boundaries

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.

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

Dong,

Xu,

Wang

2023

Hermite spectral method for the inelastic Boltzmann equation

Li,

Lu,

Wang

2023

We propose a Hermite spectral method for the inelastic Boltzmann equation, which makes two-dimensional periodic problem computation affordable by the hardware nowadays. This new approach involves utilizing a Hermite expansion, whereby the expansion coefficients for the variable hard sphere model are simplified into a series of summations that can be precisely derived. Additionally, a new collision model is built with a combination of the quadratic collision operator and a simplified collision operator, which helps us to balance the computational cost and the accuracy. Various numerical experiments, including spatially two-dimensional simulations, demonstrate the accuracy and efficiency of this numerical scheme.

Simulation of flow and debris migration in extreme ultraviolet source vessel

Meng,

Zhao,

et al. 2024

Practical extreme ultraviolet (EUV) sources yield the desired 13.5 nm radiation but also generate debris, significantly limiting the lifespan of the collector mirror in lithography. In this study, we explore the role of buffer gas in transporting debris particles within an EUV source vessel using direct numerical simulations. Our study involves a 2 × 1 × 1m3 rectangular cavity with an injecting jet flow subjected to sideward outlet. Debris particles are introduced into the cavity with specified initial velocities, simulating a spherical radiating pattern with particle diameters ranging from 0.1 to 1 μm. Varying the inflow velocity (from 1 to 50 m/s) of the buffer gas reveals a morphological transition in the flow field. At low inflow velocities, the flow remains steady, whereas higher inflow velocities induce the formation of clustered corner rolls. Upon reaching sufficiently high inflow velocities, the jet flow can penetrate the entire cavity, impacting the end wall. Interestingly, the resulting recirculation flow leads to the spontaneous formation of spiraling outflow. The distinct flow structures at various inflow velocities lead to distinct patterns of particle transport. For low-speed gas, it is efficient in expelling all particles smaller than 0.4 μm, while for high-speed gas, those fine particles accumulate near the end wall and are challenging to be extracted. Our findings highlight the significance of controlling flow conditions for effective debris particle transport and clearance in diverse applications especially in EUV source vessels.