Water flow in a nanoscale channel is known to be affected by strong water–wall interactions; as a result, the flow considerably deviates from the conventional continuum flow. Nanochannel with a sudden contraction or expansion is the most fundamental morphological nanostructure in many nanoporous systems such as shale matrix, mudrock, membrane, etc. However, the nanoconfinement effects of water flow in nanoporous systems and its effect on macroscopic flow behavior are still evolving research topics. In this work, our recently developed pore-scale lattice Boltzmann method (LBM) considering the nanoscale effects is extended to directly simulate water flow in nanoporous systems. The results show that the flow rate is dramatically decreased in hydrophobic nanopores because of additional flow resistances at the contraction and expansion junctions. This indicates that the bundle of capillary models or the permeability averaging method overestimates the water flow rate in nanoporous media if the contraction/expansion effects between different nanopores are ignored. This work highlights the importance of wettability of the nanochannel in the determination of dynamic water flow behaviors in the contraction/expansion nanosystem. Other important aspects, including velocity distribution, flow patterns, and vortex characteristics as well as pressure variation along the flow direction, are for the first time revealed and quantified. Large differences can be found comparing gas or larger-scale water flows through the same system. A new type of pressure variation curve along the axis of flow direction is found in the hydrophobic nanochannel with a sudden contraction/expansion. This work provides the fundamental understanding of water transport through the nanoscale system with contraction and expansion effects, giving implications to a wide range of industry applications.

中文翻译：

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模拟突然收缩或膨胀的纳米通道中的水流

众所周知，纳米级通道中的水流会受到强水壁相互作用的影响。因此，流动大大偏离了传统的连续流动。具有突然收缩或膨胀的纳米通道是页岩基质、泥岩、膜等许多纳米多孔系统中最基本的形态纳米结构。然而，水流在纳米孔系统中的纳米限域效应及其对宏观流动行为的影响仍在不断发展研究。话题。在这项工作中，我们最近开发的考虑纳米尺度效应的孔隙尺度晶格玻尔兹曼方法（LBM）被扩展到直接模拟纳米多孔系统中的水流。结果表明，由于收缩和膨胀接头处的额外流动阻力，疏水纳米孔中的流速显着降低。这表明如果忽略不同纳米孔之间的收缩/膨胀效应，毛细管模型束或渗透率平均方法会高估纳米多孔介质中的水流速。这项工作强调了纳米通道的润湿性在确定收缩/膨胀纳米系统中的动态水流动行为中的重要性。其他重要方面，包括速度分布、流动模式和涡流特性以及沿流动方向的压力变化，首次被揭示和量化。比较通过同一系统的气体或较大规模的水流可以发现很大的差异。在具有突然收缩/膨胀的疏水纳米通道中发现了一种新型的沿流动方向轴的压力变化曲线。