Three-dimensional CFD analysis of liquid slug acceleration and impact in a voided pipeline with end orifice

This research describes the dynamic behavior of an isolated slug driven by pressurized air in a voided line with an end orifice. A three-dimensional (3D) computational-fluid-dynamics (CFD) model is used to simulate the rapid propulsion and impact at the orifice for given slug length and driving air pressure, and is validated against experimental data. New mechanisms are observed: (<i>i</i>) the driving air pressure at the slug tail decreases with the slug motion; (<i>ii</i>) when the slug arrives at the orifice, the air fraction is almost one hundred percent in the most upstream part of the pipe, then it attenuates rapidly until an invariant eighty percent is achieved with a constant mass shedding rate; (<i>iii</i>) the velocity distribution in the radial direction of the cross-section at the midpoint of the slug length evolves from uniform to trapezoidal and then to logarithmic during slug movement; (<i>iv</i>) the initial vertical slug front changes its shape due to air intrusion at the top of the slug front; (<i>v</i>) the slug’s acceleration decreases first and then increases under the combined effects of its decreasing mass, nonlinear attenuation of the driving pressure, and increasing skin friction; (<i>vi</i>) the slug length has a constant rate of decrease.本研究针对带末端孔口的空管内，由压缩空气驱动的孤立段塞（slug）的动态特性展开分析。本研究采用三维（3D）计算流体动力学（CFD）模型，针对给定段塞长度与驱动气压的工况，模拟段塞的快速推进与孔口撞击过程，并通过实验数据对模型进行验证。研究中观测到多项新机制： （i）段塞尾部的驱动气压随段塞运动逐渐降低； （ii）当段塞抵达孔口时，管道最上游区域的含气率近乎100%，随后含气率快速衰减，最终在恒定质量脱落速率下稳定至80%； （iii）在段塞运动过程中，段塞中点横截面的径向速度分布会从均匀分布逐步演变为梯形分布，最终转变为对数分布； （iv）初始竖直的段塞前缘会因顶部发生空气侵入而改变形状； （v）在段塞质量减小、驱动气压非线性衰减以及壁面摩擦阻力持续增大的共同作用下，段塞的加速度呈现先降低后升高的变化趋势； （vi）段塞长度以恒定速率逐渐缩短。