本文提出了一种求解两相流动问题的精确稳定的界面追踪方法—ADV-VOF(accurate density and viscosity volume of fluidmethod)。该方法具有以下特征:(1)基于同位网格系统;(2)使用PLIC方法捕捉界面和计算网格边界上精确的密度和粘度值;(3)采用守恒的Navier-Stokes方程,其中对流项离散格式为有界高阶组合格式—STOIC;(4)应用分步算法求解守恒的Navier-Stokes方程;(5)利用Bi-CGSTAB方法求解压力修正方程。以上特征保证了ADV-VOF方法是一种精确、稳定、高效、简便的界面追踪方法。最后我们对ADV-VOF方法和传统的IDV-VOF(inaccurate density and viscosity volume of fluid method)方法进行了比较分析,得出ADV-VOF方法的性能远远优于传统的IDV-VOF方法。
The passive phase separation concept was proposed to modulate flow patterns for heat transfer enhancement. By the flow pattern modulation, the gas tends to be near the wall and the liquid tends to be in the tube core. Experiment has been performed to verify the fresh idea and the flow pattern modulation mechanism was analyzed qualitatively. This paper focuses on the numerical simulation of the bubble dynamics for a single bubble in the vertical phase separation condenser tube to quantitatively explore the flow pattern mechanism, based on a multiscale grid system and the volume-of-fluid (VOF) method. It is found that: (1) the modulated liquid film thickness can be decreased by 70% compared to that in the bare tube region; (2) the modulated bubble traveling velocity can be doubled, causing the increased liquid velocity and velocity gradient in the annular region to weaken the fluid boundary layer; (3) the significantly increased bubble traveling velocity in the annular region promotes the mass and momentum exchange between the annular region and the core region, and yields the self-sustained pulsating flow in the core region. The above three factors are benefit for the performance improvement of the heat transfer facilities.
