Abstract: In order to investigate the complex characteristics in the wake of Taylor bubble in cryogenic liquid tube flow, the flow field in the wake was measured using particle image velocimetry (PIV) technique. The PIV measurement results were further analyzed by snapshot proper orthogonal decomposition (POD). The criterion for determining the wake structure using inverse viscosity number Nf is applicable for liquid nitrogen if surface tension effect can be neglected in a vertical tube, but not applicable for the tube at the inclination of 60o to the horizontal. The Nf number of present experiment is much larger than the critical value for the occurrence of an open wake. However, a closed wake was observed both in instantaneous and average flow field as q being above 60o. The axial size of vortex in the wake (Lwake) increases with the increase of Taylor bubble length (LTB) in an exponential function of Lwake= 51.3-123.5exp(-LTB/2.9). POD analysis shows that the first several POD modes can effectively capture the main features in Taylor bubble wake. The first 4 modes occupy the 15.6%, 6.4%, 5.6% and 3.8% of total energy. The vortexes distribution coincides with the maximum velocity fluctuation. The present study explores the scope of the theory for normal atmospheric temperature liquid.

Key words: cryogenic liquid, Taylor bubble, particle image velocimetry, proper orthogonal decomposition, wake region, flow structure

摘要： 为了研究低温流体中Taylor气泡周围复杂流场的平均及瞬变特征，搭建了用于低温流体粒子图像测速实验平台，使用快照本征正交分解(POD)方法详细分析了Taylor气泡尾迹区内流场结构. 结果表明，在表面张力作用可忽略的情况下，使用流体逆粘度数Nf判断竖直管路中尾迹区流型的方法对液氮流体仍然适用，当管路水平倾角q≤60o时，该法不再适用. Taylor气泡尾迹区在90o≤q≤60o内由开放尾迹转变为封闭尾迹，30o倾角下尾迹区涡的轴向尺寸(Lwake)和Taylor气泡长度(LTB)呈指数函数关系，Lwake=51.3-123.5exp(-LTB/2.9). POD分析表明，尾迹区存在明显的湍流大尺度结构，前4阶模态分别占据总能量的15.6%, 6.4%, 5.6%和3.8%. 距离Taylor气泡尾部0.5~1.0D(D为管路内径)的位置存在大尺度结构中涡的交界面，此处的速度脉动也最强. 本研究拓展了常温流体中Taylor气泡尾迹区理论在低温流体中的适用范围.

关键词: 低温流体, Taylor气泡, 粒子图像测速, 本征正交分解, 尾迹区, 流场结构