Abstract: The morphological evolution and migration dynamics of discrete bubbles in variable diameter pipelines have not been clearly analyzed. The deformation dynamics behavior of discrete bubbles in a variable diameter circular tube is an important theory to guide the transport and separation of gas-liquid two-phase mixture. Therefore, high-speed camera technology, combined with numerical simulation, is used to explore the migration and fragmentation mechanism of discrete bubbles in variable diameter pipes. For the structure of a variable diameter circular pipe, a study on the migration behaviors of discrete bubbles in the variable diameter field is conducted under different Reynolds numbers and bubble sizes. The flow pattern, velocity field and bubble deformation characteristics within the variable diameter circular pipe at various inlet Reynolds numbers are analyzed. The aim is to explore the interaction patterns between the flow field characteristics and discrete bubbles and provide theoretical support for revealing the motion and deformation mechanism of discrete bubbles in variable cross-section field. The results indicate that the surrounding fluid velocity is altered by bubbles. The velocity gradients are increased, and the turbulence kinetic energy in the surrounding flow field is elevated. At the same time, the dramatic change of turbulent kinetic energy in the sudden expansion section leads to the rapid deformation or even fragmentation of discrete bubbles in the variable diameter circular tube field. Additionally, as the inlet Reynolds number increases, the fragmentation position of bubbles in the flow field tends to approach the sudden expansion section. When Re=5.16×103, the shortest bubble fragmentation distance is 16.09 mm. When the Reynolds number is constant, as the bubble radius increases from 2.5 mm to 4.5 mm, the dimensionless maximum deformation of the bubble is increased from 0.26 to 0.67. The numerical simulation results demonstrate good agreement with experimental findings.

Key words: variable diameter circular tube, bubbles, flow field characteristics, deformation characteristics, migration behavior

摘要： 离散气泡在变径圆管内的变形动力学行为是指导气液两相介质输运及分离的重要理论。针对变径圆管结构，开展不同雷诺数及气泡粒径条件下的变径场域内离散气泡的运动行为研究。分析不同入口雷诺数下变径圆管内的流型流态、速度场以及气泡变形特性，探索变径圆管内流场特性与离散气泡间的相互作用规律。结果表明，气泡使其周围的流体速度变化，造成速度梯度增加，致使其周围流场的湍动能上升；此外，随着入口雷诺数增大，气泡在流场内的破碎位置越趋向于突扩截面处，当入口雷诺数Re=5.16×103时，气泡的破碎距离最短为16.09 mm；当雷诺数不变，气泡半径从2.5 mm增大至4.5 mm时，气泡的无量纲最大变形量由0.26增加至0.67，数值模拟结果与实验结果呈现出较好的一致性。

关键词: 变径圆管, 气泡, 流场特性, 变形特性, 运动行为