Abstract: Convective heat transfer can be enhanced effectively by longitudinal vortices,thus it is meaningful to study the swirling flow induced by longitudinal vortex generator in exploring the heat transfer enhancement mechanism.The structure of the flow field and the flow characteristics in a tube with trapezoid winglets embedded were investigated by performing particle image velocimetry(PIV)measurement in both transverse and longitudinal sections.The trapezoid winglets were uniformly distributed over the circumference upstream placed at an angle of 135° to the tube wall.The experimental results showed that multi-longitudinal vortices flow was induced downstream of the winglets with four symmetrical counter-rotating vortex pairs.Two different flow patterns were in the near wall region,one was the flow towards the wall and the other was the flow away from the wall,both of which effectively increased the component of the transverse velocity perpendicular to the mainstream.In a certain range of Reynolds number involved in the experiment,it could be drawn that the maximum value of transverse velocity and the radial velocity in the sections downstream the winglets could reach 27% and 20% of the averaged mainstream velocity respectively.The continuous disturbance to the flow induced by the longitudinal vortices proceeded downstream along the tube wall,which increased the velocity of the flow near the wall significantly,by 1.0-3.6 times of that in the smooth tube.The fields of velocity and vorticity showed that the disturbance turned out to be more powerful with the increase of Reynolds number.Compared with the flow field in the smooth tube,it was found that the flow mixing generated by the winglets enhanced mass transfer between the mainstream and the fluid in the near wall region due to its enhancement of radial velocity component perpendicular to the wall,which was beneficial for convective heat transfer enhancement in the tube.

Key words: trapezoid-winglets, longitudinal vortices, tube

摘要： 诱导流体产生纵向涡结构可以有效强化对流换热,因此研究纵向涡发生器的扰流特性对于深化强化传热机理具有重要意义。利用粒子成像测速(PIV)技术,对圆管内置梯形翼片扰流后的流场进行了测量,实验中4个梯形翼片沿周向均布,与管壁夹角135°呈迎流放置,分析了横、纵截面内的流场结构和流动特点。结果显示,管内放置翼片能够诱导形成多纵向涡流动,在下游产生了4个对称的涡偶,每个涡偶对应的两个涡旋转方向相反,形成了内侧向壁流,外侧背壁流的流动结构,增大了垂直于主流方向上的速度分量。实验涉及的Reynolds数范围内,翼片下游横截面内的横向速度和径向速度分别达到主流均速的27%和20%以上;纵向涡沿壁面向下游发展可对流体产生持续扰动,使近壁流体的速度相对于光滑管提升了约1.0～3.6倍;翼片对流体的扰动作用随着Reynolds数的增大而增强,对于圆管内对流换热的强化具有促进作用。

关键词: 梯形翼片, 纵向涡, 圆管