不同翼片扰流特性的PIV对比实验

利用粒子成像测速（PIV）技术对圆管内置梯形翼片后方流场进行了测量,分析了翼片迎流（UFW）和顺流（DFW）两种放置方式对流场的扰动特性。结果表明,迎流翼片形成的涡沿周向延展范围较大,持续性好,涡偶内侧为向壁流;顺流翼片形成的涡沿径向延展范围较大,在较短距离内扰动较为明显,涡偶内侧为背壁流。两种流动结构都能有效提高壁面附近的速度分量,促进主流和壁面附近流体的质量交换。随着Reynolds数增大,纵向涡的稳定性减弱,在Re=3000时,翼片的扰流效果均较好。     

圆管内置梯形翼片的流场特性PIV实验

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。     

Experimental study by PIV of swirling flow induced by trapezoid-winglets

The characteristics of the longitudinal vortex induced by trapezoid-winglets in a circular tube are investigated by the Particle Image Velocimetry （PIV） Technique with flow Reynolds number in the range of 500-13 000. In the experimental test section, four trapezoid-winglets are fixed symmetrically on the tube wall in two different ways： up-flow and down-flow. The results show that a counter-rotating vortex pair is formed behind each winglet and they distribute as a symmetrical vortex array in the transverse section. Between the two vortexes in a vortex pair the fluid flows towards the wall in the up-flow winglet case and away from the wall in the down-flow winglet case, corresponding also to the regions of peak values of the velocity components normal to the mainstream. Both of the flow patterns enhance the velocity in the near wall region, leading to the intensification of the transverse mixing and the mass transfer in the tube. With Reynolds number increasing, the flow maintains the vortex pattern in the case of the up-flow winglets, indicating better persistence of the longitudinal vortex, while the vortexes in the case of the down-flow winglets are more scattered and tend to breaking into small eddies. The trapezoid winglet shows the preferable turbulent disturbance characteristics in the tube and the experimental results provide benchmark data for further CFD studies.