Abstract:

The flow field nearby standard Rushton impeller in a stirred vessel was investigated by volumetric three-component velocimetry (V3V) measurements, which about 18000 vectors were captured in a cubic volume of approximately 140 mm in height by 140 mm in width by 100 mm in depth. The characteristics of three-dimensional flow field and the structure of trailing vortices were derived from velocity data. The distributions of radial, axial and tangential velocities in 30° cross-section behind the impeller were analyzed and the distributions of radial and axial velocities by V3V were in good agreement with these by 2D-PIV (particle image velocimetry), especially for trailing vortices in the region of discharge streams. The 2D-PIV study on development of trailing vortices shows that the trailing vortices had an upward inclination at an angle of 10° to the horizontal line as a result of influence by the liquid free surface. The traces of movement were asymmetrical between upper and lower trailing vortices in a way that the movement and decay of the lower vortex were slightly faster than the upper one, which is consistent with the V3V results. Further, trailing vortices were still visible at 60° angle behind the impeller blade. Compared results of both V3V and 2D-PIV to that of pseudo-isotropic approximation, the normalized turbulent kinetic energy (k/V_tip) by pseudo-isotropic approximation was overestimated 25%-33% at locations of trailing vortices and the impeller, where turbulence tends to be anisotropic.

Key words: V3V, PIV, trailing vortex, Rushton impeller, flow field, isotropy

摘要：

用体三维速度测量技术（volumetric three-component velocimetry measurements，V3V）实验研究了涡轮桨搅拌槽内桨叶附近流场。通过速度数据得到三维流场特性，确定尾涡三维结构；分析了叶片后方30°截面轴向、径向和环向速度沿径向分布规律；对比了V3V和2D-PIV（particle image velocimetry）径向和轴向速度，发现速度分布吻合较好，特别是尾涡所在的射流区。用2D-PIV方法对尾涡发展规律进行研究，发现受流体自由液面影响，尾涡轨迹向上倾斜，并与水平方向成10°，上、下尾涡运动轨迹不对称，下尾涡运动比上尾涡稍快，衰减亦较快，这与V3V实验结果一致；叶片后方60°尾涡依然清晰可见。用V3V和2D-PIV方法对桨叶附近湍流各向同性假设进行了分析，发现桨叶区和尾涡所在位置湍动能被各向同性假设近似法高估了25%~33%，桨叶区和尾涡所在位置趋向于各向异性。

关键词: V3V, PIV, 尾涡, 涡轮桨, 流场, 各向同性