离心叶轮出口与无叶扩压器内部流动特性的DPIV实验研究

在叶轮转速n＝1500rpm对应的设计工况下,利用两维数字式粒子图像速度场仪(DPIV)对风机模型从叶轮出口至无叶扩压器区域内部流动状态进行整场测量,在"锁相"状态下获得沿扩压器宽度方向上三个径向面的瞬态速度场、涡量场的分布,并进一步对时均流场的流动特性进行全面讨论.测量结果表明:设计工况下,叶轮出口流动呈现典型的"射流-尾迹"流动结构,其对无叶扩压器内部流动状态具有深刻的影响.旋涡由叶轮叶片表面边界层的粘性切应力产生,并随流动被输运至下游无叶扩压器,然后随着流动的均匀化而逐渐消失.涡量的分布及变化反映了无叶扩压器内部流体相互掺混和能量传递、重新分布从而达到均匀化的过程.随着半径的增加,尽管无叶扩压器内流动在圆周方向亦逐渐趋于均匀化,但沿宽度方向的差距未得到改善,这将导致沿轴向α不变原则设计的叶片扩压器的二元叶片无法避免冲击损失.

Digital Particle Image Velocimetry(DPIV) investigation of the flow characteristics within the vaneless diffuser of a centrifugal blower

Flow field in the vaneless diffuser as well as at the impeller outlet of a centrifugal blower has been measured via 2D digital particle image velocimetry (DPIV). The whole fields of flow velocities were obtained in three planes perpendicular to the axis of impeller, in the hub to shroud direction, on design operating conditions with 1500 rpm by employing the phase-locked technique. Through further processing of the instantaneous velocity data key parameters of the phase-averaged velocity field could be derived, including the radial component of velocity, the absolute or relative flow angle. Measurements clearly shows the so-called "jet-wake" flow structure at the impeller outlet, which has significant influence on the flow characteristics in the vaneless diffuser. The vortices, produced by the viscous stresses of the blade boundary layer, cast off the blade trail, transport downstream and decay gradually with the homogenization of the flow. The distribution and transformation of vorticity obtained directly from PIV measurements offers the possibility to enhance the understanding of the phenomena of the fluid mixing, energy transfer and redistribution in the vaneless diffuser. Though the non-uniformities mix out remarkably in the circumferential direction, great differences in the spanwise direction remain throughout the diffuser.