Three-Dimensional Separation in Complex Internal Flow Fields

研究了轴向压气机转子中旋转失速单元的演化和结构。为了捕获转子和失速单元同时锁相的流场，提出了双锁相平均技术。利用斜热线获得了转子上、下游三维速度分量随时间的系综平均，利用高响应压力传感器获得了机匣壁面压力分布。短长尺度失速单元的特征是在转子前方机匣壁上向叶片吸力面形成集中涡。在长尺度失速单元中，分离涡在单元前半部分发生叶片分离时不规则上移，并在单元后半部分从轮毂侧重新进入转子。短、长尺度失速单元对应的机匣壁压力扰动随定转子间隙的不同而不同。对于大间隙和中间隙，通过短长度尺度的尖波扰动来检测失速起始，并增加尖波的数量来产生高频波。当间隙较大时，它们在轻度失速处变成短长度尺度的部分跨距失速单元；当间隙中等时，它们变成大失速单元，并伴有生长或长长度尺度的扰动。因此，在后一种情况下，失速过程被确定为“高频失速开始”。对于较小的定子间隙，失速过程被识别为“长波失速起始”，并通过表明关闭定子间隙抑制短长度尺度扰动的增长来支持最近的短波失速起始计算模型。研究发现，短长度尺度的扰动是由叶片表面的分离涡引起的，以减少循环。

Evolution and structure of rotating stall cells in an axial compressor rotor have been investigated. To capture the flow field phase-locked to both of the rotor and the stall cell, a double phase-locked averaging technique has been developed. Time-dependent ensemble averages of three-dimensional velocity components upstream and downstream of the rotor have been obtained with a slanted hot-wire, and pressure distributions on the casing wall with high response pressure transducers.The short length-scale stall cell is characterized by a concentrated vortex spanning from the casing wall ahead of the rotor to the blade suction surface. In the long length-scale stall cell, the separation vortices go upstream irregularly when blade separation develops in the front half of the cell, and reenter the rotor on the hub side in the rear half of it. Behavior of casing wall pressure disturbances corresponding to the short and long length-scale stall cells is different depending on the stator-rotor gap. For the large and middle gap, the stall inception is detected by a spiky short length-scale disturbance, and the number of spiky waves increases to generate the high frequency waves. They becomes the short length-scale part-span stall cells at the mild stall for the large gap, while they turn into a big stall cell with growth or a long length-scale disturbance for the middle gap. In the latter case, therefore, the stalling process was identified with 'high frequency stall inception'. For the small stator-rotor gap, the stalling process is identified with 'long wave-length stall inception', and supported the recent computational model for the short wave-length stall inception by showing that closing the rotor-stator gaps suppressed the growth of short length-scale disturbances. It is found that the short length-scale disturbance results from a separation vortex from a blade surface to reduce circulation.