Three-Dimensional Separation in Complex Internal Flow Fields

研究了轴流压气机转子内旋转失速单元的演化和结构。为了捕捉旋翼和失速单元的锁相流场，发展了一种双锁相平均技术。利用倾斜热线获得了旋翼上下游三维速度分量随时间变化的集合平均值，并利用高响应压力传感器获得了机匣壁上的压力分布，短尺度失速单元的特征是从旋翼前的机壳壁到叶片吸力面有一个集中的涡流。在长尺度失速气室中，当叶片分离在气室的前半部分发展时，分离涡不规则地向上游移动，并在后半部分重新进入轮毂侧的旋翼。根据定子-转子间隙的不同，短尺度失速单元和长尺度失速单元对应的机匣壁压扰动行为是不同的。对于大、中间隙，用短尺度尖峰扰动检测失速起始，尖峰波个数增加，产生高频波。对于大间隙，它们在轻度失速时成为短尺度的局部失速单体，而在中间间隙时，它们变成具有生长或长尺度扰动的大失速单体。因此，在后一种情况下，失速过程被认为是“高频失速开始”。对于小的定子-转子间隙，失速过程被归结为“长波失速起始”，并支持最近的短波失速起始计算模型，表明关闭动静间隙抑制了短长度尺度扰动的增长。结果表明，短尺度扰动是叶片表面分离涡减少环量的结果。

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.