Characteristics of Rotating stall in High Specific Speed diagonal Flow Fan and Suppression of Rotating Stall

为了阐明比速为1620的高比速对角流风机的旋转失速特性，提出抑制旋转失速的改进设计，从非定常内部流动的角度进行了实验研究。得到的主要结果如下：在低流量系数范围内，风机出现旋转失速，总压曲线呈现不稳定特征，呈正梯度。只有当叶尖间隙很小时，压力特性才会出现迟滞。旋转失速消失与增加尖端间隙。旋转失速单元数为1个，其转速约为转子转速的77%。细胞在尖端区域约占3/4周长和1/3跨度。槽内的速度波动在尖端区域非常强烈，随着半径的减小而减弱。含旋转失速的进口流场如下图所示。池外流动与高流区相似，且随叶片通过频率有规律变化。在胞区流场非常不规则，流动方向有正、负变化。采用双锁相法测量转子出口总压场。胞室区域的总压很高，其分布使得叶片压力侧的总压较高。相对于带尖涡的高压区，失速单元的衰减非常缓慢。开发并试验了两种带分离环的套管处理方法。其中一种完全抑制了旋转失速，在低流区以负梯度稳定了风机特性曲线。在设计条件下，风机效率仅下降2%。从非定常流场的角度，结合内部流量测量结果，讨论了套管处理的有效性。

In order to clarify the characteristics of rotating stall in a high specific speed diagonal flow fan with specific speed of 1620, and to propose an improved design for suppressing the rotating stall, experimental investigation has been conducted from the viewpoint of unsteady internal flow. The main results obtained are as follows.The rotating stall of the fan is observed in the range of low flow coefficient where total pressure curve indicates unstable character with positive gradient. Hysteresis appears in the pressure characteristics for small tip clearance only. The rotating stall disappears with increased tip clearance.The number of rotating stall cell is one, its speed is about 77% of the rotor speed. The cell occupies about 3/4 circumference and about 1/3 span at tip region.Velocity fluctuation in the cell is very strong at tip region and it becomes weak with decreasing radius.The inlet flow field with rotating stall is as follows. The flow outside of cell is similar to that of high flow region with regular variation by blade passing frequency. In the cell region flow field is very irregular, flow direction changes positive and negative.Total pressure field at rotor exit is measured with dual-phase-lock method. Total pressure is high in the cell region and its distribution is such that higher pressure is indicated at pressure side of the blade. Decay of the stall cell is very slow compared with decay of high pressure region with tip vortex.Two types of casing treatments with separator ring are developed and tested. With one of them the rotating stall is completely suppressed and fan characteristics curve is stabilized with negative gradient at low flow region. This is realized with little decrease in fan efficiency of 2% at design condition.The effectiveness of the casing treatment is discussed with the results of internal flow measurement from the viewpoint of unsteady flow.