Rotating Instabilities in Axial Compressors – Investigation of the Formation Mechanism in Stationary and Rotating Blade Rows

旋转不稳定性 (RI) 可能出现在轴流式压缩机以及其他涡轮机的稳定运行范围内。它们会导致叶片振动和产生噪音，因此必须避免。 RI 主要出现在叶尖间隙比大或叶片发生率高的情况下。 RI的旋流结构在时间和空间上是波动的。关于 RI 的基本机制有几种解释方法。一种假设指出端壁附近的剪切层不稳定性 (SSI) 是 RI 的原因。如果流体层彼此相对移动并因微小扰动而变得不稳定，则可能会出现这些 SSI。 该项目的目的是提高对轴流压缩机中 RI 的基本机制和特性的物理理解。将通过在无叶片通道、非旋转线性压缩机叶栅以及最终在大型轴流压缩机的转子叶片排中进行实验和 CFD 模拟，从根本上研究 SSI 作为 RI 触发器的假设。流动不稳定性将通过不同的参数变化以有针对性的方式触发，例如尖端间隙宽度和入射角。此外，使用柯安德喷嘴，将产生不稳定的剪切层，并研究其对旋转流动结构的影响。详细实验和数值研究的重点是基础机制和 RI 特性的模型概念的发展，以及从简化实验装置到旋转机器的见解的可转移性。将确定哪些参数组合 SSI 出现以及它们是否负责 RI。这将是制定一般规则的第一步，旨在预测和避免 RI，从而有助于提高涡轮机运行的可靠性。

Rotating Instabilities (RI) can appear in the stable operating range of axial compressors as well as in other turbomachines. They are responsible for blade vibration and noise generation and have therefore to be avoided. RI mainly emerge for large tip clearance ratios or high incidence of the blading. The rotating flow structures of RI are fluctuating in time and space. There are several explanatory approaches concerning the underlying mechanism of RI. One hypothesis states that shear layer instabilities (SSI) near the endwall are the reason for RI. These SSI can appear if fluid layers move relative to each other and become unstable because of small disturbances. Aim of this project is to improve the physical understanding of the underlying mechanism and the properties of RI in axial compressors. The hypothesis of SSI as trigger of RI will be fundamentally investigated by means of experiments and CFD simulations in the vaneless channel, in the non-rotating linear compressor cascade and finally in the rotor blade row of a large scale axial compressor. The flow instabilities will be triggered in a targeted manner with different parameter variations, e.g. tip gap width and incidence. Further on, using Coandă-nozzles, an unstable shear layer will be generated and its influence on rotating flow structures will be investigated.Focus of the detailed experimental and numerical investigations are on the development of model conceptions of the underlying mechanism and the properties of RI as well as the transferability of the insights from simplified experimental setups towards the rotating machine. It will be determined for which parameter combinations SSI appear and if they are responsible for RI. This will be a first step for the development of general rules with the aim to predict and avoid RI and thus contribute to an improved reliability of operation of turbomachines.