The interaction of the tip clearance vortex system of tandem blades in a high-speed linear cascade

本项目的目的是为了加深对串列叶片叶顶间隙涡系统的了解。叶轮机械中旋转元件和静止元件之间必须存在的间隙造成了巨大的损失。传统的压气机叶片采用单叶顶间隙涡，这对流动损失和稳定性有很大影响。相反，串列叶片形成了至少两个涡的系统，这是由于每个单独的串列叶片上形成了叶尖间隙涡。本项目计划对串列叶片叶顶间隙区域的流场进行详细的分析。实验研究将在高速直线压气机叶栅中进行。第一个焦点是分析两个串列叶顶间隙涡与靠近侧壁的流场的相互作用。涡系破裂(VBD)现象--从常规叶片上的叶顶间隙涡得知--应该在串列叶片上发生时进行研究。以前的研究已经揭示了一个工作点，在这个点上，最有可能的是，只有前尖部间隙涡经历了VBD。应进行动态壁压测量，以确认这一点，并允许更详细的了解。接下来，将分析是否可以识别出两个叶尖间隙涡都呈现VBD的较大冲角处的工作点，以及这对流场及其相关损失的影响。在下一步中，将分析只有串列前叶片或后叶片表现出尖端间隙的配置。因此，可以在没有其他涡旋的存在和影响的情况下评估一个涡旋的行为。结果表明，流动现象(如VBD的各个点)是否在相互影响下向不同的入射方向移动。此外，还对损耗来源和损耗发展进行了研究。计算结果将揭示某些损失是由前叶顶间隙涡、后叶顶间隙涡还是两者的相互作用造成的。实验采用现场壁面压力测量，并使用一种新型的表面栅栏探头来确定壁面剪应力。

The aim of this project is to contribute to the understanding of the tip clearance vortex system of tandem blades. The necessarily existing clearances between rotating and stationary elements in turbomachinery cause significant losses. Conventional compressor blades feature a single tip clearance vortex, which has a majorinfluence on flow losses and stability. In contrast to that, tandem blades develop a system of at least two vortices, which result due to tip clearance vortices forming on each, individual tandem blade. In this project a detailed analysis of the flow field in the tip clearance region of tandem blades is planned. Experimental investigations will be conducted in a high-speed linear compressor cascade. The first point of focus lies on theanalysis of the interaction of the two tandem tip clearance vortices with the flow field close to the side wall.The phenomenon of vortex break down (VBD) - known from tip clearance vortices on conventional blades - shall be investigated in its occurance on tandem blades. Previous investigations have revealed an operating point in which most probably, only the front tip clearance vortex experiences VBD. Instationary wall pressure measurments shall be done to confirm this and to allow for a more detailed understanding. Following, it willbe analyzed whether an operating point at higher incidences can be identified at which both tip clearance vortices are showing VBD and how this impinges on the flow field and its associated losses. In the next step, configurations will be analyzed in which only the tandem front or rear blade exhibits a tip clearance. Thus, a vortex’ behavior without the presence and influence of the other vortex can be assessed. The results willshow if flow phenomena (such as the individual point of VBD) are shifted towards different incidences by the mutual influence. Furthermore, the loss sources and loss development are investigated. The results shall reveal whether certain losses result from the front tip clearance vortex, the rear one or the interaction of the two of them. The experiments are conducted using instationary wall pressure measurements and by the application of a novel surface fence probe, which is used to determine the wall shear stresses.