Abstract
This paper presents a focusing schlieren system designed for the investigation of transonic turbine tip-leakage flows. In the first part, the functional principle and the design of the system are presented. Major design considerations and necessary trade-offs are discussed. The key optical properties, e.g., depth of focus, are verified by means of a simple bench test. In the second part, results of an idealized tip-clearance model as well as linear cascade tests at engine representative Reynolds and Mach numbers are presented and discussed. The focusing schlieren system, designed for minimum depth of focus, has been found to be well suited for the investigation of three-dimensional transonic flow fields in turbomachinery applications. The schlieren images show the origin and growth of the tip-leakage vortex on the blade suction side. A complex shock system was observed in the tip region, and the tip-leakage vortex was found to interact with the suction side part of the trailing edge shock system. The results indicate that transonic vortex shedding is suppressed in the tip region at an exit Mach number around M 2 , i s = 0.8.
Highlights
Blade tip-leakage flows of unshrouded blade tips are known to reduce stage efficiency in axial flow turbines [1]
A focusing schlieren system designed for the investigation of transonic turbine tip-leakage flows is presented
All other density gradients that occur outside of this plane are, to some extent, offset from each other and will be blurry [15]. This effect, which is similar to the focusing principle of a normal lens, lends the focusing schlieren system its main characteristic, i.e., its ability to sharply focus on a specific plane
Summary
Blade tip-leakage flows of unshrouded blade tips are known to reduce stage efficiency in axial flow turbines [1]. Far fewer studies available in the open literature have considered transonic and supersonic tip-leakage flows, as can be found in the final stages of low-pressure steam turbines, modern turbofan aero engines, and organic Rankine cycle (ORC) turbines. Schlieren photography has been used in turbomachinery applications to investigate two-dimensional phenomena such as profile loss, or the wake flow and trailing-edge shock system of transonic blade profiles [9,10,11]. For the analysis of transonic tip-leakage flows, it is desirable to have a device available which allows to sharply focus on a specific plane, i.e., a focusing schlieren system. A focusing schlieren system designed for the investigation of transonic turbine tip-leakage flows is presented. The paper is structured as follows: In the first part, the functional principle and design of the focusing schlieren setup are presented. Results of an idealized transonic tip-leakage flow and of full-scale linear cascade tests are presented and discussed briefly
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