Optical Investigation of Profile Losses in a Linear Turbine Cascade

Abstract

Two or three dimensional flow visualisation is an active research field because of its non-intrusiveness. However, its application is often restricted to ideal laboratory conditions. Only few of the multi-dimensional flow visualization techniques have been developed into quantitative measurement techniques which are applicable to turbo machinery flow conditions. This paper demonstrates the application of completely optical measurements for the estimation of the loss coefficient of turbine blades in a linear cascade wind tunnel, which represents flow conditions close to the real but geometrically simplified and therefore more accessible. Two optical means were combined for this purpose. To determine the velocity field of the wake, two-component Particle Image Velocimetry (PIV) was employed. Tomographical Background Oriented Schlieren method (BOS), using 8 cameras placed in 180° around the cascade, was used for measuring a 3-D density field in the wake. For purposes of comparison, the traditional loss estimation method with 5-hole pressure probes accompanied the optical methods. Inlet parameters were measured with a Prandtl probe and a temperature sensor. The optical techniques are, once they are installed as in the present work, much faster than any other conventional method, e.g. pneumatic probes. Moreover, optical techniques can be used for investigation of transient flows.