On the Application of Optical Measurement Techniques in Thermal Turbomachines

Abstract: In the last decades progress in optical measurement technology enabled its application in turbomachinery research. This work aims to review research work in this field performed by the Institute for Thermal Turbomachinery and Machine Dynamics at Graz University of Technology.

In the first part, attention is focussed on stereoscopic PIV, a planar non-intrusive time-resolved measurement technology. The requirement for unsteady measurement techniques suitable for turbomachinery applications became more intense, since the designers need this flow data in order to validate their advanced numerical simulations, understand the flow physics and be able to further optimize their modules in terms of unsteady component interaction.

The flow through modern thermal turbomachinery is characterized by subsonic and transonic regions with a high level of turbulence and a significant level of unsteadiness. The unsteadiness in the flow is related to the relative motion between rotor and stator with rotor blade passing frequencies up to 20 kHz and above. In multistage axial turbomachinery the unsteady mixing of wakes from stator and rotor blades during rotor-stator motion results in a complex three-dimensional (3-D) flow field especially when shock systems are present being reflected by passing blades.

The use of intrusive measurement techniques in such a sensitive regime can alter or disturb the flow and sometimes large measurement uncertainties have to be taken into account. Therefore the application of time-resolved optical non-intrusive techniques can help to provide the engine designer with more reliable results. Fast recording of three-component velocity providing ensemble averaged as well as instantaneous data is an advantage of PIV, especially in turbomachinery research with its highly unsteady flows and test rigs that are expensive in operation. Thus PIV offers a major advance for the experimenter but is not without disadvantages. Due to the high flow velocities very small tracer particles have to be used and imaged well focused through windows of special design. Also the designer is often interested in pressure losses, which cannot be quantified by PIV.

To overcome these disadvantages also other planar and pointwise measurement techniques are demonstrated and discussed, such as high-speed Schlieren visualization, Laser Interferometry and a Background-Oriented-Schlieren technique. These measurement techniques focus on the direct assessment of the density field or density fluctuations and do no longer base only on velocity measurements.

This work concludes with an outlook of future applications of optical measurement techniques for turbomachinery applications at the Institute for Thermal Turbomachinery of TU Graz.