Low coherence interferometry in turbomachinery and flow velocimetry

Abstract

The work reported in this thesis was carried out at the Institute of Fluid Dynamics, ETH Zurich, between 2002 and 2006. The work was mainly funded by ALSTOM Power Switzerland as a part of the Center of Energy Conversion CEC co-operation between ETH and ALSTOM. This thesis introduces two novel methods for sensing applications in low coherence interferometry. For the first time under operating conditions the tip clearance of an early stage of a power plant gas turbine has been measured in detail. On the other hand a new technique of laser velocimetry has been developed, allowing contactless boundary layer measurements over moving surfaces. The Tip Clearance Probe was the initial project. This measuring system has been developed to obtain temporally resolved tip clearance data from early stages of gas turbines. The working principle relies on the interference between backreflected light from the blade tips during the blade passage time and a frequency shifted reference. Common tip clearance sensors either do not have the temporal resolution or an adequate measurement range or they cannot withstand the high temperature loads. The low coherence interferometry technique adapted to tip clearance sensing allows to measure with absolute spatial accuracy of tens of microns. The probe can hence be mounted in a cooled recess without compromising accuracy. A prototype of the system, an all-fiber assembly, has been successfully applied to a laboratory cold-gas turbine and to the first stage behind the combustor of a large-scale power generation gas turbine (GT26-ALSTOM). The second part of this thesis outlines the principle of this sensor and reports on the turbine measurements. The novel Self-Referencing Boundary Layer Profiler combines flow velocimetry measurements with the spatial high resolving qualities of low coherence interferometry. The new approach is that both the object to be measured and an optical reference are outside of the interferometer, i.e., they are self-referenced to each other. Thus, the technique is applicable to contactless measurements of near surface flows, even if the surface moves irregularly. The measurement location is always selected based on its distance to an reference object. The distance can be adjusted without moving optical parts in the sensor head, but by varying the path lengths in the interferometer arms. The absolute accuracy of the measurement location and the spatial resolution depend on the properties of the low coherence light source, with typical values of tens of microns. The working principle of this new technique and proof-of-principle tests are described in the third part of this thesis.