Evolution of unsteady secondary flows in a multistage shrouded axial turbine

This work presents the results of detailed unsteady flow measurements in a rotating two stage shrouded axial turbine. The turbine was built at the Laboratory of Turbomachinery at the ETH Zurich as part of this thesis. The design, the rig engineering and the manufacturing of the turbine was one significant task of this work. The resulting very precise facility is used to experimentally study the evolution and convection of the unsteady secondary flow field and its interaction with the blade rows in a real multistage environment. The time-resolved flow field measurements in the second turbine stage are performed with highly sophisticated miniature fast response pressure probes and state-of-the-art pneumatic multi-hole probes for the steady flow field. The unique combination of this versatile research facility and the advanced measurement technology of fast response probes, makes the presented results very unique. Novel probe calibration models and probe designs are developed and validated in the turbine flow field. The engineering of highly automated data reduction systems (HERKULES) lead to a powerful software tool for the postprocessing of the fast response pressure data within a few hours for a typical area traverse of several hundred grid points. The interaction of the rotor indigenous vortices with the downstream blade rows are subject of significant loss generation due to the stretching of the vortices as they convect through the downstream blades. This highly unsteady process is measured in great detail at the exit plane of the first rotor. The unsteady flow field is classified into three time periods for one blade passing event. In the first period, the level of interaction between the vortices, the wakes and the downstream blades is moderate and the turbine losses are minimal. The second phase shows an interaction mechanism between the rotor vortical system and the rotor wake, as the vortices are pushed towards the rotor suction side due to the relative motion of the rotor and stator blades. The high loss fluid from the wake is rolled up into the passage vortex and increases loss at the rotor hub section. The turbine losses reach a maximum within the third period where the vortices are tilted and stretched in the streamwise direction. The associated increase of vorticity generates more shear and thus more loss. The measured mechanism is typical for unsteady flows and losses in this type of flow environment. The results of the labyrinth seal variation highlights the importance of the reentry path of the leakage flow into the mainstream and indicates that the hub and tip labyrinth design ought to be considered from different perspectives. The prediction of secondary flows and flow profiles is achieved by a novel flow model as shown in the thesis. The flow model is validated with experimental data wherein the results show excellent agreement with the measured data.

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