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Unsteady aerodynamic stator-rotor interaction in high pressure turbines

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VKI PHDT 2006-08, Unsteady aerodynamic stator-rotor interaction in high pressure turbines, ISBN 978-2-930389-26-5

Unsteady aerodynamic stator-rotor interaction in high pressure t

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Unsteady aerodynamic stator-rotor interaction in high pressure turbines
by  Persico Giacommo, published 2006, ISBN 978-2-930389-26-5
PhD thesis the von Karman Institute/Politecnico di Milano, Italy, 2006
Abstract

Aerodynamic research in Turbomachinery is a core topic in energy technology: the most relevant portion of electric power is still generated by gas and steam turbines and the gas turbine engine is the preferential power plant in modern and future aircraft. Hence a great amount of work have been devoted and is still required to the optimization of Turbomachinery. The flow downstream of a blade row is not uniform in space due to potential effects, wakes, secondary flows and shock waves. The relative motion of adjacent cascades and the small axial spacing (smaller than 1
2 of the blade axial chord in actual engines) make highly unsteady the interaction of the cascade-exit flow structures with the downstream blade row. In this work the aerodynamic unsteadiness due to the stator-rotor interaction in high pressure (HP) turbines is studied: the HP turbine blades are highly loaded and of low aspect ratio, and a detailed knowledge of the unsteady three-dimensional aerodynamics of HP stages is crucial to provide realistic estimates of the unsteady aerodynamic loadings and to set up design methodologies for performance optimization.

Two turbine stages were considered in the present research (one of them installed at the Laboratorio di Fluidodinamica delle Macchine of the Politecnico di Milano, the second one operating at the von Karman Institute for Fluid Dynamics), similar but working at different levels of compressibility. In the preliminary phase of the research a dedicated work on the instrumentation was required: a low-pressure shock tube for dynamic calibration was designed and a fast-response aerodynamic pressure probe was developed. In the frame of these activities, a novel 2D analytical model for the prediction of the post-shock perturbations was proposed, and a combined analytical - numerical method was setup to estimate the line-cavity system dynamics. The resulting probe revealed a promptness of 80 kHz.

The second part of the research is related to the analysis of the flow in the subsonic turbine installed in the closed-loop test rig operating at the Politecnico di Milano. Initially the steady flow upstream and downstream of the stator was investigated, then unsteady measurements were performed downstream of the rotor. A parametric study of the unsteady stator-rotor interaction was carried out: the effects of the rotational speed, the stator loading and the axial gap were considered. The research carried out at the von Karman Institute was devoted to unsteady total pressure measurements downstream of a transonic HP turbine stage installed in the short duration facility CT3.

The results showed that in subsonic conditions the relevant sources of interaction are the secondary flows: the incoming vortices, transported in the rotor channel according to simple kinematic mechanisms, dominate the generation and evolution of the rotor secondary flows. The intensity of the interaction,higher for smaller axial gap, was found to be always dominant in the hub region and strongly influenced by the rotor loading (increasing as the loading increase). At high Mach number, instead, the interaction is dominated by the stator-exit potential field and shock wave, that instantaneously modify the expansion ratio across the rotor: this induces periodic modifications of the rotor trailing edge shock wave. Despite the different phenomena, relevant similarities exist between the two cases: also in the transonic case the interaction is maximum in the hub region, while in the outer part of the channel the effect of the upstream stator is evanescent and the flow is dominated by the rotor vortices. These results suggest possible strategies for the set-up of passive techniques to control the effects of the stator-rotor interaction in turbines.

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Manufacturer von Karman Institute for Fluid Dynamics

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