Abstract

Unsteady Rotor STator Interaction in a shrouded Four stage axial turbine: an experimental investigation

Florian von Gosen, Benjamin Winhart, David Engelmann, Francesca di Mare

Chair of Thermal Turbomachines and Aeroengines,
Faculty of Mechanical Engineering,
Ruhr University Bochum, 44801 Bochum, Germany

Keywords: rotor stator interaction, multistage axial flow turbine, unsteady measurements, Adamczyk decomposition, unsteady numerical simulations

In a multistage axial flow turbine with small axial gaps, the blades are exposed to periodic aerodynamic excitations emanating from both the upstream and the downstream neighboring relatively moving blade-rows. The aerodynamic rotor stator interaction arises from the both upstream and downstream acting inviscid potential fields as well as the downstream acting viscous wake. In the inter-blade-row spaces all three effects interact and interfere with one another. These interactions contribute dominantly to the time-dependent pressure fluctuations along the blade surface and thus to the unsteady blade forces. In continuous operation these transient blade loadings can result in a significant reduction of turbomachinery blading lifetime (e.g. high cycle fatigue due to forced response in compressors and low-pressure Turbines or unsteady thermal and aerodynamic loading in high-pressure turbines). Hence there is an urgent need for an improved understanding to precisely predict these phenomena, especially in multi stage configurations.

Time-resolved experimental data is gathered in a shrouded four stage axial air turbine test rig. The test rig represents a scaled version of a high-pressure (HP) steam turbine section and is equipped with a typical HP blading with a constant kinematic degree of reaction of approximately 50 %. Unsteady hot-wire measurements and steady multi-hole-probe and temperature measurements are conducted in several two dimensionally traversable measurement planes within the inter-blade-row spaces and time-resolved pressure transducers are mounted on the surface of a stator blade exposed to the aforementioned periodic aerodynamic loading. The experimental investigations are conducted at design conditions and at off-design conditions.

The experimental data is used to validate URANS numerical simulations, which allow to trace the origins of wakes, vortices and interferences beyond the experimentally accessible regions. The spectral, temporal and spatial analysis of the time-resolved measurement data provide an insight into the contribution of the different blade rows towards the unsteady flow field phenomena by both employing the decomposition method proposed by Adamczyk and phase averaging methods. A rotating beating interaction is shown and the interference of wake and potential field in the inter-blade-row space hint at a significance of the axial blade row spacing for improved turbomachinery blading lifetime.