TY - GEN

T1 - Characterising the Unsteady Flow-Field in Low-Flow Turbine Operation

AU - Kim, Hye Rim

AU - Stania, Lennart

AU - Maroldt, Niklas

AU - Oettinger, Marcel

AU - Seume, Joerg R.

N1 - Funding Information: The project was supported by funds from the GUIde Consortium at Duke University. The findings, opinions, and recommendations expressed herein are those of the author and are not necessarily those of Duke University or the GUIde Consortium Members. The authors would like to acknowledge the great support from the GUIde Consortirum committee for the insights on the topic, especially, Professor Robert Kielb for the organization of the consortium and profound advice in the course of the close collaboration. We would also like to acknowledge the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) for the funding major equipment at the new test facility, Forschungsverbund Dynamik der Energiewandlung (DEW, Dynamics of Energy Conversion).

PY - 2023

Y1 - 2023

N2 - Current operational considerations require steam turbines to operate in a more flexible way, with more frequent and faster start-up and an increasing part-load operation. For very low mass-flow rates, the interaction of highly separated flow with the high speed rotor blades causes windage flow. This type of flow is characterized by increased temperature and highly unsteady flow, which forms vortex structures that rotate at a fraction of the rotor speed. If their magnitude is sufficiently high and the frequency is close to the blade eigenfrequency, non-synchronous vibration (NSV) can be induced. In this paper, low-flow turbine operation is investigated using a three-stage turbine rig that features an instrumentation concept focused on capturing aerodynamic and aeroelastic phenomena. Extensive steady probe, unsteady pressure, and tip-timing measurements are utilized. The experimental scope covers a wide range of operating points in terms of rotational speed and mass flow rates. Low-flow regimes are detected by a reversal in torque and increase in temperature. Unsteady measurements during transient operation identified large-scale vortical flow structures rotating along the circumference, so called rotating instabilities (RI). The onset, growth, and breakdown regimes of RI are characterized for different low-flow conditions. The quantitative characteristics of RI with regards to nodal diameter and rotational speed are derived by a cross-correlation of multiple unsteady sensors. The blade vibration measurements show a moderate structural response from unsteady aerodynamic excitation, indicating no significant NSV occurring in the present experimental setup. Later in the study, an acoustic excitation system has been applied to trigger a locked-in NSV without interrupting the coherent flow structures. From that, significant blade response has been observed, revealing a high degree of mistuning and damping of the rotor blading.

AB - Current operational considerations require steam turbines to operate in a more flexible way, with more frequent and faster start-up and an increasing part-load operation. For very low mass-flow rates, the interaction of highly separated flow with the high speed rotor blades causes windage flow. This type of flow is characterized by increased temperature and highly unsteady flow, which forms vortex structures that rotate at a fraction of the rotor speed. If their magnitude is sufficiently high and the frequency is close to the blade eigenfrequency, non-synchronous vibration (NSV) can be induced. In this paper, low-flow turbine operation is investigated using a three-stage turbine rig that features an instrumentation concept focused on capturing aerodynamic and aeroelastic phenomena. Extensive steady probe, unsteady pressure, and tip-timing measurements are utilized. The experimental scope covers a wide range of operating points in terms of rotational speed and mass flow rates. Low-flow regimes are detected by a reversal in torque and increase in temperature. Unsteady measurements during transient operation identified large-scale vortical flow structures rotating along the circumference, so called rotating instabilities (RI). The onset, growth, and breakdown regimes of RI are characterized for different low-flow conditions. The quantitative characteristics of RI with regards to nodal diameter and rotational speed are derived by a cross-correlation of multiple unsteady sensors. The blade vibration measurements show a moderate structural response from unsteady aerodynamic excitation, indicating no significant NSV occurring in the present experimental setup. Later in the study, an acoustic excitation system has been applied to trigger a locked-in NSV without interrupting the coherent flow structures. From that, significant blade response has been observed, revealing a high degree of mistuning and damping of the rotor blading.

KW - aeromechanical instabilities

KW - non-synchronous vibration

KW - rotating instability

KW - turbine low-flow operation

UR - http://www.scopus.com/inward/record.url?scp=85177465362&partnerID=8YFLogxK

U2 - 10.1115/GT2023-102813

DO - 10.1115/GT2023-102813

M3 - Conference contribution

AN - SCOPUS:85177465362

T3 - Proceedings of the ASME Turbo Expo

BT - Proceedings of ASME Turbo Expo 2023

PB - American Society of Mechanical Engineers(ASME)

T2 - ASME Turbo Expo 2023: Turbomachinery Technical Conference and Exposition, GT 2023

Y2 - 26 June 2023 through 30 June 2023

ER -