Characterizing the Unsteady Flow Field in Low-Flow Turbine Operation

authored by: Hye Rim Kim, Lennart Stania, Niklas Maroldt, Marcel Oettinger, Joerg R. Seume
Abstract: 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 flowrates, 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 flowrates. 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 (RIs). The onset, growth, and breakdown regimes of RI are characterized for different low-flow conditions. The quantitative characteristics of RI with regard 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.
Organisation(s): Institute of Turbomachinery and Fluid Dynamics
Type: Article
Journal: Journal of turbomachinery
Volume: 146
No. of pages: 11
ISSN: 0889-504X
Publication date: 10.2024
Publication status: Published
Peer reviewed: Yes
ASJC Scopus subject areas: Mechanical Engineering
Electronic version(s): https://doi.org/10.1115/1.4065243 (Access: Closed)

BibTeX

@article{dadfe60cd6cf4491981c09197984509b,
title = "Characterizing the Unsteady Flow Field in Low-Flow Turbine Operation",
abstract = "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 flowrates, 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 flowrates. 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 (RIs). The onset, growth, and breakdown regimes of RI are characterized for different low-flow conditions. The quantitative characteristics of RI with regard 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.",
keywords = "aeromechanical instabilities, non-synchronous vibration, rotating instability, turbine low-flow operation",
author = "Kim, {Hye Rim} and Lennart Stania and Niklas Maroldt and Marcel Oettinger and Seume, {Joerg R.}",
note = "Publisher Copyright: Copyright {\textcopyright} 2024 by ASME.",
year = "2024",
month = oct,
doi = "10.1115/1.4065243",
language = "English",
volume = "146",
journal = "Journal of turbomachinery",
issn = "0889-504X",
publisher = "American Society of Mechanical Engineers(ASME)",
number = "10",
}