Session: 40-07: Turbine Secondary Flows and Interactions II

Paper Number: 152403

Origin, Evolution, Decay and Break-Down of Rotating Instabilities in Low-Flow Turbine Operation – Part B: Unsteady Results 

Whereas Part A of this paper focused on the time-mean effect of rotating instabilities in low-flow turbine operation, Part B details the unsteady flow field. Both experimental and numerical data obtained from full-annulus URANS calculations are used to investigate the temporal and spatial evolution of rotating instabilities. Measurements are used to characterize the windage vortices, which show different modal structures, positions and angular velocities according to the operating point, which is solely defined by the flow coefficient. Due the nature of the flow phenomena at hand, full-annulus unsteady simulations are required to fully capture the flow field.

Mode decomposition and wavelet analysis are utilized to better understand the temporal evolution of rotating instabilities in low-flow operation. It can be demonstrated that coherent RI structures form at random intervals, travel across the circumference and decay over time. In previous measurements the extension and speed of low-flow RI could be divided into several characteristic flow regimes. Similarly, the decay behavior is also dependent on the flow regime. At very low mass flow rates, the flow structures are no longer coherent.

Using unsteady pressure measurements and numerical evaluation methods such as POD, the spatial evolution and formation regions can be identified. Rotating instabilities form in flow separation regions such as inside the diffuser, at the blade leading edged and at the stator hub-side leading edges. All of these regions are afflicted by high swirl. The experimental pressure spectra are well predicted by unsteady simulations, if the full wheel is taken into account.

In terms of the origin of rotating instabilities in low-flow turbine operation, it is a result of the interaction between the fluctuating pressure field and the incidence at the blading. As the stalled passages feature a local contraction of the streamtube, the flow at a neighboring passage is driven towards that region, reducing local incidence; the flow reattaches. As a result, the local pressure rises again, resulting in a wavelike pattern around the circumference. The dependency on the tip-gap vortex in small. Instead, the incidence-induced separation results in the formation of a radially oriented vortex, that, when incidence reduces, detaches and propagates through the passage. A formation mechanism similar to spike-type stall inception is proposed.

Based on this evaluation, the behavior of rotating instabilities in low-flow turbine operation can be further characterized and the criticality for operational conditions is assessed.

Presenting Author: Hye Rim Kim Institute of Turbomachinery and Fluid Dynamics / Leibniz University Hannover

Presenting Author Biography: 2004-2007: Sungkyunkwan University, B.Sc. Physics
2008-2010: Seoul National Univeristy, M.Sc. Mechanical Engineering
2010-2015: Samsung Techwin, Research Engineer
2016-2018: Seoul National Univeristy, Research Assistant
2019- now: Leibniz University Hannover, PhD candidate

Authors:

Hye Rim Kim Institute of Turbomachinery and Fluid Dynamics / Leibniz University Hannover
Marcel Oettinger Institute of Turbomachinery and Fluid Dynamics / Leibniz University Hannover
Joerg R. Seume Institute of Turbomachinery and Fluid Dynamics / Leibniz University Hannover