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

Paper Number: 152402

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

The increasing contribution of renewable energy sources to the power grid requires more flexible power generation from conventional power plants. This means that steam and gas turbines increasingly operate under part-load conditions to supplement power during peak demand, or in standby for quick-restart capabilities in case of a sudden shortage in other sources. Turbines in low-load operation can suffer from windage flow, so-called ventilation, which limits the operating range due to increased thermal and vibration stresses on the blades.

 

Additionally, larger-scale windage vortices form inside the turbine as a result of increased blade incidence. These vortices are unsteady in time and represent rotating instabilities (RI). Experimental data show that they rotate at 58-83% of the rotor speed and extend over multiple passages. These flow structures scale and position differently depending on the operating point. With this two-part paper, we aim to expand on our previous work by explaining the origin, evolution and temporal behavior of these RI in a three-stage turbine rig.

 

Expanding on the previously presented data, in Part A of this paper, the time-averaged characteristics of low-flow turbine operation and the rotating instabilities forming in such operating points are investigated. Steady-state and time-averaged, full-annulus unsteady flow simulations supplement the experimental data.

 

The map of the turbine rig investigated is assessed experimentally and numerically for multiple speed lines and mass-flow rates. It is found that steady-state simulations and even simplified mean-line considerations using a model introduced in this paper, are well able to predict the time-mean flow field. Only for very low flow coefficients larger deviations are observed, necessitating unsteady simulations. Using the flow coefficient, an onset condition independent of the absolute value of speed can be established for all speed lines. Flow field traverses and numerical evaluation of additional parameters such as entropy production and swirl angle give additional insight into the detailed flow field.

 

When decreasing the mass-flow rate at constant speed, local diffuser separation is observed. In low flow, suction-side incidence results in large-scale flow separation on the pressure side. In low flow, this incidence is oriented perpendicular to the direction of rotation. This induces backflow in the top-most section of the span and, by interaction with the main flow, induces a vortex structure extending from 90 % to 100 % span—the windage vortex. The high incidence also results in a change in the direction of turning in the turbine passage. As a result, the enthalpy change reverses its sign and the work process is inhibited or even reversed. Additionally, increased vibration amplitudes are identified in low-flow operation.

Presenting Author: Marcel Oettinger Institute of Turbomachinery and Fluid Dynamics - Leibniz University Hannover

Presenting Author Biography: - 2011 - 2017 University Stuttgart: Aerospace Engineering
- 2017 - 2022: Institute of Turbomachinery and Fluid Dynamics - Leibniz University Hannover: Research assistant
- Since 2022: MTU Aero Engines AG: Engineer

Authors:

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