In autumn of 2010 the Federal Government of Germany decided as part of its energy strategy that the percentage of renewably produced electricity should be increased from 17 percent of today to 80 percent by the year 2050 (1). Furthermore all nuclear power plants should be switched off by the year 2022 (2).
The demand for clean and reliable energy at acceptable costs cannot be guaranteed by renewble power plants like wind turbines and photovoltaic plants alone due their unsteady availability unless there exist appropriate storage devices .
As a result modern conventional/thermal power plants will play a major role in the energy concept. They will have to become more flexible with an higher load acceptance rate, a broader range of operation and a higher efficiency at partial loads.
The project „COOREFLEX-turbo“ is devoted to the development of appropriate methods to improve the thermal power plants with respect to the afforementioned goals.
Concrete, the goals are (3):
„Increasing interest in the aeroelastic behaviour of blades for turbo machines: Effect of flutter and forced-response on coupled shrouds neads further investigation“
One important aspect in the blade design for turbo machines is their aeroelastic bahaviour. Compressor and turbine blades oscillate when interacting with the flow field. One differs between two physical phenomena:
Oscillations close to the eigenfrequencies of the blades are very critical for the life time of the blades. While it is possible in turbo machines, which are designed for almost stationary conditins, to circumvent these frequencies (at least mostly), this is not possible when frequent load changes are required.
For low-pressure turbines, the amplitude of the oscillation, and therefore also the mechanical loading, can be limited by applying shrouds, which are connected to each other.
It is the main goal of the research to investigate whether the dissipative contact forces can lead to stable limit cycles, especially when the aerodynamic damping of the blades is negative.
„Nonlinear numerical methods for nonlinear phyical problems“: Our contribution
When performing aeroelastic problems, assumptions are made regarding the nonlinearity of the underlying equations by assuming a linear interaction between the movement of the blades and the flow field (4) to reduce the computational costs. At the Institute for Propulsion of the DLR a software tool has been developped for such simulations based on the CFD solver TRACE (5) and the CSM solver CalculiX. For investigating the non-linear effect caused by shrouds contacts, the assumption of linearity doesn't hold anymore.
Currently both solvers are coupled at runtime to take into consideration nonlinear effects of both the flow field and the structure when simulating flutter and forced-response of the blades. The numerical method is validated with experiments performed at the „Leibnitz Univerität Hannover“
In addition a strategy will be developped to model the oscillation of shrouds in the simulation of the flow without the need to explicitely mesh the frictional contacts.
The new coupling will also help to further validate the linear models.
(1) Bundesministerium für Wirtschaft und Technologie (BMWi), 2010: Energie - Energiekonzept für eine umweltschonende, zuverlässige und bezahlbare Energieversorgung.
(2) Deutscher Bundestag 17. Wahlperiode, Drucksache 17/6246 vom 22.06.2011; Gesetzentwurf der Bundesregierung: „Entwurf eines Dreizehnten Gesetzes zur Änderung des Atomgesetzes“.
(3) Gesamtzielbeschreibung des Verbundforschungsprojektes COOREFLEX-turbo.
(4) Schmitt, S., 2003.: Simulation von Flattern und aerodynamischer Zwangserregung in Turbomaschinenbeschaufelungen, DLR Forschungsbericht 2003-22, DLR-Köln, Institut für Antriebstechnik.
(5) Nürnberger, D., Eulitz, F., Schmitt, S. und Zachcial, A., 2001: Recent Progress in the Numerical Simulation of Unsteady Viscous Multistage Turbomachinery Flow. ISOABE 2001-1081, Bangalore, September, 2001.
(6) Dhondt, G., 2004: The Finite Element Method for Three-Dimensional Thermome-chanical Applications, Wiley, Hoboken. ISBN 0-470-85752-8.
(7) Kersken, H.-P., Frey, C., Voigt, C. und Ashcroft, G., 2010: Time-linearized and Time-accurate 3D RANS Methods for Aeroelastic Analysis in Turbomachinery. Journal of Turbomachinery, 2012 (134).