The intention of the ADTurB project and a description of the work packages may be found on the following web site of the Research Project at KTH: http://www.energy.kth.se/adturb2/
In a high pressure turbine stage potential effects, shocks and wakes from the stator blades have a strong influence on the flow field around the dowstream rotor blades. The passing stator disturbances cause a high degree of aerodynamic unsteadiness and results in aerodynamic forces which can lead to forced vibration of the rotor blades. Unfortunately such forced vibration of the turbine blades causes High Cycle Fatigue failures which are very difficult to monitor and cannot be scheduled for. The goal of the project was to reduce the risk of these failures by understanding the mechanism, establishing what factors are important, knowing how to solve existing problems and how to predict blade vibration amplitudes before an engine is built. The objectives were to investigate the key factors which allow an accurate prediction of the response of a rotor blade.
A previous Framework 4 project (ADTurB BRPR-CT95-0124) was restricted to examining response of High Pressure Turbine Blading to upstream vane excitation only. The current project builds on this experience extending the technology to other excitation sources such as "Low Engine Order (LEO) Excitation" which is caused by non-uniformites in flow around the annulus due to differences in nominally identical vanes.
DLR was involved in ADTurBII with stage tests executed in the Wind Tunnel for Rotating Cascades at Göttingen. Aerodynamic investigations were carried out by the Institute of Propulsion Technology in cooperation with KTH, Stockholm whereas investigations concerning aeroelasticity were carried out by the Institute of Aeroelasticity.
First a rotor with rigid blades was used to enable the identification of the aerodynamic forcing functions on the rotor blades. The second experiment with a new designed rotor with flexible blades then allowed to study the aeroelastic interaction between the blade motions (vibrations) and the aerodynamic forcing functions. Both experiments also provide an experimental database for the validation of forced response prediction codes for high pressure turbines.
The stage tests with the rigid blades provided experimental data for the prediction of the unsteady aerodynamic forcing functions for a high pressure turbine environment. The forcing functions were identified on the basis of unsteady pressure measurements on the rotor blades with fast response pressure transducers (Kulites) and of the velocity field mapping inside and outside of the rotor using laser anemometry.
In order to allow the blades to respond to the aerodynamic forcing functions the stage tests were continued with flexible turbine blades. Therefore a new rotor had to be built with flexible blades. The aero design of this blades was identical to the rigid ones, but the dynamic behaviour was designed to simulate a typical high pressure turbine blade with a firtree foot. To detect the modeshapes the blades were heavily instrumentated by the use of strain gauges. Additionally some blades had unsteady pressure instrumentation (Kulites) to check the forcing functions against the rigid blade tests and to detect any changes relating to the blade vibrations.
Authors: Fritz Kost, Hans-Jürgen Rehder