Nicolas Vortmeyer

Application of Active Combustion Instability Control to a Heavy Duty Gas Turbine

During the prototype shop tests, the Model V84.3A ring combustor gas turbine unexpectedly exhibited a noticeable “humming” caused by self-excited flame vibrations in the combustion chamber for certain operating conditions. The amplitudes of the pressure fluctuations in the combustor were unusually high when compared to the previous experience with silo combustor machines. As part of the optimization program, the humming was investigated and analyzed.To date, combustion instabilities in real, complex combustors cannot be predicted analytically during the design phase. Therefore, and as a preventive measure against future surprises by “humming”, a feedback system was developed which counteracts combustion instabilities by modulation of the fuel flow rate with rapid valves (Active Instability Control, AIC). The AIC achieved a reduction of combustion-induced pressure amplitudes by 86%.The combustion instability in the Model V84.3A gas turbine was eliminated by changes of the combustor design. Therefore, the AIC is not required for the operation of customer gas-turbines.Copyright

Experimental Investigation of Shock-Induced Blade Oscillation at an Elastically Suspended Turbine Cascade in Transonic Flow

Experimental investigations on shock-induced flutter in a linear transonic turbine cascade are presented. To examine the relation between trailing edge shock oscillations on adjacent blades in transonic flow and observed turbine blade vibrations, an elastic suspension system has been developed so that only aerodynamic coupling occurs in the system.The experimental investigations have been performed on a linear test rig with superheated steam as working fluid. The test facility enables Mach and Reynolds numbers to be varied independently.The investigated cascade consists of seven blades which are taken from the tip section of a transonic low pressure steam turbine blade. Each blade is attached by an elastic spring system which allows the respective blade to vibrate in a mode equal to the real blade’s first bending mode. By varying the individual spring stiffness it is possible to either get a tuned or mistuned cascade.The examinations mainly deal with the oscillatory behavior of the blades with respect to a variation in the isentropic outlet Mach number. In addition, the complex shock-boundary-layer interaction on the blades’ suction sides is described.An important result is that the maximum blade oscillation amplitude can be related to a specific outlet Mach number. At this Mach number all seven blades are vibrating with exactly the same frequency. This phenomenon is observed at both the tuned and the mistuned cascades.Spectrum analysis shows that one of the major shock oscillation frequencies corresponds to the flutter frequency.In addition to this frequency the spectrum analysis of the blade oscillation shows the dominant frequencies of the shock oscillation which are not natural blade frequencies.The experimental results show that oscillating shocks in a linear cascade give high potential for aeroelastic excitation of transonic blades under certain flow conditions. Blade oscillations and shock characteristics are discussed in detail.Copyright

Experimental Investigations on the Thermal Load and Leakage Flow of a Turbine Blade Tip Section With Different Tip Section Geometries

The gap flow across the tips of cooled rotor blades of combustion turbines is of crucial importance for the thermal load of the tip section and the stage efficiency. Various blade tip section designs with different cooling concepts were tested in a two dimensional steam cascade test rig with superheated steam as working fluid. The blade tip surface temperature distributions were measured with a thermography system and the gap flow was determined by 105 static wall pressure measurements at the opposite wall. The experimental program included systematic variations of decisive flow parameters like Mach and Reynolds numbers of the main and leakage flow as well as variations of the gap width in a range of 1.6% to 4.8% of the chord length. The performance of a simple flat blade tip served as a baseline for the comparison with advanced grooved tip designs developed in earlier studies. In addition attention was given to a systematic investigation of the influence of the coolant mass flow ejected out of the blade into the grooved tip section. The paper presents an overview of the experimental program and results as well as a discussion of the influence of the main flow and geometry parameters on the leakage flow characteristics and the accompanying thermal load of the tip section.Copyright

Aerodynamic Excitation of Transonic Turbine Cascade; Description of the Experimental Method

This paper presents the design of a cascade and the experimental method to investigate shock inducted flutter. To examine the interaction between a shock wave and blade oscillation at transonic flow, a cascade with 7 blades, each of them separately elastically mounted, has been developed so that only an aerodynamic coupling can occur in the system. The experimental investigations have been performed at a two-dimensional test rig with superheated steam as working fluid.