These design trends together with the low structural frequency in last stage steam turbines increase the susceptibility of the turbine blades to flutter. Flutter is the self-excited and self-sustained aeroelastic instability phenomenon, which can result in rapid growth of blade vibration amplitude and eventually blade failure in a short period of time unless adequately damped. To prevent the occurrences of flutter before the operation of new steam turbines, a compromise between aeroelastic stability and stage efficiency has to be made in the steam turbine design process.
It is a significant problem for the manufacturers of large-scale turbomachinery such as gas turbines, steam turbines and aircraft engines. It can lead to dramatic blade loss in the short-term and high-cycle fatigue HCF in the long-term.
Flutter is a problem for turbomachinery because of the possible phase differences between the blades when they are vibrating. If the blades are identical, the aeroelastic modes coupled structural and aerodynamic system are patterns of blade vibration with a constant phase angle between adjacent blades.
Each aeroelastic mode has a different inter-blade phase angle. The Blade flutter thesis turbine phase angle affects the phase between the local unsteady flow and local blade motion which in turn affects the unsteady aerodynamic work done on the blades.
Adverse phase angles can lead to positive work being performed on the blades which results in flutter. Flutter can be identified as the cause of blade vibration by examining the frequency of the vibration.
If the frequency is very close to a blade mode frequency and is not near an engine order frequency then the vibration is probably due to flutter. This can be shown graphically in a Campbell diagram see below. Turbomachinery flutter occurs at frequencies very close to a blade mode frequency because the structural forces dominate and the aerodynamic forces do not significantly affect the vibration frequency.
Schematic of Campbell Diagram In order to evaluate the flutter risk, it is necessary to determine the unsteady flow due to the blade mode shape for each possible inter-blade phase angle. The blade mode shapes can be determined by standard FE analysis and the aerodynamic forces can be neglected.
The calculated unsteady aerodynamic work is used to calculate the logarithmic decrement log-dec of the blade motion for each aeroelastic mode. The log-dec is the rate of decrease in the blade vibration amplitude per cycle.
Negative log-dec values indicate growth in the blade amplitude and a possible flutter problem if there is insufficient mechanical damping. Unsteady Flow Modelling The most difficult and computationally expensive component of flutter prediction is the determination of unsteady aerodynamic forces.
Flutter usually occurs at off-design operating conditions where flow separation occurs, for example, stall flutter and choke flutter. In the past, 3D unsteady viscous flow modelling has been too computationally expensive to use during the design phase. Manufacturers have had to rely on simplified flow models e.
It has been shown that simplified models and empirical data fail to capture the physics of unsteady flow, and hence have a limited range of applicability. However, due to the continuing development of numerical methods and the constant increase in the speed and memory of computer hardware, it is now possible to perform 3D viscous flutter analysis during the design phase.
Linear Flow Analysis While other unsteady flow frequencies may exist in the flow, only the unsteady flow component at the structural frequency contributes to the work that feeds flutter. Linear flow analysis can be used to accurately predict the unsteady flow at the blade mode frequency.
The unsteady flow perturbations are assumed to be small and the unsteady flow response is assumed to be linear. This assumption is valid for many flutter cases in turbomachines.
Linear flow analysis is 10 to times faster than conventional time domain methods and just as accurate for most cases. The plot on the right shows the aerodynamic damping for a blade torsion mode for various flow models: The results of the 3D viscous analysis are significantly different from the 3D inviscid and 2D viscous analyses with the 3D viscous model predicting some unstable cases while the other flow models predict that all cases are stable.
The reason for the difference is the presence of corner separation near the hub and suction surface of the blade. The corner separation can be seen in the figure below left of a contour slice of flow Mach number at ten percent blade height.
The corner separation is only captured by the 3D viscous simulations. This highlights the importance of 3D viscous effects on flutter. Economic Benefits of Flutter Analysis Early identification of potential flutter problems can have significant economic benefits: The occurrence of flutter can also cause delays in development of steam turbines.This thesis consists of a detailed analysis of different blade testing methods and improvements to a novel concept for tri-axial testing of large wind turbine blades.
As the. 1 NUMERICAL ANALYSIS OF COMPRESSOR BLADE FLUTTER IN MODERN GAS TURBINE ENGINES Vasily Vedeneev*, Mikhail Kolotnikov**, Pavel Makarov** * Faculty of mechanics and mathematics, Lomonosov Moscow State University, Russia ** Gas-turbine Engineering Research and Production Center “Salut”, Russia ABSTRACT Blade flutter of the modern gas turbine engines is .
Statement of Originality The work presented in the thesis is, to the best of the candidate’s knowledge and belief, original and the candidate’s .
blade. The trend of larger turbines, as well as some current design trends in the wind industry increases the risk of aeroelastic instabilities such as utter, so this is given special consideration. Purpose The purpose of this thesis is to create a tool that will aid the design of new wind turbine blades.
Flutter is controlled by the blade’s structure and the relative wind speed (rotational speed and oncoming wind speed). To the best of my knowledge, there have not been any flutter related failures of commercial wind turbines to date. Long flexible wind turbine blades can lose their stability to flow induced instabilities such as coupled-mode flutter.
This type of flutter occurs when increasing wind speed causes a coupling of a bending and a torsional mode, which create limit cycle oscillations that can lead to blade failure.