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Improved Fidelity Predictions for Resonant Stress in Turbine Components


OBJECTIVE: To improve physics-based design systems for turbine-component durability by providing a means to predict both aerodynamic forcing and aerodynamic damping in relevant geometries via a single high-fidelity calculation. DESCRIPTION: Turbine engine performance increases result in substantial savings in aircraft life cycle costs. Thrust specific fuel consumption (TSFC) decreases with increases in both turbine inlet temperature and the overall pressure ratio of the cycle, but this combination often results in decreased turbine durability. Turbine components are subjected to exceptionally harsh environments and failure due to high cycle fatigue is common in the development of new engines. Resonant stresses that lead to fatigue failure can arise as a consequence of the interaction between a turbine blade and airfoil wakes, potential fields, and/or shocks that can travel downstream and/or upstream through the engine. Turbine airfoil surfaces constantly encounter fluctuating flowfields induced by such flow structures. These can manifest as pressure fluctuations that impart time-varying forces that generate cyclic rotor vibratory stresses that can in turn reduce the life of the airfoil. Design methodologies are constantly being improved to predict these airfoil vibratory stresses, and computations of some level of fidelity are now performed in the design cycle at many companies. Additionally, the motion of the airfoil as it vibrates can act to reduce or increase the motion while at the same time affecting the force on the airfoil. The fidelity of predictions of this fluid/structural interaction is critical to making effective design decisions relative to flutter and/or the magnitude of resonant stresses prevalent in the airfoil. Accordingly, a decrease in the level of empiricism in resonant stress predictions would have substantial benefits to the development cost of future systems and components. Typical practice for determining resonant stresses involves substantial simplification of the physics of the problem. For example, computation of aerodynamic forcing functions is usually performed assuming rigid components. That is, the fluid and solid models are assessed in an uncoupled manner. Additionally, aerodynamic damping is usually estimated based on past experience and only occasionally are aerodynamic damping calculations performed. On top of this, there are well known means of reducing aerodynamic forcing like asymmetric airfoil spacing that necessitate full-wheel unsteady aerodynamic analyses for accurate predictions of resonant stress. Often, empiricism or at best simple Fourier analysis is used to determine the design of an asymmetric vane ring. To this is added the geometric variation between components around the wheel that can increase or decrease the resonant stress experienced by the blade. So, an increase in the fidelity of physics-based design systems with respect to aeromechanical predictions is sought that will take into account the variability of turbine airfoils in operating engines as well as the nonlinear effects that airfoil motion has on aerodynamic forcing functions. PHASE I: Demonstrate the feasibility of an improved physics-based analysis tool in accordance with the requirements stated in the description. A preliminary design of a turbine component accomplished with the analysis tool and a plan for experimental validation will be accomplished during the Phase I effort. PHASE II: Fully develop, demonstrate, and validate the software tool proposed in Phase I. A demonstration will be conducted at the end of Phase II. It is desired that the software and supporting documentation be delivered at the end of the Phase II effort for additional evaluation by U.S. Government personnel. It is encouraged that the small business team with turbine engine manufacturers to ensure suitability of the tool for use in a commercial environment. PHASE III: The code would be used by both commercial and military engine manufacturers to design airfoils for advanced demonstrator engines, and be adopted as standard work, thereby resulting in life cycle cost reductions. REFERENCES: 1. Ekici, K., Kielb, R. E., and Hall, K. C., 2010,"Aerodynamic Asymmetry Analysis of Unsteady Flows in Turbomachinery,"ASME Journal of Turbomachinery, Vol. 132, pp. 011006-1-011006-11. 2. Kielb, J. J. and Abhari, R. S., 2001,"Experimental Study of Aerodynamic and Structural Damping in a Full-Scale Rotating Turbine,"ASME Paper No. 2001-GT-0263. 3. Montgomery, M., Tartibi, M., Eulitz, F., and Schmitt, S., 2005,"Application of Unsteady Aerodynamics and Aeroelasticity in Heavy Duty Gas Turbines,"ASME Paper No. GT2005-68813. 4. Schennach, O., Pecnik, R., Paradiso, B., Gttlich, E., Marn, A., and Woisetschlger, J., 2007,"The Effect of Vane Clocking on the Unsteady Flowfield in a One and a Half Stage Transonic Turbine,"ASME Paper No. GT2007-27848. 5. Weaver, M. M., Manwaring, S. R., Abhari, R. S., Dunn, M. G., Salay, M. J., Frey, K. K., and Heidegger, N., 2000,"Forcing Function Measurements and Predictions of a Transonic Vaneless Counter-Rotating Turbine,"ASME Paper No. 2000-GT-0375.
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