Abstract
A computational method of fluid-structure coupling is implemented to predict the fatigue response of a high-pressure turbine blade. Two coupling levels, herein referred to as a “fully coupled” and “decoupled” methods are implemented to investigate the influence of multi-physics interaction on the 3 D stress state and fatigue response of a turbine blade. In the fully-coupled approach, the solutions of the fluid-flow and the solid-domain finite element problem are obtained concurrently, while in the decoupled approach, the independently computed aerodynamic forces are unilaterally transferred as boundary conditions in the subsequent finite element solution. In both cases, a three-dimensional unsteady stator-rotor aerodynamic configuration is modelled to depict a forced-vibration loading of high-cycle failure mode. Also analyzed is the low-cycle phenomenon which arises due to the mean stresses of the rotational load of the rotating turbine wheel. The coupling between the fluid and solid domains (fully-coupled approach) provides a form of damping which reduces the amplitude of fluctuation of the stress history, as opposed to the decoupled case with a resultant higher amplitude stress fluctuation. While the stress amplitude is higher in the decoupled case, the fatigue life-limiting condition is found to be significantly influenced by the higher mean stresses in the fully-coupled method. The differences between the two approaches are further explained considering three key fatigue parameters; mean stress, multiaxiality stress state and the stress ratio factors. The study shows that the influence of the coupling between the fluid and structures domain is an important factor in estimating the fatigue stress history.
Highlights
Gas turbine blades operate in a severe condition of high-pressure and temperature and as such, resonant vibrations are likely to occur with the newer designs incorporating the use of thinner airfoil profiles rotating at near sonic tip speed.[1]
This observation was drawn by evaluating the sign of the aerodynamic work coefficient, which is defined as the normalized work done by the unsteady pressure on the blade for each cycle of oscillation
The stress and fatigue properties of a turbine blade are analyzed by comparing the response from two simulation approaches of a fully-coupled and decoupled simulation approach
Summary
Of interest in this study is the influence of a coupled field simulation methodology on the stress and fatigue life evaluation of a gas turbine blade. On the solid-side finite element model, the problem was formulated following a forced response procedure with the bounding equation of motion of the form: 1⁄2Mfx€g þ 1⁄2Cfx_ g þ 1⁄2kfxg 1⁄4 1⁄2R This represents a system of differential equation of second-order with an applicable standard solution procedure of either the direct integration or the mode superposition techniques as described in Bathe.[19] In equation 7, the terms 1⁄2M; 1⁄2k are the mass and stiffness matrices, respectively, for a blade nodal number, n; of a certain model eigenvector, f/i; ng. The corresponding influence of the timestep selection is evident in the solution accuracy and does not affect the numerical stability of the solver, as opposed to its effect in explicit schemes While this is true generally for implicit schemes, the current implementation with multi-physics interaction shows some instabilities in the predicted solution of the fully-coupled case, where a strong coupling is ensured between the fluid and solid-side equations. The corresponding integration functions in terms of the displacement and velocity variable at time t þ Dt are given as;
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