Fatigue life estimation of turbine blades due to transient thermal, vibratory, and centrifugal stresses

Dhar, Deepak (1994) Fatigue life estimation of turbine blades due to transient thermal, vibratory, and centrifugal stresses. Masters thesis, Memorial University of Newfoundland.

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Abstract

The present work involves the study of the three dimensional transient heat transfer and vibratory analysis of turbine blades with mixed and non-linear boundary conditions. The equations derived for the heat transfer analysis are general (three dimensional) in nature and are new contributions in this field. The mathematical model for both the heat transfer and the vibration analysis is formulated using curved, solid, C° continuity, quadratic, serendipity, twenty noded isoparametric finite elements. -- The equations which are non-linear in nature for the transient temperature distribution within the turbine blade are derived using the finite element analysis. The non-linearities arise due to the radiative heat transfer and also due to the change in the material properties of the turbine blade with temperature. Using a finite difference scheme, the non-linear differential equations are transformed to non-linear algebraic equations in the time domain. The transient temperatures obtained from the heat transfer analysis are used in the calculation of the temperature gradients and transient thermal stresses. The vibratory analysis is done at first for finding the undamped natural frequencies of the turbine blade. These free vibration studies include the effect of the non-linearity in the stiffness matrix caused by the rotation of the turbine blade. The frequencies arising from the free vibration analysis also change because of the change in the material properties of the turbine blade as the temperature of the turbine blade changes with time. Thereafter, the stresses due to (a) the nozzle excitation and (b) the centrifugal stresses at different rotor speeds are calculated. The nozzle excitation forces are modelled as a sinusoidal pulse. Finally, the total effect of all the three different types of stresses (transient thermal, centrifugal, and vibratory due to the nozzle excitation) on the fatigue life of the turbine blade is studied.

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