Usually, a large steam turbine-generator unit has itself a blade vibration mode that is close to its double electrical frequency. This mode of vibration will easily be excited by electrical load unbalance, thereby the turbine blades will be affected by this kind of vibrations, especially for the last three rows of blades. In fact, turbine generators operate in corrosive environment and undergo the statistical stress impact due to the randomly unbalanced currents. In this paper, the blades are subjected to corrosion fatigue, thereby small stress still may cause damage significantly. On the other hand, the damage caused by system unbalance is so small that people could neglect it usually. Nevertheless, for the long-term operation with lasting system unbalance, its influence on reliability may no longer be omitted. According to the gamma distribution in unbalanced negative phase current (I2), the probability level of fatigue life, the reliability against fatigue failure and crack growth of turbine blades are evaluated for three turbine-generator system in the paper. The blades with various materials, safety factors and stress concentration factors are considered in the simulations. The influence of extreme value distribution of I2 to the reliability is also investigated. According to the results, we have reason to believe that corrosion fatigue is one of causes that led to crack initiation or damage of blades under normal operation.

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Thesis (M.S. in Aeronautical Engineering) Naval Postgraduate School, June 1997. / Thesis advisor, Raymond P. Shreeve. AD-A333 428. Includes bibliographical references (p. 49-50). Also available online.

Compressor blades often have a small 'spike' in the surface pressure distribution at the leading edge. This may result from blade erosion, manufacture defects or compromises made in the original design process. In this thesis it is shown that these spikes will increase the loss generated by a blade only when they become large enough to initiate boundary layer transition at the leading edge through a separation bubble; this process increases profile loss by about 30%. A criterion is presented, based on the spike diffusion, which can be used to determine whether leading edge transition will occur or not; this criterion is simple and quick to determine and has to potential to be used on a production line to reject those leading edges that would otherwise be detrimental. The spikes are also shown to have a significant effect on the flow close to the endwalls. If they cause leading edge transition in this region then they will cause a growth in the size of the three-dimensional separations that exist in the corner between the blades' suction surfaces and the endwalls. On the low speed stator tested this process increased hub loss by around 100%. Thus to prevent spikes becoming large a new method for producing a 'spikeless' leading edge has been developed; this leading edge can be attached easily to the thickness distribution of any blade and was found to be sharp, that is with very high curvature at the leading edge point. This spikeless leading edge was also found to be the best when the effects of real manufacture deviations, measured off of a production line, were considered. Asymmetry was found to be detrimental and bluntness was only beneficial when unrealistically large deviations were considered. The best leading edge geometry is therefore sharp and symmetric.

Tip leakage losses were studied in a linear turbine cascade with a tip clearance gap equal to 2.1 percent of blade height. The blades of the cascade have a turning angle of 109.4 degrees, an aspect ratio of 1.0, and an axial chord length of 235.2 mm. The cascade was located at the exit of a low speed wind tunnel; the blade exit Reynolds number based upon blade axial chord was 4.5x10⁵. The flow was measured at a plane 0.96 axial chords downstream from the blade leading edge. Barlier studies performed at the tip gap exit and at a downstream plane 1.4 axial chords from the blade leading edge were utilized with the present study to understand loss development better. The effect of tip leakage and the corresponding loss production mechanisms involved as the flow mixes out were analyzed. As part of the objective of the study, a computerized data acquisition system was developed which acquires pressure data and controls movement of a five hole pressure probe. The flow properties at the measurement plane were numerically integrated. To estimate the maximum potential loss of the cascade, the flow was mixed-out through a momentum analysis. The loss at the measurement plane due to tip leakage was found to be equal to the sum of the total pressure loss within the tip gap and the dissipated tip gap secondary kinetic energy. As the flow proceeded downstream, losses were attributed to dissipation of secondary kinetic energy, trailing edge wake mixing, endwall losses, and primary flow mixing. / Master of Science