Abstract
Steam turbines are an integral part of power generating systems. The durability and reliability of steam turbines depend heavily on the integrity of their blades. In the last stages of a steam turbine, steam condenses and produces droplets. The effect of these droplets on the efficiency of the steam turbines, as well as erosion and corrosion of blades has been studied by several researchers. However, these studies fall short in these areas; (a) the effect of droplets sizes and increased condensation on the vibration and fatigue of rotating blades has not been established, (b) the erosion models treat droplets as inert particles and disregard that droplets react with the blade materials and finally that (c) erosion and corrosion are treated as two separate processes instead of simultaneous processes which influence one another. This research, therefore, looks at how increased wetness impacts the vibration of the last stage rotating blade. The work presents the results of numerical calculations and characterization of vibrations of low-pressure steam turbine last stage’s rotating blades. A Fluid-Structure Interaction (FSI) study is carried out using ANSYS Fluent 18.1 and ANSYS Mechanical 18.1. Using a one-way coupling between ANSYS Fluent and ANSYS Mechanical, it is possible to link the two systems and allow fluid forces from Fluent to be used for the blade excitation in ANSYS Mechanical. An equation of motion for the blade tip displacement along the axial, radial, and tangential direction is developed through curve fitting the data points from the time history extracted from ANSYS Mechanical. The resulting equations show that the rotating blade exhibits vibrations which are characterized by Amplitude Modulation (AM). The modulating signal is found to oscillate at 15±0.3𝐻𝐻𝐻𝐻 and the carrier signal is found to oscillate at 137±3𝐻𝐻𝐻𝐻. The frequency of these AM signals is found to be further away from the calculated natural frequencies of the rotating blade. As a result, the risk of resonance due to fluid forces is found to be very low...
M.Eng. (Mechanical Engineering)