Steam turbine rotor is a very complicated assembly, typically consists of several rotor rows. Due to design limitations and increasing demands on the efficiency of the steam turbines, it is practically impossible to avoid all of the resonant states. The significant vibrations can occur, for example, due to passing resonance state during turbine start up or run out. In the worst case the turbine operates state is close to the resonance state of the rotor row. This leads to the significant oscillation of the bladed disk, and may results in the blade (or blade to disk joints) high cycle fatigue. These parts are highly loaded components and any cracks are unacceptable. Therefore it is absolutely necessary to damp vibration by using, for example, passive damping elements. The damping element analyzed in this thesis is a strap with an isosceles trapezoidal cross section, which is placed in the circumferential dovetail groove in the blade segmental shrouding. The sliding between the contact surfaces leads to the dissipation of energy which causes decreasing of undesirable vibrations. The main aim is to design the optimal dimensions of the strap cross-section with a view to the most effective damping of vibration for a particular turbine operating state. Considered bladed disk has 54 blades which are coupled in 18 packets by segmental shrouding. The damping element is paced in circumferential dovetail groove created in the shrouding. This type of damping element is suitable especially for damping vibrations in the axial direction and only with the mode shape with the nodal diameters. The modal properties of the bladed disk are influenced by the sliding distance. Since the friction force depends on centrifugal force acting on the damping element and on the angle of the side walls of the strap and groove, the sliding distance can be influenced by the damping element dimensions. During the optimization process the best possible size of middle width, height and angle of damping element cross-section is searched. The strap weight, contact area size and flexural stiffness of damping element can be influenced by these parameters. Their change has also impact on the size of the contact pressure and thus on the size of relative motion as well. As stated previously, the damping efficiency is influenced by the relative motion between the damping element and shrouding. Numerical simulation in time domain is very time-consuming, especially for systems containing nonlinearities. In order to verify dynamic behavior of the computational model with the passive friction element in numerical simulations, the simplified model is created. The model is created in the ANSYS environment. The main requirement imposed on this model is to have as small number of degrees of freedom as possible, so the time needed to perform the simulation is reduced to a minimum. To satisfy this requirement the simplified model is a cantilever beam with rectangular cross section. The dovetail groove is created in this model in longitudinal direction. In this groove is damping element. In addition to damping element dimensions optimization, the influence of each design variable on model dynamic behavior is studied. The results are verified experimentally. Experiment also shows other interesting results that confirm the damping element influence on the modal characteristics. The gained knowledge is used to optimize the dimensions of the damping element in the model of the bladed disk.
| Identifer | oai:union.ndltd.org:nusl.cz/oai:invenio.nusl.cz:233951 |
| Date | January 2011 |
| Creators | Lošák, Petr |
| Contributors | Malenovský, Eduard |
| Publisher | Vysoké učení technické v Brně. Fakulta strojního inženýrství |
| Source Sets | Czech ETDs |
| Language | Czech |
| Detected Language | English |
| Type | info:eu-repo/semantics/doctoralThesis |
| Rights | info:eu-repo/semantics/restrictedAccess |
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