Finite Element Analysis,. To The Heart Of The Turbine Impeller Vibration Strength

The impeller of radial turbine is one of the key components in gas turbine. The strength and vibration influences the life cycle and the running quality of the turbomachine. Practice of the turbine's running indicates that the damage of the impeller is caused predominantly by dynamical fatigue. It is important to investigate the characteristics of its strength and vibration.In this paper, the static stress on the impeller caused by the centrifugal force, bolt tension and thermal field is analyzed by means of the FEM of the linear elastic theory. Results reveal that the stresses don't exceed the permitted stress; thermal stress and tension stress are concentrated locally.With the model of large deformation and small strain, the static stress and strain of the impeller is computed. Results indicate that the displacement field is similar to that from the linear analysis. Therefore, the dynamical problem of the impeller can be regarded as nonlinear problem of small deformation and small strain; the impeller 's rest position of the vibration with tiny amplitude can be solved by linear analysis.The impeller is looked upon as a spinning beam, in order to obtain a simple dynamical equation of the spinning impeller. Compared with the equation of other spinning body, for example blade of turbine, the dynamical equation of the spinning impeller differs in that no initial stress stiffness matrix is enclosed while centripetal stiffness and initial stress stiffness matrices are same. The eigenfrequency of the impeller's first bending mode must fall with the rise of impeller rotating speed until zero frequency happens.The dynamical equation of the a radial inflow turbine impeller is founded upon the dynamical equation of spinning elastic body in the form of FEM. Referring to the results of the nonlinear static analysis, the initial displacement matrix in the dynamical equation is neglected. Modal analysis has been undertaken for vibration characteristics of spinning impeller. Conclusion follows as: the major vibration forms are of bending and torsion mode. The mode shape after 11^th mode is the vibration of blade. Approaching critical rotation speed, the frequencies of each modes fall to alternative extends, especially the first bending mode is more evident. Then after, the frequency of the first bending mode rises greatly. The results accord with the simplified dynamical equation of the impeller as spinning beam. This conclusion is important to design high-speed cantilever impeller shaft.In addition, the effect of thermal field upon the impeller is investigated in two aspects of material characteristics and thermal stresses. The material elastic characteristics of impeller vary with temperature field, and thermal stresses are taken in the form of initial...