Study On Impeller Strength And Vibration Feature Of Variable Speed Hydrodynamic Coupling

The variable speed hydrodynamic coupling mainly depends on the kinetic energy which is made by the liquid in the impeller to transfer the power. Its operation relys on the structural integrity of rotating parts, while the integrity of structure mainly relys on the capabilities of vibration resistance and intensity of the key components. The impeller of hydrodynamic coupling is one of the key components for transfering power, and it holds the conversion between liquid kinetic energy and mechanical energy. The stress state of the impeller is the most fundamental parameters for the Impeller designing process and the analysis of malfunction. Accurate analysis of the stress field is crucial. In the process for solving it, the stress analysis of blade is the major question. Therefore, whether the result of the stress calculation is accurate or not directly influences the stress analysis of impeller. When the impeller of hydrodynamic coupling is working, it stands centrifugal force, and the combined effect of liquid pressure, its stress condition is very complex. Under these conditions, the impellers of hydrodynamic coupling are often affected by the stable and nonstable fluid exciting force and the variational centrifugal force. Because of that, the vibration of the impeller appears. When the hydrodynamic coupling is working, the vibration of hydrodynamic coupling not only affects the performance and life of machinery, but also the vibration caused by mechanical malfunction and damage, which result in major economic losses. The vibration of the impeller may cause the fatigue damage to the structure after a long time working, and if the excitation source frequency is close to or equal to the natural frequency of impeller, the vibration will cause resonance which can easily destroy the structure of the impeller. And the dynamic stress which is caused by resonance is the key factor when the impeller blades cracke. Therefore, the strength, vibration and reliability of impellers are more and more important. And the strength and vibration analysis are important to the impeller of hydraulic coupling, both in theoretical and actual fields, such as optimum design, stable operation and damage preventing fields. It is the aim of this thesis that developing an analysis model and calculation method which are suitable to the strength and vibration of the impeller. The main contents are as follow:(1) The three-dimensional solid element which is used to calculate the impeller of variable speed hydrodynamic coupling is analyzed. The finite element equation of impeller is set up in the centrifugal force field, and the finite element equation for static analysis of impeller is derived. The three-dimensional solid 8-node hexahedral element is used to simulate the real shape of impeller of hydrodynamic coupling. The element stiffness matrix and mass matrix calculation model of the impeller are achieved in finite element analysis. It provides the theoretical foundation for further analysis and calculation.(2) Three-dimensional model of the YOCQz420 variable speed hydrodynamic coupling is established with three-dimensional modeling software. Then the model is guided into the finite element analysis software and establishes the impeller and single blade finite element model with the free mesh technology. The traditional numerical method is used to solve the pressure of the impeller, and the pressure is loaded into the finite element model by the way of function, which makes a great contribution to impeller of hydrodynamic coupling strength and vibration research.(3) Considering the influences of rotating centrifugal force and hydrostatic pressure, the blades is analyzed under the traction condition and starts working condition separately at different circumstances of fluid-filled rates. The stress of pump and turbine under traction conditions is larger than the start working condition, and the stress of blade in 80% of liquid-filled rate is larger than the 40% of liquid-filled rate. The strength of the impeller is analyzed with FEM, and the calculation can directly display the locations of stress concentration and solve the maximum stress. Study shows that in the pump, maximum equivalent stress appears at the entrance and exit point of the blades;maximum equivalent stress of turbine appears at the entrance of blade. The gradients of stress in these areas change greatly and they tend to cause fatigue damage; the most distortion area of the pump blades appears at the outlet of the runner blade; and the most distorted area of the turbine appears at the inlet of the runner blade. These areas are the sensitive areas of blade vibration.(4) According to the stress characteristics of impeller vane in operation and the influencing factors needed to consider, the modal analysis with pre-stress is conducted to vane based on the static analysis by using the finite element software. The modal analysis of vane is carried out by using the method of cyclic symmetric to solve the vibration frequency and vibration mode. According to the calculation results, the possibility of the resonance vibration which appeares when turbine blades and pump blades are working is analyzed.(5) The effects of the centrifugal force and the fluid pressure on the vibration characteristics of the impeller blades are detailedly discussed. The results show that with the increase of impeller speed, the vibration frequency of impeller has an upward trend, but does not change significantly, and the influence of centrifugal force to the vibration characteristics of the impeller can be neglected. As the influence of liquid pressure, with the increases of the rate of liquid filled, the frequency of the trend decreases and everyone is different form others, so we can consider the influence of liquid pressure on the vibration characteristics.