Stability Analysis And Experimental Investigation On High-speed Turbomachinery Shafts

High-speed turbomachinery shafts supported by gas bearing are widely used in high speed micro-turbomachinery, such as air refrigerating machines, high speed permanent magnet motors, micro gas turbines and so on, as it has the advantages of compact structures, low power consumption of gas bearings, high speed, high power density, et al. Gas whip induced by the nonlinear gas film force in high speed state is one of the key issues which affects on the stability of rotor-bearing system. In the following, the thesis presented here concentrates on theoretical analysis and experimental investigation on the stability of rotor-bearing system and the main work of the thesis is as follows:1. A qualitative analysis on stability of gas bearing-rotor system is carried out. The analytical model of gas-film force for finite length journal bearing including the items of rotary inertia, whirling, squeezing and bearing supply pressure is proposed. Engineering stability criterion and its inference are given by function of the eccentricity ratio and it means, when the eccentricity ratio ε≤εmax and dε/dt≤0, rotor-bearing system can be in stable operation state in the engineering and when the eccentricity ratio of rotor meets ε≤εmax and dε/dε≤0at a certain speed, the rotor speed can continue to rise up. According to engineering stability criterion, it presents the methods and measures for improving the stability of rotor-bearing system form the design of bench to the on-line adjustment of stability of rotor-bearing system.2. Based on the spectrum analysis, nonlinear vibration analysis method on the stability of rotor-bearing system is proposed. It means the different spectrum plots are used to analyze the dynamic characteristic of rotor system before or after the speed of bifurcation and gives the analysis methods of spectrum plots of synchronous whirl induced by the residual unbalance before the speed of bifurcation point and whirling and whip induced by nonlinear of gas film force after the speed of bifurcation point. This method is agreement with the experimental data.3. The bench of high-speed aerostatic bearing-rotor was built which included the design and building of unit of power element, test rig, the vibration signal testing and remote monitoring. The bench aims to achieve speeds to60000rpm, control and measure the parameters of bearing gas, monitor the vibration of rotor online and test rig remotely, store and analyze the data of vibration.4. Experimental modal analysis (EMA) was presented in gas film constraint state. It details analysis process including experimental scheme, selection of incentive and arrangement of measuring points, analysis on signal feature of incentive and response, estimation and evaluation of function for frequency response. The results show that the natural frequency of the cylindrical, conical and first bending modes increases along with supply pressure in design speed range and the design speed staggers the speed of first bending while the speed of second bending mode is above the design margin of safe speed. The results of EMA provide evidences for the regulation of run-up or coast-down experimental scheme and the analysis on characteristics of low frequency vibration.5. Experimental investigations of the effect of speed rate, bearing supply pressure and metal rubber on stability of rotor-bearing system were carried out. By analysis method of spectrum plots, the research concentrates on the dynamic characteristics of rotor system in/before/after the speed value of bifurcation state and the results show that when more than twice the cylindrical critical speed is reached, gas whip (i.e. a large vibration) will appear and the speed of bifurcation increases(i.e. gas whip onset speed)with the supply pressure increasing reasonably, thus improving the margin of stable operation of rotor system. The metal rubber pad can achieve to further postpone gas whip occurrence and reduces the cylindrical, conical and first bending critical speed as well as the vibration amplitude, thus improve the stability of rotor system.6. The application of stable adjustment measures of high-speed micro-turbomachinery in engineering field was verified. The analysis of spectrum plots on stability of rotor-bearing system is agreement with experimental results, thus it means the validity and practicability of engineering stability criterion.