Study On Nonlinear Dynamics Of Turbo Expander Rotor-bearing System

Recently, with the development of world air separation equipmentmanufacturing industry and the increasing requirements of domestic users,the capacity of air separation equipment manufacturing in our country hasa great-leap-forward development. However, the key drivers of the deviceis still heavily dependent on imports, and the core technology is still vacant.Turbo expander provides a large amount of cold, as one of the majorrotating machinery and equipment used in air separation process. Itsperformance directly impacts on energy consumption and operatingreliability of the whole sets of air separation equipment. Shafting vibrationis the highlighted problems of the rotor and the design bottleneck. Theprominent features of shafting of the turbo expander used in large airseparation equipment is: high-speed, large temperature difference andstrong nonlinearity. The traditional rotor dynamics mainly focuses onlinear dynamics, while present study suggests that, nonlinear factors ofrotor have a great influence on the dynamics of the system, mostparticularly nonlinear oil film force.In this effort, this dissertation involves the mathematical modeling oftilting pad bearing oil film force and turbo expander rotor as well as thenonlinear dynamics analysis of a rotor-bearing system, which combines thetheoretical modeling, the numerical simulation with experimentalinvestation. The research content is given in detail as follows:A four-pad tilting pad bearing nonlinear oil film force analyticalmodel is established based on the short bearing theory assumption. The oilfilm force model is corrected by the oil film pressure modification functionbased on the oil film pressure boundary conditions. The swing angle ofeach pad is considered in this dissertation, while ignoring the radialdisplacement of the pad. The differences between tilting pad journal bearing and general bearing are discussed through this oil film model, aswell as the effect on oil film force caused by changes of bearing parameter.The linear dynamic model of the turbo expander bearing-rotor isconstructed for the modal analysis and critical speeds calculation, so thatthe working speed of turbo expander can be far away from the criticalspeeds. The effect on the critical speed caused by the asymmetric lubricanttemperature difference is investigated. The results show that, the criticalspeed increases with the reduction of the temperature but keeps unchangedwhen the temperature continues going down. The declining temperaturenearly has effect on the lower modes and critical speeds while has greatimpact on higher modes and critical speeds.The nonlinear rotor dynamics equations of turbo expander rotor arecombined with tilting pad bearing nonlinear oil film force. Thisdissertation analyzed the influence of speed, asymmetric oil temperature,eccentricity distance and tilting pad bearing preload on the rotor-bearingstability, which are potentially destabilizing factors. The results show that,as the rotor speed increases, it appears single-cycle motion, double periodmotion and quasi-periodic motion respectively, accompanied by oil whirland oil oscillation phenomenon. When the oil temperature is allowed, thelower oil temperature is, the more stable the system will be. The rotoreccentricity has little effect on the stability of the system. If the preload istoo low, it will cause instability. But when the speed is low, the preloadchange has little effect on stability of the rotor.The turbo expander rotor-bearing system with tilting pad bearingdynamics test rig is designed, including rotor-bearing system, powercontrol system, lubrication system and test system. This test rig can beused to simulate the characteristics of the turbo expander rotor, measuretilting pad bearing oil film pressure and oil temperature, as well as theinfluence on the rotor vibration caused by the asymmetric lubricanttemperature.Finally, the dissertation is summarized, and the prospect is presented.