| This study is mainly studied on the dynamic properties of rotor system supported by magnetic-gas bearings used for turbo expander.Literature research shows that electromagnetic bearings and gas bearings,as two kinds of typical support way with high speed,both have the advantages of nearly frictionless,no abrasion,and non-pollution,and work under high temperature and low temperature.However,the alternating coils of stator of electromagnetic bearings need to be powered and electromagnetic force is produced between stator of electromagnet and rotor.Therefore,the electromagnetic bearings have the power consumption,and the heat of coils would have a negative effect on the refrigeration turbo expander,especially for the cryogenic refrigeration turbo expander.The gas bearings operate with low stability and strong frictional characteristics during the low speed stage and have smaller load capacity than that of oil bearings.Electromagnetic bearings and gas bearings have some deficiencies when used separately.When magnetic bearings used in combination with gas bearings and utilize the strengths of each to overcome the weakness of each,its performance is more ascendant than that of individual magnetic bearing and gas bearing.Firstly,the rotor is supported by electromagnetic bearings during start-up and shut down or low speed stage,the dry friction of gas bearings could be improved during start-up and shut down.Sceondly,when rotating speed increases to a certain air-bearing speed,the gas bearings start to bear the load,the electromagnetic bearings doesn't bear the load or only bear a small part of load for the hybrid magnetic-gas bearing system,the power consumption of electromagnetic bearings and heat of coils could be reduced.Furthermore,for several turbo expanders that employ a fan brake(e.g.,small cryogenic turbo expander),the fan sucks and discharges air from and to the atmosphere directly;hence,the power provided by the turbo expander is wasted completely.To recycle this part of energy,we recommend replacing the fan impeller with a booster and supplying pressurized gas to the gas bearing to enhance the environmental pressure at both ends of the bearing.As a result,the load capacity of the aerodynamic bearing can be improved.In order to better study the performance of magnetic-gas bearings,the load characteristics of electromagnetic bearings and aerodynamic gas bearings are conducted a separate study in this paper at first,which lay the foundations for the study on performance of magnetic-gas hybrid bearings.The main research work is as follows:First of all,the structure of active electromagnetic bearing is designed and ANSYS software is used to analyze magnetic field distribution of active electromagnetic bearings against the performance of turbo-expander rotor.It is revealed that there is inevitably magnetic flux leakage on both sides of rotor and axial electromagnetic bearing.In addition,a deviation is noted between the centers of the rotor and the axial bearing because of machining precision,installation error,or change of running state,and the axial electromagnetic bearing produces unbalanced radial forces on the rotor.The influence of axial electromagnetic bearing on the electromagnetic bearing system was studied.Meanwhile,the dynamic characteristics of rotor system supported by electromagnetic bearing were also studied.The effects of the bearing stiffness and damping on the critical speed of rotor were analyzed.Moreover,the prototype of turbo-expander supported by electromagnetic bearing was tested.The tested results were compared to theoretical results;the running reliability of rotor system was verified.It lays the foundation for the subsequent study on the rotor system of magnetic-gas hybrid bearing.Secondly,aiming at the shortcoming of low capacity in aerodynamic gas bearing,one method to enhance the load capacity of gas bearing by increasing the environmental pressure of aerodynamic gas bearing was proposed.The influence of the environmental pressure of bearing end on the dynamic characteristics of aerodynamic gas bearing was analyzed.Results show that the load capacity of the aerodynamic gas bearing increases with the increase in environmental pressure.The advantage that the environmental pressure can enhance the gas bearing performance is able to expand the application range of aerodynamic gas bearing.Consequently,this bearing may not be limited only to high-speed and light-load conditions.Moreover,the rotor easily occurs instability at a small eccentricity ratio,the increase of environmental pressure improves the stability of the rotor system at small eccentricity ratio.Therefore,considering that the other conditions are invariant,an increase of the environmental pressure not only improves the load capacity of bearing and shortens the length of rotor,but also enhances the stability of the rotor system,which helps improve the dynamic characteristics of the system.Furthermore,it is confirmed that increasing a gas bearing's environmental pressure can increase its load capacity by analyzing the effect of environmental pressure on the load capacity of gas bearing.For several turbo expanders that employ a fan brake(e.g.,small cryogenic turbo expander),the fan sucks and discharges air from and to the atmosphere directly;hence,the power provided by the turbo expander is wasted completely.To recycle this part of energy,we recommend replacing the fan impeller with a booster and supplying pressurized gas to the gas bearing to enhance the environmental pressure at both ends of the bearing.As a result,the load capacity of the aerodynamic bearing can be improved.The design of the fan brake often focuses on simplified structure and improved mechanical strength;efficiency is often a secondary consideration.This study investigates the fan impeller as a booster,which not only consume the mechanical work of the turbo expander but also compresses the gas and does work on it to increase the gas pressure from the outlet nozzle.Booster impeller with high pressure ratio was designed and the flow field distribution was analyzed on the premise that the power of turbo expander is coupled.The feasibility of increasing the environmental pressure of aerodynamic gas bearing by booster impeller was verified.Finally,the combination of magnetic-gas bearing would lengthen the shaft length and changes the original dynamic characteristics of rotor system.The dynamic characteristics of hybrid magnetic-gas bearing was analyzed;modal analysis,critical speed analysis,unbalancing response analysis and transient dynamic analysis on rotor system supported by hybrid magnetic-gas bearing were studied base on the performance study of electromagnetic bearing and gas bearing.Analysis results show that it will not produce resonance for this rotor system during operation speed,and it has little unbalancing response.It would not produce large response and collision under the constant acceleration state and impact load.The rotor system was safe and reliable;it lays the foundation for the subsequent study on the rotor system of hybrid magnetic-gas bearing. |
PDF Full Text Request