On The Non-linear Dynamic Performance Of Micro Gas Bearing-rotor System For Microengine

Micro-gas-lubricated bearings are the best candidates for supporting the high speedrotors in microengine. The implementation of micro-gas-lubricated bearings encounterschallenges because of their characteristics which are different from their conventional onesas follows: the extremely low length-to-diameter ratio and film thickness, the very highrotating speed and operating temperature. However, influences of those above factors oncharacteristics of micro gas bearings have not been fully understood yet. In this dissertation,influences of the gas rarefaction effects, the operating temperature, the viscous frictionforce and the microfabrication defects are taken into account to study the charcteristics ofthe bearing, including the the stiffness and damping coefficients, the whirl stable region,and the non-linear dynamic performance.Based on the Molecular Gas-film Lubrication model valid for arbitrary inverseKnudsen numbers, the numerical models to obtain the stiffness and damping coefficients,the threshold of whirl stability and the bifurcation behavior of the system are developed,and the numerical methods are established to study the gas rarefaction effects on theperformance of the micro gas bearings.Gas film thickness equation of the tapered or bowed bearing is combined with theMolecular Gas-film Lubrication model so that both the gas rarefaction effects and the Taperor Bow effects are introduced simultaneously to study the steady-state performance and thestability charts of micro bearings. Compared with the ideal bearing with the same minimumclearance, the Taper and Bow effects weaken the load capacity, increase the required stableminimum eccentricity and narrow the stable static load region.The operating temperature of the micro gas bearing system for microengine is up to1300K～1700K. As the operating temperature goes higher, the gas rarefaction effectsbecome stronger and the gas viscosity increases at the same time. Thus the final impact oftemperature on the steady-state and dynamic performance of the system should beinspected. The results show that the influence of temperature on micro gas thrust bearingcould be divided into the viscosity effect domain region and the rarefaction effects domainregion. As temperature increases, the load capacity becomes larger in the viscosity effectdomain region and weaker in the rarefaction effects domain region. In the junction zonebetween the two regions, the load carrying capacity is the highest, but the stability is the weakest of all. Influences of temperature on the non-linear dynamic performance of themicro gas journal bearing is rather complex, and when the temperature increases, theamplitute of the rotor goes larger in general.It is found that the viscous friction force is comparable to the load carrying capacityfor the micro gas bearing system with low L/D ratio. The rotor kinetic equations, whichconsidering the effects of viscous friction force, are derived. The center orbits, phaseportraits, Poincaré maps, and FFT spectra of the system response and the correspondingbifurcation diagrams are studied. The results indicate that, when the viscous friction force istaken into account, the rotor motion shows more stable, for the low-frequencylarge-amplitude self-excited whirl motion would not appear so frequently. The reason isthat the viscous friction force absorbs the energy of the whirl motion and thus decreases thewhirling amplitude and a more stable rotor motion then appears.The flow fields of the micro turbine and the micro seal, which could influence thebalance of the microengine rotor, are simulated by CFD software. The pressure distribution,the velocity distribution and the mass flow rate of those fields are studied to estiblish theconnection between the supply pressure and the corresponding force on the rotor.The idea of adopting the conical bearings to support the high-speed microengine rotorin both axial and radial direction is proposed, and the simplified bearing system forcompactive micro engine is designed and manufactured. Numerical method is developed toinspect the load carrying capacity and the gas consumption law of the conical bearings. Theresults show that thiner gas film thickness implies larger load capacity but less gasconsumption.An integrated in-situ measurment system is developed to measure the rotor speed, thesupply pressure and the mass flow rate of the micro gas bearing system. Curves of massflow rate versus supply pressure are tested and show good agreement with the numericalresults. Mass flow rate, supply pressure and axial vibration in start up process are studiedand analysed.