Hybrid Gas Bearings Research Articles

Flexure Pivot Tilting Pad Hybrid Gas Bearings: Operation With Worn Clearances and Two Load-Pad Configurations

Abstract Gas film bearings enable the successful deployment of high-speed microturbomachinery. Foil bearings are in use; however, cost and lack of calibrated predictive tools prevent their widespread application. Other types of bearing configurations, simpler to manufacture and fully engineered, are favored by commercial turbomachinery manufacturers. Externally pressurized tilting pad bearings offer a sound solution for stable rotor support. This paper reports measurements of the rotordynamic response of a rigid rotor, 0.825kg and 28.6mm in diameter, supported on flexure pivot tilting pad hybrid gas bearings. The tests are performed for various imbalances, increasing supply pressures, and under load-on-pad (LOP) and load-between-pad (LBP) configurations. Presently, the initial condition of the test bearings shows sustained wear and dissimilar pad clearances after extensive testing reported earlier, (Zhu, X., and San Andrés, L., 2007, “Rotordynamic Performance of Flexure Pivot Hydrostatic Gas Bearings for Oil-Free Turbomachinery,” ASME J. Eng. Gas Turbines Power, 129, pp. 1020–1027). In the current measurements, there are no noticeable differences in rotor responses for both LOP and LBP configurations due to the light-weight rotor, i.e., small static load acting on each bearing. External pressurization into the bearings increases their direct stiffnesses and reduces their damping, while raising the system critical speeds with a notable reduction in modal damping ratios. The rotor supported on the worn bearings shows an ∼10% drop in first critical speeds and roughly similar modal damping than when tested with pristine bearings. Pressurization into the bearings leads to large times for rotor deceleration, thus demonstrating the little viscous drag typical of gas bearings. Rotor deceleration tests with manually controlled supply pressures eliminate the passage through critical speeds, thus paving a path for rotordynamic performance without large amplitude motions over extended regions of shaft speed. The rotordynamic analysis shows critical speeds and peak amplitudes of motion agreeing very well with the measurements. The synchronous rotor responses for increasing imbalances demonstrate the test system linearity. Superior stability and predictable performance of pressurized flexure pivot gas bearings can further their implementation in high performance oil-free microturbomachinery. More importantly, the measurements show the reliable performance of the worn bearings even when operating with enlarged and uneven clearances.

Finite-volume solution of the compressible Reynolds equation: Linear and non-linear analysis of gas bearings

The present work deals with several aspects of the finite-volume numerical solution of the Reynolds equation for a compressible fluid. First, the upwind procedure used for stabilizing the solution is given a new interpretation. It is shown next how film thickness discontinuities can be handled in a mathematically correct way and finally, the finite-volume numerical solution is presented for unstructured grids. After validating these numerical aspects, the static and dynamic characteristics of cylindrical hydrodynamic and externally pressurized hybrid gas bearings are presented. The dynamic coefficients are obtained by using both a small perturbation and a non-linear approach.

Experimental Response of Simple Gas Hybrid Bearings for Oil-Free Turbomachinery

Gas film bearings offer unique advantages enabling successful deployment of high-speed microturbomachinery (<0.4 MW). Current applications encompass micropower generators, air cycle machines and turbo expanders. Mechanically complex gas foil bearings are in use; however, their excessive cost and lack of calibrated predictive tools deters their application to mass-produced systems. The present investigation provides experimental results for the rotordynamic performance of a small rotor supported on simple and inexpensive hybrid gas bearings with static and dynamic force characteristics desirable in high-speed turbomachinery. These characteristics are adequate load support, stiffness and damping coefficients, low friction and wear during rotor startup and shutdown, and most importantly, enhanced rotordynamic stability. The test results evidence the paramount effect of feed pressure on early rotor lift-off and substantially higher threshold speeds of rotordynamic instability. Higher supply pressures also determine larger bearing direct stiffnesses, and thus bring an increase in the rotor-bearing system critical speed albeit with a reduction in damping ratio.

Hybrid Flexure Pivot-Tilting Pad Gas Bearings: Analysis and Experimental Validation

Gas film bearings offer unique advantages enabling successful deployment of high-speed microturbomachinery. Current applications encompass micro power generators, air cycle machines, and turbo expanders. Mechanically complex gas foil bearings are in use; however, their excessive cost and lack of calibrated predictive tools deter their application to mass-produced oil-free turbochargers, for example. The present investigation advances the analysis and experimental validation of hybrid gas bearings with static and dynamic force characteristics desirable in high-speed turbomachinery. These characteristics are adequate load support, good stiffness and damping coefficients, low friction and wear during rotor startup and shutdown, and most importantly, enhanced rotordynamic stability at the operating speed. Hybrid (hydrostatic/hydrodynamic) flexure pivot-tilting pad bearings demonstrate superior static and dynamic forced performance than other geometries as evidenced in a high-speed rotor-bearing test rig. A computational model including the effects of external pressurization predicts the rotordynamic coefficients of the test bearings and shows good correlation with measured force coefficients, thus lending credence to the predictive model. In general, direct stiffnesses increase with operating speed and external pressurization, whereas damping coefficients show an opposite behavior. Predicted mass flow rates validate the inherent restrictor-type orifice flow model for external pressurization. Measured coast-down rotor speeds demonstrate very low-friction operation with large system time constants. Estimated drag torques from the gas bearings indirectly validate the recorded system time constant.

Hybrid Gas Bearing Effectively Stabilized by Water Evaporation from Ultra-Fine Porous Medium

A hybrid bearing effectively stabilized by water evaporation from ultra-fine porous medium is proposed and its basic characteristics are clarified by numerical simulation and experiment. The proposed bearing aims to be applied to MGT (Micro Gas Turbine) and has mainly three advantages suitable for application to MGT. The first is the stability improved by water evaporation from ultra-fine porous medium. The second is the effective lubrication by liquid water at the start and stop of the journal rotation. The third is the cooling effect on the high-temperature journal due to water evaporation. A fundamental experiment from water lubrication to vapor lubrication via transition state has been carried out. The experiment indicated the potential of vapor lubrication and confirmed the stable start-up at water lubrication.

Optimisation of slot entry hybrid gas bearings

Optimisation of hybrid gas bearings is considered from the view-point of minimum total power dissipation on load per unit power of the bearing. The optimisation techniques presented in this paper are derived on a varyin clearance basis. The selection of bearing design parameters for the optimum condition varies according to the restraints and constraints of the design. Therefore, the bearing design parameters are related and selected in order to achieve the optimum performance. The optimum design condition for minimum total power dissipation occurs when the friction power is three times the pumping power ( K = 3).

Design of One-Piece Jet-Engine Compressor End Seals

Performance of advanced air-breathing propulsion systems can be significantly improved through the utilization of effective sealing devices. One of the areas in which immediate benefits are realizable from improvements in sealing efficiency is the high-pressure compressor end area. This paper describes the design of a face-type flexible seal and a face-type rigid seal for a compressor end seal based upon the cruise conditions expected in one version of the supersonic transport (SST) jet engine. The seals, non-contacting during operation, operate on air films achieved through the employment of hybrid gas-bearing geometries on the seal faces. The design procedure discussed consists primarily of the selection of the most applicable seal-face geometries force and moment balance, determination of seal tracking capability, establishment of leakage characteristics, thermal gradients, and stress and deflection calculations. The design results indicate that satisfactory seal performance can be obtained with either of the two configurations. Leakage reductions of an order of magnitude compared to that encountered with labyrinth seals commonly used in existing jet-engine systems can be achieved.