Thermal Management Techniques for Oil-Free Turbomachinery Systems

Tests were performed to evaluate three different methods of utilizing air to provide thermal management control for compliant journal foil air bearings. The effectiveness of the methods was based on bearing bulk temperature and axial thermal gradient reductions during air delivery. The first method utilized direct impingement of air on the inner surface of a hollow test journal during operation. The second, less indirect method achieved heat removal by blowing air inside the test journal parallel to the shaft axis to simulate air flowing axially through a hollow shaft. The third method emulated the most common approach to removing heat by forcing air axially through the bearing's support structure. Internal bearing temperatures were measured with three type K thermocouples embedded in the bearing that measured general internal temperatures and axial thermal gradients. Testing was performed in a 1 atm, 260°C ambient environment with the bearing operating at 60 krpm, and supporting a load of 222 N. Air volumetric flows of 0.06, 0.11, and 0.17 m³/min at approximately 150 to 200°C were used.

The tests indicate that all three methods provide thermal management but at different levels of effectiveness. Axial cooling of the bearing support structure had a greater effect on the bulk temperature for each air flow and demonstrated that the thermal gradients could be influenced by the directionality of the air flow. Direct air impingement on the journal's inside surface provided uniform reductions in both bulk temperature and thermal gradients. Similar to the direct method, indirect journal cooling had a uniform cooling effect on both bulk temperatures and thermal gradients but was the least effective of the three methods.     

Air Injection as a Thermal Management Technique for Radial Foil Air Bearings

A thermal management technique for radial foil air bearings was experimentally evaluated. The technique is based on injecting air directly into the internal circulating fluid-film to reduce bulk temperatures and axial thermal gradients. The tests were performed on a single top foil, Generation III, radial foil bearing instrumented with three thermocouples to monitor internal temperatures. A through hole in the bearing shell coincident with the gap between the top foil's fixed and free ends provided entry for the injection air. The tests were conducted at room temperature with the bearing operating at speeds from 20 to 40 krpm while supporting 222 N. Two different mass flow rates of injection air were evaluated for this method, 0.017 and 0.051 kg/min. Test results suggest that the air injection approach is a viable thermal management technique capable of controlling bulk temperatures and axial thermal gradients in radial foil air bearings.     

Journal Design Considerations for Turbomachine Shafts Supported on Foil Air Bearings

Foil air bearings can offer substantial improvements over traditional rolling element bearings in many applications and are attractive as a replacement to enable the development of advanced oil-free turbomachinery. In the course of rigorous testing of foil journal bearings at NASA Glenn Research Center, shaft failure was repeatedly encountered at high ambient temperature and rotational speed, with moderate radial load. The cause of failure is determined to be excessive non-uniform shaft growth, which increases localized viscous heating in the gas film and eventually leads to a high-speed rub and destruction of the bearing and journal. Centrifugal loading of imbalance correction weights and axial temperature gradients within the journal due to the hydrodynamic nature of the foil bearings, determined by experiment and finite element analysis, are shown to be responsible for the non-uniform growth. Qualitative journal design guidance is given to aid in failure prevention.     

Effect of cooling flow on thermal performance of a gas foil bearing floating on a hot rotor

This paper presents the measurements of the thermal behavior of a gas foil bearing (GFB) floating on a hot rotor in a tangential air injection cooling scheme. The cooling air was tangentially injected against rotor spinning into the inlet mixing zone of the test GFB. The hollow rotor was heated by a cartridge heater. The GFB temperatures were measured at intervals of 30 deg along the circumference of the axial center except for at 45 deg, where the cooling flow is injected. The rotor temperatures were measured near the GFB side ends using an infrared thermometer, which was calibrated with a thermocouple. Load cells measure the static load and bearing torque. The baseline rotor temperature was measured without GFB over the axial length at rotor speeds up to 15 krpm and for increasing heater temperatures up to 400 °C. The results showed relatively uniform rotor temperatures at the test journal GFB section, and severe heat convections on the rotor surfaces. The GFB and rotor temperatures were measured under a static load of 80 N for increasing heater temperatures of 100 °C, 200 °C, 300 °C and 400 °C and with increasing cooling flow rates of 100 liter/min, 150 liter/min, and 200 liter/min. The circumferential GFB temperatures showed the maximum temperatures at the loaded zone and the minimum temperatures in the unloaded zone. The increasing cooling flow effectively reduced both the rotor and GFB temperatures, showing a dramatic decrease with the smallest amount of cooling flow. GFB friction torque was measured for two test cases for the static load of 80 N at a rotor speed of 10 krpm: 1) A lift-off and touch-down operating cycle for increasing heater temperatures without the cooling flow, and 2) a continuous operation for the heater temperature of 400 °C with increasing cooling flows. In test case 1, the GFB friction torque decreased for higher heater temperatures due to a larger thermal expansion of the bearing housing than the rotor’s. In test case 2, the GFB friction torque decreased with increasing cooling flows due to strong cooling effects on the rotor temperature. The results imply that the tangential air injection increased the GFB clearance by directly cooling the rotor and effectively alleviating the rotor expansion; hence, the scheme is capable of an effective cooling for high temperature GFB applications, such as micro gas turbines.     

Pressure and Temperature Field Measurements of a Lightly Loaded Circumferential Groove Journal Bearing

The current study presents the experimental determination of the lubricant film pressure and temperature distributions on one land of a symmetrical two-land circumferential groove journal bearing (CGJB) operating under low-load steady-state conditions. Film pressure and temperature measurements are done both in axial and circumferential directions. Five axial planes, with three equally spaced pressure tappings per plane, construct the base pressure measurements grid. The pressure tappings circumferential location is shifted 20° from one plane to the neighboring plane. Similarly, the base temperature measurements grid consists of four axial planes, with three equally spaced thermocouples per plane. Hence, by rotating the bearing sequentially, with an angular step of 10°, it is possible to create fine measurement grids. This method of measurement is valid, because the circumferential groove journal bearing's operation is independent on the angle of loading. Two supply pressures were considered for the tests. The higher supply pressure leads to a smoother bearing operation. In the diverging zone, subatmospheric regions were measured. Superposing pressure and temperature fields reveals that the maximum temperatures are encountered in the sharp pressure drop region.     

应用于燃料电池的高速单级空气压缩机波箔轴承技术（英文）

The motivation to use air foil bearings in fuel cell compressors is driven by the demand for oil-free and high-power density system to reduce system volume and weight. The characteristics of air foil bearings that realize this demand are its independency on auxiliary system and no scheduled maintenance as well as their superb performance at high speeds. However, integration of the foil bearings to the compressor needs rigorous developmental tests for the bearing to withstand high g-load during vehicle maneuver and to remain stable in rotordynamics under external destabilizing forces. This paper presents multi-pads foil bearing technology applicable to single stage high speed fuel cell air compressors.Two different multi-pad air foil bearing designs(two-pad vs three-pad) were tested using a high-speed spin test rig to identify the differences in rotordynamics responses. The two-pad bearing is superior in rotordynamics without any sub-synchronous vibration while three-pad bearing provides more uniform load capacity in all directions with less rotordynamics stability. Frequency-domain modal analyses verify the experimental observations. Axial foil bearings with 38 mm outer diameter was designed and tested up to 140 krpm with load capacity of 90 N(1.4 bar specific load capacity).Finally, a platform design of single stage 15 k W fuel cell compressor with rated speed of 130 krpm is proposed using the multi-pad foil bearings and axial foil bearings developed through this paper.     

空气箔片轴承技术在高速单级燃料电池空压机上的应用（英文）

The motivation to use air foil bearings in fuel cell compressors is driven by the demand for oil-free and high-power density system to reduce system volume and weight. The characteristics of air foil bearings that realize this demand are its independency on auxiliary system and no scheduled maintenance as well as their superb performance at high speeds. However, integration of the foil bearings to the compressor needs rigorous developmental tests for the bearing to withstand high g-load during vehicle maneuver and to remain stable in rotordynamics under external destabilizing forces. This paper presents multi-pads foil bearing technology applicable to single stage high speed fuel cell air compressors.Two different multi-pad air foil bearing designs(two-pad vs three-pad) were tested using a high-speed spin test rig to identify the differences in rotordynamics responses. The two-pad bearing is superior in rotordynamics without any sub-synchronous vibration while three-pad bearing provides more uniform load capacity in all directions with less rotordynamics stability. Frequency-domain modal analyses verify the experimental observations. Axial foil bearings with38 mm outer diameter was designed and tested up to 140 krpm with load capacity of 90 N(1.4 bar specific load capacity).Finally, a platform design of single stage 15 k W fuel cell compressor with rated speed of 130 krpm is proposed using the multi-pad foil bearings and axial foil bearings developed through this paper.     

Analysis of gas foil bearings integrating FE top foil models

Gas foil bearings (GFBs) find widespread usage in oil-free turbo expanders, APUs, and micro gas turbines for distributed power due to their low drag friction and ability to tolerate high-level vibrations. The performance of GFBs depends largely on the support elastic structure, i.e. a smooth foil on top of bump strips. Conventional models include only the bumps as equivalent stiffnesses uniformly distributed around the bearing circumference. More complex finite element (FE) models couple the elastic deformations of the 2D shell or 1D beam-like top foil to the bump deflections as well as to the gas film hydrodynamics. Predictions of journal attitude angle and minimum film thickness for increasing static loads and two journal speeds are obtained for a GFB tested decades ago. For the GFB studied, 2D FE model predictions overestimate the minimum film thickness at the bearing centerline, while underestimating it at the bearing edges. Predictions from the 1D FE model compare best to the limited tests data, reproducing closely the experimental circumferential wavy-like film thickness profile. Predicted stiffness and damping coefficients versus excitation frequency show that the two FE models result in slightly lower direct stiffness and damping coefficients than those from the simple elastic foundation model.     

考虑润滑油黏温效应的动压滑动轴承性能分析

滑动轴承的相关研究很多都基于等黏度的情况下,这与轴承的实际工作情况有较大的出入。使用计算流体力学FLUENT通过编写的黏温方程UDF程序进行动压滑动轴承润滑油黏度的计算,并考虑黏温效应对动压滑动轴承性能的影响,比较等黏度与变黏度情况下动压滑动轴承的油膜压力与承载力、油膜的轴向与周向温度分布。结果表明:在考虑黏温效应条件下,轴承的承载力、油膜压力、摩擦力均小于定黏度条件下,这是由于温度升高导致黏度降低,从而减小了油膜静压力和承载力;在轴承轴向方向上,从油膜中心位置向两端部,油膜温度逐渐升高;在轴承圆周方向上,从收敛区到发散区,油膜温度先升高后降低,油膜温度峰值出现在轴承发散区的端部位置。     

弹性箔片动压径向气体轴承起飞转速和承载能力的实验研究

在建立的气体轴承性能测试实验台上对新型弹性箔片气体动压径向轴承进行了起飞转速和承载能力的实验研究,并分别通过摩擦力矩和径向位移响应频谱两种方法分析了轴承的起飞转速。结果表明:轴承起飞后摩擦力矩逐渐减小并趋于稳定,且径向位移响应频谱图上只有较大的低倍频分量出现,高倍频分量和其它频率分量要小得多;两种分析方法得到的轴承起飞转速基本吻合;载荷越大,轴承和转子中心的偏心距越大;由于箔片弹性变形使卸载过程存在能量损失,同样载荷下卸载时的偏心距比加载时大。     

On the Fluid Flow and Thermal Analysis of a Compliant Surface Foil Bearing and Seal

Foil gas bearings have been applied successfully to a wide range of high-speed rotating machinery such as air cycle machines (ACMs) and auxiliary power units (APUs). The performance of these bearings are based on the high pressure gas in a very thin layer between the journal and the bearing governed by the Reynolds equations. Generation of heat in these bearings especially at high journal rotating speed and high loads or at high ambient temperature directly affect their performance. Thermal and fluid flow analysis of an advanced compliant foil journal bearing/seal are presented. The side flow (known as leakage) and the approximate temperatures are the results of this analysis. The result of preliminary analysis shows that the major portion of the heat is carried through conduction and using the modified Couette flow approximation for the present working fluid, air, helped in analysis of the temperature magnitude, which can be related to the gas viscosity behavior and thin gas film thicknesses.     

The Role of Radial Clearance on the Performance of Foil Air Bearings

Load capacity tests were conducted to determine how radial clearance variations affect the load capacity coefficient of foil air bearings. Two Generation III foil air bearings with the same design but possessing different initial radial clearances were tested at room temperature against an as-ground PS304 coated journal operating at 30000 rpm. Increases in radial clearance were accomplished by reducing the journal's outside diameter via an in-place grinding system. From each load capacity test the bearing load capacity coefficient was calculated from the rule-of-thumb (ROT) model developed for foil air bearings.

The test results indicate that, in terms of the load capacity coefficient, radial clearance has a direct impact on the performance of the foil air bearing. Each test bearing exhibited an optimum radial clearance that resulted in a maximum load capacity coefficient. Relative to this optimum value are two separate operating regimes that are governed by different modes of failure. Bearings operating with radial clearances less than the optimum exhibit load capacity coefficients that are a strong function of radial clearance and are prone to a thermal runaway failure mechanism and bearing seizure. Conversely, a bearing operating with a radial clearance twice the optimum suffered only a 20% decline in its maximum load capacity coefficient and did not experience any thermal management problems. However, it is unknown to what degree these changes in radial clearance had on other performance parameters, such as the stiffness and damping properties of the bearings.     

评价箔片径向空气轴承载荷性能的一种经验方法

介绍一种简单的“经验法则”来评价箔片空气轴承的载荷性能,它是一种可挠面的动压气体轴承,正在研究其在无油透平机械中的应用。经验法则是基于基本原理和文献中报道的可靠实验数据得出的,通过一个经验值——载荷系数D,将轴承的载荷性能和轴承的尺寸、速度联系起来。在经验法则中,轴承承载力是轴承转速和轴承设计面积的线性函数。轴承载荷系数D和轴承弹性支承结构的设计特点和轴承运行工况(温度、速度)有关。     

Experimental investigation of air spindle unit thermal characteristics

This paper presents a report on the first stage of a research on the thermal analysis of spindle units with aerostatic bearings and an experimental investigation of temperature distributions. The objective of the present paper is to provide background information for further analysis. A test machine was used running to a maximum rotational speed of 20000 min⁻¹ with a 60 mm diameter spindle supported by aerostatic journal bearings of 20 μm radial clearance, and the temperatures of the housing, bush and the interface between the bush and air film were measured. In addition, the inlet and outlet air temperatures, the air film pressures and the deformations of the housing and spindle were measured. The experimental results show that the heat flow pattern is essentially radial flow, although axial heat flow was observed. The circumferential temperature distribution can be considered to be uniform, and the temperatures are proportional to the square of the spindle speed.     

Performance of a Complaint Foil Seal in a Small Gas Turbine Engine Simulator Employing a Hybrid Foil/Ball Bearing Support System

Operation of a non-contact compliant gas foil seal (CFS) in a high temperature hybrid dynamic simulator representative of a small gas turbine engine spool is discussed. At the hot section of the simulator two oil-free components, a CFS and a compliant foil bearing (CFB) were mounted and at the cold (compressor) section of the simulator, an oil-mist lubricated ball bearing was installed. The preliminary numerical study on the fluid flow and thermal analysis of a CFS was discussed in the previous work by the authors. The experimental results for successful operation of the foil bearing and foil seal at temperatures up to 560 °C and speeds up to 55,000 rpm are presented. The surface of the CFS and CFB journals for high temperature tests were coated with PS304 solid lubricant film, developed by NASA The CFS performance at different operating speeds and temperatures and differential pressures was investigated. In a similar test, a leakage flow comparison was made among a labyrinth seal, a brush seal and a CFS. The experimental results indicate superior performance of the CFS over the two other types of seals. Unlike brush seal, CFS showed no evidence of rub or induced wear on the journal or seal surface.     

Three-Dimensional Thermohydrodynamic Analyses of Rayleigh Step Air Foil Thrust Bearing with Radially Arranged Bump Foils

A three-dimensional (3D) thermohydrodynamic (THD) model for air foil thrust bearings (AFTBs) is presented. The nonisothermal Reynolds equation is solved using pressure boundary conditions at the cooling air plenum considering local temperature-dependent viscosity and density. Air film temperature is calculated using the 3D energy equation with thermal boundary conditions at the top foil, thrust runner, and top foil’s leading edge. The cooling air plenum distributes the cooling air to multiple radially arranged cooling channels. The plenum temperature and pressure are found from mass and energy balance equations applied to the plenum. Temperature fields of the top foil, bump foils, thrust disc runner, bearing plate, and cooling air channels are also solved through appropriate energy balance equations with their surroundings. A robust computational algorithm with multiple iteration loops was developed to find all the temperature fields. THD analyses were performed for AFTB with outer radius of 50 mm up to 100,000 rpm. As the cooling air source pressure is increased, the plenum pressure also increases and its temperature decreases due to more cooling capacity. However, cooling effectiveness is not necessarily proportional to the pressure because the flow residence time inside the cooling channels is inversely proportional to the pressure. The analyses show that the thrust disc temperature is a parabolic function with speed, and thermal expansions of the thrust disc and thrust plates contribute to the most significant driving force of thermal instability. Optimum cooling air pressure was found around 12,500 Pa for the proposed AFTB design at the reference simulation condition.     

波箔动压空气径向轴承实验台设计

现有的波箔动压空气径向轴承实验台存在最大工作转速达不到轴承实际工作转速,无法在不影响轴承正常工作的情况下对轴承施加径向载荷,无法测量轴承的阻力距等缺点.为了全面满足波箔动压空气径向轴承的实验需要,设计一种最高转速为60 000 r/min的波箔动压空气径向轴承实验台,可以在不影响轴承正常工作的情况下对轴承施加径向载荷,可以同时测量转轴转速、轴承阻力距、转轴水平和竖直方向的位移、轴承工作温度、冷却空气的压力.     

Minimum Film Thickness in a Gas Foil Journal Bearing with an Unbalanced Rotor

A gas-lubricated foil journal bearing consists of a compliant metal shell structure that supports a rigid journal or rotor by means of a gas film. The response of this system to the periodic forces of an unbalanced rotor supported by a single bearing is predicted using perturbation analysis. The foil structure and the gas film are modeled with an analytically perturbed finite element approach to predict the rotor dynamic coefficients. A dynamic model of the rotor is used to predict periodic journal motion. The perturbation analysis is then used with the periodic response of the rotor to calculate periodic changes in the gas film thickness. Other quantities such as the gas film pressure and the foil deflection can also be calculated. The model includes bending and membrane effects in the top foil, coupled radial and circumferential deflections in the corrugated sub-foil, and the equivalent viscous dissipation of Coulomb friction effects in the foil structure. The approach is used to investigate the effects of top-foil thickness on minimum film thickness in a bearing.     

The Experimental Study of Aerodynamic Plate-Foil Journal Bearings for High Speed Cryogenic Turboexpander

Experiments have been conducted on the application of a new type of gas lubricated self-acting journal bearing—plate-foil bearing to a high speed cryogenic turboexpander. The 25.0 mm diameter bearing consists of a rigid housing and a flexible foil element; different structure parameters of the bearing are selected in the experiments. Experimental results indicate the foil bearing presented here offers better system stability performance and high durability, and it confirmed that the application of this type of plate-foil bearing to the cryogenic turboexpander is feasible. The experiment results, including the effect of the plate-foil bearing structure parameters on the rotor-bearing system stability, are described in this paper.     

Spatially Sampled Pressure Profile Measurements in Externally Pressurized Gas Journal Bearings

Abstract

Externally pressurized gas journal bearings (EPGJBs) are widely adopted to support high-speed rotors. This article presents novel experimental benchmark test data that enable EPGJB model validation. Axial and circumferential pressure profile measurements are presented for a 40-mm-diameter annular restrictor EPGJB operating at speeds up to 25?krpm. The corresponding rotor position, air mass flow rate, and load capacity are also measured. The effects of supply pressure, rotational speed, and load on the measured variables are presented. The measurements are in good agreement with numerical model computations. The cases in which the model deviates from the measured data are discussed in terms of compressible flow theory.