eJournals International Colloquium Tribology 23/1

International Colloquium Tribology
ict
expert verlag Tübingen
125
2022
231

Static Performance Analysis of Porous CMC Journal Bearings for Cryogenic Applications

125
2022
Artur Schimpf
Helge Seiler
Markus Ortelt
Dennis Gudi
Martin Böhle
Highly performant and efficient rocket propulsion is widely used with cryogenic propellants. The associated high-speed turbopumps, exposed to high mass flow and high-pressure rise, are primarily provided with ball bearings. The use of porous bearings is now intended to reduce friction, increase durability and enable an increase in running cycles. This study investigates an innovative micro-porous journal bearing design made of ceramic matrix composites (CMCs) in a specific fiber-layup, with liquid nitrogen as a representative cryogenic lubricant. A static load is applied to the bearing, and the radial clearance is examined. A porous liner made of layered carbon fiber-reinforced carbon (C/C) was used for this investigation.
ict2310259
23rd International Colloquium Tribology - January 2022 259 Static Performance Analysis of Porous CMC Journal Bearings for Cryogenic Applications Artur Schimpf Technical University of Kaiserslautern/ Chair for Fluid Mechanics and Fluid Machinery, Kaiserslautern, Germany Corresponding author: schimpf@mv.uni-kl.de Helge Seiler German Aerospace Center/ Institute of Structures and Design BT, Stuttgart, Germany Markus Ortelt German Aerospace Center/ Institute of Structures and Design BT, Stuttgart, Germany Dennis Gudi Technical University of Kaiserslautern/ Chair for Fluid Mechanics and Fluid Machinery, Kaiserslautern, Germany Martin Böhle Technical University of Kaiserslautern/ Chair for Fluid Mechanics and Fluid Machinery, Kaiserslautern, Germany Abstract Highly performant and efficient rocket propulsion is widely used with cryogenic propellants. The associated high-speed turbopumps, exposed to high mass flow and high-pressure rise, are primarily provided with ball bearings. The use of porous bearings is now intended to reduce friction, increase durability and enable an increase in running cycles. This study investigates an innovative micro-porous journal bearing design made of ceramic matrix composites (CMCs) in a specific fiber-layup, with liquid nitrogen as a representative cryogenic lubricant. A static load is applied to the bearing, and the radial clearance is examined. A porous liner made of layered carbon fiber-reinforced carbon (C/ C) was used for this investigation. 1. Introduction The turbopump (TP) is a crucial part of a reusable rocket engine. In order to increase the longevity of a turbopump, alternatives to rolling element bearings are being developed. Hydroand aerodynamic lubrication offers the advantage of less wear and higher rotational speeds to improve performance. Xu et al. [1] present a review of promising bearing technologies for the next generation of turbopumps. Bearings based on a fluid film offer the advantage that wear is minimized or even negated during the start-up or shut down phase. Childs et al. [2] carried out ramp measurements to simulate the run-cycles. The measurements were carried out with pressurized air on orifice bearings. Hannum et al. [3] experimented with a hybrid of rolling and fluid film bearing with liquid nitrogen (LN2) lubrication. This study presents the first results of the further work of [4]. Measurements with air lubrication have already been carried out on pressurized porous CMC for suitability in turbomachines. No instabilities (pneumatic hammer) occurred due to the compressibility of the gas. In this report, the first experiments with liquid nitrogen are evaluated. This measurement series is intended to simulate operation with cryogenic propellants. 260 23rd International Colloquium Tribology - January 2022 Static Performance Analysis of Porous CMC Journal Bearings for Cryogenic Applications 2. Experimental method Figure 1: Test setup for LN2 bearing experiments The porous liner (see Figure 1) has an inside diameter of 28 mm, an outside diameter of 40 mm and a length of 35 mm. At room temperature, the radial clearance between the “Zero-Expansion-CMC” liner and the shaft is 30 μm ± 5 μm. The liner is pressurized with liquid nitrogen and evaporates at the axial ends in the atmosphere. The evaporated nitrogen is pumped out as best as possible at both ends. An electric motor drives the shaft. Two aerostatic support bearings are pressurized with filtered and dried compressed air of 7.35 bar. The radial load on the test bearing is examined with a stepper motor. The load is determined with an S-shaped force sensor. The resulting eccentricities are determined with laser triangulation sensors. 3. Results Three series of experiments were tested with pressurized liquid nitrogen (9 bar). Prior to the tests, the test bearing was aligned with the shaft, while compressed air was supplied. At the beginning of each series of tests, the ball valve of the compressed air supply was closed and the LN2 line opened. The measurements were started as soon as the lubrication consisted of liquid nitrogen. The rotational speed was increased to 2000 rpm and the radial force increased gradually. The measurement was terminated as soon as the fog began hindering the optical distance sensors (see Figure 1). At the end of each series, the LN 2 line was closed and the airline opened. The compressed air prevented the shaft from icing up on the liner and made further measurements possible within 30 minutes. The vertical displacement of the shaft was determined with the sensors VS 1 and VS 2 . An increase in the radial load increased the tilting of the shaft. A rigid shaft was assumed, and the tilting at the z-level of the test bearing housing V t was calculated. Figure 2: Experimental results Figure 2 shows the radial load and the resulting eccentricity. The eccentricity was assessed using the difference between the measured vertical displacement V and the tilt Vt. 4. Discussion Prior measurements [4] with pressurized air (up to 6 bar) and rotational speeds of up to 8000 rpm showed 23rd International Colloquium Tribology - January 2022 261 Static Performance Analysis of Porous CMC Journal Bearings for Cryogenic Applications a mainly vertical eccentricity. Accordingly, the focus for this measurement was placed on a vertical determination of the eccentricity. Optical measurements instruments could be used for the determination of the eccentricity due to the suction of the evaporated nitrogen. The measurements were terminated depending on the formation of fog. The gap width during the series of measurements was not specified due to the lack of information about the temperature distribution of the shaft and the resulting thermal deformation. The series of measurements carried out the correlation between the increase in the load-carrying capacity and the vertical displacement. Exp. 2 and 3 show a tendency to reduce the standard deviations due to an increase in the radial load. A comparable trend became precise with air measurements, too (see [4]). The first test series shows higher fluctuations in the measured values, possibly due to a premature start of the measurement with a gas-liquid N2 mixture. 5. Conclusion This study determined the load-carrying capacity and the resulting eccentricity of a new type of micro-porous CMC journal bearing design. A functionally representative porous C/ C bearing was lubricated with pressurized LN 2 . The data provide preliminary evidence about the suitability of the C/ C liner for cryogenic application under the tested operating conditions targeting further stepwise verification effort in more realistic high-performance rocket TP environment. References [1] Xu, J., Li, C., Miao, X., Zhang, C., and Yuan, X., 2020, “An Overview of Bearing Candidates for the Next Generation of Reusable Liquid Rocket Turbopumps,” Chinese J. Mech. Eng.English Ed., 33(1). [2] Childs, D. W., Klooster, D., Borchard, H., Pavelek, D., and Phillips, S., 2016, “Transient Lift-off Test Results for an Experimental Hybrid Bearing in Air, Simulating a Liquid Hydrogen Turbopump Start Transient,” Proceedings of the ASME Turbo Expo. [3] Hannum, N. P., and Nielson, C. E., 1983, “The Performance and Application of High Speed Long Life Hybrid Bearings for Reusable Rocket Engine Turbomachinery,” NASA Tech. [4] Schimpf, A., Ortelt, M., Seiler, H., Gu, Y., Schwarzwälder, A., and Böhle, M., 2021, “Experimental Investigation of Aerostatic Journal Bearings Made of Carbon Fiber-Reinforced Carbon Composites,” J. Tribol.