Development of a High Speed Gas Bearing Test Rig to Measure Rotordynamic Force Coefficients

Abstract

The use of gas bearings has increased over the past several decades to include microturbines, air cycle machines, and hermetically sealed compressors and turbines. Gas bearings have many advantages over traditional hearings, such as roiling element or oil lubricated fluid film bearings, including longer life, ability to use the process fiuid, no contamination of the process with lubricants, accommodating high shaft speeds, and operation over a wide range of temperatures. Unlike fluid fllm bearings that utilize oil, gas lubricated bearings generate very little damping from the gas itself. Therefore, .mccessful bearing designs such as foil bearings utilize damping features on the bearing to improve the damping generated. Similar to oil bearings, gas bearing designers strive to develop gas bearings with good rotordynamic stabiiity. Gas bearings are challenging to design, requiring a fully coupled thermo-eiastic, hydrodynamic analysis including complex nonlinear mechanisms such as Coulomb friction. There is a .•surprisingly low amount of rotordynamic force coefficient measurement in the literature despite the need to verify the model predictions and the stability of the bearing. This paper de.icribes the development and testing of a 60,000 rpm gas bearing test rig and presents measured stiffness and damping coefficients for a 57 mm foil type bearing. The design of the rig overcomes many challenges in making this measurement by developing a patented, high-frequency, highamplitude shaker system, resulting in excitation over most of the subsynchronous range.

Foil Bearing Design Guidelines for Improved Stability

“…Diagonalization of Z CYL, where the two diagonal terms become z CYL-j = u CYL-j +iv CYL-j , yields the condition for neutral stability (v CYL-j = 0 at Ω Crit-j ). The corresponding critical mass m Crit is then given by u CYL-j /(Ω Crit-j ω Rot ) 2 . In this paper a non-dimensional form of the critical mass for cylindrical motion is introduced as follows: Applying the same procedure for the rotor tilting motion the critical transversal inertia can be found in nondimensional form.…”

“…Over the past three decades foil bearings have found a wide range of applications such as in air cycle refrigeration units [1], microturbine generators [2,3] and turboexpanders for cryogenic application [4]. Foil bearings allow oil-free operation as lubrication is provided by a gas film.…”

“…The goal of the investigation is to define a set of unified design guidelines for stable foil bearing supported rotor operation. The objectives are to: (1) implement a comprehensive model for foil bearing supported rotors based on existing, state of the art component models, (2) investigate the role of Coulomb friction in subsynchronous vibration, (3) quantify the impact of the axial foil bearing feedline orientation on static load, and (4) characterize the stability of foil bearing supported rotors and define the conditions under which stable operation is guaranteed.…”

Thermal Management and Dynamic Performance of a Cooler Design for the Supercritical CO2 Turbine Shaft Cooling

“…Experimental and numerical investigations have been carried out on several different configurations to study the dynamic performance of gas film, such as gas foil bearings (Basumatary et al, 2020;Zhang et al, 2018;Zywica et al, 2021), self-acting gas journal bearings (Feng et al, 2017;Zhang et al, 2014), and other externally pressurized gas bearing (Moore et al, 2011;Han et al, 2014;Ise et al, 2014). Although the pressure distribution of the gas film obtained numerically with standard Reynolds equation is approximately to determine the stiffness and damping, the thermally coupled analysis of the bearing may be impeded.…”