An Experimental Study of Gas Lubricated Foil Journal Bearings Using an Instrumented Rotor with Wireless Telemetry

In order to increase their efficiency and power-density, turbomachines are continuously pushed to run faster, and hotter rotors. These requirements create enormous engineering challenges that affect the design of turbomachines down to the component level. Among these challenges is the choice of an adequate bearing technology. Gas lubricated foil bearings showed competency to support several high-speed turbomachinery applications.

The foil bearing performance is governed by the properties of the gas film and the underlying compliant structure. A significant amount of research is dedicated to analyze the latter. However, the gas film was addressed only once in the experimental research efforts on foil bearings extending from the 1960s. This gap in the literature is due to the complexity of the foil bearing structure that hinders the placement of sensors through the bearing surface. As a consequence, the pressure profile inside the gas film of compliant foil journal bearings were never measured. The lack of such experimental data is hampering the conclusive validation of foil bearing models using pressure as the fundamental variable.

The goal of this thesis is to provide pressure profile measurements within the gas film of compliant foil journal bearings at different rotational speeds. The experimental data will be a step towards the validation of foil bearing models using gas film measurements. An instrumented rotor with embedded pressure probes and a wireless telemetry is used to execute that mission. The designed rotor is capable of measuring the pressure profile at two different axial planes inside the bearing.

The developed embedded pressure probes consisted of pressure transducers, and pneumatic channels to connect them to the measurement point on the surface of the rotor. Such layout required a special calibration procedure in order to account for the dynamics of the pneumatic channel that influences the pressure signal. A Siren Disk was designed and manufactured to produce periodic pressure signals with a controlled frequency and amplitude. Such signal was used to excite the pressure probes, and consequently identity their transfer functions, which are used to correct the pressure signals afterwards.

As a proof of concept, the instrumented rotor was tested on externally pressurized gas journal bearings up to a speed of 37.5 krpm. The test bearings were equipped with pressure taps to measure the spatially sampled pressure profiles from the bearing side. The two measurements were compared and were in good agreement at quasi-static conditions. The bearing side measurement was considered as a reference signal (input), and once compared to the rotor side measurement (output), an in-situ calibration and system identification is performed. The pressure measurements were used to validate an externally pressurized bearing model based on the compressible Reynolds equation at different rotational speeds and supply pressures.

The developed transfer function was subjected to several fitness tests before placing the instrumented rotor on foil bearings and measuring the pressure profiles at different rotational speeds. The developed transfer functions were used to correct the measured signal within the gas film of the foil bearing. Finally, the pressure profiles were compared to a foil bearing model based on the compressible Reynolds equation.