Thermal Management and Rotordynamic Performance of a Hot Rotor-Gas Foil Bearings System-Part II: Predictions Versus Test Data

Abstract

Implementation of gas foil bearings (GFBs) in microgas turbines relies on physics based computational models anchored to test data. This two-part paper presents test data and analytical results for a test rotor and GFB system operating hot. A companion paper (Part I) describes a test rotor-GFB system operating hot to 157 degrees C rotor OD temperature, presents measurements of rotor dynamic response and temperatures in the bearings and rotor, and includes a cooling gas stream condition to manage the system temperatures. The second part briefs on a thermoelastohydrodynamic (TEHD) model for GFBs performance and presents predictions of the thermal energy transport and forced response, static and dynamic, in the tested gas foil bearing system. The model considers the heat flow from the rotor into the bearing cartridges and also the thermal expansion of the shaft and bearing cartridge and shaft centrifugal growth due to rotation. Predictions show that bearings' ID temperatures increase linearly with rotor speed and shaft temperature. Large cooling flow rates, in excess of 100 l/min, reduce significantly the temperatures in the bearings and rotor. Predictions, agreeing well with recorded temperatures given in Part I, also reproduce the radial gradient of temperature between the hot shaft and the bearings ID, largest (37 degrees C/mm) for the strongest cooling stream (150 l/min). The shaft thermal growth, more significant as the temperature grows, reduces the bearings operating clearances and also the minimum film thickness, in particular, at the highest rotor speed (30 krpm). A rotor finite element structural model and GFB force coefficients from the TEHD model are used to predict the test system critical speeds and damping ratios for operation at increasing shaft temperatures. In general, predictions of the rotor imbalance show good agreement with shaft motion measurements acquired during rotor speed coastdown tests. As the shaft temperature increases, the rotor peak motion amplitudes decrease and the system rigid-mode critical speed increases. The computational tool, benchmarked by the measurements, furthers the application of GFBs in high temperature oil-free rotating machinery. [DOI: 10.1115/1.4001827]