Instability studies of an 'O'-ring flexibly supported, gas bearing, mounted, cool air unit.

摘要

A Cool Air unit (C. A. U. ) is powered by air bled from the engine andsupplies cool air to the avionics and cockpit of the aircraft. Essentiallya small turbo machine, the rotor is mounted upon two plain (gas) airbearings, which are flexibly supported by elastomeric '0'-Rings.The project objectives were to investigate, experimentally andtheoretically, the mechanism of dynamic instability, half frequency whirl(HFW) observed in the C. A. U design, and hence evaluate methods by which itcan be best controlled. Two main areas of study were embarked upon.Firstly a steady state aerodynamic study, based on an existing singlebearing rig, to evaluate the extent of aerodynamic operation and singlebearing instability (HFW). This Test Rig did not have '0'-Ringflexibility. Bearing L/D tested were 2,1.5 and 1.0, with variable loadingof 18-1 -º 51.2N and speeds of 6000 -º 40000 Rpm in approximately 5000 Rpmintervals. the theoretical study investigated non-linear effects of airfilm pressure distribution.Secondly a Dynamic instability study of HFW was carried out on a newlydesigned Test Rig, simulating small turbo machines and the C. A. U.. Threebearing types were evaluated, Aerodynamic, Hybrid and Hybrid porous. WithL/D of 1.0. '0'-Ring Centres varied from 0.17 4 0-83 of bearing length.Viton and Silicon materials were tested with 70 shore hardness. Rotormass, inertia, asymmetry and unbalance were investigated along with '0'-Ring stiffness and damping variation from air pressure (0 4 120psi).Theory was developed to determine whirl onsets and effects of unbalance,and damping of the support.Non-linear (stagnant areas) of air film detected experimentally, are notexplained by Raimondi's theory. Experiments show that Raimondi's theoryseriously over estimates the applicable area of fully developedaerodynamic operation. Pressure Profile at onset of Instability tends toa Sommerfeld condition. Temperature was a good indication of lubricationregime. Linear temperature rise curve denotes Aerodynamic operation, andtransition to non-linear curve represents onset of Instability (HFW).Authors theory considers non-linearity of air film and suggests a newmethod of evaluation to improve convergence.Dynamic study of dual bearing rig concludes, first instability onset speed(RSW) can be passed through due to residual unbalance and damping in the'0'-Ring support. Theory developed shows relationship of RSW and HFWeffects due to unbalance and damping and results agree well. Viton offeredbetter damping than Silicon. RSW not present in Viton at'0'-Ring centresabove 10 mm. HFW not present with Viton below 55,000 rpm. RSW observedwith ui /cu = 1.0 and HFW occurred with m /m = 0-493 4 0.58. hybrid porousbearings had lower performance than hybrid, but better than aerodynamic.Hybrid bearings mounted in Viton '0'-Rings offer best performance longterm. Aerodynamic bearings can be concluded to be inherently unstable andhave limited aerodynamic operation, so should see little use in high speedturbo machinery, including C. A. U. or aircraft applications where zero gloading likely.