|Institution:||University of Texas – Arlington|
|Full text PDF:||http://hdl.handle.net/10106/24099|
Kim, Daejong Recently, Air Foil Bearings (AFB) are gaining higher industrial acceptance in micro-turbo machinery. AFB show high speed operating capability and environmental friendly operation in comparison to rolling element bearings. Utilization of ambient air as a lubricant eliminates maintenance of a pressurized oil lubricating system. AFBs are therefore a feasible alternative to oil lubricated bearings in small machines. High speed operation results in thermal management issues which can be solved through a suitable cooling mechanism. Axial cooling by passing air through bearing support structure (called bump foils) is the most commonly used. In this method, cooling is achieved in the heat exchange channels in corrugated foil structure.A large thermal gradient exists on the journal shaft in the turbine hot section. Regardless of the cooling method, turbine hot section bearings have problem of maintaining uniform operating clearance along the axial direction due to uneven expansion of shaft along the shaft toward the turbine impeller.This work presents an alternative method to reduce thermal gradient of bearing journal in turbine hot section by thermal isolation of the bearing journal from the main shaft. A high temperature test rig mimicking the turbine hot section was designed and the concept of thermal isolation of the bearing journal was implemented into the high temperature test rig.Three-pad AFB was chosen for this experiment. The bearing journal was press fit on the main shaft but through a narrow ring adapter. The ring adapter reduces the contact surface area between the main shaft and journal; small contact area reduces direct heat transfer to the journal. Axial cooling was provided through the bump foil channels along the length of bearing by applying pressure drop across the bearing. Thermocouples measure temperatures of the bearing sleeve, bearing holder, ball bearing housing, heating cartridge, and support rolling element bearings in the rig pedestal.Temperature distribution of stationary shaft along its length was recorded at stationary condition, and journal temperature was found to be substantially less than main shaft. Experiments were also carried out at cartridge heater temperature of 800o C to reach steady operating condition at speeds up to 25krpm with a 45N load on the bearing.Preliminary test results confirm that thermal isolation of the bearing journal is very effective and very uniform journal temperature could be achieved. The test should be further improved to achieve higher operating temperature and speeds.