Dynamic performance of turbocharger rotor-bearing systems
The objectives of this investigation were to design and construct a high speed turbocharger test rig (TTR) to measure dynamics of differing turbocharger rotor bearing systems and to develop a coupled rotor-cartridge model for the ball bearing rotor system to corroborate the experimental and analytical results. The ball bearing rotor is supported by an angular contact ball bearing cartridge. In order to achieve the objectives of the experimental aspect of this study, a TTR was designed and developed with the capability of reaching speeds in excess of 100,000 rpm driven by compressed air. The TTR was used to compare and contrast the whirl and friction characteristics of two identical turbochargers differing only by the support structure of the rotor system; one containing a floating ring bearing turbocharger (FRBT) and the other a ball bearing turbocharger (BBT). A pair of displacement sensors was installed to measure the whirl of the rotor near the end of the compressor. The BBT was shown to be significantly more rigid and stable as compared to the FRBT with an average reduction in radial rotor motion of 47%. The motion of the BBT consisted of mainly synchronous motion whereas the FRBT was dominated by subsynchronous motion throughout the entire range of speeds. The TTR was also used to compare frictional losses within the bearings. A study of run-down times after the pressurized air supply was removed indicated that the BBT has significantly lower frictional losses under all operating conditions tested with an average increase in run-down time of 14.1%. A wireless telemetry based temperature sensor was designed specifically for the turbocharger ball bearing system to monitor the internal bearing temperature located on the cage during operation. It was shown to be able to withstand the harsh environments of turbocharger fnapplications operating at high speeds. The sensor accurately monitored transient bearing cage temperature due to changes in operating speed. Custom sensors were developed in order to measure the axial forces acting on the rotor due to aerodynamic effects. The sensors utilized cantilever beams outfitted with strain gages to measure the applied load from the bearing cartridge. Results of dynamic testing indicated the magnitude and direction of the axial force is dependent on the operating conditions of the turbine and compressor. To achieve the objectives of the analytical investigation, the explicit finite element method (EFEM) and the discrete element method (DEM) were coupled to investigate dynamics of flexible rotor systems supported by deep groove ball bearings. DEM was used to develop the dynamic bearing model (DBM) in which all of the components of the bearing (i.e. races, balls, and cage) have six degrees-of-freedom. The flexible shaft was modeled with a full 3D elastic formulation using the EFEM. Rotor and inner races of the bearings were fully coupled such that both translation and rotation of the flexible rotor are transmitted to the bearings. The resulting reaction forces and moments calculated in DBM were in turn applied to the nodes of the shaft. The combined rotor-bearing model was used to investigate the motions of the inner races at low speeds and the resulting reaction forces and moments from the supporting bearings due to a large applied load on the shaft. In the current coupled modeling approach, the deformation of the shaft affected the internal components of the bearing by altering the orientation of the inner race which results in ball spin and slip. The preceding rotor-bearing model was extended to represent the turbocharger rotor-cartridge system that is under consideration. A DEM angular contact ball bearing cartridge model was coupled with an EFEM shaft to simulate the dynamics of the turbocharger test rig. The bearing cartridge consists of a common outer ring, a pair of split inner races, and a row of balls on each end of the cartridge. The coupled rotor-cartridge model was used to investigate the shaft motion and bearing dynamics as the system traverses critical speeds. The analytical and experimental shaft motion results were in close agreement. The cartridge model allowed for thorough investigation of bearing component dynamics. Effects of ball material properties were found to have a significant impact on turbocharger rotor and bearing dynamics.
Sadeghi, Purdue University.
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