Rotordynamic Behavior of ORC Micro Turbine Generator Supported by Gas Foil Bearings

Rotordynamic Behavior of ORC Micro Turbine Generator Supported by Gas Foil Bearings
Organic Rankine Cycle; Turbine Generator; Gas Foil Bearing; Optimal Design; Rotordynamics
Issue Date
13th Asian International Conference on Fluid Machinery
, 1-9
High-speed 10 kW class micro turbine generators (TG) was designed as double stage centrifugal type turbine, and generated the electric power of 10 kW at 50 krpm rotating speed. The rotor is composed of a permanent magnet in which that is supported by two gas foil journal bearings (GFJBs) and one pair of gas foil thrust bearings (GFTBs). The total weight and the axial length of rotor are 2.6 kg and 310 mm, respectively. The GFJBs had a diameter (D) of 37.8 mm. Analysis and test of supported gas foil journal bearings were performed according to the change of aspect ratio (gamma = L/D) 0.58, 0.72 and 0.85 to optimize of the system or to reduce the instability due to the hydrodynamic effects of bearings and the aerodynamics of turbines. Inherently, air foil bearing as a gas bearing has an insufficient damping force and stiffness. So, the aspect ratio of length to diameter is important fact related with the high speed turbomachinery of gas foil bearings. When the too small ratio of length to diameter of bearings, sometimes journal is contact to the bearing surface because of low load carrying capacity. Also, there is rubbing phenomenon and journal is excited such as sub-synchronous vibration near rigid mode of journal. On the other hand, the excessive ratio of length to diameter of bearings tends to move journal to the centered position and make an unstable situation. In case of the proper aspect ratio, 0.72, there is very small resonance sub-synchronous vibration and small orbit vibration. So, it is important to be designed and analyzed with the dynamic characteristics of bearing in high-speed turbomachinery.
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