Analysis and Experimental Test of a 200 kW Oil-Free Micro Gas Turbine Simulator

Abstract

This paper presents the measured rotordynamic performance and model predictions of a 200 kW oil-free micro gas turbine simulator (MGTS). The MGTS has a single spool configuration, and a gas generator rotor (GTR) and an electric motor rotor (EMR) connected by a diaphragm coupling and supported on gas foil journal bearings (GFJBs). A couple of double acting gas foil thrust bearings (GFTB) are used inside the gas turbine. The MGTS is a full-size replica of a 200 kW micro gas turbine, with the exception of the impeller blades of the compressor and turbine. The design of the diaphragm coupling determines the first and second bending critical speeds of the MGTS. A speed-up test was conducted up to 40 krpm to evaluate the rotordynamic stability of the rotor-bearing system. The test results showed that the synchronous motion had several peaks, which were due to the rigid modes of the rotor-bearing system. The highest peak occurred at ~10 krpm and was produced by the second bending mode of the coupling. The results for 40 krpm revealed the occurrence of small amplitude synchronous motions. The predicted critical speeds of the rotor-bearing system and synchronous rotor motions were in good agreement with test data.

Appendix: Effect of Thermal Expansion on Bearing Clearance

The measured temperature of the turbine side of the bearing housing is ~400 °C, and that of the other side is ~300 °C. The hollow shaft temperature is assumed to be the same as that of the bearing housing. Equation (2) expresses the linear thermal expansion. The initial circumferences of the hollow shaft and the bearing housing are respectively 267 and 273 mm. The initial temperature is 20 °C, and the middle-point temperature of the hollow shaft and the bearing housing is assumed to be 350 °C. The hollow shaft was made from Inconel-718, which has a thermal expansion coefficient of 13.3 µm/m/°C; and the bearing housing was made from SUS-630, which has a thermal expansion coefficient of 12.2 µm/m/°C. The difference between the thermal expansion coefficients of the materials causes variation of the bearing radial clearance. The initial bearing clearances of the hot section bearings on the turbine and compressor sides are respectively 270 and 250 µm. The actual bearing radial clearances are thus reduced to ~150 µm.

$$ L = \alpha_{t} \times L_{1} (T_{2} - T_{1} ) $$

(2)

L:

Linear expansion

L₁ :

Initial length

α_t :

Coefficient of linear expansion of the material

T₁ :

Initial temperature

T₂ :

Final temperature