## 4 Results and Discussion
### Pump-Drive Turbine Operating Test  
Figure 11 shows the bearingss and rotor of the manufactured pump-drive turbine. The internal flow path was designed such that pressurized liquid CO\({}_{2}\) supplied from the outside can be supplied to the radial and thrust bearings and discharged to the outside. As shown in the figure, the thrust bearing consists of six recesses; five supply holes were made in each recess. The radial and thrust bearings were made of brass and the rotor was made of STS 420. The pump and turbine impeller were attached to both sides of the rotor through a tie rod, as shown in Figure 11b. In addition, to measure the shaft vibration, proximity probes were installed vertically and horizontally near the turbine-side radial bearing. This manufactured pump-drive turbine was installed on the 250 kW sCO\({}_{2}\) test cycle facility in KIMM, as shown in Figure 12, and an operation test was conducted. To supply pressurized liquid CO\({}_{2}\) to the bearing, a CO\({}_{2}\) pump was also installed in the test cycle and piping was constructed to the pump-drive turbine. The supply pressure of the liquid CO\({}_{2}\) for the bearing is 70 bar and bearing discharge pressure is maintained at 60 bar.  
In the test, high-temperature, high-pressure CO\({}_{2}\) (84 bar, 136 \({}^{\circ}\)C) generated through a pump and a heater (boiler) was supplied to the drive turbine; the flow rate was adjusted through a valve to increase the rotating speed. The discharged hightemperature CO\({}_{2}\) from the drive turbine flowed into the cooler (condenser). After flowing through the cooler, CO\({}_{2}\) passed through the liquid separator and flowed into the pump.  
Figure 13 shows the rotor vibration measured by the displacement sensor installed near the turbine side radial bearing with increase in rotating speed to the rated speed of 21,000 rpm. In the figure, waterfall diagrams of the vibration amplitude in the vertical and horizontal direction are presented.  
Figure 11: Pump-drive turbine assembly. (**a**) Radial and thrust bearing; (**b**) Rotor and impeller.  
Figure 12: Test loop for pump-drive turbine. (**a**) Test loop for supercritical CO\({}_{2}\) cycle; (**b**) Pump-drive turbine module.  
No subsynchronous vibration is found in the entire speed range, indicating that the hydrostatic bearing is operating in a stable state, as predicted in Figure 8. It was also observed that 1X synchronous vibration was dominant for the entire test speed range and no critical speed was present under the rated speed. This is consistent with the predicted rotordynamic analysis results shown in Figure 9. In addition, the measured vibration amplitude in the x-direction was similar to that in the y-direction at entire speeds, indicating that the stiffness of the manufactured bearing is symmetrical, as predicted in Figure 8.  
Using unbalance response prediction, Figure 14 compares measured and predicted 1X vibration amplitude at the turbine side bearing. As can be seen in the figure, predicted rotor vibration increases with rotating speed and amplitude is around 1 \(\upmu\)m at the rated speed. However, measured rotor vibration maintains a value of 2 \(\upmu\)m at all rotating speeds. These test results are believed to have occurred because the rotor unbalance is relatively small, and the measured 1X vibration was mainly caused by shaft runout occurring due to a machining error in the rotor.