## 4 CFD Validation and Discussion
### Engine Performances Validation at Sea-Level Conditions  
Comparing the CFD results with the test results in sea-level conditions shows that the CFD results predict the engine performance well, with most of the performances having errors within 5%, as shown in Figures 18 and 19. As illustrated in Figure 18, the error in the EGT value is minimal in the high-RPM region. However, in the low-RPM region below 70,000 RPM, the difference in EGT reaches up to 200 K, which results in a maximum error of 24%. This error is mainly due to the difference in EGT measurement locations. In the experimental test, the temperature at a point near the exhaust nozzle wall is measured by a K-type thermocouple mounted on the engine. On the other hand, when calculating the EGT value of CFD, the average temperature value is used. As illustrated in Figure 20, the temperature field at CFD revealed a temperature gradient in the radial direction. Because of this, in the case of low RPM (30,000 RPM), there is a significant discrepancy of over 200 K between the temperature value between the sensor location and the average temperature value. So, this contributed to the difference in EGT at low RPM. Conversely, the increased swirl resulted in a higher temperature near the wall in the region of high RPM. Consequently, the discrepancy between the mass flow rate average temperature and the temperature near the wall is reduced.  
Figure 19: Engine Performances Comparison According to Shaft Rotating Speed: (**a**) Fuel Flow Rate; (**b**) TSFC.  
Figure 18: Engine Performances Comparison According to Shaft Rotating Speed: (**a**) Air Flow Rate and Thrust; (**b**) Pressure Ratio and EGT.  
As shown in Figure 19, the fuel flow rate showed an error of up to 7%. Similarly, TSFC, a performance metric related to fuel flow rate, also showed an error of up to 10%. Following an investigation into the source of the fuel flow rate error, it was determined that the error was caused by the lubrication. As shown in Figure 21, one of the fuel lines is used for lubrication purposes; therefore, approximately 92% of the measured fuel flow rate at the test is used for combustion. After correction of the fuel flow rate, the error between CFD and experimental results was reduced after calibrating the test results, as shown in Figure 22. This reduced the error in the fuel flow rate and TSFC from 7% and 10% to 1% and 2.8%, respectively.  
Figure 21: Fuel Lubrication System.  
Figure 20: Temperature Profile and Streamlines around EGT Sensor Location: (a) 30,000 RPM; (b) 104,000 RPM.  
However, contrary to what was described earlier, the error tended to increase in the low-RPM region below 50,000 RPM. To determine the underlying cause of this phenomenon, the analysis varied the bleed air flow rate to 0%, 1.5%, and 3% at 50,000 RPM conditions and compared these. As a result, as the bleed flow rate decreases, the fuel flow rate decreases by approximately 3%, as shown in Figure 23. In the analytical model presented in this paper, the bleed air flow rate is set to be 3% for all the RPM ranges. However, it should be considered that this bleed flow rate changes with RPM due to the differential pressure varying with RPM.