## 4 Results
### Condensation Analysis  
Blade leading-edge condensation is a unique phenomenon of sCO\({}_{2}\) centrifugal compressors operating near critical point, which may have an impact on a compressor's operation boundary and the fatigue life of blades. Generally, it is necessary to avoid leading-edge condensation as much as possible. For the condensation of the blade leading-edge, [PERSON] proposes the acceleration margin to condensation (AMC) Mach number, which calculates the isentropic static point where the saturation conditions would be met by modelling an isentropic expansion of the flow down to the saturation pressure/temperature [20]. However, due to the sudden change in the calculation of sound velocity in the two-phase region, the criterion of AMC Mach number may be distorted near pseudo-critical line. In this paper, the speed of  
Figure 15: Comparison of dimensionless isentropic head coefficient performance curve. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Design** & **Leading Edge (Exp)** & **Training Edge (Exp)** \\ \hline Tip clearance & 0.25 mm & 0.43 mm & 0.45 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 8: The tip clearance of the experimental impeller. condensation acceleration velocity (\(V_{limit}\)) is used as shown in Equation (5), where \(h_{limit}\) is the enthalpy value corresponding to isentropic expansion to saturation state starting from inlet condition. \[V_{limit}=\sqrt{h_{01}-h_{limit}} \tag{5}\]  
Figure 17 shows the condensation acceleration velocity (\(V_{limit}\)) near the critical region. In the supercritical-gas-like region, the condensation acceleration velocity (\(V_{limit}\)) is mainly influenced by both inlet temperature and inlet pressure; meanwhile, in the supercritical-gas-like region, condensation acceleration velocity (\(V_{limit}\)) is mainly affected by inlet pressure, the influence of inlet temperature becomes negligible, and the condensation acceleration velocity (\(V_{limit}\)) is the lowest near the critical point. Under certain inlet temperatures near critical temperature, the increase in condensation acceleration velocity (\(V_{limit}\)) in the supercritical-liquid-like region is much lower than the decrease in condensation acceleration velocity (\(V_{limit}\)) in supercritical-gas-like region, as is the increase in inlet pressure. Next, condensation at the experimental inlet is analyzed using Figure 17. The inlet pressure near the choke boundary (@ 29,000 RPM) is lower than that near surge boundary (@ 31,000 RPM) and all inlet conditions are located at supercritical-liquid-like region; therefore, the condensation acceleration velocity (\(V_{limit}\)) near the choke boundary (@ 29,000 RPM) is lower, and only 50 m/s. In addition, near the choke boundary, the relative speed of sCO\({}_{2}\) in the inner flow channel increases, which further aggravates the condensation phenomenon. Figure 18 shows the dryness distribution of test point near the choke boundary (@ 29,000 RPM) and of the test point near the surge boundary (@ 31,000 RPM), which is obtained using a numerical simulation method [21; 22; 23]. It can be seen that a large area of condensation appears, extending from the leading-edge of the blade top to 50% of the downstream section for the condition near the choke boundary (@ 29,000 RPM), because of the suction effect of the blade tip clearance; meanwhile, the figure only shows a small area of condensation at the blade leading-edge for the condition near the surge boundary. Figure 16: Comparison of dimensionless isentropic efficiency performance curve. [PERSON] at.al analyzed the condensation process of sCO\({}_{2}\), where the inlet pressure is set to 10 MPa. As is shown in Figure 19, the density drop rate of supercritical-liquid-like condition is significantly higher than that of the supercritical-gas-like condition [24]. Therefore, the condensation of supercritical-liquid-like condition will lead to a sudden increase in volume flow in the inner flow channel, and a rapid increase in relative velocity, which may aggravate the throat blockage.