# Investigation on Unsteady Flow Characteristics of a SCO\({}_{2}\) Centrifugal Compressor
## 5 Conclusions  
In this paper, a SCO\({}_{2}\) centrifugal compressor of large input power and mass flow rate was designed and numerically investigated. A thorough numerical analysis of the unsteady flow field in the centrifugal compressor was performed in ANSYS-CFX. Streamlines, temperature, pressure, and Mach number distributions at different time steps in one revolution period were covered to present the unsteady effect of turbomachinery. Meanwhile, the force on a single rotor blade was monitored to investigate the frequency components of exciting force, thus providing foundation for vibration analysis. Moreover, the torque, output power, pressure ratio, and isentropic efficiency in the steady and the unsteady time-averaged condition were calculated and compared with the design condition to measure the validity of the design.  
The preliminary design provided the key geometry parameters of impeller and diffuser. For instance, the inlet diameter of diffuser was 220 mm. In the meantime, multiple tests were conducted to ensure the validity of the impeller and the diffuser. We finally chose the geometry of best performance and least vortex after several rounds of improvements.  
A thorough numerical analysis of the unsteady flow field in the centrifugal compressor was performed in ANSYS-CFX. The computation adopted hexahedral mesh, finite volume method, and the RNG \(k\)-\(\varepsilon\) two-equation turbulence model. In the unsteady computation, the monitored parameters revealed obvious periodic characteristics. Comparing the calculation result of four different time steps in a revolution, the result revealed obvious unsteady flow characteristics. To be specific, the flow at 0.75T had the worst flow characteristic while the flow at 0.25T was the best due to the different location of the diffuser towards the impeller. The largest amplitude of exciting forces was found in the frequency of 3750 Hz and the exact value of circumferential and axial forces were respectively 160.2 N and 59.3 N. The pressure ratio, input power, and isentropic efficiency mainly met the design requirement, although the efficiency was slightly lower in the viscous CFD computation owing to the inaccurate calculation of flow loss in the design.  
In conclusion, the unsteady flow characteristics are prominent in the centrifugal compressor and unsteady CFD simulation is able to provide a more accurate estimation of compressor performance. On the whole, the design of the 5 MW centrifugal compressor in this paper can achieve a satisfactory aerodynamic performance. In the actual operation, the air exciting-vibration force caused by unsteady effect should be considered in the strength assessment. Also, it has been concluded that this design  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Design** & **Steady CFD Result** & **Unsteady CFD Result** \\ \hline pressure of inlet (MPa) & 8.80 & 8.72 & 8.74 \\ pressure ratio & 2.15 & 2.17 & 2.16 \\ input power (MW) & 5.00 & 5.10 & 5.06 \\ isentropic efficiency (\%) & 76.30 & 75.70 & 75.54 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Comparison of design and CFD results.  
method is in accordance with the CFD result, which suggests that it can be used in centrifugal compressor designs with other power levels.  
[PERSON] and [PERSON] conceived and designed the analysis method for this SCO\({}_{2}\) centrifugal compressor; [PERSON] and [PERSON] performed the numerical experiments; [PERSON] analyzed the data and wrote the paper.  
The authors declare no conflict of interest.