# Analysis and Improvement of a Two-Stage Centrifugal Compressor Used in an MW-Level Gas Turbine
## 4 Optimized Design of the High-Pressure-Stage Centrifugal Compressor
mass flow of the unit unchanged, when the diameter ratio \(D_{3}/D_{2}\) is reduced. Considering all factors, the original vaned diffuser was replaced by a tandem vaned diffuser as shown in optimized design 2 in Figure 10c. In the first row, 15 small vanes were selected, which is less than the 17 of the original design. In this way, the throat area of the vaned diffuser slightly decreases when the diameter ratio decreases. The second row combines 15 large vanes and 15 splitter vanes. In this way, the divergent angle of the vaned diffuser can be reduced to prevent separation. Meanwhile, the friction loss on the inner surface of the vaned diffuser with the increase in the vane number would not be significant. By using the optimized method, the ratio of design \({A_{d}}^{*}/{A_{i}}^{*}\) to theoretical \({A_{d}}^{*}/{A_{i}}^{*}\) is 1.037, which is closer to 1 and indicates the better matching of the diffuser and impeller. The throat area of the vaned diffuser is slightly larger than that of the impeller, which makes the choke mass flow of the optimized design consistent with that of the original design. The steady simulations were performed using the commercial code Numeca. The S-A turbulence model was used. Grids were generated by AutoGrid. The grid number of the original design was 1,180,000; after optimization, it was 1,740,000. Total pressure of 101,300 Pa was set at the inlet and the back-pressure was set at the outlet for boundary conditions. The wall was adiabatic with a sliding surface. The calculation results are shown in Figure 11, and the 1D calculations are also shown for comparison. The calculation results show that the pressure ratio of the original design was 2.838, and the efficiency was 80.68%. The pressure ratio of the optimized design was 2.967, and the efficiency was 85.03%. Compared with the pressure ratio and efficiency of the original design, those of the optimized design 2 represented a significant improvement. The trend of the 1D calculation results was similar to that of the Numeca calculation results. In the design condition, the pressure ratio and efficiency of the original design were 2.894% and 82.29%, respectively, whereas the pressure ratio and efficiency of the optimized design were 2.952% and 84.52%, respectively. Compared with the values in the original design, the pressure ratio and efficiency of the optimized design increased by 2.01% and 2.23%, respectively. Figure 11: Performance map of the original and optimized designs for the high-pressure stage: (**a**) Reduced mass flow vs. total pressure ratio; (**b**) Reduced mass flow vs. isentropic efficiency. Figure 12 shows the inlet attack angle distribution of the vaned diffuser. Compared with the attack angle in the original design, the angle in the optimized design was reduced to a certain extent. Specifically, the attack angle decreased by approximately \(5^{\circ}\) near the blade root, which changed the range of the attack angle from the blade root to blade tip less than \(8^{\circ}\). In the original design, the range of the attack angle was close to \(15^{\circ}\).