This paper describes the computational results on the performance of a centrifugal compressor stage with twisted vaned diffuser by varying the speed of the impeller and direction of twist for the diffuser vane. The centrifugal compressor stage configuration consists of a 2-D impeller (no twist is provided for the impeller vanes) with various configurations of diffusers. Diffuser configurations considered are Vaneless Diffuser (VLD), Low Solidity Vaned Diffuser (LSVD) and Twisted Vaned Diffuser (TVD). The analysis was carried at four different rotational speeds with corresponding mass flow rates of the impeller. 9° twist is given to the diffuser vane from hub to shroud by providing rotation at the leading edge, keeping the profile at hub as reference in the direction of rotation of the impeller and opposite to the direction of rotation of the impeller. The off-design cases considered includes operation at 80%, 90%, 110% and 120% of the design mass flow rate. CFD results are validated with experimental results for stages with VLD and LSVD for certain chosen performance parameters such as head coefficient, stage input power and exit flow angle. The computational results indicate that variations in impeller speed will cause changes in all significant performance parameters like the total pressure rise, power coefficient and efficiency of the stage and static pressure recovery coefficient of the diffuser. Contour plots were generated from CFD results and analyzed for better understanding of effect of rotational speed of the impeller on the performance of the centrifugal compressor. As a result of this study, it can be concluded that twisted vaned diffuser improves the performance in comparison to low solidity vaned diffuser for all the chosen impeller rotational speeds. The performance of the compressor stage is superior when diffuser vane twist is provided in the direction opposite to the rotation of impeller.

Computational Fluid Dynamic (CFD) analysis is conducted to improve the diffuser performance on an industrial centrifugal compressor stage. The present work is concentrated on twisted vaned diffusers(TVD) with different shapes like constant thickness flat plate (FP), cambered curved arc plate (CCAP), uncambered aerofoil (NACA 0010) and cambered aerofoil (NACA 2410). For constant thickness plates, the leading and trailing edges have been rounded to have a smooth entry and exit. The aerofoil diffuser vane is formed from a standard section profile with minor modifications at the trailing edge. The twist is given to the diffuser blade by rotation at its leading edge resulting in different stagger angles from hub to shroud. The analysis was conducted at an impeller tip Mach number of 0.35. The overall stage performance is evaluated in terms of head coefficient, stage efficiency, power coefficient of the stage and static pressure recovery coefficient of diffuser with varying flow coefficients. The TVD NACA 2410 provides best performance for the chosen compressor stage.

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