An examination of the condition of the flow leaving the impeller exit kinetic energy often accounts for 30–50% of the shaft work input to the compressor stage; for energy efficiency, it is important to recover as much of this as possible. This is the function of the diffuser, which follows the impeller. Effective pressure recovery downstream of an impeller is very important in order to realize a centrifugal compressor with a high efficiency and a high pressure ratio, and an appropriate selection of a diffuser for a specific impeller is a critical step in order to develop the compressor accordingly. The purpose of this study is to investigate the sensitivity of how compressor performances change as the vaned diffuser geometry is varied. Three kinds of vaned diffusers were studied and compared with its results. The first vaned diffuser type is based on a modified NACA airfoil, the second is a channel diffuser, and the third is a conformal transformation of NACA 65-(4A10)06 airfoil. A mean-line prediction method was applied to investigate the performance and stability for three kinds of diffusers. Computational fluid dynamic (CFD) analyses and a detailed interior flow pattern study have been done. In this study, the off-design behavior of three different types of diffusers, given by the mean-line prediction, was investigated using CFD results and the NACA 65 diffuser geometry, which satisfies a wider operating range and has a higher pressure recovery than the others, was selected. The numerical results were compared with experimental data for validation and showed good agreement.

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