A Method for Preliminary Design of Variable Geometry Vaned Diffuser for Centrifugal Compressor

Centrifugal compressors require a wide operating range as well as high efficiency. Typical active control forms to enhance the operating range are suction throttling with a valve in a suction line, discharge throttling with a ring installed in a vaneless diffuser, variable inlet guide vanes, and variable geometry vaned diffuser (VVD). This study focuses on the design of a VVD whose vane rotates around a pivot to adjust its opening and proposes a method to estimate the main parameters of the VVD at the preliminary design stage. The design method here is based on matching the diffuser vane inlet angles with the flow angles at the diffuser inlet. If the designed VVD can maintain low incidence at the diffuser vane leading edge over the whole movable range of the vane is evaluated. A graph, which indicates the relation between the diffuser vane inlet angles and the flow angles at the diffuser inlet at diffuser choking and diffuser stalling, makes the evaluation easy. The proposed procedures were applied to select the number of vanes of the VVD for a turbocharger compressor. Tests for the validation of the design procedures were conducted by using fixed vaned diffusers that simulated the VVDs installed on the turbocharger with an 85 mm-diameter impeller. The results showed the effectiveness of the proposed design procedures. This paper first presents examples of applications of the typical active methods extending centrifugal compressors' operating range for real machines and then discusses the design of VVD.