Numerical Research on Influence Factors of Flow Separation near Centrifugal Impeller Outlet

The flow behavior near the centrifugal impeller outlet determines the impeller aerodynamic performance significantly and also influences the performance of the downstream components.The three factors affecting the impeller outlet flow separation were numerically investigated.For the shrouded centrifugal impeller,the impeller diffusion ratio has great influence on its outlet flow behavior;there is a large separation zone near the impeller outlet at the design flow rate when the centrifugal impeller inlet/outlet relative velocity ratio is above 2.0,which agrees with the conventional impeller design theories.For the unshrouded centrifugal impeller,there always exists a separation zone near the impeller outlet at the design flow rate when the inlet/outlet relative velocity ratio is high or low.The tip clearance has a significant influence on the scale of the impeller outlet flow separation zone.For the impeller studied here,the smaller tip clearance does not correspond to the smaller separation zone,and the separation zone is smallest when the tip clearance equals 7.5% of impeller outlet width.In addition,increasing the diffuser pinch angle is effective in controlling the impeller outlet flow separation,but an excessive pinch angle can enlarge the recirculation flow zone near the impeller tip clearance,which will lower the aerodynamic performance of the entire compressor stage.