On the Design Criteria for Suppression of Secondary Flows in Centrifugal and Mixed Flow Impellers

In this paper, for the first lime, a set of guide-lines are presented for the systematic design of mixed flow and centrifugal compressors and pumps with suppressed secondary flows and a uniform exit flow field. The paper describes the shape of the optimum pressure distribution for the suppression of secondary flows in the impeller with reference to classical secondary flow theory. The feasibility of achieving this pressure distribution is then demonstrated by deriving guide-lines for the design specifications of a 3D inverse design method, in which the blades are designed subject to a specified circulation distribution or 2πrV¯θ. The guide-lines will define the optimum choice of the blade loading or ∂rV¯θ/∂m and the stacking condition for the blades. These guide-lines are then used in the design of three different low specific speed centrifugal pump impellers and a high specific speed industrial centrifugal compressor impeller. The flow through all the designed impellers are computed numerically by a 3D viscous code and the resulting flow field is compared to that obtained in the corresponding conventional impeller. The results show consistent suppression of secondary flows in all cases. The design guide-lines are validated experimentally by comparing the performance of the inverse designed centrifugal compressor impeller with the corresponding conventional impeller. The overall performance of the stage with the inverse designed impeller with suppressed secondary flows was found to be 5% higher than the conventional impeller at the peak efficiency point. Exit flow traverse results at the impeller exit indicate a more uniform exit flow than that measured at the exit from the conventional impeller.