The paper describes the design of the blade geometry of a medium specific speed mixed flow pump impeller by using a three-dimensional inverse design method in which the blade circulation is specified. The design objective is the reduction of the impeller exit flow nonuniformity by reducing the secondary flows on the blade suction surface.
The efficiency and stability of mixed and radial flow turbomachines and adversely affected by the presence of impeller exit flow nonuniformity. In recent years, as a result of improvements in experimental techniques and numerical methods, it has been possible to obtain a better understanding of the basic mechanism behind this phenomenon.
Inverse Design Method
In this design method the blades are represented by sheets of vorticity whose strength is determined by a specified distribution of circumferentially averaged swirl velocity or directly related to the bound circulation. The partially differential equations for the flow field are solved numerically by using a finite difference approximation on a body-fitted coordinate system. Once a new estimate for the velocity field has been obtained, it is then possible to compute the new blade shape by using Eq.6 (Figure 1). This iterative process is then repeated until changes in blade shape between two iterations fall below a certain given tolerance, usually taken as 10¯5 radians.
Figure 1: Equation 6
The design conditions used for the design of the mixed flow pump were based on the medium specific speed impeller, which has been studied extensively by Goto. This impeller was designed by using conventional techniques involving the use of curve fits to connect the blade angles smoothly between the leading and training edges.
The impeller geometry of a medium specific speed mixed flow pump was designed by using the three dimensional inverse design method of Zangeneh (1991).The main objective of the design was to suppress secondary flows in order to obtain a uniform exit flow field from the impeller. Two different impellers were designed using the inverse design method with the same circulation distribution, but different stacking conditions.
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