Axial and Mixed Pump Theory

Axial Pump
Axial Pump

Unlike the centrifugal pump, the performance in axial machines is a function of the action of the blade profiles. Because of this, the main approach in design of axial pumps is focused on blade performance.

Impeller blades of axial flow pumps have a double curvature form at the inlet and at the outlet due to the change in diameter from hub to periphery. Absolute flow before and after the impeller and relative flow along the impeller passage are axisymmetric and potential. There is no radial mixing. Under this condition, each streamline is parallel to the axis of the pump. Fluid passes parallel to the pump axis i.e., along the streamline.

The basic principle of operation in pumps is the interaction of force and energy that transfer from the impeller to the fluid.

For axial pumps there is no centrifugal force, and energy transfer takes place only from kinetic to pressure energy (diffuser effect). Diffuser patter of flow has its own limitations. The angle of divergence should not exceed 8° to 10° and a strictly smooth streamlined flow passage must exist, otherwise, flow separation at the boundary layer may exist.

As a consequence, a flow mixing between streamline takes place and axisymmetric and potential flow no longer prevails. Only in mixed-flow pumps with many blades is the fluid dynamic in the passage dominant, as in centrifugal machines.

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