The design of axial inducers for turbo-pumps

Abstract

In many turbopump applications it is desirable to run the pump at the highest possible speed to minimise the size and weight of the unit and facilitate matching with a drive turbine. Frequently, a limit on rotational speed is imposed by pump cavitation with its associated deterioration in performance and structural damage. For conventional single sided centrifugal pumps cavitation occurs when the suction specific speed ... exceeds 8-10,000 (1) so that for such pumps the maximum rotational speed without cavitation is determined by the flow and the suction head available. To permit operation at higher speeds an inducer or boost pump may be mounted in front of the main pump (f ig. 1). A typical axial inducer is simply a very lightly loaded axial pump which raises the pressure of the fluid sufficiently to avoid cavitation in the main pump. It has been found possible to operate inducers successfully at suction specific speeds up to 30,000, so that considerable reductions in total pump weight can be achieved when they are employed. Of course, since the inducer is simply a lightly loaded pump which is capable of handling a cavitating fluid, the functions of inducer and main pump may be combined by designing the main pump so that the inlet is lightly loaded. However, it is not always convenient to do this and in many applications conventional centrifugal pumps are still used, preceded by a separate inducer. The objective of this report is to put the design of such inducers on a rational basis by developing a method of calculating blade shapes for the optimum pressure distribution.