Centrifugal pumps have been utilized in various industrial fields with the diversification of applications and operating fluids due to the simple structure and the easy maintenance. A centrifugal pump with tandem arrangement of double rotating cascades and stationary cascade downstream of the impeller has been confirmed as a preferable structure for air-water two-phase flow pump. In this case, the smaller impeller yields lower pump head in single-phase flow, without increasing the rotational speed. Therefore, it might be desired to install other devices, instead of vaned diffuser (stationary cascade), downstream of the larger impeller if two-phase flow performance would be improved under the restricted pump size. In the present study, a double volute passage is focused downstream of larger impeller consisting of tandem arrangement of rotating cascades. Then its effects on two-phase flow performances are experimentally examined. The guiding principles of selecting a preferable diffuser structure on designing a air-water two-phase flow centrifugal pump with higher performance are discussed in comparison with case of vaned diffuser downstream of smaller impeller.
