Abstract: A centrifugal pump for a first fluid has a housing with an inlet and includes a rotatable impeller which is adjacent to a stationary plate. The inlet communicates with a space defied between the impeller and the plate. An outlet adjacent to the periphery of the impeller also communicates with the space. The space is divided into a plurality of zig-zag pumping passages which extend from the inlet to the periphery of the impeller. Each passage has undulating sidewalls defined by complementary undulations in the facing surfaces of the plate and the impeller. Each passage also has endwalls defined by vanes mounted to the impeller. The vanes generally conform to the undulations in the impeller and are also complementary to the undulations in the plate. An injection port in the plate communicates with the space downstream of the inlet at a point where the lowest absolute pressure within the passages exists. A second fluid may be injected through the port and into the passages.