Experimental investigation of the unsteady flow in a centrifugal turbomachine vaned diffuser

The unsteady effects caused by the interaction of the rotor wakes with the flow within the vaned diffuser of a centrifugal turbomachine stage have been experimentally investigated. The experiment has been carried out on the simplified model of a centrifugal pump operating in air at the nominal point. The diffuser flow has been surveyed by means of a two-colour four-beam laser Doppler velocimeter on a blade-to-blade surface at midspan and over two cylindrical measuring surfaces located just upstream of the diffuser vanes leading edges and in proximity of the inlet throat of the diffuser. Instantaneous data taken in phase with the shaft rotation have been averaged and used to construct images of the flow field for different relative positions of impeller blades and diffuser vanes. The sequences are detailed enough to show the evolution of the rotor wakes within the forward part of the diffuser. The rotor blade wakes entering the diffuser are chopped by the vanes leading edges into individual segments which are convected through the diffuser passages where they mix out rapidly.