Impeller Volute and Diffuser Interaction

The flow in centrifugal pumps is exceedingly complex, involving curvature, system rotation, separation, turbulence, and secondary flows. Moreover, the geometry is often asymmetric due to the volute shape. As a result, the relative motion between impeller and volute generates an unstableness which affects not only the overall pump performance, but is also responsible for pressure fluctuations, hydraulic noises and unforeseen hydrodynamic forces. These fluctuations not only generate noise and vibration that cause unacceptable levels of stress and reduce component life due to fatigue, but also introduce unfavourable characteristics of pump performance even at or near the design point. Experimental and numerical approaches contributed to the understanding of the highly complex flow interactions that occur in a centrifugal pump. Unsteady phenomena in diffuser pumps become more complicated at off-design operating conditions. At reduced flow rates, the flow rate and pressure of a pump become increasingly unstable. When substantial flow fluctuations are propagating at a low frequency along the circumference, but are limited to a part of the components (e.g., rotor, diffuser, or volute), the phenomenon is typically referred to as rotating stall. However, the knowledge about the unsteady pressure fluctuations and the unsteady blade loading is still not satisfying. Furthermore, the design of the centrifugal pumps has already reached a level that only through a detailed understanding of the internal flow an increase of the overall performance can be achieved. Due to the curved passages inside the impeller and the volute the flow is to be considered as three-dimensional. Additionally, since the flow following blade passages as well as the volute casing interacts with viscous boundary layers, secondary flows are generated. Therefore, a correct simulation of the impeller/volute interaction requires the simultaneous solution of the three-dimensional unsteady Navier-Stokes equations in both the impeller and volute.