Pump sump designs play an important role for vertical pumps since they are responsible for the flow conditions at the impeller inlet. To first foresee and then prevent possible damages resulting from uneven flow conditions, computational fluid dynamics (CFD) can support model tests to reduce at least costs, but, with increasing computer power, also time. This paper compares different approaches to model the flow in pump sumps: The conventional single phase fixed water surface method and the multiphase (air+water) free water surface method. Both methods are compared with unsteady state computations and two different geometries (original and modified geometry) regarding differences in water surface, flow distribution in the pump wells, measurements, and computational time. All CFD results compare well with the experiments and are in good agreement with the vortex formation, type, and location as well as flow pre-rotation. Most of the time, the main interest lies in flow, vortices, forces, and frequencies in the attached pump, and additional computations need to be undertaken. Due to different timescales between the flow in the pump sump and in the pump itself, a decoupled technique is conventionally used in which firstly the flow field of the sump and secondly in the vertical pump is computed. We compare this decoupled method with a fully coupled one regarding computational time needed and time discretisation required as well as resulting flow structures and radial forces. Results of both methods are mainly in a good agreement, show slight differences in the velocity profile at the impeller inlet, and larger ones in magnitude of radial forces and force modulus. Decoupled and coupled method show practically the same computational effort, which recommends the application of the fully coupled approach not only in special cases where the vortices in the sump are triggered by the impeller rotations, especially at low flow rate operating points and existence of pre-swirl vortices.

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