Conventional single stage centrifugal pumps are mostly designed with a spiral volute. Due to relative movement between impeller and the asymmetric volute the flow at the outlet of the impeller is strongly interacting with the volute flow. The unsteady phenomenon leads to unbalanced radial dynamic forces as well, especially at off-design points, giving rise to pump vibration and hydraulic noises.
In order to weaken the unsteady phenomenon, a special kind of slope volute is designed. Different from the spiral one, it keeps the radial size of the volute casing unchanged, but increasing the axial size to make sure the sectional area changing regularly from the tongue to pump outlet. CFD analysis has used to solve the unsteady 3D viscous flow in both conventional and special designed centrifugal pumps with the same impeller for several flow rates.
Compared to the spiral volute pump, there’s secondary flow with only one vortex existing in the slope volute. The average pressure and amplitudes of pressure fluctuations keep nearly unchanged along the slope volute wall due to the symmetrical shape in the radial direction. The pressure distributions for both pumps at fBPF are also very different. Interaction of the impeller flow with the tongue is weaken due to the different kind of tongue shape. It has little influence on the interaction flow field in the pump with slope volute. The radial load component always reaches a minimun magnitude at nominal conditions for both pumps. But the traces calculated for the pump with slope vloute are less stretched in the radial direction than those computed for the pump with spiral volute. Results indicate that a lower pulsation of the radial force is expected, and hardly affected by the flow rate for the pump with slope volute.