Study of Flow Field in Rectangular Sump Models and Performance Analysis of a Mixed Flow Pump

Abstract

It is an accepted fact that quite a number of problems faced in a pumping station are related to the design of sump or intake rather than pump design. Head-capacity curves provided by the pump manufacturer are obtained on the condition of no vortices flowing into the pump intake. The efficiency and performance of pumping stations depend not only on the performance of the selected pumps but also on the proper design of the intake sumps. A faulty design of pump sump can lead to the occurrence of swirl and vortices, which reduce the pump performance and induce vibration and additional noise. Therefore, sump model test is necessary to check the flow condition around intake structure. Numerical simulation is a good facility for reducing the time and cost involved throughout the design process. In this study, the commercial software ANSYS CFX-13.0 has been used for the CFD analysis of the sump models.

In a scaled sump model for air entrainment simulation, numerical analysis of single phase and two-phase with SST turbulence model was carried out to predict vortex (both free surface vortex and submerged vortex) occurrence and location. The effectiveness of curtain walls and square bars to eliminate the free surface vortex is evaluated. Meanwhile, the experiment including PIV system was performed in KMOU to investigate the flow conditions around pump intake structure.

In a full size sump model, overall numerical analysis for the sump model with a mixed flow pump installed was carried out. Hydraulic performances of the mixed flow pump for head rise, shaft power, pump efficiencies versus flow rate changed from 50% to 140% of the design flow rate were studied by the performance curves. A trident shaped anti vortex device (AVD) composed of three wall fillets and one center splitter was installed under the pump intake. The effectiveness of the AVD for the suppression of submerged vortex was evaluated. In addition, numerical simulation of cavitation phenomenon in the mixed flow pump has been performed by calculating the full cavitation model with k-ε turbulence model.

According to the results, details of the location, size and strength of vortices were predicted in the numerical simulation. For the scaled sump model, although there is a little difference between the single phase and two-phase simulation, all the results predict the free surface vortex and submerged vortex formation and location. The CFD simulations of flow condition show good agreement with the PIV results. The effectiveness of curtain walls installed to inhibit the free surface vortex is confirmed in the CFD results

while the square bars installed in the channel failed to suppress the free surface vortex. For the mixed flow pump with sump, the best efficiency point (BEP) is at the design flow rate with the corresponding efficiency of 89.6%. The effectiveness of AVDs installed for the submerged vortex is confirmed by comparisons of the sump model with and without AVD results.

Cavitation phenomenon analysis shows the pump operating at the BEP condition has a considerable extent over the requirement of cavitation performance. With numerical simulation, the inception of cavitation in the blade passage is observed on the suction surface where the leading edges meet the tips, and then as the inlet total pressure decrease, the cavitation zone is spread out over the suction surface as well as the leading edge of the impeller blade.