Numerical Analysis of Vortices Behavior in a Pump Sump

Abstract

Recently, designers and engineers of pump stations realized that the efficiency and performance of a pumping station does not depend only on the performance of selected pumps, but also on proper design of intake structure. Most recurring problems faced in a pumping station are related to the sump or intake design rather than pump design. International design standards for a pump sump restrict undesirable flow patterns up to a certain extent implication of which does not guarantee a problem free sump but provides a basis for initial design. A faulty design of pump sump can lead to form of swirl and vortex, which reduce the pump efficiency and induce vibration, noise, and cavitation. To reduce these problems and for the advanced pump sump design with high performance, it is essential to know the detailed flow behavior in sump system. Swirl angle parameter and vortices formation are important parameters that determine the quality of flow ingested by sump. According to the swirl angle parameter, the hydraulic institute prescribes the method that needs to be employed for estimating this parameter.

In this study, numerical analysis and experimental test of pump sump were carried out to predict vortex formation (free surface vortex, side wall and submerged vortex) occurrence, location, and air entrance in details. A four blade zero-pitch traditional swirl meter was installed at the suction pipe to measure the flow intensity by swirl angle calculations; the key point is to obtain the average tangential velocity at different suction pipe diameter. The other part of this study is overall numerical analysis for sump model with a mixed flow pump installed. Hydraulic performance of the mixed flow pump for head rise, shaft power, and pump efficiencies versus flow rate changed from 50% up to 150% of the design flow rate were studied by performances curves. In addition, a basic numerical simulation of cavitation phenomenon in the mixed flow pump has been performed by calculating the full cavitation model with k-ε turbulence model.

Swirl angle and average tangential velocity estimated by CFD simulation was in agreement with experimental results obtained. The results also show that submerged vortex strength was almost proportional to the flow rate in the sump. The free surface vortex had an unsteady behavior as its location and duration drastically varied. In addition, post processing results showed the tangential velocity behavior and the four types of free surface vortex (Surface swirl, Surface simple, air bubbles and full air core to intake) by changing the air volume fraction values. In the mixed flow pump performance study, the efficiency without and with sump model was 83.4% and 80.1% respectively at the design flow rate.