A History of Vortices
Detecting and mitigating critical flow structures in water pumping stations is a complex engineering task, that has always been based on experimental activities.
Vortex generation in pumping stations must be prevented or limited to superficial and low intensity vortices, because deep and intense ones can severely damage the pumps or / and affect their performance (flow, head & efficiency) due to non uniform velocity distribution at the pump impeller inlet and their air content. This kind of dangerous vortices might appear in some operational conditions related for example to low water levels or high rotational speeds and high flow rate values.
Vortex generation must be carefully studied and verified during the design phase of pumping stations, and measures aimed at preventing vortices or reducing their intensity must be taken.
One of the key factor to prevent vortices is the submergence, that is the minimum immersion depth required by the pump to prevent vortices. It varies according to the size of the pump and the rotational speed. As an indication, it is typically 2.5 times the diameter of the hydraulic part, but it must be adapted to the pump NPSHr conditions as well as the pump and pumping station geometry since this rule is very general.
Common practice is to build scaled prototypes of a portion of the pumping station, typically one single pump room is built, in order to simulate the real working conditions and to verify the presence and nature of vortices. Vortices are detected visually, from the observation of the free surface curvature and with the support of colored tracers, that can make the flow structures visible and help the classification of vortices according to the HIS vortex classification shown in Figure 1. This classification is essentially based on visual identification and characterization of structures such as swirls, ripples and air bubbles. This kind of activity requires experienced engineers to distinguish between Type1-2, that are not dangerous, and the other types, that must be avoided. The complexity of the classification is also associated to the transient nature of these vortices that tend to be unstable and to appear and disappear. In this frame the use of Computational Fluid Dynamics (CFD) can support and supplement the experimental activities by providing detailed information about the flow behavior in the whole pumping station and about vortices and fluid structures in the proximity of the pumps.
It must be pointed out that also the CFD simulation of pumping stations and pump rooms is not a simple task. The accurate solution of multi-phase flows on complex geometries, with air entrainment and transportation in unsteady conditions requires high quality computational mesh, high order accuracy and parallel computing. Moreover the identification and classification of flow structures requires the definition of quantitative criteria during the post-processing of CFD results.
Nonetheless GE Power and EnginSoft accepted the challenging task of developing and running CFD models of a pumping station with the aim of comparing the simulated vortices with the ones detected during experimental tests on a scaled model. If CFD demonstrated to be able to identify and classify vortices with the same accuracy of experimental tests, it could be used to speed up the design process, for example to detect critical operating conditions or to compare alternative designs to mitigate vortices intensity.