Algorithm for estimating swirl angles in multi-intake hydraulic sumps

Hydraulic Pump sumps are designed to provide a swirl free flow to the pump. The degree of swirl is measured in physical model tests using a swirl meter and a quantity known as swirl angle is generally measured. The present paper presents a novel method to compute the bulk swirl angle using the local velocity field obtained from computational fluid dynamics data. The basis for the present method is the conservation of angular momentum conservation. By carrying out both numerical and experimental studies the novel swirl angle calculation method is validated. Further the effect of vortex suppression devices in reducing the swirl angle is also demonstrated. Introduction Hydraulic pump sumps commonly used in circulating water cooling systems for power generation plants are required to be designed appropriately as it greatly influences the performance of the pumps and their efficiency. A non-uniform approach flow can cause the fluid in the pump intake pipe to swirl. This swirling flow if ingested by the pump can cause noise and vibration, thereby increasing the operational and maintenance costs [1]. A typical multi-intake pump sump consists of multiple inlet channels, a transition region, forebay and pump bay areas. Standards have been evolved for the design of the pump intake by Hydraulic Institute [2] for its satisfactory performance. Hydraulic Institute [2] has also suggested a procedure to perform scaled model tests and assess the quality of flow in the intake pipe using a swirl meter and recommends the measured swirl angles to be limited to less than a prescribed limit. Often due to some constraints in the construction site, certain compromises have to be made that may require some deviations from the suggested dimensions for the pump intake. In such cases, scaled physical model tests are carried put to ensure the adequacy of the design. The pump sump has been studied experimentally using scaled model laboratory tests [3-7]. Such laboratory tests have helped in understanding the vortex formation process, but they are expensive and take time to construct and operate. Further, detailed velocity fields, which would help the engineers to evolve better design of pump sumps, cannot be obtained in experiments. The advancement in high speed computing and development of robust turbulence modelling techniques as well as evolution of accurate numerical algorithms in the last few years have led to the development of Computational Fluid Dynamics (CFD) codes. These have provided an alternative means to study the hydraulic flow characteristics of pump sumps. There have been several studies [8-12] in the application of CFD to model flows in pump sumps. These studies have been conducted for idealized single intake rectangular geometry. Practical pump sumps however have multiple-intakes and the disturbances in the forebay diffusion area may be carried to the pump thereby altering the flow patterns and inducement of swirls at the pump inlet. While the studies [8-12] have identified the regions of vortex formation and compared them with experiments and described swirl qualitatively (with streamlines), quantification of the swirl in terms of swirl angles that enables comparison of the numerical results with the field data has not been made. Chen and Guo [14] have conducted simulations in the pump sump where the forebay diffusion area was also modeled. The results were benchmarked by comparing the velocity profiles at various locations with the experimental results. The flow was qualitatively analyzed by studying the streamlines and vector plots at various locations in the pump sump. However quantitative results in terms of swirl angles at the pump inlet were not presented. Desmukh et al [15] have conducted simulations on multi-intake hydraulic structures and qualitatively analyzed the flow structure. However rigorous benchmarking of the simulation with experiments was not carried out. From the current literature, it is observed that there is a gap between the way pump sumps are qualified by physical model tests and computation methods. While the former gives a simple elegant quantitative measure for swirl in the intake the later appear to use qualitative measures. It is also surprising that such a void exits considering that these techniques have been coexisting for over two decades. The objective of the present work is to close this gap by evolving a method to quantify the swirl angle as used by Hydraulic Institute from the velocity field obtained from CFD. The CFD analysis is carried out in the commercial code Fluent. This has been made from first principles by using conservation of angular momentum. This method has been qualified using field tests carried out at Indian Institute of Technology, Bombay. The paper also provides some of the guidelines on the corrective actions that can be implemented to minimize the swirl angles.