Computational Fluid Dynamics Based Design of Sump of a Hydraulic Pumping System CFD Based Design of

Solids and Structures (SAS) Volume 5, 2016 doi: 10.14355/sas.2016.05.002

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Computational Fluid Dynamics Based Design of Sump of a Hydraulic Pumping System CFD Based Design of Sump Prithvi Shukla, Samay Bhatnagar, Shankar Sehgal*, Harmesh Kumar Mechanical Engineering Department, University Institute of Engineering and Technology, Panjab University, Chandigarh, India *

sehgals@pu.ac.in

Abstract In this paper, the design of an existing sump of a hydraulic pumping system has been analyzed in order to evaluate its performance. Computational fluid dynamics based analysis of the flow pattern inside the sump has been carried out using ANSYS software. Maximum value of the swirl angle has been evaluated for water flow inside the sump. It is observed that the maximum value overshoots the permissible limit as per hydraulic standards. The study highlights the importance of evaluating the maximum swirl angle in sump design.

Keywords Computational Fluid Dynamics; Sump Design; Hydraulic Pumping System; Swirl Angle

Introduction Computational fluid dynamics (CFD) expertise helps in improving the product design and its working behaviour (Anderson, 1995; Dupont & Cugal, 2006). CFD can be used in numerical simulation of three dimensional turbulent flows in the multi-intakes sump of any pump station (Chen & Guo, 2007; Chuang & Hsiao, 2011). Sump is an important part of a hydraulic pumping system. It is a reservoir or a large storage area in which liquid is stored for its quick and easy access. Sump is an intermediary between the source and the sink of pumping system.

Fig. 1 shows the two inlet domains of the sump from which the water enters into the basin. Water then flows towards the four separate areas, wherein suction pipes are installed for enabling quick movement of water towards the inlet of the pump. The pumping system under consideration consists of three main pumps and one auxiliary pump. This system has been modeled using Solidworks software. The solid model is then imported into ANSYS workbench for further CFD based analysis (ANSYSÂŽ , 2003). In this work, simulation studies have been carried out to investigate the nature of fluid flow from inlet to outlet of the sump. Outlet domain of the sump has been drawn in Fig. 2. From here on, water goes into the pump.

FIG. 1 INLET DOMAINS OF THE SUMP

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FIG. 2 OUTLET DOMAIN OF THE SUMP

Problem Formulation This paper discusses about the CFD based design of sump of a particular hydraulic pumping system. The sump in the proposed system has two sluice gates as shown in Fig. 1 . The sump contains four pump inlets, out of which, three are the main inlets while the fourth is an auxiliary inlet, which is used only during emergency.

According to the sump geometry, water is entering from cooling towers through two sluice gates. These two water streams meet in the fore bay and create non-uniformity. Due to this, the whole width of the fore bay is not fully utilized and a lot of recirculation zone and dead zone can be seen clearly during analysis. This non-uniformity of flow continues in the pump chamber, which results in high swirl angles in the pump. Since the back wall clearance behind the pump is slightly more, the water gets this extra space for recirculation.

The quantitative results are given in the form of swirl angles. The maximum swirl angle is found 17.6° which is much above the acceptable limit of 5° as per Hydraulic Institute Standards. In order to have uniform flow distribution in the fore bay and to minimize the swirl angles, the sump geometry needs modification.

Modeling and Meshing A 3D model similar to the original sump is created followed by surface meshing, in order to perform further analysis. Both these steps are paramount during the process as a flaw in the design would lead to a variation to the expected results. A step by step process flow is followed to attain a final meshed surface.

Design of Sump Geometry The geometry of the sump of Fig. 1 shows the water domain which is the active domain in our study. A wireframe model shown in Fig. 3 shows that the suction pipes are tapered from the bottom, this is done to assure smooth entry of water. The diameter of the three main inlets is 700mm and that of the auxiliary pipe is 300mm.

FIG. 3 WIREFRAME MODEL OF THE SUMP

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Creating Named Selections For creating the fluid simulation, firstly the boundary conditions need to be defined which show the areas from where the fluid will enter and leave the domain. The faces on which boundary conditions are applied, are named in the meshing phase of the analysis, and also predefine the locations, makes the meshing more accurate as the software adapts to the changes made in by the user. The mesh controls are kept unchanged to reduce complexity.

Fig. 1 and Fig. 2 show the two set of faces named as OUTLET and INLET respectively as the depicted the face from where the water will exit and enter.

Meshing of the Model Meshing is a crucial part of the analysis in which the whole geometry is divided or broken down into manageable shapes or elements, whose study is far simpler than the original body. These small units are studied individually and the compilation of these results gives the changes introduced in the domain. Elements of any shape can be used, for example square triangle or tetrahedral. Here, 333677 tetrahedral elements are used, and polyhedral, hexahedral or pyramidal elements are avoided for simplicity of the analysis. The meshes have a total of 64938 nodes. Nodes are the points on a meshed surface/body where the different elements meet.

There are few terms such as maximum face angle, which defines and sets the maximum allowable angle between input surface normal’s at neighboring mesh vertices. If the angle between surface normal’s is greater than this setting, the mesh is further refined (more vertices are inserted) and the mesh is made denser. Two vertices are neighbors if they are at the opposite ends of a single facet edge. The value here is 174.786°. Solid meshing of the fluid volume has been drawn in Fig. 4, while corresponding wireframe meshing has been shown in Fig. 5.

FIG. 4 SOLID MESHING OF THE FLUID VOLUME

FIG. 5 WIREFRAME MESHING OF THE FLUID VOLUME

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Applying Boundary Conditions The solver requires some initial values for initializing the finite element analysis procedure. These initial values are approximation of the required conditions. At the inlet, the velocity of water is kept around 2.5m/s, using a high intensity turbulence model, this model is so chosen to keep the frictional, turbulence errors in consideration. At the outlet face named (OUTLET), a relative pressure of 200kPa is assumed, averaging over the entire face. There is no slip in the wall and the surfaces of the sump are kept smooth to reduce friction losses as much as possible. It can be seen in Fig. 6 that arrows are used to show the direction of water at the inlet and outlet face.

FIG. 6 DIRECTION OF INLET AND OULET OF WATER

User Defined Plane for Calculation of Swirl Angle To calculate value of the swirl angle just below the entry of the suction pipes, a user defined plane seen in Fig. 7 is created. The green surface shown is the desired surface.

Before Swirl angle is calculated, its cause and effects need to be known. Swirl angle is the angle between the axial and the radial component of the velocity of water in any plane. Swirls is formed in any flow where turbulence is high and it is necessary to keep this value at minimum, for the calculation of this angle, a plane is defined where this angle needs to be calculated.

This is done in CFD-post Insert>Locations>user surface>name of surface>offset this surface from (OUTLET) by 5000mm. The green circled surface represents the plane, where the swirl angle needs to be calculated, which has been discussed in the next section.

FIG. 7 USER DEFINED PLANE

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Results and Discussion After the solver runs its cycle of 100 iterations, the profile of the flow followed by water is generated in the given case for specified boundary conditions. As seen from Fig. 8 and Fig. 9 , there is no regularity in the flow, and also there is liquid recirculation and non uniform flow. All these factors arise due to the sudden changes in the flow of the liquid at high speed. A key seen on the left side of the Fig 10. shows the velocity of the liquid using colour coding, for example the colour blue signifies low speed and red shows high speed, this is how the study of the flow of liquid can be studied using ANSYS. The figure also shows us that at the entry to the suction pipes, there is sudden change in the velocity of water.

FIG. 8 TWO DIMENSIONAL VIEW OF THE FLOW PATTERN

FIG. 9 THREE DIMENSIONAL VIEW OF THE FLOW PATTERN

The swirl angle has been calculated in two steps, the first one is the input formulae, and the second is running those formulae, where the software picks up the input velocities and compiles it to give the required result. There are three directions u, v and w which correspond to the velocities in the x, y and z direction. The first step is to calculate the velocity u in the user defined plane named Swirl1. These velocities have been evaluated to be 3.41, 1.04 and -0.18 m/s respectively. The Swirl angle obtained using ANSYS software is 17.2 degrees. This angle is much higher than the prescribed limit of 5. Thus, the study shows that sump design needs to be modified in order to have a better fluid flow pattern.

Conclusions The design of an existing sump system has been analyzed using CFD feature of ANSYS. Results show that the value of the swirl angle is way beyond the permissible limit , which can be attributed to the fact that there was recirculation and irregular flow during the entry to the suction pipes and high turbulence. Hence, it is concluded that the design of the sump system needs to be modified in order to have more efficient pumping system.

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REFERENCES

[1]

Anderson, J. David. “Computational fluid dynamics: the basics with applications.” USA: McGraw-Hill, Inc, 1995.

[2]

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[3]

Chen, Hong-xun and Guo, Jia-hong. “Numerical simulation of 3-D turbulent flow in the multi-intakes sump of the pump station.” Journal of Hydrodynamics, 19(1), (2007): 42–47.

[4]

Chuang, Wei Liang and Hsiao, Shih Chun. “Three-dimensional numerical simulation of intake model with cross flow.”

Journal of Hydrodynamics, 23(3), (2011): 314–324.

[5]

Dupont, Philippe and Cugal, Mike. “CFD expertise improves product design and operational behaviour.” World Pumps, (2006), 22–25.

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