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The 11th

Asian International Conference on Fluid Machinery and Paper number xxxxxxxxxThe 3rd Fluid Power Technology Exhibition November 21-23, 2011, IIT Madras, Chennai, India

Original Paper

Validation of Hydraulic design of a Metallic Volute Centrifugal

pump using CFD

Jaymin Desai1, Vishal Chauhan

1and Shahil Charnia

1, Kiran Patel

2

1 Executive, CFD Analysis Center, R & D Hydraulics, Jyoti Ltd.2Manager, CFD Analysis Center, R & D Hydraulics, cfx@jyoti.com, kiran2810@gmail.com

Jyoti Ltd., Nanubhai Amin Marg, Industrial Area, P.O.Chemical Industries,

Vadodara-390003, Gujarat, India.

Abstract

The present paper validates the hydraulic design of a metallic volute centrifugal pump (Nsq=76). CFD is used for numerical

investigation of the entire pump and every component of the pump like metallic volute; diffuser, impeller, and draft tube areincluded in the analysis. Single phase, steady state incompressible flow is selected for the CFD analysis. Fluid flow behavior isstudied in all components and losses are estimated. Pump performance is predicted at BEP (Best Efficiency Point) as well as atpart load operations. The validation is done by comparing the CFD analysis results with the experimental test results and it shows

a good agreement between both the results.

Keywords: CFD, Metallic concrete volute pump, BEP, Blade loading.

1.IntroductionFor any pump industry, the performance prediction of the pump at its duty point and off duty points is necessary to predict its

performance for its entire operating range. Traditional approach for this is to perform pump testing at different values of the

discharge. The performance is obtained by calculating head, power, and efficiency for various varying discharge values. The

testing process is very much time consuming and pump performance is obtained after the actual pump is manufactured.Computational fluid dynamics (CFD) is a method for fluid flow investigation combining the fluid mechanics principles,

computational sciences, and computation power of the modern computing facilities. CFD is used at the initial pump design stageto predict its performance and to perform necessary modifications in the pump geometry. In the present time, CFD is most widelyused as an alternative for the testing to predict the flow behavior through Navier-Stocks equations and various turbulence modelsalong with visual effects for the results obtained. CFD is used to optimize the pump geometry before manufacturing to save thetime and cost and to validate the pump performance with bench marking between the CFD results and testing results.

2.ObjectiveThis paper includes numerical investigation of a metallic volute pump (MVC) with commercial CFD softwares. Metallic volute

pump is a type of centrifugal pump, which contains metal volute casing, diffuser, and impeller and draft tube. The objective forthis investigation is to predict the pump performance with CFD at three different values of total discharge, one for BEP, and other

two for off duty points with lower and higher discharge values then the discharge for the BEP. In addition, losses in variousregions of the pump are to be observed using CFD results. The performance predicted with CFD analysis is to be validated withthe testing results.

3. Numerical investigation

CFD is the numerical method for solving the fluid flow problems. To observe the flow in the pump using numerical analysis, theNavier-Stokes equations are solved in pump components, which are supplemented by a turbulence model according to [1].

Numerical investigation for any problem comprises of geometrical modeling followed by meshing or in other words, domaindecomposition into small cells. There are various commercial softwares available with the capability for both the modeling andmeshing processes. During the preprocessing stage, the physics is defined on the modeled and meshed geometry of the fluiddomain. It also includes assignment of the boundary conditions to various domains of the pump geometry and initial valuesassignment at various boundaries of the domain. After preprocessing, solving is done with commercial solver. Results are obtained

from this solving process and they are post processed to predict the flow behavior inside the domain.

Paper ID: AICFM_TM_002

Accepted for publicationCorresponding Author:Jaymin Desai, cfx@jyoti.com

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Fig.1 Pump System

3.1 Geometric modeling

Problem definition in the CFD starts with 3D geometric modeling for the fluid region to be investigated. The geometric model

should have good quality of the surfaces in order to get desired quality of mesh. Many commercial softwares available today arecapable to model the impeller geometry and also to predict its hydraulic behavior at the initial design stage. ANSYS Bladegen issuch software used for the impeller geometry creation with initial hydraulic behavior prediction and the facility to modify thegeometry. 3D models for the other components of the pump geometry shown in Fig.1 are created in the Pro/ Engineer modelingsoftware.

3.2 Computational grid

Meshing is very important stage in numerical investigation as the accuracy of the CFD analysis depends on the quality of themesh created. To capture the flow behavior more accurately, block structured meshing is used for the impeller. The mesh in theimpeller domain is entirely structured mesh with O-grid blocks in the near wall region of the blades. It is created taking into

consideration the Reynolds no. value ranges from 70000 to 100000 and such as to keep the y+ value below 80. The O-grid iscreated near the wall is at the most orthogonal to the blade and there is no any aspect ratio error. The inflation layers are alsocreated at the hub and shroud wall. The O-grid and the inflation layers help predict the wall shear stresses and boundary layerlosses accurately with proper selection of the turbulence model.

Table 1. Meshing details in various domains of the pump

Domains No. of nodes Elements

Draft Tube 282447 959139

Suction Cone 109090 358590

Impeller 1707576 1576818

Volute Casing with Diffuser 1692780 4108338

The other domains i.e. draft tube and suction cone, diffuser, and volute casing are meshed using unstructured grid elementswith inflation layers on their wall. The use of inflation layers in the unstructured mesh elements takes care of the flow behaviorcapturing in the near wall region. Both the structured and unstructured meshes are created to satisfy the quality criteria within the

specified range.Table. 1 shows the meshing details in the various pump domains and it is observed from the table that the number of nodes is

high enough in each domain to increase the accuracy of flow behavior prediction. To check the grid independency, two different

meshes with coarse and fine elements were created. According to [4], grid dependency of the accuracy for the CFD results is veryless. Table.1 shows only the node count for the finer mesh.

Structured meshing in the impeller is visible in Fig.2 and 3. It is created using Ansys Turbo grid commercial software. Fig. 4and 5 shows unstructured meshing in the draft tube, diffuser, and volute casing. Their mesh is created in the Ansys ICEM CFD

software.

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Fig.2 Impeller Fig.3 Impeller and Diffuser

Fig.4 Diffuser & volute casing Fig.5 Draft Tube

3.3Boundary conditions

The flow inside the pump is incompressible and continuous and hence steady state is assumed. The draft tube with suction

cone, diffuser, and the volute casing are stationary domains while impeller is rotating domain. Draft tube inlet is Inlet boundarywith initial condition given as 1 atm. total pressure and medium turbulence intensity. Volute casing outlet is defined as Outletboundary and total discharge is given there as initial condition. The other components are taken as wall boundary with no slip

boundary condition. SST Turbulence model is taken with automatic wall function treatment. All the domains are connected witheach other by domain interfaces with general connection interface models. Impeller is connected with suction cone outlet anddiffuser inlet with Frozen Rotor as frame change/ mixing model. High resolution advection scheme is selected for the solution.The Convergence criterion is defined for residual type MAX with the target value for the convergence as 10

-4. Three different

values for the total discharge are specified at volute casing outlet. One value is for the BEP (or rated point) and two others are 80%and 120% of the rated discharge. Inlet is kept constant at total pressure of 1 atm. for all three cases.

4. Results and discussion

As the flow is assumed as steady and the water is incompressible media, the amount of the turbulence in the flow will be less.The flow enters the domain from the draft tube, passes through the impeller, diffuser and then through the volute casing it leaves

the domain. The pressure and velocity gradually rises from inlet to the outlet. Thus, the net head increases at the outlet of thepump. The pressure and velocity values should be carefully examined in order to find the hydraulic losses and to calculate theefficiency of the pump. For careful investigation, every quantity should be calculated in each individual domains of the pump.

In order to analyze the effect of individual domains on the overall performance of the entire system, the problem domain isdivided into three different regions for individual investigation:

1. Impeller region2. Diffuser and volute casing and3. Entire pump containing draft tube, impeller, diffuser, and volute casing.

The following section includes investigation of the flow through above three domains to approximate their effects on the entirepump performance. The CFD results are the bases for the analysis. In the last section, the quantities obtained by CFD arecompared with testing results in order to find co relation and accuracy of the results.

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4.1 Impeller domain analysis

Impeller increases total pressure of the incoming water through the draft tube. The draft tube is converging region and flow isaccelerating. The impeller increases the static pressure of the incoming flow by diffusion and centrifugal action. Because of thispressure, difference between pressure and suction side of the blade is generated. This pressure difference between the pressure and

suction side is expressed with blade loading chart. Fig. 6 to 8 shows static pressure blade loading for various discharge valuesnon-dimensionlised in terms of the rated discharge for the BEP.

For positive incidence of the flow, blade loading shows stagnation on the pressure side and low-pressure pick on the suction

side [1]. Pressure distribution at the BEP and at the 80% discharge show the same behavior as can be seen from the Fig. 6 and Fig.7. Therefore, the flow incidence is positive on the entire blade span for both of these discharges.

Fig.6 Blade loading for (Q/Qrated) =1

At 120% of the rated discharge, there is violation from this observation as shown in Fig. 8 at impeller shroud meaning thatthere is negative incidence of the flow at the shroud for the discharge value 120% of the rated discharge.

Fig.7 Blade loading for (Q/Qrated) =0.8 Fig. 8 Blade loading for (Q/Qrated) =1.2

The work transfer by the impeller depends upon the non-uniformity of the flow over the pitch of the blade and this non-uniformity increases with blade loading [1]. The torque supplied to the impeller increases with the increase in the mass flow.From the above figures, blade loading and non-uniformity of the flow and hence the hydraulic losses increases with increase in thedischarge value.

To approximate the performance of the impeller, it is necessary to observe the losses in the impeller and its hydraulic efficiencywith respect of the mass flow rate through the system.

To perform this analysis, hydraulic efficiency and the losses in the impeller are calculated at three different discharges namely therated mass flow rate at the BEP, 80% and 120% of the rated discharge.The hydraulic efficiency decreases with increasing the discharge. It decreases sharply up to BEP with increase in the dischargeand then it decreases slowly up to Q/Qrated=1.2. The nature of the hydraulic losses in the impeller as shown in the Fig. 10 is toincrease up to BEP and then it decreases with further increase in the discharge.

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Fig. 9 Hydraulic efficiency of the impeller Fig. 10 Losses in the impeller

Velocity in the impeller and diffuser domain has influence on the total pressure variation in both the domains. With increase in

the discharge values, the velocity variation in both of the domains should be examined. Very low values of the velocities tend toflow separation and circulation in the flow. Flow separation causes losses in the domain. Fig. 11 shows velocity contours for theimpeller domain. It shows that the amount of low velocity zones and flow separation on the impeller vanes decreases with

increase in the discharge.

(Q/Qrated) =0.8 (Q/Qrated) =1 (Q/Qrated) =1.2Fig.11 Velocity contours on the impeller vanes

4.2 Diffuser and Volute domain analysis

The overall pump performance depends on the pressure, velocity, and head variation in each domain of the pump. The impellerincreases the static pressure and the amount of the whirl in the flow and because of that, the flow downstream the impeller is

having high value of the circumferential velocity.

Fig.12 Diffuser and Volute casing

This circumferential component is to be gradually converted into the axial component in order to reduce the kinetic energylosses and to recover the static pressure. The diffuser and volute casing are downstream of the impeller and their objective is torecover the static pressure and to reduce the total pressure losses due to high kinetic energy downstream the impeller. Both ofthem contribute in the net head rise because they further increase the static pressure downstream the impeller.

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H

Q/Qrated

H vs. Q/Qrated

0.02

0.22

0.42

0.62

0.82

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La

Q/Qrated

La vs. Q/Qrated

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Fig.12 shows diffuser and volute domains. The diffuser and volute are with the diverging sections. To analyze the effect ofthese two domains on the overall performance of the pump, the static pressure rise and head loss between domain inlet and outletare found for the diffuser and volute domains and they are non dimensionalised with the corresponding values of the percentage

change in the quantities at the BEP.

Fig.13 Static pressure rise Fig.14 Head loss

Static pressure rise between inlet and outlet of the diffuser and volute domains is shown in Fig. 13. With increase in the

discharge, percentage change in the static pressure decreases in the diffuser and increases in the volute casing. This shows that asthe discharge increases, the amount of static pressure recovered by the diffuser decreases. This reduction in the static pressurerecovery in the diffuser is taken care by the volute casing. The pressure recovery in the volute casing increases with increase in the

discharge. Fig. 14 shows head loss in diffuser and volute casing domain. Its value is more on both sides of the BEP. This showsthat at the static pressure and the kinetic energy recovery in the diffuser and the volute casing are higher at the BEP than thecorresponding values at the off duty points. It means that at the BEP, the flow reaction in the diffuser and volute domain is

comparatively better than the off duty points.Fig. 15 shows velocity contours for the diffuser domain. The low velocity zones tend to increase with increase in the flow rate.

It shows that the amount of flow separation increases in the diffuser with increase in the discharge. Thus, the nature of the velocitywith increase in the discharge is opposite in the diffuser than that in the impeller. These low velocity zones increase separationzones on the diffuser vanes with increasing discharge. Hence low static pressure rise in diffuser.

(Q/Qrated) =0.8 (Q/Qrated) =1 (Q/Qrated) =1.2

Fig.15 Velocity contours in the Diffuser

4.2 Overall domain analysis

The overall domain analysis is performed in order to analyze the performance of the entire pump with the reaction of the flowwithin each domain of the pump. It includes calculation for the efficiency, head, and power at varying discharge values. Fig. 16 to18 show velocity contours in the hub region of the flow channel formed by impeller, diffuser and casing. The flow separation at

impeller leading edge decreases with increase in the discharge as can be seen from the figures. The low velocity zones and henceflow separation at part loads in both diffuser and volute casing are more than those at the BEP are. The flow separation occurs atthe pressure side of the diffuser vanes at part loads. The velocity downstream the impeller, i.e. in the diffuser and volute casingincreases with increase in the discharge. The flow is accelerating in the periodic region between two vanes. This acceleration of

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the velocity is visible by the high velocity contours in the region between the pressure and suction sides of the diffuser vanes. Thevelocity in the volute casing increases with increase in the discharge.

Fig.16 Velocity contours for (Q/Qrated) =1

Fig.17 Velocity contours for (Q/Qrated) =0.8 Fig. 18 Velocity contours for (Q/Qrated) =1.2

Accuracy of the CFD analysis is verified with the bench marking process between the CFD results and the results obtainedthrough testing. To establish co relation between both of the results, efficiency of the pump, head, and the input power arecalculated using CFD results. These results are then converted into dimensionless form with respect to the corresponding values atthe BEP. The head, efficiency, and power obtained through the testing are also converted into dimensionless form. The pumpcharacteristics obtained with these values are shown in Fig. 19 to 21. They show good agreement between them.

Fig. 19 Efficiency at various discharges Fig. 20 Head at various discharges

Efficiency at the BEP is obviously higher than the off duty points as shown in the Fig. 19. Pump head decreases with theincrease in the discharge. Its value is higher at lower discharge and gradually decreases with the increase in the discharge as

shown in the Fig. 20. Input power increases with increase in the discharge, as the pump has to work more in order to increase theamount of the hydraulic energy to be delivered. This variation of the power is shown in the Fig.21.

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Efficiency

Q/Qrated

Efficiency Vs. QND

CFD Results

Testing Results0.7

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H/Hrat

Q/Qrated

HND Vs. QND

CFD Results

Testing Results

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From all these three figures, it is observed that the variation between the CFD results and the testing results is reasonably small.

Fig. 21 Power at various discharges

5. Conclusions

Blade loading on the impeller increases with the flow rate. This reduces the amount of effective work transfer by the impellerdue to improper momentum transfer. The amount of stall and shock losses due to flow separation increases in the impellerwith increase in the discharge. Hence, losses in the impeller domain increase as shown in the impeller domain analysis. As aresult, hydraulic efficiency of the impeller decreases with the increase in the discharge.

The kinetic energy loss downstream the impeller is recovered in the diffuser and volute casing in the form of the staticpressure recovery. At part loads, the low velocity zones are formed on the pressure side of the diffuser vanes. It indicates flowincidence angle for the diffuser vanes is negative [1]. Because of this, separation regions on the diffuser vanes are there at partloads and also at the BEP. These separation zones block the part of the throat area and they are smaller for the flow ratesabove Q/Qrated =1. This negative incidence increases stalling effect in the flow and the flow is accelerating through the

diffuser channel without proper momentum transfer. Because of this, increasing values of the velocities are found in thediffuser near the suction side of the vanes and its value increases with increase in the mass flow rate. This acceleratingvelocity results in the part of the kinetic energy unutilized downstream the diffuser. This energy is recovered in the volute

casing by some amount.

For the diffuser and volute casing, the losses are comparatively small at the BEP compared to the part loads. It is justified bythe head loss in the diffuser and volute domain, which is comparatively less at the BEP.

CFD results under predicts the values by a small amount. The reasonable small variation between the CFD and tested results

confirms the accuracy of the CFD analysis results and hence validates the pump performance predicted by CFD analysis.

Acknowledgments

The CFD analysis of the pump along with testing is done entirely within the premises of Jyoti Ltd., Vadodara. The authors ofthis paper are thankful to the organization for its total support.

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P/Prat

Q/Qrated

PND Vs. QND

CFD Results

Testing Results

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Nomenclature

Nsq Specific speed Q Pump discharge [m3/sec]

BEP Best efficiency point Qrated Rated discharge at BEP[m3/sec]

CFD Computational Fluid Dynamics QND Non dimensionalised discharge with the QratedH Net head [m] y

+ Non dimensionalised distance from wallHrat Rated head at BEP [m] La Hydraulic losses in the impellerHND Non dimensionalised head with Hrat H Hydraulic efficiency of the impeller

P Power [kW] ND efficiency non dimensionalised with the efficiency at BEPPrated Rated power at BEP [kW]PND Non dimensionalised power withPrated

References

[1] Glich, J.F., 2010, Centrifugal Pumps, second edition, Springer-Verleg, Berlin, Germany.[2] Stepanoff, A.J., 1957, Centrifugal & Axial flow pumps, John Wiley & Sons, New York.[3] Lazarkiewicz, S., Troskolnski, A.T., Impeller Pumps, 1965, Pergamon Press, London.

[4] Asuage, M., 2005, "Numerical modelization of the flow in centrifugal pump: Volute influence in velocity and pressure fields",International Journal of Rotating Machinery, vol. (3), pp 244-255.

[5] Zhou, W., Lee, T.S., 2003, "Investigation of the flow through centrifugal pump impellers using computational fluid dynamics",International Journal of Rotating Machinery, vol.9 (1), pp 49-61.