double suction centrifugal pump

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Cavitation simulation and NPSH prediction of a double suction centrifugal pump

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2012 IOP Conf. Ser.: Earth Environ. Sci. 15 062025

(http://iopscience.iop.org/1755-1315/15/6/062025)

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Cavitation simulation and NPSH prediction of a double

suction centrifugal pump

P Li1,2

, Y F Huang2 and J Li

2

1National Key Laboratory of Science and Technology on Aero Engine Aero

Thermodynamics, Beihang University, Xueyuan Rd. No.37, Beijing, 100191, China 2 CAE Department, HiKey Technology Inc.,Xueyuan Rd. No.35, Beijing, 100191,

China

E-mail: [email protected]

Abstract. This paper illustrates the flow field numerical analysis of the double-suction

centrifugal pump. For the study of the cavitation flow inside the double-suction centrifugal

pump, the professional pump/motor simulation software PumpLinx and its Full Cavitation

Model has been employed. According to the PumpLinx calculation result and the Cavitation

damage index, the cavitation position, level and the cavitation characteristics of the double-

suction centrifugal pump has been predicted. For the further objective, the simulation of the

flow field in the double-suction centrifugal pump under different inlet conditions has been

carried out. By the result analysis, NPSHr has been predicted; the reliability of the results has

been verified by comparing with the experimental data. At the same time, this practice can

provide guidance for the optimal design of double-suction pump.

1. Introduction

Phenomenon of cavitation influences the performance of centrifugal pump during its running, it can

always cause a series of problems, including abnormal flow, cavitation damage, vibration, noise, head

decline, and even halt of operation sometimes. Thus, researches on the cavitation of centrifugal pumps

must be carried out to ensure the safety, stabilization and high efficiency of performance. With the

development of computer technology, numerical simulation has become an effective method in

cavitation research. General CFD tools, like Fluent, StarCD, CFX, CFDesign etc. are applied and

developed broadly. While in sorts of branch fields, professional codes for special use have been

widely developed also. For example, Airpak and Icepak is the electronic thermal analyzer. AVL FIRE

focuses on engine gross performance. PumpLinx is the professional tool for pump and valve modeling.

In pump simulation, the key points are the cavitation and turbulence prediction in flow domain. But

for most general CFD tools, refer to different cavitation and turbulence models, it is always very hard

to achieve a steady state, and simulation results always deviate severely from test data also. Besides, it

takes too long in mesh building, parameters setting and calculating. To solve these problems, Simerics,

an American company, developed a professional hydraulic numerical simulation software for pumps

and valves which is called PumpLinx. It utilize the full cavitation model that is put forward by Ashok.

Singhal and Jiang Yu, which can solve complicated cavitation problems with good convergence.

Beside numeric Cartesian meshing technique which is based on binary tree method, together with

plentiful pumptemplates embedded in the software can ensure the accuracy and effectiveness of

simulation. This paper takes one type of double suction centrifugal pump for example, uses

26th IAHR Symposium on Hydraulic Machinery and Systems IOP PublishingIOP Conf. Series: Earth and Environmental Science 15 (2012) 062025 doi:10.1088/1755-1315/15/6/062025

Published under licence by IOP Publishing Ltd 1

PumpLinxas the tool of simulation, cavitation and turbulence in flow domain are analyzed, prediction

of NPSH by simulation is compared with test data, the result turns out with excellent agreement,

which verified the reliability of the numerical simulation in centrifugal pump.

2. Simulation

2.1. Governing equations

2.1.1. Basic Equations. The fluid motion in the pump is dominated by basic laws of mass, momentum

and energy conservation. In this paper, heat transfer in the pump is not considered, thus, its basic

equations consist of continuity and momentum equations.

Continuity equation:

0

td v v nd

t

(1)

Momentum equation:

tvd v v n vd nd pnd f d

t

(2)

where ( )t and are the Volume and surface area of the control volume, n means the normal vector

of point on surface, is the fluid density(kg/m3), P is the fluid pressure(Pa) , v and v are the

velocity vector and surface kinetic velocity(m/s), and the shear stress tensor can be expressed by an equation related to velocity and viscosity, for Newton fluid, it can bewritten as

2

3

ji kij ij

j i k

uu u

x x x

(3)

In which iu means a component of velocity v and ij is the Kronecker delta function. For centrifugal

pump simulation, turbulence model and cavitation model are needed also.

2.1.2. Turbulence model. Appropriate turbulence model can describe the flow situation inside the

centrifugal pump accurately. Choosing a proper turbulence model can decrease the simulation errors,

and enhance the accuracy of performance prediction. Standard k and RNG are the most widely adopted models for turbulence modeling in pumps. This paper uses standard k model, which can satisfy the need of centrifugal pumps turbulence prediction, not only in convergence, but also in accuracy.

2.1.3. Cavitation model. The most difficulty of cavitation prediction is that density at the interface

between gas and fluid varies extensively. Besides, the position where cavitation generates, the domain

and the shape of bubbles are influenced by pressure field, and pressure filed is determined by

geometry, boundary and operating conditions. The full cavitation model in PumpLinx is based on two-

phase flow theory, considering the compressibility of fluid and vapors vaporization and condensation processes in the meanwhile. It takes vapor and undissolved gas into account by introducing the

conception of mixing density, thus, the prediction of cavitation is more reliable. Fornon-condensable

gas, PumpLinx provides four kinds of simulation models for choices, they are:

Constant gas mass fraction;

Equilibrium dissolve gas model;

Variable gas mass fraction;


2

Dissolved gas model.

To simplify the issue, this paper uses the first model, which calculates the dynamic process of

bubbles generation, flow and dissolution, its convergence and accuracy have been verified by lots of engineering application instances. The equation of the model is shown inequation(4):

( )( ) ( )tf e c

tf

fd v v n fd D f n d R R dt

(4)

where fD is the diffusivity of the vapor mass fraction, f is the turbulent Schmidt number.The vapor

generation rate eR and the vapor condensation rate cR are shown as

1 1

2 22

[ ] 13

vl ve e v g

l l

P PR C k f f

(5)

1 1

2 22

[ ]3

vl vc c v

l l

P PR C k f

(6)

In which the model constant eC and cC equal to 0.02 and 0.01, l and v are the density of fluid

and vapor, l is the surface tension, vf and ean the mass fraction of vapor and non-condensable gas.

The mixed fluid consists of pure fluid, vapour and non-condensable gas, The calculation of the

mixture density is modeled as

11 g v gv

v g l

f f ff

(7)

2.2. Flow domain preparation

2.2.1. Flow domain model. Flow domain should be extracted from the assembly model of the double

suction centrifugal pump for analyzing. The format is STL. The main parameters include: Diameter of

impeller outlet D2=730mm; Width of impeller outlet b2=370mm; Number of blades z=6; Volumetric

flow rate Q=9400 m3/h; Rotational speed n=745rpm; Head H=21m. According to the actual

performing condition, we choose water at 20 as the working material, its detailed properties are shown in table 1.

Table 1.Properties of working material

Parameter Value

Temperature 20

Density 998kg/m3

Dynamic Viscosity 0.001003Pa.s

Vapour Pressure 3610Pa

Vapour Density 0.0245kg/m3

Gas mass fraction 2.310-5

Liquid Bulk Modulus 2.15109 Pa

2.2.2. Mesh generation. Mesh the flow domain by using the embedded mesher in PumpLinx. The

special binary tree mesher can create cartesian cellswhich has excellent orthogonal, split automatically

at clearances, sharp edges etc. with thicker cells, so that the flow domain can be described with less


3

cells but higher precision. Besides, the operation is simple, efficient and timesaving. For this double

suction centrifugal pump, we use this one key meshing method to generate meshes for each part of the

flow domain. Figure1 shows a binary tree mesh on a cutting plane passing through a centrifugal and

Figure2 shows a partial scheme of the binary tree mesh on the leading edge of the blades.The whole

job can be finished in several minutes, and the number of cells is about 500000.

Figure 1. Sectional meshing scheme of the pump volute.

Figure 2. Partial scheme of the binary tree meshes on the

leading edge.

3. Results

3.1. Pressure distribution

The pressure distribution under design condition in the flow domain of the double suction centrifugal

pump is shown below. The pressure increases gradually along the radial direction in impeller with a

reasonable distribution and smooth transition. When the high pressure fluid is induced to the diffuser,

the pressure increases in a wide range along the volute, then reaches the maximum value at volute

outlet. The head obtained is 21.4m, it matches the test value of 21m very well. Overall, the pressure

distribution in the flow domain is reasonable. In the meanwhile, there are apparent low pressure

regions formed at the back side of the leading edges on blades, so its necessary to investigate the cavitation characteristics for future optimization.


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Figure 3. Pressure distribution on impeller. Figure 4. Pressure distribution on volute.

3.2. Cavitation performance analysis

In order to analyze the cavitation characteristics of the pump, different conditions of inlet pressure

varying from 105kpa up to 160kpa are modeled. Results are shown in Figure5. Total volume of vapour

and undissolved gas decreases with inlet pressure increases. And no matter the what is the level of

inlet pressure, cavitation always generates from the back side of leading edges. Total volume fraction

Figure 5. Total volume fraction of cavitation in impeller under different inlet conditions.

describes the bubbles forming, flowing and deforming. While, cavitation does not cause damage itself unless bubbles are fractured on the surface of walls. The cavitation damage model in PumpLinx can

help engineers to predict the probable region of cavitation damage, and make improvements in design

to prevent damage happening in real.


5

Figure 6. Contrast of cavitation and cavitation damage region on pump.

Figure 6 shows the contrast of cavitation and cavitation damage region. Cavitation happens at the

back side of leading edgeon blades where bubbles form, while damage happens somewhere

downstream that bubbles burst because of high pressure aroud them. Damage is determined by

fracture rate of bubbles, latent heat of vaporization and hardness of flow surface etc. In PumpLinx, it is

judged by cavitation damage power. By comparing the test and simulation results, we get that damage

happens where cavitation damage power surpasses 5108W.

3.3. NPSHr prediction and comparison

Cavitation influences pump performance extensively, and the bigger cavitation volume is, the more

pump head decreases. With the pressure at inlet decreases, take the condition as the basis for the

cavitation when pump head decreases by 3% at constant flow rate. Figure 7 shows the Net Positive

Suction Head curve at the designed flowrate and speed. From the curve, we get the value of NPSHr

Figure 7. Head and NPSHr prediction.

11.5m, which has high agreement with the tested value of NPSHr=12.2m. As clarified in the former

part, if the actual inlet pressure is lower than NPSHr, cavitation will happen, pump efficiency will

decline, and damage may destroy the structure of the pump in some extreme conditions. Double

suction centrifugal pumps are widely used in irrigation operations in areas of high altitude, so its very important to predict NPSHr precisely for safety use.


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4. Conclusions Based on the analysis of numerical simulation of the double suction centrifugal pump with PumpLinx,

the results turn out:

The head result of the pump under design condition deviates 1.8% from the test data, the precision is acceptable.

The pressure distribution in the impeller is reasonable, however, there is apparent lowpressure region generated, cavitation performance needs to be improved.

By comparing the cavitation damage prediction with the test data, we find that damage happens where cavitation damage power surpasses 5108W, this is very important for

structure and optimization.

The calculated NPSHr by PumpLinx is about 11.5m, it is about 6% lower than the test data of 12.2m.The probable sources of the inaccuracy include: a) The position of the inlet in

PumpLinx does not match the corresponding monitor in test equipment perfectly. b) Heat

exchange is not considered in simulation, while heat created by friction and ambient transfer

may influence the flow course as well. c) The material properties in PumpLinx are simplified

comparing with the actual situation, especially the value of gas mass fraction, its really hard to estimate the actual value in fact.

Overall, PumpLinx is a reliable and efficient tool for centrifugal pumps flow and cavitation analysis. Its embedded pump templates, binary tree mesher, and advanced full cavitation model can

ensure high precision and convergence. In the meanwhile, simulation time needed is shortened greatly,

not only in meshing, setting, but also in calculating. Thus, engineers can spend more time on pumps performance assessment and optimization.

References

[1] Liu Y, Zhao X F, Qi X Y, Hui W A and Zhang W J 2008 Journal of Lanzhou University of Technology 34(3) 44-47

[2] Ma F Y, Yang G P and Wu W W 2011 Fluid Machinery 4 30-34. [3] Samuel A L, Jiang Y and Michal F 2007 High Fidelity Modeling for Liquid Pump Design (USA:

SAE Int.)

[4] Singhal, Athavale, Li H and Jiang Y 2002 Journal of Fluid Engineering 124(3) 617-624 [5] Ding H, Jiang Y, Visser F C and Furmanczyk M 2009 Demonstration and validation of 3D CFD

simulation tool predicting pump performance and cavitation for industrial applications 2009

ASME Fluids Engineerings Division Summer Meeting (Vail, USA, 2009)

[6] Su Y S, Wang Y S and Duan X Y 2010 Thansactions of the Chinese Society of Agricultural Machinery 41(3) 77-80

[7] Pan Z Y, Ni Y Y, Li H and Cao Y J 2008 Drainage and Irrigation Machinery 26(4) 35-38


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double suction centrifugal pump