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“The Design & Analysis of a Low

NPSH Centrifugal Pump

Featuring a Radial Inlet and Axial

Inducer Using STAR-CCM+”

Edward M Bennett

Travis A JonasMechanical Solutions

11 Apollo Drive

Whippany, NJ 07981

March 19, 2013

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AGENDA

� Introduction and problem definition

� Design challenges

� Multiphase CFD analysis

� Design optimization

� Results

� Conclusions

Problem Statement

• Mechanical Solutions, Inc., an engineering

consultancy specializing in turbomachinery

engineering that is based in New Jersey, was

commissioned to design a two-stage centrifugal

pump for a major manufacturer

• This presentation will be concerned with the first

stage, specifically the radial inlet and inducer

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Fluid Dynamic Challenge

�The pump design is required

to operate at low NPSH(Net

Positive Suction Head) at two

different flow rates.

�As the inlet pressure is

lowered , vapor will form in

the pump passages, and

eventually the performance

will deteriorate. Cavitation

erosion and vibration can

cause serious damage to

blading.Cavitation head breakdown curve

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Fluid Dynamic Challenge

• First stage pump

components

• Radial inlet

• Axial inducer

• Centrifugal impeller

• Vaneless diffuser

• Crossover and de-swirl

cascade

First stage flowpath

Radial Inlet

• The radial inlet guides and turns the flow from

radial to axial parallel to the rotating shaft.

• The challenge of the radial inlet design is to

symmetrically distribute the flow through the pump

• Non-symmetric flow can destroy range, stability, and

performance.

6Typical pump radial inlet

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Axial Inducer

�Axial inducers are used in

rocket turbopumps and high

energy density industrial

pumps

�Inducers act as boost pumps

to the main head producing

impeller. They can operate at

higher cavitation levels since

their power level is only a

fraction of the main impeller.

The large axial inlet facilitates

lower velocities at the throat

and can swallow more

cavitating vapor

Cavitating Inducer (Brennen(1994)

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Introduction

�High energy industrial pumps

� “Reducing the size and firstcost of pump and driver bymeans of durable higher-speedmachines with demonstrateddurability.” (Cooper, 2009 TexasA&M Pump Symposium)

� Industrial pumps generally havefar greater life cyclerequirements compared torocket turbopumps and usuallyhave much greater rangerequirements

Typical inducer and centrifugal pump stage

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Challenges of Inducers

� General performance

� Inducers have generally had reasonable design

performance but unsatisfactory off-design

performance. This can create problems,

particularly in industrial applications where

range is important. For the present case, the

NPSH requirement is fixed on two flows, that

are different by 37%. High performance rocket

inducers have a maximum flow range from 65 to

125%.

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Challenges of Inducers

�Low flow recirculation and

instability

� Inducers often exhibit

large tip recirculation at

low flow rates. These

instabilities can cause

unsteady vibrations and

damage to the system,

in addition to reducing

range.

From AIAA 2001-3001, Ferguson, et alDesign flow coefficient = 0.08

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Challenges of Inducers

�In order to design inducers

with lower NPSH capability,

the flow coefficient must be

reduced. The flow coefficient

is the ratio of the incoming

axial velocity/tip speed of

inducer. This creates a

compromise with the

problem of flow instabilities

and recirculation.

Brumfield Criterion for inducers (Lobanoff and Ross)

Extra Inducer Challenges

• This pump design has a requirement for a large

rotating shaft that forces the inducer design to

utilize a large inlet hub/tip ratio and a higher inlet

flow coefficient (0.156), which is not ideal for low

NPSH operation.

• The inducer design will have a smaller throat

passage area and thus internal velocities will

rise. This lowers static pressure and can lead to

premature cavitation.

• Radial inlets and inducers have normally not been

as efficient due to the non-symmetries in the flow

created by the radial inlet.

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MSI Inducer and Pump Design

Flowchart

Preliminary Design

CFturbo

Detailed Blading Design

CFturbo, BladeModeler

Detailed Volute Design

CFturbo

Inducer NPSH Analysis

Cooper Streamline Code

Component CFD and Cavitation Analysis

STAR-CCM+

Detailed Stage Performance Analysis

STAR-CCM+

CAD and FEA

Radial Inlet Design

• MSI started the pump design by creating a radial

inlet that minimized circumferential distortions

around the inlet

14MSI radial inlet

Axial Inducer Design

• MSI designed an axial inducer that was skewed to

the highest flow operating point, in order to

achieve the necessary pressure boost.

• The design was optimized to reduce recirculation

at lower flows

15MSI 4-vaned axial inducer

Physics of the Pump Stage

• Full unsteady stationary/rotating components non-

symmetric flow interaction

• Unsteady multiphase cavitation

• Turbulent, separating, recirculating flows

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CFD Requirements

• Unsteady stator/rotor interaction

• Advanced turbulence models

• Sophisticated unstructured meshing capabilities

of complex passages

• Unsteady cavitation models

• Massive parallelization

• STAR-CCM+ possessed all of the

CFD requirements!

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CFD Model

• Full 360 degree model was meshed in STAR-CCM+

• Mesh created using polyhedrals and prismatic

boundary layer elements

• Total mesh size was 23577286 vertices and

8245730 cells

• High order unsteady segregated model used for

discretization

• Realizable k-epsilon model used for turbulence

• Unsteady cavitation model employed

• Calculation solved on 132 Linux cluster

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CFD Model

�Boundary Conditions

�Mass flow inlet

�Mass flow exit

�Reference pressure

set and sequentially

lowered to meet

NPSH requirements

�RPM selected

�Two volumetric

flows

�2700 gpm

�3700 gpm

Computational Mesh

First Prototype Results

• The first stage analysis was conducted at 2700

gpm. Cavitation breakdown began at NPSH = 22

feet(Goal is 10 feet). There is an non-symmetric

interaction between the inducer and impeller

created by the non-multiple impeller vane count.

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Non-symmetric cavitation present in

impeller

4 inducer vanes

7 impeller vanes

Impeller Revision

• The impeller vane count was changed to match the

inducer vane count. The impeller was positioned

circumferentially such that the leading edge of the

impeller is situated between the trailing edges of

the inducer to minimize wake /cavitation

interaction.

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2700 GPM Analysis

22Streamlines(No Inducer Recirculation)

2700 GPM Analysis

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Vapor Fraction at NPSH = 7.81 feet

(Target is to be less than 10 feet)

2700 GPM Analysis

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0

50

100

150

200

250

300

350

400

450

0 10 20 30

To

tal

He

ad

(ft

)

NPSH (ft)

Total Head vs NPSH (rKE, 2700 gpm)

Target NPSH is Achieved (10 feet)

3700 GPM Analysis

25Streamlines(No Inducer Recirculation)

3700 GPM Analysis

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Vapor Fraction at NPSH = 8.54 feet

(Target is to be less than 10 feet)

3700 GPM Analysis

27Target NPSH is Achieved (10 feet)

0

50

100

150

200

250

300

350

400

0 10 20 30 40

To

tal

He

ad

(ft

)

NPSH (ft)

Total Head vs NPSH (rKE, 3700 gpm)

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Conclusions

� All fluid dynamic targets and goals have beenachieved.

� Cavitation behavior successfully predicted across awide range of flows.

� STAR-CCM+ was able to accurately mesh theflowpath using unstructured polyhedrals andprismatic elements, resulting in an efficient,economic workflow.

� STAR-CCM+ was able to facilitate the requiredunsteady, multiphase physics.

� The parallel capabilities of STAR-CCM+ enabledsolution turnaround in quick order.