Cavitation in PUMP & NPSH detailed (FLUID machinery, centrifugal pumps, reciprocating pumps)
“The Design & Analysis of a Low NPSH Centrifugal Pump ... · NPSH Centrifugal Pump Featuring a...
Transcript of “The Design & Analysis of a Low NPSH Centrifugal Pump ... · NPSH Centrifugal Pump Featuring a...
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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.