cfd simulations of condensation induced water hammer · what is a water hammer? (i) water hammer =...
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CFD Simulations of Condensation Induced
Water Hammer
Sabin Cristian Ceuca
Gesellschaft für Anlagen-und Reaktorsicherheit (GRS) gGmbH, München
46th Annual Meeting on Nuclear Technology, 07.05.2015
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CIWA – Research Alliance
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Source: google
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Presentation Outline
Introduction – how do Water Hammers appear?
Computational simulations – a vital tool to analyze Two-Phase Flow Dynamics
Implementation of the newly developed Heat Transfer Coefficient model into Computer
Codes
Assessment of the newly developed Heat Transfer Coefficient model
Summary and Outlook
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What is a Water Hammer? (I)
Water Hammer = event with impact load resulted from a sudden change in fluid velocity
• Triggering mechanisms:
pump start / pump cost-down, operation of a valve, Direct Contact Condensation
Phenomenon not (entirely) known in depth
Can yield catastrophic results
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What is a Water Hammer? (II)
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Source: dvau
Liquid
Impact
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CIWH – a particular case of WH
Direct Contact Condensation (two-phase flow, one component) + Kelvin-Helmholtz
Instability
The risk of Condensation Induced Water Hammer often underestimated
misunderstood
Highly stochastic
Especially dangerous for intermediate system pressure levels!
WH-Water Hammer 6
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Condensation at the two-phase interface
Condensation rate
Typical closure law based on empirical
correlation
C1, C2,m,n – influenced by experimental apparatus (flow channel, inclination),
BC+IC…
WH-Water Hammer, HTC-Heat Transfer Coefficient, Ai-Interfacial Area, BC-Boundary Conditions, IC-Initial Conditions 7
i
liq
sat
vap
subcoolingl
mass Ahh
THTCS
NM
m
gl
n XxCCNugl ,PrRe 2,1 ,
Direct Contact Condensation – WH driving force
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DCC in a NPP (ex. PWR)
DCC-Direct Contact Condensation, NPP-Nuclear Power Plant, PWR-Pressurized Water Reactor 8
Source: epr-reactor
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Engineering Best Practices - Avoiding CIWH
Horizontal Pipes (segments)
Water subcooling < 20 K
Pipe (segment) L / D < 24
Liquid velocity “high enough”, Fr > 1
Avoidance of steam nearby sub-cooled water
System Pressure < 1 MPa
CIWH-Condensation Induced Water Hammer, L-Pipe Length, D-Pipe Inner Diameter Fr-Froude Number 9
gD
vFr l
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Two-Phase Flow Dynamics – FV Method
termsourcetermdiffusivetermconvective
termtransient
Sgraddivvdivt
FV-Finite Volume, CFD-Computational Fluid Dynamics 10
Source: td.mw.tum
.,etcEnergy
Momentum
Mass
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Surface Renewal Theory (Higbie 1935)
Gas absorption into liquids
Interfacial Mass Transfer limited by the liquid-side Molecular Diffusion
Unknown = Surface Renewal Frequency
HTC directly linked with the turbulent character of the flow (i.e. TKE, epsilon and the
thermo-physical properties of the liquid)
-> mechanistic model !
“continuous smooth” transition between different flow regimes
SRT-Surface Renewal Theory , HTC-Heat Transfer Coefficient , TKE-Turbulent Kinetic Energy
Development of the SRT based Hybrid HTC (I)
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Variable Hughes and Duffey
(H&D) (1991) Shen et al. (2000)
Eddy Length
Scale (Lt) [m]
Eddy Velocity
Scale (Vt) [m/s]
C [-] 2 1.407
n [-] 1/2 2/3
SRT-Surface Renewal Theory, CFD-Computational Fluid Dynamics
n
l
tn
tlll
VL
CHTC
12/1Pr
4/13
l
2/3kC
4/1 l
4/12/1 lC
Development of the SRT based Hybrid HTC (II)
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SRT-Surface Renewal Theory, HTC-Heat Transfer Coefficient 13
Can a single / constant SRT be
representative for „all“ flow regimes
(turbulent intensities)?
gas
liquid
Development of the SRT based Hybrid HTC (III)
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Accounts for two different eddy length scales: macro (Shen) and micro scale (H&D)
Dynamic switch between the two models
SRT-Surface Renewal Theory, HTC-Heat Transfer Coefficient 14
2
Rek
turbulent
gas
liquid
Development of the SRT based Hybrid HTC (IV)
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CFD Codes
• ANSYS CFX (VoF-Model) – commercial code
• OpenFOAM (VoF-Model) – open source code
CFD-Computational Fluid Dynamics, VoF-Volume of Fluid 15
2500 0 Returbulent
H&
D
Sh
en
Source: td.mw.tum
Implementation of the Hybrid HTC into Computer Codes
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The PMK2 Experimental Facility-Hungary
L-Pipe Length, D-Pipe Inner Diameter Fr-Froude Number
Sensor
#
Sensor type
8 Void-Temp.
9 Wire Mesh
10 Pressure
Pipe must be horizontal
Subcooling > 20 K
L / D > 24
Fr < 1
Existence of steam nearby
System Pressure > 10 bara
Source: Prasser et al., 2004
Initial
Conditions Water Steam
p [bara] 14.5 14.5
T [°C] 25 ~200
[kg/s] 1.01 0.0 m
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PMK2 - CFD vs Experiment: T1 & T2
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VT1 VT2
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PMK2 - CFD vs Experiment: T3 & T4
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VT3 VT4
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WMS
PMK2 - CFD vs Experiment: WMS
CFD-Computational Fluid Dynamics, WMS-Wire Mesh Sensor 19
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PMK2 CFD Simulation w. ANSYS CFX
CFD-Computational Fluid Dynamics 20
Problem Time 6.8 s, 7.0 s, 7.2 s, 7.25 s, 7.3 s, 7.5 s
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Dimensions of test section:
∅51.3 x 2469 (mm)
Water inlet: T-junction
p1…p5: pressure sensors
Tf/g1…Tf/g4: thermocouples
Source: TUHH
Source: Urban and Schlüter, 2014
Source: Urban, TUHH
The TUHH Experimental Facility-Germany
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TUHH CFD Simulation w. OF
CFD-Computational Fluid Dynamics, OF-OpenFOAM 22
Void Fraction
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Summary and Outlook
A new mechanistic Hybrid HTC model was developed, based on two individual SRT
• suit-full for system code simulations!
Calibration of the Ret switch
CFD Simulations with the Hybrid HTC model offer a valuable insight into the mechanisms
triggering CIWH – steam entrapment
• Rolling Wave and Steam Pocket Collapse
• Steam Bubble Collapse
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