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

CIWA – Research Alliance

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Source: google

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

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

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

DCC in a NPP (ex. PWR)

DCC-Direct Contact Condensation, NPP-Nuclear Power Plant, PWR-Pressurized Water Reactor 8

Source: epr-reactor

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

Two-Phase Flow Dynamics – FV Method

termsourcetermdiffusivetermconvective

termtransient

Sgraddivvdivt

FV-Finite Volume, CFD-Computational Fluid Dynamics 10

Source: td.mw.tum

.,etcEnergy

Momentum

Mass

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)

12

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)

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)

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

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

PMK2 - CFD vs Experiment: T3 & T4

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VT3 VT4

WMS

PMK2 - CFD vs Experiment: WMS

CFD-Computational Fluid Dynamics, WMS-Wire Mesh Sensor 19

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

TUHH CFD Simulation w. OF

CFD-Computational Fluid Dynamics, OF-OpenFOAM 22

Void Fraction

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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