wp2.2 boiling flow and departure from nucleate boiling · cfd model options for dnb rans is ok ; no...

WP2.2: Boiling flow andDeparture from Nucleate Boiling

D. Bestion, C. Morel, S. Mimouni, E. Krepper, A. Badillo, Y. Sato, B. Niceno, M. C. Galassi, A. Del

Nevo, F. Moretti, B. Koncar , M. Matkovič, L. Vyskocil, J. Macek, D. Tar , G. Mayer, G. Hazi, A.

NURISP SEMINARApril2-3, Karlsruhe

1

Vyskocil, J. Macek, D. Tar , G. Mayer, G. Hazi, A. Markus

OBJECTIVES

1. Better understanding of local flow processes in boiling flow

– non-uniform heat flux– grid effects, – channel shape/size impact

2. Improvement of the subchannel approach– decrease of conservatisms through more general

and accurate CHF correlations


2

3. Help for design/optimization of fuel assemblies– parametric studies on design– optimization of CHF test procedures, reduction of

CHF tests

4. Development of a CHF “Local Predictive Approach”

– DNB correlations based on local parameters instead of cross-sectional averaged parameters

type 1 type 2 type 3

type 4 type 5

type 6

MULTI-SCALE ANALYSIS OF DNB

DNSLBM, VOF, LS PF

NucleationBubble detachment

Separate Effect Testsadiabatic

Air –water & steam-waterDEDALE, LiNX, TOPFLOW

CHAPTAL

CFD models

Rod Bundle tests

KFKI, BFBT, PSBT, LWL

modeling M

odel

ing

valid

atio

n

validationvalidation

ModelingSubchannel codes

DNSLBM, VOF, LS PF

NucleationBubble detachment

Separate Effect Testsadiabatic

Air –water & steam-waterDEDALE, LiNX, TOPFLOW

CHAPTAL

CFD models

Rod Bundle tests

KFKI, BFBT, PSBT, LWL

modeling M

odel

ing

valid

atio

n

validationvalidation

ModelingSubchannel codes


3

CFD models• LES

•RANS

Separate Effect TestsBoiling flow

ASU, DEBORA, TESS

Modeling

Fuel design

Modeling

validation

Subchannel codes

CHF prediction

CFD models• LES

•RANS

Separate Effect TestsBoiling flow

ASU, DEBORA, TESS

Modeling

Fuel design

Modeling

validation

Subchannel codes

CHF prediction

CFD MODELLING OF BOILING FLOW

Identification of important flow processes

Exp. Data BASIS+ DNSMODEL OPTIONS


4

CLOSURE LAWS

+ DNSMODEL OPTIONS– Nb of fields– Space and time resolution

SET OF EQUATIONS

CFD MODEL OPTIONS FOR DNB

� RANS is OK ; no added value with LES� 2-Fluid at least necessary to model all interfacial forces� Turbulence modeled by k-ε, SST, or Rij-ε� Efforts to predict bubble diameter:

� Monodispersed assumption: transport of n or Ai� Polydispersion modeling by:

NURISP Mid-term Review Meeting, October 7 th, 2010 - BRUSSELS:Progress of SP2 activity at Mid-term

5

� Polydispersion modeling by:• MUSIG approach• MSM (statistical moments)

� Main efforts devoted to:� Interfacial forces & H&M transfers� Wall transfers : momentum, energy, DNB criterion� 2-phase effects on turbulence� polydispersion

Interfacial forces

Drag force :

Lift force :

Added Mass force :

( )lglgDliDl

Dg VVVVCa

8

1MM

vvvvvv−−ρ−=−=

∇⋅+∂

∂−

∇⋅+

∂∂

ρα−α+α−=−= ll

lgg

glMA

MAl

MAg VV

tV

VVt

V

121

CMMvv

vvv

vvv

L

TD


6

Lift force :

Turbulent Dispersion force :

( ) ( )lT

llglLLl

Lg VVVVCMM

vvvvvv∇−∇⋅−αρ−=−=

α∇ρ−=−= llDTDTl

DTg KCMM

vv

+ possibly a Wall lubrication force

CL may change sign when size increases (Tomiyama)

D

WB

Wall heat transfer & Interfacial heat transfers

Convection :

Vaporization :

Quenching :

( )lwcc TThAq −= log +=T

uCh pll

*

log ρ

( )lwlqqq

ta

TTftAq

πλ −= 2

LNd6

fq g3

nuce ρπ=

Models required for f, dnuc, & N


7

Quenching :ql

qqqta

ftAqπ

=

Liquid to Interface heat transfer

( )lsatilili TTahq −=

Nud

hs

lli

λ= 33.05.0 PrRe6.02 +=Nu

Turbulence models

• Sato (1981): turbulence viscosity of the liquid phase + Increasing of the liquid viscosity by bubbles:

• Sources and sinks in turbulent equations

6.0=SC LGGLSL UUCk

Crr

−++= αρε

ρµµ µ

2

( ) ( ) kturbl SPkkvk +−+

∇⋅∇=⋅∇+∂ ερµαραρα v


8

( ) ( ) klllll

k

lllllllll SPkkvk

t+−+

∇⋅∇=⋅∇+

∂∂ ερ

σµαραρα v

LGgd

kkl UUFS

rrv−⋅−= α

( ) ( ) ( ) εεε

ε

ερεεσ

µαεραερα lllll

ll

turbl

llllllll SCPCk

vt

+−+

∇⋅∇=⋅∇+

∂∂

21

v

τεε

kl

l

SCS 3=ε

ρµ µ

2kCT =

Bubble diameter in monodispersed approachBubble diameter in monodispersed approach

Bubble number density :

Interfacial area density :

( ) BK

n

Coal

n

Coll

n

Nuc

nnVndiv

t

n φφφφ +++=+∂∂ v

63sd

nπ

α=with :


9

Interfacial area density :

6

63

i

sa

dα=with :

( ) BK

a

Coal

a

Coll

nColl

Nuc

nnuc

g

ig

g

i

ii

i

iidd

dt

daVadiv

t

a φφφπφπρ

ααρ

++++

−Γ=+∂∂ 22

,3

2v

BK

n

i

BK

a

CO

n

i

Coal

a aa iiφαπφφαπφ

22

3

36

3

36

=

=

Data base for DNB

visualisations

Boiling flow Simple geometry

LWL BFBT, PSBT

Boiling flowreactor geometry

CHF testsreactorgeometry


10

Adiabatic bubbly flow

DedaleTopflow

DeboraASU

Boiling flow Simple geometry

Turbulencepromoter

AGATE

KFKI

Review about DNB mechanisms and models

1. Hydrodynamic instability (e.g. Zuber, 1958)2. Leidenfrost T°reached (e.g. Unal, 1992; Bricard, 1995; Le Corre, 2007)3. Dry spot spreading (e.g. Unal, 1992; Bricard, 1995; Ha & No, 2000; Le Corre, 2007)4. Macro-layer vaporization (e.g. Haramura, 1983; Celata, 1994; He, 2001)5. Micro-layer vaporization (Zhao, 2002), 6. Recoil force instability (Beysens, 2003); 7. Micro-layer rupture (Théofanous, 2002)

CEA


11

Conclusion of the review

� No local DNB criterion for CFD for convective boiling

� No consensus on the DNB mechanism itself

� The very simple switch to film boiling based on a limiting void fraction (NEPTUNE) performs not so bad compared to existing models but is not sufficiently precise

� Two different & complementary ways are proposed for future

CEA


12

� Two different & complementary ways are proposed for future activities regarding the DNB prediction by CFD:– Long term activity: identification of the DNB mechanism using new

experiments and the use of DNS simulations– Short term activity: establish a semi-empirical local DNB criterion

with some free parameters to fit on tube CHF data. Then this DNB criterion could be confronted to a large data base including CHF in complex geometry.

Direct Numerical Simulation of Boiling

Numerical Method

• Navier-Stokes solver : PSI-BOIL

- Finite Volume method on Cartesian grids

- Projection method

• Interface tracking method

CIP-CSL2 (color function) with

Phase change rate (kg/m3s):

S

V

lq

vq

PSI


13

local interface sharpening scheme

• Phase change model

Sharp Interface Model

• Features

- Mass conservative scheme

- Simple phase change model

Phase change rate (kg/m3s):

l vq q Sm

L V

+=&

Latent heat Cell volume

Interface area

Heat fluxes

Verification: 3D Bubble Growth in Superheated Liquid

• Condition of simulation- Water and steam at 1 bar - Liquid superheat 5 C°- Unbounded domain -Thermal boundary layer: about 10 µm - Grid size: 8µm (Coarse), 4µm (Medium), 2µm (Fine)

0.0002

PSI


14

Time (s)

Rad

ius

(m)

0 0.0001 0.0002 0.00030

5E-05

0.0001

0.00015

AnalyticalCoarseMediumFine

Validation: Saturated Pool Boiling

EXP.*

Water at 1 (bar), Wall superheat = 9.44 (K), Contact angle =47°PSI


15

0.000 (s) 0.006 (s) 0.016 (s) 0.043 (s)

0.0007 (s) 0.006 (s) 0.016 (s) 0.045 (s)

PSI-BOIL

*: R. Siegel, E.G. Keshock, Effects of reduced gravity on nucleate boiling bubble dynamics in saturated water, AIChE J., 10 (1964) 509-517.

106.0105.6105.2104.8104.4104.0103.7103.3102.9

Sample Calculation: Subcooled Pool Boiling

Hea

tflu

x(W

/m2 )

0 0.1 0.2 0.3 0.4

10000

15000

20000

SaturatedSubcooled

Time (s)

Water at 1 (bar), Wall superheat = 6.17 (K) , Contact angle =38° PSI


16Saturated boiling Subcooled boiling (97℃)

102.9102.5102.1101.7101.3100.9100.5100.1

99.799.399.098.698.297.897.497.0

Motivation:– key parameters of CHF are based on simple theoretical considerations (e.g. force balance

calculations) – for the application of theories the problem has to be oversimplified– numerical simulation can be used to approach real situations

Objective: to determine the functional relationship between thermophysical, geometrical parameters and the bubble detachment diameter, bubble release frequency taking into account more and more realistic situations

αα -- modelmodel ββ -- modelmodel γγ -- modelmodel

Use of LBM for boiling simulations

KFKI


17

αα -- modelmodel

qq

P, P, TsatTsat

periodicperiodic

nono--slipslip

ββ -- modelmodel

qq

P, P, TsatTsat

periodicperiodic

nono--slipslip

γγ -- modelmodel

qq

P, P, TsatTsat

periodicperiodic

nono--slipslip

heat conduction in the wall heat conduction in the wall + cavity in the wall

qq

P, P, TsatTsat

periodicperiodic

nono--slipslip

interaction between cavities

� without cavity and without heat conduction in the wall, simple theories work fine ( )gl

b gD

ρρσ−

~( ) 4/1

21 ~

−−

−

l

glb

gDf

ρρρσ

� Db vs. g � power function (exponents depends on the heating method, part of the heat escapes by natural convection)

� Db vs. θ� linear trend (slope lower with cavities)

� Db vs. heat flux � linear trend (slope influenced by cavity)

Use of LBM for boiling simulations KFKI

30 40 50 60 70 80 90

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

-model, homogeneous heating -model, inhomogeneous heating, q = 42900 W/m -model, inhomogeneous heating, q = 50050 W/m

Depa

rture

Diam

eter

[mm

]

Contact Angle [°]

0.4 0.6 0.8 1.0 1.2 1.4 1.60.4

0.5

0.6

0.7

0.8

0.9

1.0

-model, inhomogeneous heating -model, inhomogeneous heating -model, inhomogeneous heating -model, homogeneous heating a = 0.6307, b = -0.5109 a = 0.6275, b = -0.6423 a = 0.6750, b = -0.4899 a = 0.6620, b = -0.4888

Dep

artu

re D

iam

eter

[mm

]

g, gravitation (x 9.81 [m/s2])

Dd=a gb


18

30 40 50 60 70 80 90

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

-model, homogeneous heating -model, inhomogeneous heating, q = 42900 W/m -model, inhomogeneous heating, q = 50050 W/m

Depa

rture

Diam

eter

[mm

]

Contact Angle [°]

0.4 0.6 0.8 1.0 1.2 1.4 1.60.4

0.5

0.6

0.7

0.8

0.9

1.0

-model, inhomogeneous heating -model, inhomogeneous heating -model, inhomogeneous heating -model, homogeneous heating a = 0.6307, b = -0.5109 a = 0.6275, b = -0.6423 a = 0.6750, b = -0.4899 a = 0.6620, b = -0.4888

Dep

artu

re D

iam

eter

[mm

]

g, gravitation (x 9.81 [m/s2])

Dd=a gb

44000 46000 48000 50000 52000 54000 56000 58000

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

-model -model D

d = 9 10-6 +0.1716

Depa

rture

Dia

met

er [m

m]

Heat Flux [W/m]

30 40 50 60 70 80 90

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

-model, q = 42900 W/m-model, q = 50050 W/m-model

Rel

ease

Per

iod

[ms]

Contact Angle [°]

�f vs. θ� with flat surface the release period decreases sharply with increasing θ until it reaches minimum. With cavity the release period is a monotone increasing function of the θ and this function can be described well by a parabola.


KFKI


19

30 40 50 60 70 80 90

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

-model, q = 42900 W/m-model, q = 50050 W/m-model

Rel

ease

Per

iod

[ms]

Contact Angle [°]

� Increasing φ, Tw does not change at beginning then starts increasing (increase described by a cubic function of φ in line with data). φ where Tw starts to increase depends on modeling conduction or cavities.

� Modeling conduction, Tw starts increasing at higher φ for flat surface and even higher with cavities. Increasing φ, the Tw fluctuations increase, too.

5000 10000 15000 20000 25000 30000 35000 40000 45000300

350

400

450

500

550-model-model-model

Sur

face

Tem

pera

ture

[°C

]

Heat Flux [W/m]


KFKI


20

5000 10000 15000 20000 25000 30000 35000 40000 45000300

350

400

450

500

550-model-model-model

Sur

face

Tem

pera

ture

[°C

]

Heat Flux [W/m]

� Cavities can interact with each other, resulting in cancellation or freezing of bubble production in a neighbouring cavity (energy partitioning between cavities)

Is the wall boiling model able to detect the CHF value?

• Following the analysis of Kurul (1990), the heat flux at the wall is splitinto three terms:

• convective heat flux to liquid qc at the fraction of the wall area unaffected by the presence of bubbles,

• a quenching heat flux qq where bubbles departure bring cold water in contact with the wall periodically,

EDF


21

• a vaporisation heat flux qe needed to generate the vapour phase.

Convective heat flux

vaporisation

quench

the wall surface fraction A occupied by bubble nucleation reaches 1 in 8 CHF tests

DEBORA cases under CHF conditions-

• Values of A under critical heat flux conditions are between 10%and 70%.

• A was calculated as a function of the heat flux : A tends to 1 insome cases at CHF value but not in all cases.

• Various authors (Zuber) postulate that CHF occurs when theHelmholtz instability appears in the interface of the large vaporcolumns leaving the heating surface � bubble diameter atdetachment when the CHF is reached. In some cases, the Unal

EDF


22

detachment when the CHF is reached. In some cases, the Unaldiameter and the Rayleigh-Taylor diameter tend towards thesame value when the heat flux tends towards the CHF value. Butthe accuracy levels should be improved to prove the efficiency ofthe criterion.

� The wall model for nucleate boiling is not able to detect the CHFvalue.

Uncertainty of Modeling: Bubble Dep. Diameter

( )6

1

3

3

coscos32

sin864

−+⋅⋅

⋅−=

φφφ

ρρσ

gD

vlbubble

( ) 2n

1ii

i

ibubble(bubb) x

x

xDD ∑

=

⋅

∂∂= δδ

0.1

1

U_Dbubb [m]

d(Dbubb)/d(fi)*U(fi)

d(Dbubb)/d(sigma)*U(sigma)

d(Dbubb)/d(Drho)*U(Drho)

JSI


23

=D

dBarcsinφ

( ) ( ) gD

vl ⋅−⋅

−+⋅=

ρρσ

φφφ

3coscos32

sin24

Calculated uncertainty values forbubble departure diameter Dbubble

0.0001

0.001

0.01

0 20 40 60 80 100 120 140 160 180fi [°]

U_D

bubb

[m]

d(Dbubb)/d(Drho)*U(Drho)

Characteristic diameters are deducedfrom the equilibrium of the surfacetension and the buoyancy forces.

Uncertainty of Modeling: Bubble dep. diameter

10

12

14

16

18

20

D, D

bubb

[mm

]

D [mm]

Dbubb

Dfritz [mm]

10

100

1000

Ur_

D, U

r_D

bubb

[%]

Consideration of uncertainties of measured parameters may lead to significant deviation in prediction of D and Dbubb even for a simple equilibrium of buoyancy and surface tension.

JSI


24

Calculated diameters with error bars for virtual growing bubble D and departing bubble Dbubb vs. Fritz correlation (DFritz).

0

2

4

6

8

0 20 40 60 80 100 120 140 160 180fi [°]

D, D

bubb

[mm

]

0.1

1

10

0 20 40 60 80 100 120 140 160 180fi [°]

Ur_

D, U

r_D

bubb

[%]

Ur_Dbubb [%]

Ur_D [%]

( )vlFritz g

Dρρ

σφ−⋅

⋅⋅= 0208.0

Relative uncertainty for D and Dbubb

strongly depends on the wetting angle.

Wall-to-fluid heat transfer in boiling model

• “Two-phase” shear velocity affects the heat partitioning in boiling model

1. Modified single-phase heat transfer coefficient hlog

2

)ln(1

++ By

κ ( )TThA −=Φ

JSI


25

2. Characteristic velocity, appears in Unal’s model for dbw

,)ln(

1wuuByu

∆−+= ++δδ κ

250=+δy

log2log,

)ln(1

)ln(

∆−+

+=

++ uBy

Byhh ph

κ

κ ( )δTThA wphCphC −=Φ 2log,2,1

0

20000

40000

60000

80000

100000

0 1 2 3 4

Sin

gle

-phase

heat f

lux

(W/m

2)

z (m)

Wall func

Temp wall func

Wall-to-fluid heat transfer in boiling model: Results – ASU case

0

0.1

0.2

0.3

0.4

0.5

0 0.2 0.4 0.6 0.8 1

Vo

id fr

actio

n

(r-Ri)/(Ro-Ri)

Exp_tp6

base

Wall func

Temp wall func

1 70000

0.E+00

1.E-04

2.E-04

3.E-04

4.E-04

5.E-04

6.E-04

7.E-04

0 1 2 3

dbw

(m

)

z (m)

dbw_wf1

dbw_tp1

370

JSI


26

Negligible effect on evaporation, some influence only in PDB region.

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

Liq

. Vel

oci

ty (

m/s

)

(r-Ri)/(Ro-Ri)

Exp_tp6

base

Wall func

Temp wall func

Does not affect radialprofiles at measuringplane

0

10000

20000

30000

40000

50000

60000

70000

0 1 2 3

Evap

ora

tio

n h

eat f

lux

(W/m

2)

z (m)

Wall func

Temp wall func

350

352

354

356

358

360

362

364

366

368

0 1 2 3

TW

(K)

z (m)

Tw_wf1

Tw_tp1

Minor effect on dbw and Tw due to changed uδ

Texas A&M experiment

(Estrada-Perez and Hassan, IJMF 2010)

� Rectangular vertical channel W/H~1 approximates BWR flow channel

� PTV technique: velocity fluctuations and average velocities measured,

� Fluid HFE-301 at ambient pressure.

m. p.

475 mm

TAMU, JSI


27

� Heated surface: 7mm wide, 175mm high,

� 455 mm from the channel inlet,

� fully developed region,

� Re [3309, 16549];

qw [0, 64 kWm-2].

0.120

1]

Re3309_q640_u'

Texas A&M: HFE 301 & Turbulence data

800

900

1000

HFE 301 Water

HFE 301 vs. water• Density ratios at given p/pc are preserved.• Experiment: pw/pHFE … 9.36 bar / 1 bar• BWR: pw/pHFE … 70 bar / 8 bar• PWR: pw/pHFE … 140 bar / 16 bar

Experimental turbulence� Fluctuations over the channel depth (w’)

were not measured, they are assumed to have the same values as v’

� kexp was defined from measured velocity fluctuations in one plane (u’, v’)

( )22exp '2'

2

1vuk ⋅+⋅=

TAMU, JSI


28

0.000

0.020

0.040

0.060

0.080

0.100

0.000 0.002 0.004 0.006 0.008

velo

city

flu

ctu

atio

n [m

s-1

]

width [m]

Re3309_q640_u'

Re3309_q640_v'

u’

v’

w’0

100

200

300

400

500

600

700

800

0 0.02 0.04 0.06 0.08 0.1

p/pc

ρρ ρρ L/ ρρ ρρ

v

P=1 bar

Tsat= 37 C

p=9.36 bar

Tsat = 177 C

Texas A&M: 2D vs 3D mesh

0,2

0,3

0,4

0,5

0,6

axia

l vel

ocity

[m s

-1] .

3Dfine

2Dfine

TAMU, JSI


29

0

0,1

0 0,002 0,004 0,006 0,008width [m]

2Dfine

� Due to the small W/H of the channel the geometry cannot be reduced to a 2D case.

� A full 3D domain with medium grid refinement 20x20x370 was used

Texas A&M: Results –Velocity profiles (Re= 9926)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.002 0.004 0.006 0.008

Liqu

id V

eloc

ity [m

s-1

]

width [m]

LiqVel (CFX)

LiqVel (NEPTUNE)

LiqVel (Exp)

Re= 9926

qw= 3.9 kWm-2

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.002 0.004 0.006 0.008

Liqu

id V

eloc

ity [m

s-1

]

width [m]

LiqVel (CFX)

LiqVel (NEPTUNE V7)

LiqVel (Exp)

Re= 9926

qw= 42.3 kWm-2

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.002 0.004 0.006 0.008

Liqu

id V

eloc

ity [m

s-1

]

width [m]

LiqVel (CFX)

LiqVel (NEPTUNE)

LiqVel (Exp)

Re= 9926

qw= 64 kWm-2

Liquid velocity TAMU, JSI


30

width [m] width [m]

Turbulent kinetic energy

0.000

0.002

0.004

0.006

0.008

0 0.002 0.004 0.006 0.008

k l[J

kg-1

]

width [m]

k_eff (CFX)

k_eff (NEPTUNE)

k_eff (Exp)

Re= 9926

qw= 3.9 kWm-2

0.000

0.002

0.004

0.006

0.008

0 0.002 0.004 0.006 0.008

k l[J

kg-1

]

width [m]

k_eff (CFX)

k_eff (NEPTUNE)

k_eff (Exp)

Re= 9926

qw= 42.3 kWm-2

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0 0.002 0.004 0.006 0.008

k l[J

kg-1

]

width [m]

k_eff (CFX)

k_eff (NEPTUNE)

k_eff (Exp)

Re= 9926

qw= 64 kWm-2

CHAPTAL Experimental Program :

Adiabatic two-phase flows (water-R116 gas)

• The CHAPTAL test section is a 5 m long vertical pipe with an inside diameter of 38 mm.

• Local measurements are performed at : 7.5D, 54.5D and 155.5D, where D = 38mm

qq qq

outlet

EDF


31

38mm

• Strategy for validating separately the forces modelling and the bubble size modelling:

– Calculation with imposed bubble diameter

– Calculation with from the moment method

qq

qqinlet

Liquid flowrate (kg/s) Gas flowrate (g/s) Case 1 1 14 Case 2 1 7 Case 3 1 20 Case 4 2 14

CHAPTAL calculations by NEPTUNE_CFD

EDF


32

First set of computations on 3 meshing :1. a coarse grid (10 cells in the radial direction and 100 cells in the axial direction) , 2. a medium grid (20 cells in the radial direction and 200 cells in the axial direction)3. a fine grid (30 cells in the radial direction and 400 cells in the axial direction).

Results are similar

focus on forces modelling, not on the bubble diameter D*except for simulations called “polydispersion”, the experimental D* is imposed in the whole computational domain.

Case 1

Case 2


EDF


33

Unfortunately, surface tension for bubbles of Freon 116 immerged in liquid water is still unknown.several values between 0.01 and 0.07 N/m were tested. However, it is planned to measure σ in a near future.

Case 2

Case 3

Case 4


EDF


34

Case 4

• Unfortunately, we currently ignore the value of the surface tension forbubbles of Freon 116 immerged in liquid water, but the air-water surfacetension gives a satisfactory explanation of the concordance betweencalculations and experimental values

• Under this assumption, the maximum horizontal dimension of thedeformed bubble calculated using an empirical correlation given byWellek et al. (1966), is underestimated. Thus, considering the current setof models and available measurements, the tests featuring small bubbles


EDF


35

of models and available measurements, the tests featuring small bubblesshould be privileged in order to minimize the error on the Wellek’scorrelation.

Extension of the MUSIG approach to bubbly flow with condensation

Coalescence & break up not taken into account

•The best agreement was obtained with Hughmark model

0.30

0.40

mm

]

K16.118_6F: 0.608 m

ExpCFX

0.10

0.15

ctio

n [-]

F: 0.608 mExperimentCFX (total)dB<4.5 mm

0 10 20 30DB [mm]

0.00

0.10

0.20

0.30

0.40

H [%

/mm

]

K16.118_6I: 1.552 m

ExpCFX

0.000 0.020 0.040 0.060 0.080 0.100Radius [m]

0.00

0.03

0.05

0.08

0.10

Gas

vol

um

e fr

actio

n [-]

I: 1.552 mExperimentCFX (total)dB<4.5 mmdB>4.5 mm

HZDR


36

with Hughmark model•Ranz and Marshall model undererestimated the heat transfer•Tomiyama model overerestimate the heat transfer

0 10 20 30DB [mm]

0.00

0.10

0.20

0.30

0.40

H [%

/mm

]

K16_118.6A: 0.221 m

ExpCFX

0.000 0.020 0.040 0.060 0.080 0.100Radius [m]

0.00

0.03

0.05

0.08

0.10

Gas

vol

um

e fr

actio

n [-]

A: 0.221 mExperimentCFX (total)dB<4.5 mmdB>4.5 mm

0 10 20 30DB [mm]

0.00

0.10

0.20

H [%

/m

0.000 0.020 0.040 0.060 0.080 0.100Radius [m]

0.00

0.05

Gas

vol

um

e fr

ac dB>4.5 mm

Extension of the MUSIG approach to bubbly flow with condensation

0 10 20 30 40 50DB [mm]

0.00

0.10

0.20

0.30

0.40

H [%

/mm

]

K16.118_6.0K: 2.538 m

ExpCFX

0.000 0.020 0.040 0.060 0.080 0.100Radius [m]

0.00

0.05

0.10

0.15

0.20

0.25

Gas

vo

lum

e fr

actio

n [-

]

K: 2.538 mExperimentCFX (total)dB<4.5 mmdB>4.5 mm

0.30

0.40

m]

K16.118_6H: 1.495 m

ExpCFX

0.15

0.20

0.25

ract

ion

[-]

H: 1.495 mExperimentCFX (total)dB<4.5 mmdB>4.5 mm

In a test with larger bubbles small deviations of the cross sectional averaged void fraction from the experiments can be explained by the

HZDR


370 10 20 30 40 50

DB [mm]

0.00

0.10

0.20

0.30

0.40

H [%

/mm

]

K16_118.6B: 0.278 m

ExpCFX

0.000 0.020 0.040 0.060 0.080 0.100Radius [m]

0.00

0.05

0.10

0.15

0.20

0.25

Gas

vol

um

e fr

actio

n [-

]

B: 0.278 mExperimentCFX (total)dB<4.5 mmdB>4.5 mm

0 10 20 30 40 50DB [mm]

0.00

0.10

0.20

H [%

/mm

0.000 0.020 0.040 0.060 0.080 0.100Radius [m]

0.00

0.05

0.10

0.15

Ga

s vo

lum

e fr dB>4.5 mmexperiments can be explained by the

bubble sizes calculated too large. Here the neglected bubble fragmentation seems to play a role.

Polydispersion modelling with MUSIG

0.0000

0.0005

0.0010

d B [m

]

Expmonodispersed approachMUSIG

HZDR

DEBORA tests: Radial bubble size profile


38

• Coupling wall boiling with population balance model: Better description of bubble size profile (interfacial area)

• blue line: bubble size dependent on liquid temperature• red line: (MUSIG) the increase of bubble size by bubble coalescence with

increasing distance from the wall can be described• P=2.62 MPa, G=2.103 kg m-2 s-1, Tin = 68.5 °C

0 0.002 0.004 0.006 0.008 0.01r [m]

0.0000

0.4

0.6

0.8

1.0

[-]

TSUB [K]13.8918.4322.526.9429.58

0.4

0.6

0.8

1.0[-

]

TSUB [K]13.8918.4322.526.9429.58

measurement calculation


volu

me

frac

tion

HZDR

DEBORA tests: Radial void fraction profile


39

0 0.002 0.004 0.006 0.008 0.01R [m]

0.0

0.2

0 0.002 0.004 0.006 0.008 0.01R [m]

0.0

0.2

• P=1.4MPa, G=2.103 kg m-2 s-1

• test series with decreasing subcooling at the inlet� With decreasing subcooling in the radial volume fraction distribution shift from

wall to core peak – can be described by inhomogeneous MUSIG model

volu

me

frac

tion


bubble size distribution

at different positions x and RP1P2P3

Gas2 Gas1

Gas2Gas1

200

300

400

B [m

m-1]

x=3.5 mP : r=0.0095 m

HZDRCoupling wall boiling model with population balance model


40

• P=1.4 Mpa, subcooling at the inlet TSUB = 14 K• two dispersed phases � description of flow with core

peak

0.00 0.75 1.50 2.25 3.00dB [mm]

0

100

200

dαG/d

d B P1: r=0.0095 mP2: r=0.0045 mP3: r=0.001 m

Polydispersion effect in subcooled boiling

Typical DEBORA pdf

CEA


41

Average bubble dBubble population in

saturated layer

Bubble population in subcooled core flow

after some condensation

Calculating condensation with a unique

diameter

Validation of NEPTUNE_CFD against PSBT by UPISA

Selected test:Sub-channel test section(PSBT test facility)

UPISA


42

Selected test:

Subcooled boiling in PWR-type heated channels

(PSBT test facility)


Computational domain: 1/8 of cross section (by symmetry)

Meshes:10 000 ÷ 220 000 cells

UPISA


43

Simulation set-up• Code: NEPTUNE_CFD V1.0.7

• Bubble diameter: uniform or aiequation (Wei Yao)

• Turbulence modelling– Liquid: κ−ε or Rij

– Vapour: Tchen or none

• Drag coefficient: incl. (edf) or Ishii

• Non drag coefficient� Lift, added mass (std. edf or Zuber), turb.

dispersion

• Heat transfer modelling� Liquid: bulk model (Astrid) or Grenoble model

� Vapor: Relax. time or Grenoble model

• Wall boiling: EDF or GRE model

Reference calculations results

“st edf_1249”

Case ααααEXP 0.22

st edf_1249 0.26(+ 18 %)

gre_1257 0.30(+ 36 %)


UPISA


44

“gre_1257”

• Reference results:– Overestimation of average void fraction in both cases– Fail to predict correct void distribution (too much vapour close to

wall, too little in the bulk)• Sensitivity analyses:

– Rij turbulence model (liq. phase): still overpredicts void fraction;



45

– Rij turbulence model (liq. phase): still overpredicts void fraction; qualitative distribution closer to exp

– Bubble diameter (ai eq.) and lift coeff., for “st edf” case � failed due to convergence problems

– Bubble diameter (ai eq.) � failed due to convergence problems– Constant bubble diameter � no noticeable effect– 1.5% higher inlet velocity � void = 0.27 (overprediction reduced)– Grid refinement (axial) � no effect– Grid refinement (cross sectional) � slight increase in void

fraction � grid convergence not yet achieved• CFD is not yet more accurate than system code for average void

fraction prediction

DEBORA tests close to DNB

DEBORA tests with increasing Xth

A void peak appears close to the wall just before CHF

Is it a layer of detached bubbles?

NRI


46

Is it a coalescence of attached bubbles?

NEPTUNE simulation

Simulation of tube and bundle CHF tests LWL

Example results: Case 4(vertically “shrunk” domain - 1:50)

Void fraction [ -]

centralrod

NRI


47

Void fraction [ -]

� Step 1: looking for a local DNB criterion using Tube data� Step 2: verify the criterion in actual rod bundle (LWL)

� αlim = 0.8 provides 10% accuracy in most cases in tubes� not good for low G & low P� grid spacer effects qualitatively predicted (higher mixing and higher CHF)

NRI simulations of CHF tests in tubes

Series 1: Variable X eq Series 2: Variable GNRI


48

Series 3: Variable D Series 4: Variable p

DNB: Main achievements

� New data were used for modelling and validation:• TAMU rectangular boiling channel• KFKI data• DEBORA-TESS• CHAPTAL adiabatic bubbly flow• CHF in pipes• PSBT

� The main advances in understanding flow processes and in


49

� The main advances in understanding flow processes and in modelling are on the following:

– Wall function laws for momentum– Mechanical laws (interfacial forces and turbulence) consolidated

by CHAPTAL– Polydispersion effects in presence of boiling and condensation– CFD was validated in real rod bundle geometry (KFKI & PSBT)

DNB: CONCLUSIONS

� The state of the art in CHF prediction with two-phase CFD is the following:

– CHF in heated tube with steam-water is predicted with a 10% accuracy in the domain: 15< P <20MPa, -0.5 < X < 0.1, 1800 < G < 5000Kg/m2/s, 4 < D < 16mm

– CHF in rod bundle was predicted with a 20% accuracy and the effect of the spacer grids was qualitatively correct

– Larger errors were found for CHF in heated tube with Freon


50

� Accuracy of CFD not yet sufficient for DNB prediction butalready useful for:� parametric studies for fuel design,� Improving subchannel models� more mechanistic CHF models

� Need of a DNB criterion: new experiments and DNS simulations

wp2.2 boiling flow and departure from nucleate boiling · cfd model options for dnb rans is ok ; no...

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