Cfd Modelling of TBR 2

7/31/2019 Cfd Modelling of TBR 2

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CFD MODELING OF TRICKLE BEDREACTORS: Hydro-processing Reactor

Prashant R. Gunjal, Amit Arora and

Vivek V. RanadeIndustrial Flow Modeling Group

National Chemical Laboratory

Pune 411008

Email: [email protected]
mailto:[email protected]:[email protected]


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CFD Modeling of Trickle Bed Reactors

OUTLINE

Trickle Bed Reactors

Applications/ flow regimes Experiments

CFD Modeling of Trickle Bed Reactors Inter-phase momentum exchange/ capillary terms

Estimation of fraction of liquid suspended in gas phase

Simulating Hydro-processing Reactor Reaction kinetics/ other sub-models Influence of reactor scale

Concluding Remarks


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TRICKLE BED REACTORS

Wide Applications

Hydro-de-sulfurization Hydro-cracking/ hydro-treating

Hydrogenation/ oxidation

Waste water treatment

Key Characteristics Close to plug flow/ Low liquid hold-up

Suitable for slow reactions

Poor heat transfer/ possibility of mal-distribution

Difficult scale-up


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TRICKLE BED REACTORS

Gas & Liquid Flow through Void Space

= 26-48 %

GasLiquid


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FLUID DYNAMICS OF TBR

Characterizationof Packed Bed

Wetting of SolidSurface by

Liquid

Flow Regimes &Global FlowCharacteristics

Mal-distribution,Channeling &Mixing

, P, L, ,RTD


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FLOW REGIMES


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EXPERIMENTS AT NCL

Hydrodynamics

Pressure Drop

Liquid Hold-up

Conductivity Probes

Residence Time Distribution

Liquid Distribution at Inlet Uniform

Non-uniform

Experimental Parameters Bed Diameter: 0.1 & 0.2 m

Particle Diameter: 3 & 6 mm

Liquid Velocity: < 24 mm/ s

Gas Velocity: < 0.50 m/ s

Tracer : NaCl

Data

Acquisition

From CompressorLiquid Tank

Column with

3mm or 6mm

Glass Beads

SeparatorConductivity

Probe

101

PressureIndicator

Pump


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TYPICAL PRESSURE DROP DATA

0

5

10

15

20

25

30

35

0 10 20 30

Liquid Flow Rate, Kg/m 2s

Pressuregradie

nt,KPa/m

Trickle f low regime

Pulse flow regime


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PUBLISHED EXPERIMENTAL DATA

0

0.2

0.4

0.6

0.81

1.2

1.4

1.6

1.8

2

0 5 10 15 20 25

Liquid Velocity VL, (Kg/m2s)

Sup

erficialGasV

elocityVg,(m

/s)

L= ?

Trickle Flow

1

3

1. Szady and Sundersan (1991)

2. Rao et. al (1983)

3. Specchia and Baldi (1977)

Pulse Flow

Spray Flow

Bubbly Flow

L=L

L=?

L L

2


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MODELING OF MACROSCOPIC FLOW

Bed Porosity: Scale of Scrutiny

Bed diameter/ length: overall

Intermediate scale: Gaussian distribution

Smaller than particle diameter: Bi-modal distribution

Approach Used:

Experimentally measured or estimated radial variation ofaxially averaged bed porosity

Specify porosity by drawing a sample assuming Gaussiandistribution with specified variance


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CHARACTERIZATION OF PACKED BED

Radial Variation of Porosity: Mueller (1991)

( )

( )

( )

FunctionBesselorderzeroisJandr/Dr

D/d0.724-0.304b

D/d13.0for9.864D/d

2.932-7.383a

13.0D/d2.61for

3.156D/d

12.98-8.243a

)e(ar)J(1r

th

o

*

P

P

P

P

P

br*

oBB

=

=

=

=

+=


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CHARACTERIZATION OF PACKED BED

Muellers Correlation

Three data sets from literature Our experiments

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.2 0.4 0.6 0.8 1

Dimensionless Radius

Porosity

D=0.194m, dp=3mm

D=0.194m, dp=6mm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.2 0.4 0.6 0.8 1


Porosity

Szady and Sundersan (1991)

Rao et.al. (1983)

Spacchia and Baldi (1977)


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VARIATION OF BED POROSITY

Selection of appropriate value is not straight forward: RTD may help

r/R

0.70

0.56 With std dev=0

With std dev=5%

With std dev=10%


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CFD MODEL

Multi-fluid Model (Eulerian-Eulerian)

Continuity Equation

0=+

KKK

KK Ut

Momentum Balance Equation

( ) ( )RkRKKKK

KKKKK

KKK

UUFgU

PUUt

U

++

+=+

,

)()(


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CFD MODEL

Inter-phase Coupling Terms (Attou & Ferschneider, 2000)

( )

+

=333.0

2

667.0

22

2

1

)1(

)1(

)1(

)1(

G

L

pG

GLGP

G

L

pG

GGGL

d

UUE

d

EF

+

=333.0

2

667.0

22

2

1

)1(

)1(

)1(

)1(

G

S

pG

GGP

G

S

pG

GGGS

d

UE

d

EF

+=

pL

SGP

pL

sLLS

d

UE

d

EF

2

22

2

1


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CFD MODEL

Capillary Terms

Attou and Ferschneider (2000)

=

21

112

ddPP

LG

=L

G

PG

LG Fd

PP

5416.0

1

12

333.0

( )

+

=

L

GG

G

ss

G

/

G

s

p

LG

F

z

z

d

.

z

P

z

P2

32

11

1

1

4165

3

2


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CFD MODEL

Capillary Terms

Extent of Wetting, f (Jiang et al., 2002)

Scalar Transport

iimiKKiKKKiKK SCDCU

t

C+=+

)( .

cLG PfPP )1( =

025.01.881


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SIMULATED RESULTS

0

5

10

15

20

25

3035

40

45

0 0.02 0.04 0.06 0.08 0.1Velocity M agnitude, m/ sec

Non-prewetted Bed

Prewetted B ed

0

5

10

15

20

25

30

35

0 0.1 0.2 0.3Liquid Holdup

Non-prewetted Bed

Prewetted Bed

Pre-wetted Un-wetted

VL= 6 mm/ s, VG=0.22 m/ s, D=0.114 m, dP= 3 mm, =5%

ContoursofL

iquidHold-up

Distributions within the bed


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SIMULATED RESULTS

Column Diameter = 0.114 m Particle Diameter = 3 mm, VG=0.22 m/ s


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SIMULATED RESULTS

VG= 0.22 m /s, D = 0.114 m VG= 0.22 m /s, D = 0.194 m


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GAS-LIQUID FLOW IN TBR

Estimation of Frictional Pressure Drop & Fraction ofLiquid Supported by Gas

Simulate at two values of g (9.7, 9.9 m/s2)

21

1

2

2

1

gg

L

Pg

L

Pg

L

Pf

=

( )21L12

Lgg

L

P

L

P

=

Fraction of Liquid Hold-upSupported by Gas Phase < 1

Solid

Liquid

Gas

( )

LL

L

f

=

Q

gL

P

L

PL

GLGL


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SIMULATED RESULTS

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.00 0.20 0.40 0.60 0.80 1.00

Liquid Flow Rate x10-1

, kg/m2s

LiquidSaturation

and

L

Holub et. al. (1993) -Experimental data

Szady and Sundersan (1991)-Exp. Data

CFD Results

Supported Liquid Saturation

Calibrate Model

Parameters from P

Can Predict Total andSupported LiquidVolume Fraction

0

2

4

6

8

10

12

14

0.00 0.20 0.40 0.60 0.80 1.00

Liquid Flow Rate x10-1, Kg/m2s

PressureGradient

KPa/m

Saez and Corbonell (1985)-Exp. dataSzady and Sundersan (1991)-Increase in liquidSzady and Sundersan (1991)-decrease in liquidCFD Simulations w ithout Capillary ForceCFD Simulations

Operating conditions:VG=0.22m/s, D/dp =55, dp=3mm, Std. Dev.=5%, E1=215, E2=1.75


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SIMULATED RESULTS

0

10

20

30

40

50

0 0.2 0.4 0.6 0.8 1

Gas Flow Rate, (Kg/m2s)

PressureDrop,

(KPa/m)

CFD ResultsExperimental Data

0

0.1

0.2

0.3

0.4

0.5

0 0.2 0.4 0.6 0.8 1Gas Mass Velocity, kg/m2s

LiquidSaturationand

L

Liquid Saturation- CFD ResultsSupported Liquid SaturationLiquid Saturation- Exp. Data

Data of Spachio & Baldi (1977)

As gas velocity increases,

more & more liquid issupported by gas

Operating conditions: VL=2.8x10-3 m/s, dp=3mm, D/dp=30, Std. Dev.=5%, E1=500, E2=3


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SIMULATED RESULTS

0

0.04

0.08

0.12

0.16

0.2

0 2 4 6 8

Gas mass velocity, Kg/m2s

LiquidSaturationa

nd

L

Liquid Saturation- CFD ResultsSupported Liquid SaturationLiquid Saturation Exp. Data

0

40

80

120

160

200

0 2 4 6 8

Gas Mass Velocity, (kg/m2s)

PressureDrop,

(KPa/m) Experimental Data

CFD Results Data of Rao et al. (1983)

As gas velocity increases,

more & more liquid issupported by gas

Operating conditions:VL=1x10-3m/s, D/dp =15.4, dp=3mm, Std. Dev.=5%, E1=215, E2=3.4


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RESIDENCE TIME DISTRIBUTION

0

0.02

0.04

0.06

0.08

0.1

0.12

0 20 40 60 80 100 120

Time, (sec)

E(t),sec

-1

Pre-wetted Bed

Non Pre-wett ed Bed

Simulation Results

1

1

1

Ex erimental Results2

2

2

Operating conditions: VL=2.06x10-3m/s VG=0.22m/s, D/dp =18, dp=6mm, Std. Dev.=5%, E1=180, E2=1.75


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MODELING OF HYDRO-PROCESSING REACTOR

Reaction and Kinetics (Chowdhury et al. 2002)

Hydro-desulfurisation (HDS)

SH,LAd

6.1S,L

56.0H,L

HDS

2

2

cK1

cckr

+=SHArH2SAr 22 ++

De-aromatisation of Mono-, Di- and Poly- Aromatics

NaphMono_MonoMonoMono CkCkr +=NapthenesH3ArMono 2 +

MonoDi_DiDiDi CkCkr +=ArMonoH2ArDi 2 +

DiPoly_PolyPolyPoly CkCkr +=ArDiHArTri 2 +


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SIMPLIFICATIONS/ ASSUMPTIONS

Pressure drop is insignificant compared to the operating

pressure

Trickle bed reactor is operated isothermally (efficient heat

transfer)

Ideal gas law is applicable

Liquid phase reactants are non-volatile (negligible vapor

pressure) Gas-liquid mass transfer is the limiting resistance.

The catalyst particles are completely wetted


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MODEL EQUATIONS

Mass Balance of Species i

Source Terms for Gas Phase

Source Terms for Liquid Phase

( ) ( ) kikkikmikkikkkkikkk

SCDCUt

C

,,,.,

,

+=+

= Lii

Gi

GLGLii CH

C

aKS

=

=+

=

nrj

1j

ijB rLii

GiGLGLii CH

C

aKS

=

=

=nrj

1j

ijB riS


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MODEL EQUATIONS

Mass Transfer Coefficient (From Goto & Smith, 1975)

Solubility of Hydrogen/ H2S (Korsten et al. 1996)

2/1

1

2

= L

iL

L

L

LL

i

L

L

i

DG

Dak

Li

NiH

.=

220

4

2

320

210H

1

.aTa

T

aTaa2 ++++=

).008470.03670.3exp(2

TSH =835783.0a

1094593.1a

1007539.3a

1042947.0a

559729.0a

4

63

32

3

1

0

=

=

==

=

: molar gas volumeN


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CASE STUDIES

Gas and Liquid Superficial Velocities Increase with

Scale Wetting gets Better with Scale

Parameters Laboratory Scale Reactor

(Chowdhury et al. 2002)

Commercial Scale Reactor

(Bhaskar et al. 2004)

Reactor Diameter, mBed Length

Particle Diameter, mBed Porosity

LHSV, h-1Operating Pressure, MpaOperating Temperature, K

Initial H2S Conc., v/v %

0.0190.5 m0.00240.50

1-520-28

573-6930.5-8

3.816 m

0.00150.36

1-520-80

573-6930.5-8


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KINETICS/ OIL COMPOSITION

From Chowdhury et al. (2002)

AdKKinetic Constants Values

, m3/kmol 50000

K, Dimensionless 2.5 X 1012 exp(-19384/T)

k*mono, m3/kg.s 6.04 X 102 exp(-12414/T)

k*Di, m

3

/kg.s 8.5 X 10

2

exp(-12140/T)k*poly, m3/kg.s 2.66 X 105 exp(-15170/T)

Component Percentage

Ar-S % 1.67

Poly-Ar % 2.59

Di-Ar % 8.77

Mono-Ar % 17.96

Naphthenes % 19.25


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SIMULATED RESULTS-1

Temperature Initial Concentration of H2S

0

25

50

75

100

573 583 593 603 613 623 633 643 653

Temp, K

Con

version,

Ar-S%

Varying Porosity

Uniform Porosity

Ar-S Exp

0

30

60

90

0 2 4 6 8

Initial H2S Conc Gas Phase ( %, V/V)

Conversion,x

(P=4 MPa, LHSV=2.0 h-1, QGNTP/QL=200 m3/m3, TR=320

oC, yH2S=1.4% )Symbols denote experimental data of Chowdhury et al. (2002)


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SIMULATED RESULTS-2

(P=4 MPa, LHSV=2.0 h-1, QGNTP/QL=200 m3/m3, yH2S=1.4%)

Symbols denote experimental data of Chowdhury et al. (2002)

0

10

20

30

40

50

60

70

80

90

573 593 613 633 653

Temperature, K

Conversion,

Ar-P

totaltotal

poly

0

10

20

30

40

50

60

573 593 613 633 653

Temperature, K

C

onversion,

%

Ar-D

Ar-M

Ar-Di-Expt

Ar-Mono-Exp


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SIMULATED RESULTS-3

0

10

20

30

40

50

60

2 2.4 2.8 3.2 3.6 4

LHSV, h-1

Conversion,

%

Total-Ar

Ar-Poly

Ar-DiAr-Mono

(P=4 MPa, TR=320 oC, QGNTP/QL=200 m3/m3, yH2S=1.4%, filled symbols

are for experimental data of Chowdhury et al., 2002)


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INFLUENCE OF REACTOR SCALE

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.2 0.4 0.6 0.8 1


Dim

ensionlessSolidHold-up

0

5

10

15

20

25

30

35

DimensionlessLocalAxialVelocity

Solid hold-up

Local Axial Velocity

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.2 0.4 0.6 0.8 1


DimensionlessSolidHold-up

0

1

2

3

4

5

6

7

DimensionlessLocalAxialVelocity

Solid hold-up

Local Axial Velocity

Industrial (LHSV=2)Laboratory (LHSV=3)

(P=4 MPa, TR=320oC, QGNTP/QL=300 m

3/m3)


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25

50

75

100

573 593 613 633 653

Temperature, K

Conversion,

%Lab-scale Reactor

Commercial Reactor

0

2000

4000

6000

8000

10000

12000

573 593 613 633 653

Temperature, K

Ou

tletConc.

Ar-S,ppm

Lab Scale Reactor

Commercial Reactor

(Plab & com=4 & 4.4 MPa, LHSVlab=2.0 h-1, QGNTP/QL=200 m

3/m3, yH2S=1.4%)


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-20

0

20

40

60

80

100

573 593 613 633 653

Temperature, K

Conversion,

%

Poly-Ar

TotalPoly-Ar-Commercial

Total-Commercial

50

55

60

65

70

75

80

85

90

95

100

2 2.5 3 3.5 4

LHSV, h-1

Conversion,

%

Ar-S-Lab Scale

Ar-S-Commercial

(Plab & com=4 & 4.4 MPa, LHSVlab & com=2.0 & 3.0 h-1,

(QGNTP/QL)lab & com= 200 & 300 m3/m3, yH2S=1.4%)

(Plab & com=4 & 4.4 MPa, Temperature= 593 K,(QGNTP/QL)lab & com= 200 & 300 m

3/m3, yH2S=1.4%)


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0

20

40

60

80

100

120

2 3 4 5 6 7 8

Pressure, MPa

Conversion,

Poly-Ar

Di-Ar

Mono-Ar

Total

0

10

20

30

40

50

60

70

80

90

100

2 2.5 3 3.5 4

LHSV, h-1

Conversion,

%

Poly-Ar

Di-Ar

Mono-Ar

Total

(QGNTP/QL)lab & com= 200 & 300 m3/m3 T=593 K

LHSVlab & com=2.0 & 3.0 h-1 ,yH2S=1.4%

Plab & com=4 & 4.4 Mpa, T=593 K yH2S=1.4%(QGNTP/QL)lab & com= 200 & 300 m

3/m3

Continuous lines: commercial; Dotted lines: laboratory scale


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CONCLUDING REMARKS

Macroscopic CFD Model Reasonably Simulates Gas-liquid Flow in Trickle Beds

Liquid hold-up

Residence time distribution

May be used to estimate fraction of suspended liquid

Further Work is Needed for Understanding Wetting/Hysteresis to Make Further Progress

Despite Limitations, CFD Models can be Used toUnderstand Key Issues in Reactor EngineeringIncluding Scale-up & Scale-down


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MODELING OF MESO-SCALE FLOWS

Single Phase Flow through Packed Bed

Unit cell approach

Simple cubic, FCC, rhombohedral ..

Inertial flow structures, pressure drop, heat transfer

Interaction of Liquid Drop/ Film with Solid Surface

Regimes of interaction

Dynamic contact angle/ surface characteristics

VOF simulations

Insight into capillary forces???


42/52


SINGLE PHASE FLOW

Single Phase Flow through Packed Bed

Periodic Flow

Wall

Wall

Periodic Flow

Wall

Operating Parameters Cell Length 28mm, 3mm Fluid Water Particle Reynolds Number 12-6000


43/52


SINGLE PHASE FLOW

Experiments:

Suekane et al.AIChE J., 49, 1

Simulations

0

5

0

4.5


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SINGLE PHASE FLOW

0

1

2

3

-1 -0.5 0 0.5 1

x/r

Vz/Vm

ean

Experimental

Simulation-28mm

Simulation-3mm

-1

0

1

2

3

4

-1 -0.5 0 0.5 1

x/r

Vz/Vm

ean

Experimental

Simulation-28mm

Simulation-3mm

-1

0

1

2

3

4

5

-1 -0.5 0 0.5 1x/r

Vz/Vm

ean

Experimental

Simulation-28mm

Simulation-3mm

-1

0

1

2

3

4

5

-1 -0.5 0 0.5 1x/r

Vz/V

mean

Experimental

Simulation-28mm

Simulation-3mm


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SINGLE PHASE FLOW

E

S

Vz=8.66mm

/s

Max arrow

length :

A=0.26 mm/s

B=0.29 mm/s

C=0.44 mm/s

A: z/r=0.14 (experimental) B: z/r=0.28 (experimental) C: z/r=0.42 (experimental)A: z/r=0.14 (experimental) B: z/r=0.28 (experimental) C: z/r=0.42 (experimental)A: z/r=0.14 (experimental)A: z/r=0.14 (experimental) B: z/r=0.28 (experimental)B: z/r=0.28 (experimental) C: z/r=0.42 (experimental)C: z/r=0.42 (experimental)


46/52


SINGLE PHASE FLOW

Inertial Flow Structures were Captured Accurately byCFD Models

Velocity Distribution in Unit Cells Resembles that in aPacked Bed

Ergun Parameters may be Obtained through Unit CellApproach for Semi-structured Packed Bed

Unit Cell Approach may be Extended to SimulateTwo Phase Flows to Understand Wetting


47/52


LIQUID SPREADING ON SOLID SURFACES

Computer

CCD Camera

Monitor

Light Diffuser

Light

Drop Flow Over Pellet

Drop Rest on Flat Plate

8.79mm

1mm

10.4mm


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DROP IMPACT ON SOLID SURFACE


49/52


DROP IMPACT ON SOLID SURFACE


50/52


FLAT SURFACE

(f)

(c)

(d)

(e)

(b)

(a)

t=0ms

t=16ms

t=20ms

t=36ms

t=48ms

t=240ms

t=0ms

t=10ms

t=210mst=25ms

t=45ms

t=45ms


51/52


DYNAMICS OF DROP IMPACT

MODELS FOR INTERPHASE COUPLING?


52/52


MODELS FOR INTERPHASE COUPLING?

Wall Shear Stress onSolid Surface, red~100 Pa

Drop Shape at 12 ms

ShearStrain onGas-liquidInterface

Documents

Cfd Modelling of TBR 2