Hua Bai Dow Chemical

CFD Simulation of Flashing and Boiling Flows

Using FLUENT

Hua Bai and Paul GillisThe Dow Chemical Company

FLUENT UGM 2004

Liquid/Gas Phase Changefound in many industrial chemical processes involves complex physicsrepresents challenges for CFD simulations

multi-phase flowturbulence flowheat transfer evaporation and condensation mixingchemical reactions

Flashing versus BoilingFlashing

Rapid and sudden evaporationCan be caused by sudden pressure drop

BoilingModerate or steadyLiquid continuous phase

Physically, same process/phenomenaPhase change from liquid to gas

Numerically, different modeling approaches

Flashing Reactive FlowA

BA

Flash due to reactionsLiquid streams A and B mix and reactExothermic reactions raises temperatureSome reactants/products start to flashFlashing reduces density, causing local high velocities, which lowers pressure resulting in additional flashing

Boiling models in FLUENT

FLUENT 4.5 built-in Evaporation-Condensation model

simple phenomenological model evaporation rate

condensation rate sat

satLLLGLG T

TTrm

−= ρε satL TT ≥if

sat

GsatGGLGL T

TTrm

−= ρε satG TT ≤if

Model limitations

Phase change rate depends on Tsat onlyTsat can only be specified as a constant Independent on local pressure

Industrial problems require bothAbility to handle complex mixtures (10+ species)Pressure induced phase change (VLE = f(T,P,xi)

Unable to simulate flash

Flashing/Boiling CFD Modeling Efforts in Dow

Started 1990 with applications for evaporative crystallizationInitial vigorous approach based on Eulerian multiphase modelSubsequent simplified approach based on mixture/single phase modelSuccessfully used in a few industrial applications

Boiling model developmentBased on Eulerian multiphase model

Continuous liquid and dispersed bubblesThermodynamic VLE (Vapor Liquid Equilibrium) model to calculate mass transfer rate for each species

Calculate mole fraction in each phase for each species, from thelocal temperature, pressure, and concentrations

Source term for continuity equations (interphase mass transfer) Source terms for enthalpy equation to account for latent heat effect Source term for species equationsSource term for momentum equationsShear-dependent bubble sizes (Jameson 1993)Implemented in FLUENT4 via UDS (2000)

Test #1Simplified interphase mass transfer rate calculation

Based on Tsat = 100°C

Compared to simulation with FLUENT 4.5 built-in Evaporation-Condensation model Validated UDS implementation

Boiling model test run --- Water boiling in a container

P=1 atm

T t=0=99°C

Adiabatic wall

T=300°C

T sat=100°CWater boiling in a container (Test #1)

t=0 t=2.5s t=5s t=7.5s t=10sVapor vol. fraction Match well with 4.5 built-in Evaporation-Condensation model

Water boiling in a container (Test #1)

t=10s

T=300°C

T t=0=99°C

t=0300°C 107°C

Barely-changed water temperature after boilingvalidates implementationof latent heat effect

99°C99°C

Temperature (K) Temperature (K)T=300°C

Test #2VLE flash model for interphase mass transfer rate calculation Mass transfer rate is function of local temperature, pressure and concentration

Boiling model test run #2 --- Water boiling in a container

P<1 atm

Tt=0=99°C

Adiabatic wall

T sat=VLE

T=300°C

Wat

er v

olum

e fr

actio

n 0

--1

Wat

er v

eloc

ity c

onto

urs

0--

0.32

m/s

Water boiling in a container (Test #2) T sat=VLE

3.5s 4.0s 4.5s 5.0s2.5s 3.0st=0 1.5s 2.0s0.25s 0.5s 1.0s

Water boiling in a container (Test #2)velocity vectors

m/s 0.25s 0.5s 1.0s 1.5s 2.0s 2.5s 3.0s 3.5s 4.5s

Boiling Model Limitations

Hard to converge for Rapid boilingMultiple speciesReactions

Sensitive to grid qualitySingle-block structured grid required (FLUENT4)

Those limitations make it difficult for industrial applications, especially for flash

Flashing model developmentBased on single phase reactive flow model

Exothermic reactionsThermodynamic VLE model to calculate mole/mass fractions in liquid phase and vapor phase for each species,

from the local temperature, pressure, and concentrations iso-thermal flashing calculation

Mixture density is then obtained from the flashing calculation, implemented as UDS for density Source terms for enthalpy equation to account for latent heat effect Optional UDF also developed to modify reaction rates

To fit cases such as vapor phase species do not react.Initially implemented in FLUENT4 via UDS, later migrated to FLUENT5/6 via UDF

VLE Flashing Calculation

TCBAPi

ii

si +

−=logVapor pressures calculated with Antoine expression

PPKsi=VLE determined Raoult’s law (Poynting correction negligible) i

Fraction vaporized within the multi-component mixture calculated using the Rachford-Rice procedure

{ } ) 0)1(1

1(1

=−+

−= ∑

=

C

i i

ii

KKzf

ψψIteration for the value of ψ

Individual component liquid mole fractions (xi) and vapor mole fractions (yi)

)1(1 −+=

i

ii K

zxψ iii Kxy =

Flash Simulation ExampleCo-current flow mixing and reaction

ReactionsA+B=C+DA+C=U+E….

12 species, 15 reactions

B

A

A

concentration contours of species A

Flash Simulation

Flashing depicted by increased vapor volume fraction and decreased density

Vapor Vol. fraction

Density

Temperature increase caused by heat formation of reactions

Temperature

Flash model validationPressure drop comparison

Measured pressure drop across the tube: 50psiModel predicts less than 1 psi pressure drop if flash model is turned offModel predicts 47 psi pressure drop if flash model is turned on

Outlet temperature comparisonModel predicts slightly higher temperature than measurementTemperature prediction is affected by kinetics model

Summary

Flashing and boiling model have been developed in Dow and implemented in FLUENT via UDS/UDFFlashing model has been used in a few industrial applications.With the new features and more robust solver available in FLUENT6.2, it appears worthwhile to migrate the boiling model from UDS for FLEUNT4 to UDF for FLUENT6.2

New features needed for boiling model: Mass transfer capability in Eulerian multiphase modelSpecies flow and reaction in Eulerian multiphase model