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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