CFD Simulation of Flashing and Boiling Flows Using FLUENT

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Loraine O’Brien’
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1 CFD Simulation of Flashing and Boiling Flows Using FLUENT Hua Bai and Paul Gillis The Dow Chemical Company FLUENT UGM 2004

2 Liquid/Gas Phase Change found in many industrial chemical processes involves complex physics represents challenges for CFD simulations multi-phase flow turbulence flow heat transfer evaporation and condensation mixing chemical reactions

3 Flashing versus Boiling Flashing Rapid and sudden evaporation Can be caused by sudden pressure drop Boiling Moderate or steady Liquid continuous phase Physically, same process/phenomena Phase change from liquid to gas Numerically, different modeling approaches

4 Flashing Reactive Flow A B A Flash due to reactions Liquid streams A and B mix and react Exothermic reactions raises temperature Some reactants/products start to flash Flashing reduces density, causing local high velocities, which lowers pressure resulting in additional flashing

5 Boiling models in FLUENT FLUENT 4.5 built-in Evaporation- Condensation model simple phenomenological model evaporation rate m LG condensation rate m GL = r ε ρ G L = r ε ρ L G G L T T L sat T T T sat T sat sat G if if T T T L G T sat sat

6 Model limitations Phase change rate depends on T sat only T sat can only be specified as a constant Independent on local pressure Industrial problems require both Ability to handle complex mixtures (10+ species) Pressure induced phase change (VLE = f(t,p,x i ) Unable to simulate flash

7 Flashing/Boiling CFD Modeling Efforts in Dow Started 1990 with applications for evaporative crystallization Initial vigorous approach based on Eulerian multiphase model Subsequent simplified approach based on mixture/single phase model Successfully used in a few industrial applications

8 Boiling model development Based on Eulerian multiphase model Continuous liquid and dispersed bubbles Thermodynamic VLE (Vapor Liquid Equilibrium) model to calculate mass transfer rate for each species Calculate mole fraction in each phase for each species, from the local 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 equations Source term for momentum equations Shear-dependent bubble sizes (Jameson 1993) Implemented in FLUENT4 via UDS (2000)

9 Boiling model test run --- Water boiling in a container P=1 atm Test #1 Adiabatic wall Simplified interphase mass transfer rate calculation Based on T sat = 100 C Compared to simulation with FLUENT 4.5 built-in Evaporation-Condensation model Validated UDS implementation T t=0 =99 C T=300 C

10 Water boiling in a container (Test #1) T sat =100 C Vapor vol. fraction t=0 t=2.5s t=5s t=7.5s t=10s Match well with 4.5 built-in Evaporation-Condensation model

11 Water boiling in a container (Test #1) 300 C 107 C t=0 t=10s Barely-changed water temperature after boiling validates implementation of latent heat effect T t=0 =99 C 99 C Temperature (K) T=300 C T=300 C 99 C Temperature (K)

12 Boiling model test run #2 --- Water boiling in a container P<1 atm Test #2 VLE flash model for interphase mass transfer rate calculation Mass transfer rate is function of local temperature, pressure and concentration Adiabatic wall T t=0 =99 C T sat =VLE T=300 C

13 Water boiling in a container (Test #2) T sat =VLE Water velocity contours m/s Water volume fraction t=0 0.25s 0.5s 1.0s 1.5s 2.0s 2.5s 3.0s 3.5s 4.0s 4.5s 5.0s

14 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

15 Boiling Model Limitations Hard to converge for Rapid boiling Multiple species Reactions Sensitive to grid quality Single-block structured grid required (FLUENT4) Those limitations make it difficult for industrial applications, especially for flash

16 Flashing model development Based on single phase reactive flow model Exothermic reactions Thermodynamic 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

17 VLE Flashing Calculation Vapor pressures calculated with Antoine expression log P s i = A i Bi C + T i VLE determined Raoult s law (Poynting correction negligible) K i = s Pi P Fraction vaporized within the multi-component mixture calculated using the Rachford-Rice procedure C zi (1 Ki { } ) Iteration for the value of ψ f ψ = i= 11+ ψ ( K 1) i = 0 Individual component liquid mole fractions (x i ) and vapor mole fractions (y i ) x i zi = 1+ ψ ( K 1) i y = x K i i i

18 Flash Simulation Example Co-current flow mixing and reaction Reactions A+B=C+D A+C=U+E. 12 species, 15 reactions concentration contours of species A A B A

$fraction Flashing depicted by$ $increased vapor volume fraction$ Temperature increase caused by

19 Flash Simulation Vapor Vol. fraction Flashing depicted by increased vapor volume fraction and decreased density Density Temperature increase caused by heat formation of reactions Temperature

20 Flash model validation Pressure drop comparison Measured pressure drop across the tube: 50psi Model predicts less than 1 psi pressure drop if flash model is turned off Model predicts 47 psi pressure drop if flash model is turned on Outlet temperature comparison Model predicts slightly higher temperature than measurement Temperature prediction is affected by kinetics model

21 Summary Flashing and boiling model have been developed in Dow and implemented in FLUENT via UDS/UDF Flashing 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 model Species flow and reaction in Eulerian multiphase model

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