CFD Lecture (8/8): CFD in Chemical Systems

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CFD in Chemical Systems
CFD Simulation for examples of flows in chemical,
process and oil/gas industry

2
©ZeusNumerixPvtLtd:ConfidentialDocument
Definitions
 Multiphase flow is simultaneous flow of:
 Materials with different states or phases (i.e. gas, liquid or solid)
 Materials with different chemical properties but in the same state or phase (i.e. liquid-liquid
systems such as oil droplets in water)
 The primary and secondary phases:
 One of the phases is continuous (primary) while the other(s) dispersed within the continuous
phase
 A diameter has to be assigned for each secondary phase to calculate its interaction (drag)
with the primary phase
 A secondary phase with a particle size distribution is modeled by assigning a separate phase
for each particle diameter
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Definitions
 Dilute versus dense phase:
 Refers to the volume fraction of secondary phase(s)
 Volume fraction of a phase = (Volume of the phase in a cell/Volume of the cell)
 Laminar versus turbulent:
 Each phase can be laminar or turbulent
 Fluid flow (primary phase) may be turbulent with respect to the secondary phase but may be
laminar with respect to the vessel
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Why Model Multiphase Flow?
 Multiphase flow is important in
many industrial processes:
 Riser reactors
 Bubble column reactors
 Fluidized bed reactors
 Scrubbers, dryers, etc
 Typical objectives of a modeling
analysis:
 Maximize the contact between the
different phases, typically different
chemical compounds
 Flow dynamics
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Better Mixing
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Multiphase Flow Regimes
 Bubbly flow: discrete gaseous bubbles in a
continuous liquid
 Droplet flow: discrete fluid droplets in a
continuous gas
 Particle-laden flow: discrete solid particles
in a continuous fluid
 Slug flow: large bubbles in a continuous
liquid
 Annular flow: continuous liquid along walls,
gas in core
 Stratified and free-surface flow: immiscible
fluids separated by a clearly-defined
interface. 5
Bubbly flow
droplet flow
particle-
laden flow
Free-surface
flow
Slug flow
Annular flow
Gas ->
interface
^|
Gas
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6
Category Description Application
Bubbly Flow Discrete gas in
continuous fluid
Evaporators,
Aeration,
Cavitation
Droplet Flow Discrete fluid in
continuous gas
Atomizer,
Combustor, Dryer
Slug Flow Large Bubbles in
continuous fluid
Pipe flow
Stratified Flow Immiscible fluids
across an
interface
Sloshing,
Condensation
Particle-laden
Flow
Discrete solid
particle in gas
Cyclone
Separator,
Environmental
Flow
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Flow Regimes: Vertical Gas-Liquid Flow
Evaporator
Liquid flux (0.1 to 100 fps)
airflux(1to1000fps)
Superficial velocity Vsg = Q/A,
liquidbubblyBubblyslugslugannulardrop-annulardrops
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 User must know a priori the characteristics of the flow
 Flow regime, e.g. bubbly flow, slug flow, annular flow, etc
 Only model one flow regime at a time
 Predicting the transition from one regime to another possible only if the flow
regimes can be predicted by the same model. This is not always the case.
 Laminar or turbulent
 Dilute or dense
 Secondary phase diameter for drag considerations
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Modeling Approach
 Lagrangian
 Track individual point particles
 Particles do not interact
 Algebraic slip model
 Dispersed phase in a continuous phase.
 Solve one momentum equation for the mixture
 Two-fluids theory (multi-fluids)
 Eulerian models
 Solve as many momentum equations as there are phases
 Discrete element method
 Solve the trajectories of individual objects and their collisions, inside a continuous phase
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Coupling Between Phases
 One-way coupling:
 Fluid phase influences particulate phase via aerodynamic drag and turbulence transfer
 No influence of particulate phase on the gas phase
 Two-way coupling:
 Fluid phase influences particulate phase via aerodynamic drag and turbulence transfer
 Particulate phase reduces mean momentum and turbulent kinetic energy in fluid phase
 Four-way coupling:
 Includes all two-way coupling
 Particle-particle collisions create particle pressure and viscous stresses
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 What is the goal of the simulation?
 Which effects are important?
 Controlled by which hydrodynamic effects?
 Controlled by which other transport phenomena effects?
 All these factors influence which model to choose for the analysis.
Modeling Multiphase Flows
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Flow Regime
Bubbly / droplet / particle-laden / slug annular /
stratified/free surface / rapid granular flow
Model Specific
Lagrangian Dispersed Phase / Algebraic Slip / Eulerian /
Eulerian Granular / Volume of Fluid
Process Specific
Separation / Filtration / Suspension / evaporation /
Reaction
Simulation
Model
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 Hydrodynamics:
 Change in shape
 Diameter
 Particle-wall collision
 Particle-particle collision
 Coalescence
 Dispersion and breakup
 Turbulence
 Inversion
 Other transport
phenomena:
 Heat transfer
 Mass transfer
 Change in composition
 Heterogeneous reactions
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Physical Effects in Dispersed Systems
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Hydrodynamics: Particle Relaxation Time
 When does the particle follow the flow?
 Time when particle reaches about 60% of the initial difference velocity
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Dg
pp
p
f
d



18
2

24/Re
101
/1000
1.0
5
3
DpD
g
p
p
Cf
sPa
mkg
mmd







sp
2
105.5 

1-Mar-2009

14
14
Hydrodynamics: Particle Relaxation Time
Typical relaxation times in process applications
Dp p g p (sec)
Coal
combustion
0.001 to 0.5
microns
600 to 2000 2 x 10-5 Pa s 1.0x10-6 to
1.4 s
Bubble columns 1 to 5 mm 1 1 x 10-3 Pa s 5.5x10-5 to
1.4x 10-3 s
Sand air 0.1 to 5 mm 2000 2 x 10-5 Pa s 5.5x10-2 to
140 s
Sand water 0.1 to 5 mm 2000 1x10-3 Pa s 1.0 x10-3 to
2.7 s
In general : p = dp
2p/(18g)
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Multiphase formulation
Two phases vs. three phases
Fluid
Solids
Solids - 1
Solids - 2
Fluid
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Model Overview
 Eulerian/Lagrangian dispersed phase model
 All particle relaxation times
 Particle-wall interaction always taken into account, particle-particle usually not
 Algebraic Slip Mixture Model (ASM)
 Particle relaxation times < 0.001 - 0.01 s.
 Neither particle-wall interaction nor particle-particle are taken into account
 Eulerian-Eulerian model (EEM)
 All particle relaxation times.
 Particle-wall interaction taken into account, particle-particle usually not
 Eulerian-granular model (EGM)
 All particle relaxation times.
 Both particle-wall and particle-particle interaction are taken into account. 16
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Algebraic Slip Model (ASM)
 Solves one set of momentum equations for the mass averaged velocity and tracks
volume fraction of each fluid throughout domain
 Assumes an empirically derived relation for the relative velocity of the phases
 For turbulent flows, single set of turbulence transport equations solved
 This approach works well for flow fields where both phases generally flow in the
same direction
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ASM Equations
 Solves one equation for continuity of the mixture:
 Solves for the transport of volume fraction of one phase:
 Solves one equation for the momentum of the mixture:
18
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1-Mar-2009

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ASM Equations (2)
 Average density:
 Mass weighted average velocity:
 Velocity and density of each phase:
 Drift velocity:
 Effective viscosity:
19
2211  m
2211
222111
αραρ
uαρuαρ
um




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21,21 ,, uu
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r
uuu

 11
2211  eff
1-Mar-2009

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Slip Velocity and Drag
 Uses an empirical correlation to calculate the slip velocity between phases
 fdrag is the drag function
20
prel auu 


))((
t
u
uuga m
mm





dragf
ppm
p
f
d



18
)( 2








1000Reif
1000Reif
Re0175.0
Re15.01 687.0
dragf
1-Mar-2009

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Closure
 CFD modeling is possible only after knowing flow morphology, e.g. slug flow, annular
flow, etc.
 Simplifications & additional assumptions are required due to unknown detailed &
local physics, e.g. transport of interfacial area, flow regime transition, etc.
 Many simplifications for reducing computational effort are possible. Only ASM was
explained
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1-Mar-2009

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www.zeusnumerix.com
+91 72760 31511
Abhishek Jain
abhishek@zeusnumerix.com
Thank You !

CFD Lecture (8/8): CFD in Chemical Systems

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CFD Lecture (8/8): CFD in Chemical Systems