absorption of gas by a falling liquid film · 2012. 6. 26. · 30.11.2011 | tokyo | mathematical...
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30.11.2011 | Tokyo | Mathematical Modeling and Analysis | Christoph Albert |
Absorption of gas by a falling liquid film
Christoph Albert Dieter Bothe
Mathematical Modeling and Analysis
Center of Smart Interfaces/
IRTG 1529
Darmstadt University of Technology
4th Japanese-German International Workshop
on Mathematical Fluid Dynamics , 30.11.2011
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Wavy Falling Films
● Advantages
● Good heat transfer due to small thickness
● Large Interface
● Applications
● Evaporation
● Cooling
● Absorption
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Hydrodynamic Model
Assumptions
● Incompressible, Newtonian two-phase flow
● No phase transition
● Constant surface tension
∂t (ρu)+∇⋅(ρu⊗u)+∇ p=∇⋅S+ρ g , Ω∖Σ
∇⋅u=0, Ω∖Σ⟦p I−S⟧⋅nΣ=σ κnΣ , Σ⟦u⟧=0, Σ
S=η(∇ u+(∇ u)T)
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The Volume of Fluid Method
One Fluid Formulation:
● Density and viscosity depend on volume fraction
● Volume fraction has to be transported
● Additional surface tension force term f Σ
∂t (ρu)+∇⋅(ρu⊗u)+∇ p=∇⋅S+ρ g+δ f Σ
∇⋅u=0∂t f +u⋅∇ f =0
0 0 0 0 0
0 0
0 0
0 01 1
1 1
0.08
0.53
0.52
0.95
0.87
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Parasitic Currents
Parasitic Currents in a VOF simulation of a static droplet
● VOF simulations suffer from unphysical oscillation of velocity, so-called parasitic currents
● Stem from numerical treatment of interfacial jump condition for stress:
● Especially serious in stagnant flow situations
● Also problematic in simulations of falling films, which are convection dominated
⟦ p I−S⟧⋅nΣ=σ κnΣ
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Numerical Setup
u (t , y)∣x=0=(1+ϵsin (2πω t )) ( ρlμL
gδ02[yδ0
−12
(yδ0
)2
] ,0 )
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Continuum Surface Stress (CSS)
Body Force ,
n⃗Σ● Approximate by differentiating a smoothed f-field
● Momentum conservative
● Standard Surface Tension model in FS3D; delivers good results in many two-phase flow situations.
∇⋅(∥∇ f∥σ(I−n⃗Σ⊗n⃗Σ))
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Falling Films with CSS
Water/Air, Re 60
● 16 cells per mean film thickness (0.265mm)
● A = 30%, f = 20 Hz
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Continuum Surface Force (CSF)
Body Force
Renardy, J. Comput. Phys. 183 (2002)
● In the case of a sphere and constant curvature, an exact balance between surface tension and pressure can be achieved: „Balanced Force“
● Greatly reduces parasitic currents
● Relies on „good“ curvature information
● Easy for a falling film:
σ κ∇ f
i−1 i i1
hi
κ=
hxx
(1+hx2)
32
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Falling Films with CSF
Water/Air, Re 60
● 16 cells per mean film thickness (0.265mm)
● A = 30%, f = 20 Hz
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Comparison to experiment: Dietze
● Experimental data from
● DMSO/Air
●
● Re = 8.6
● f = 16 Hz
● A = 40%
● Resolution
=2.85⋅10−6m2
s,=1098.3
kgm3 ,=0.0484
Nm
16
G.F. Dietze, Flow Separation in Falling Liquid Films, 2010
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Comparison to experiment: Film thickness
● Film thickness at x = 56 mm over a time span of 0.3 s● CSS overshoots
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Species Transport in Falling Films
Data from Yoshimura et.al., 1996
Transport of Oxygen into a water film at 18°C
Sc=νL
D=570
Sh=kLδ0
D
k L=ΓL
ln (CS−C in
CS−Cout
)
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Species Transport in Falling Films: Model
Assumptions:
● Dilute system => species bears no mass or momentum, and does not affect viscosity● No adsorption at the interface => surface tension stays constant● No chemical reaction ● Local thermodynamic equilibrium at the interface => Henry's law holds● Constant Henry coefficient
∂t c+u⋅∇ c=DΔ c , ΩG(t )∪Ω
L(t)
⟦−D ∇ c⟧⋅nΣ=0, ΣcL=H cG , Σ
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Numerical Approach
Two scalar approach with one-sided concentration gradient transfer flux
● Concentration is advected in the same way as volume fraction● Two concentrations stored in each interfacial cell● Both concentrations and Henry's law yield interfacial concentration● Concentration gradient in liquid phase computed between interface and some value in bulk by subgrid model● Diffuse flux from gas to liquid according to Fick's Law
ϕG=c χΩG
ϕL=cχΩL
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Calculations
Water film at 18°C in „Oxygen“ atmosphere
Resolution
Re=31Sc=50kH ,cc=0.0270cG=4.19e-5cL ,0=2.37e-7D=2.14e-4
δ0
16
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Validation: Method
Solve stationary advection diffusion problem
on domain
with boundary conditions.
Solved with Matlab ODE Solver, by defining x as pseudotime.
u( y )∂x c=D∂y2 c
[0, xmax ]×[0,δ0]
c∣x=0=cL ,0
∂ y c∣y=0=0
∂x c∣x=10cm=0
∂ y c∣y=δ0=kH ,cc cG
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Validation: Result
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0 Hz
Planar InterfacePure Diffusion
0mm
0.46mm
5.90cm 8.53cm
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20 Hz
0mm
0.53mm
5.25cm 9.19cm
● Time-periodic wave structures appear● Large Wave humps, preceeded by several smaller capillary waves
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20 Hz
0mm
0.53mm
6.33cm 6.70cm
● Filaments of high concentration in the large wave humps● Develop along the streamlines of the large vortex● Touch the interface at a hyperbolic point
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20Hz
● Highly non-monotonous concentration profiles
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20 Hz
● Strong contribution from the capillary wave region
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20 Hz
● Flow up the wall in the reference frame of the wall
0mm
0.53mm
6.59cm 6.77cm
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20 Hz
0mm
0.33mm
6.55cm6.79cm
Pressure in film according to Young-Laplace:
Increase in pressure large enough to drive water up the wall
Compare
Δ p∼σ κ
Dietze et al., J. Fluid Mech., 637, 2009
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30 Hz
● Less capillary waves● Wave length, peak height, and wave velocity decrease
0mm
0.53mm
7.22cm 9.84cm
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40 Hz
● No capillary waves; waves become sinusoidal● Wave length, peak height, and wave velocity decrease further
0mm
0.53mm
7.39cm 9.65cm
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40 Hz
● At this Reynolds Number, there exists backflow even when Capillary Waves are absent
0mm
0.39mm
7.91cm 8.03cm
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Re 15 / 15Hz
● At this Reynolds Number, concentration profiles are monotonous
0mm
0.43mm
5.84cm 7.87cm
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Re 15 / 15Hz
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Re 15 / 15Hz
6.98cm 7.12cm
0mm
0.43mm
● No Vortex relative to wave velocity
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Thank you for your attention!