See T&K Chapter 9
“Unsteady” MassTransfer Models
ChEn 6603
Wednesday, April 4, 12
1
Outline
Context for the discussion
Solution for transient binary diffusion with constant ct, Nt.
Solution for multicomponent diffusion with ct, Nt.
Film theory revisited (surface renewal models)
Transient diffusion in droplets, bubbles
Wednesday, April 4, 12
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Context
Film theory - so far we assumed steady state, no
reaction in bulk (only potentially at interface)
• Mass Transfer Coefficients used to simplify problem - don’t fully
resolve diffusive fluxes.
• Bootstrap problem solved (via definition of [β]) to obtain total
species fluxes.
Unsteady cases?
• What if we want to know the transient concentration profiles?
• What if we want to consider the effects of transient (perhaps
turbulent) mixing near the interface?
Here we consider unsteady film-theory approaches...
Hung Le and Parviz Moin.
http://www.stanford.edu/group/ctr/gallery/003_2.html
Wednesday, April 4, 12
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Solution Options
ci
=
t
⇤ · Ni + r i
Describes evolution of ci at all points in
space/time, but requires Ni, which may involve
solution of the momentum equations...
Solve the problem numerically
• Allows us to incorporate “full” description of the physics
‣ may be quite complex (particularly if we must solve for a non-trivial velocity profile...)
• Could also simplify portions (constant [D], ct, etc.)
• Can solve this for a variety of BCs, ICs
Make enough assumptions/simplifications to solve this analytically
• Different BC/IC may require different form for analytic solution
• We already did this for effective binary and linearized theory for a few simple
problems (2-bulb problem, Loschmidt tube)
‣ T&K chapters 5 & 6.
• Here we show a few more techniques, based on unsteady film theory
‣ Don’t resolve mass transfer completely - get a coarser description of the diffusive fluxes...
(J)
(N )
Wednesday, April 4, 12
=
=
ct [k • ]( x)
[ ](J)
Approximation for
diffusive flux (total flux).
4
See T&K 9.2
Binary Formulation (1/3)
ci
=
t
⇤ · Ni + r i
What are the assumptions?
ct
xi
=
t
⇤ · Ni
What happened here?
ct
xi
+ Nt · ⇤xi =
t
⇤ · Ji
One-dimensional...
ct
xi
xi
+ Nt
=
t
z
z
Ji
2
x1
N t x1
x1
+
=D 2
t
ct z
z
Wednesday, April 4, 12
Problem statement:
semi-infinite diffusion
(binary)
z ≥ 0,
t =0,
z = 0,
t >0,
z → ∞, t >0,
BCs & ICs
xi=xi∞.
(Initial condition)
xi=xi0. (Boundary condition)
xi=xi∞. (Boundary condition)
valid for “short”
contact times
(more later)
5
Binary Formulation (2/3)
2
x1
N t x1
x1
+
=D 2
t
ct z
z
ct
✓
◆
✓ ◆
dx
dx
+ Nt
2t d
z d
✓
◆
Nt z dx
2 +
ct
d
d2 x
dx
D 2 + 2(
⇥)
d
d
Nt p
⌘
t
ct
x1 = x1,0
=0
x1 = x1,1
=1
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Observation: since x is dimensionless, z, t, D must appear
in a dimensionless combination in the solution.
z
=p
4t
chosen for convenience, ⇥
d ⇥
=
=
ζ2/D is dimensionless
⇥t
d ⇥t
1 d
2td
⇥
d ⇥
d
=
=
⇥z
d ⇥z
zd
✓ ◆2
2
2
2
⇥
⇥
d
d
= 2
= 2
⇥z 2
d
⇥z
z d
2
d2 x
= ct Di 2 2
z d
d2 x
= D 2
d
= 0
Solve using
order
reduction
1
erf
⇣
p ⇥
D
⌘
x1 x1,0
⇣
⌘
=
x1,1 x1,0
1 + erf p⇥D
Note that this
is a function of
both z and t.
6
Binary Formulation (3/3)
⇣
p ⇥
D
⌘
1 erf
x1 x1,0
⇣
⌘
=
x1,1 x1,0
1 + erf p⇥D
Calculate J1
at z=0,
J1,0 = ct
What happens to J1 as z→∞?
r
⇣
2
⌘
D exp D
⇣
⌘ (x1,0
t 1 + erf p
x1,1 )
D
Mass Transfer Coefficients (binary system):
Low-flux limit (as Nt→0)
r
D
J1,0 = ct
(x1,0
t
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x1,1 )
J1,0
=
ct k (x1,0
N1,0
=
ct
•
k
(x1,0
0
k=
r
D
t
x1, ) = ct k • (x1,0
exp
=
⇣
1 + erf
2
D
⇣
p
⌘
D
⌘
x1, )
x1, )
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See T&K 9.3.1-9.3.2
Multicomponent System
(x) Nt (x)
+
= [D]
t
ct z
2
(x)
z2
Nt p
⌘
t
ct
z
=p
4t
This has the analytic solution (see T&K 9.3.1-9.3.2):
ii h
h
ii
h
h
1
1
[I] + erf ⇥ [D] 2
(x x1 ) = [I] erf (
⇥) [D] 2
h
ih
h
ii 1
1
1
1
ct
(J0 ) = p [D] 2 exp [D] 2 [I] + erf [D] 2
(x0
⇡t
Low-flux limit (as Nt→0)
1
ct
J0 = p [D] 2 (x0
t
[k] = ( t)
1
2
[D]
1
2
(J0 ) = ct [k][ ](x0 x⇥ ) = ct [k • ](x0
(N0 ) = ct [ 0 ][k • ](x0 x⇥ )
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x1 )
[ ] = exp
x⇥ )
h
1
[D]
(x0
x1 )
1
2
x1 )
remember that these
are matrix functions!
ih
[I] + erf[ [D]
Possible approaches:
1
2
i
1
• Solve the transient problem (beware of
the “short” time assumption)
• Use this information to formulate other
steady-state models (e.g. turbulent
mixing from bulk to surface)
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See T&K §9.1
Surface Renewal Models
(J0 ) = ct [k][ ](x0 x⇥ ) = ct [k • ](x0
(N0 ) = ct [ 0 ][k • ](x0 x⇥ )
x⇥ )
[k] = ( t)
1
2
[D]
1
2
Idea: develop a model for k that
approximates the effects of
transients near the surface.
(J), (N) are functions of time
since [k] is a function of time.
How would we handle this problem?
Concept:
“Fresh” fluid from bulk is transported to the interface, where diffusion occurs for some
time, t. Then this is transported back to the bulk and replaced by more “fresh” fluid.
Initial &
Boundary
conditions:
z = 0,
z 0,
z ⇥ ⇤,
xi = xi0
xi = xi
xi = xi
t>0
t=0
t>0
Assumes that the “bulk” is unaffected by mass
transfer (“short” contact times)
Age distribution function, ψ(t), determines how long a fluid
parcel is at the interface. (will affect expression for [k])
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See T&K §9.1
Surface-Renewal Models
Age distribution function, ψ(t), determines how long a fluid
parcel is at the interface. (will affect expression for [k])
kij (t) =
Higbie model
(1935)
(t) =
Danckwerts
model (1951)
Dij
t
kij =
kij (t) (t)dt
0
Attempts to model a statistically
stationary process (fast mixing, no
saturation to bulk) by a steady state [k].
Assumes that all fluid parcels stay at the
interface for a fixed amount of time, te.
1/te
0
t te
t > te
→
[k] =
2
1/2
[D]
te
Note typo in T&K 9.3.33 (t vs. te)
Fluid parcels have a greater chance of being
replaced the longer they are at the interface.
(t) = s exp( st)
→
[k] =
s[D]1/2
s - rate of surface renewal (1/sec)
(fraction of surface area replaced
by fresh fluid in unit time)
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10
See T&K §9.4
Bubbles, Drops, Jets
δ
δ
δ
Spheres
Channels
[Fo] =
Cylinders
[D]t
2
For “small” Fo (Fo«1), we are safe to use surface renewal
concepts. What happens at “large” Fo (Fo→1)?
xi = xiI
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⇥xi ⇤ 〈xi〉 - “mixing cup” average
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See T&K §9.4
Fractional Approach to EQ
(J0 ) = ct [k][ ](x0 x⇥ ) = ct [k • ](x0
(N0 ) = ct [ 0 ][k • ](x0 x⇥ )
⇥
F
(x0
(x10
(x10
⌅x1 ⇧)
x1I )
⇤
⌅x⇧) = [F ](x0
xI )
x∞ is changing!
x⇥ )
(this solution is not valid)
For a spherical droplet/particle:
Binary
[F ] =
⇤
⇥
⇧
6
[I]
2
exp
1
m2
x0 - initial/boundary composition (t=0)
xI - interface composition (constant in time)
〈x〉 - average composition (changing in time), use in place of x∞.
m
m=1
1
[D ] =
[D]
Dref
Multicomponent
2 2
Foref
⇥
6
note:
2
m
2
⇥
Foref [D ]
⌅
Dref rt2
0
=1
m=1
[Sh] =
2
3
2
⇤
⇤
⇧
Sh ⇥ [k] · 2r0 [D]
exp
m=1
Limiting cases:
m
2 2
⇥
Foref [D ]
⇤
⇧
Sherwood number related to ∂F/∂Fo.
1
m2
exp
m
2 2
m=1
Foref ⇥ ⌅ =⇤ Sh =
Foref
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⇥
⌅⇤
1
2
3
2
⇥
Foref [D ]
[I] ⇤ [k] =
⇥
2
3r0
⌅
[D]
1
remember that these
are matrix functions!
See fig. 9.7 (L’Hopital’s rule)
2
1 =⇤ [k] = ⇧ [D]1/2
t
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