Evaporation/boiling Phenomena on
Thin Capillary Wick
Chen Li
G. P. Peterson
Rensselaer Polytechnic Institute
Department of Mechanical, Aerospace & Nuclear Engineering, Troy, NY 12180
Yaxiong Wang
Foxconn Thermal Technology Inc., Austin, TX 78758
How good is the performance of the
evaporation/boiling on the thin capillary wick?
1000000
250000
Heat transfer coeffcient [W/m2K]
100000
100000
50000
10000
15000
10000
5000
3000
1000
200
100
100
50
25
10
15
10
3
1
Free conv(air)
Free conv(water)
Forced conv(air)
Forced
conv(water)
condensing steam
boiling water
evaporation/boiling
on sinteredcopper-mesh
 First 6 sets of data are from A. F. Mills Heat Transfer 1992 Richard D. Irwin, Inc. pp. 22.
 Last set of data is from our experiments
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
The porous media coating dramatically
improves the Critical Heat Flux
Comparisons among plate-surface pooling boiling,
600
copper-mesh-coating surface pooling boiling and
copper-mesh-coating surface evaporation
367
500
2
Heat transfer coefficient [W/m K]
250000
2
Heat Flux [W/m ]
200000 400
150000 300
152
100000 200
217
50000 100
0
0
0
112
Pool boiling on plain surface
PB145-8
E145-8
100 on plane200
300
Pool boiling
2
surface
Heat Flux [W/cm
]
Evaporation/boiling
on
capillary wick
 All data are from our experiments
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
400
Why use a THIN capillary wick?
 Bubble departure diameter
 Infinite fin length
Infinite Fine Thickness [mm]
25
20
15
10
5
2
0
0
1 10
4
2 10
4
4
h [ W/m^2 K]
Dd(mm)
Fritz’s model [1935]
2.884
Cole and Rohsensow’s model [1969]
2.426
July 18, 2005
3 10
Two-Phase Heat Transfer Lab @ RPI
4 10
4
5 10
4
Objective
Heat Transfer Coefficient and CHF of
Evaporation/boiling on thin capillary wick
Experimental study
Visual Study
Parametric study
t
ε
pore size or
dwire
theoretical study
Contact
conditions
Locate
positions
of bubble
&meniscus
Geometric
& thermal
properties
Heat transfer
regime
Keff
Obtain physical understanding of
this phenomena
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
ε
β
Properties
of fluid
and flow
σ, hfg, f, etc.
Predict heff and CHF
What we could gain from perfect
contact conditions?
 reduce the heat flux density on the heated
wall due to the fin effect;
 contact points connecting the wick and wall
could interrupt the formation of the vapor film
and reduce the critical hydrodynamic
wavelength;
 significantly increase the nucleation site
density and evaporation area; and
 improve liquid supply through capillary force.
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
Sintering process development
 The use of a sintering process to fabricate the test articles was
employed to reduce or eliminate the effect of the thermal contact
resistance between the porous wick material and the heating
450
block
K [W/mK]
400
350
Kcu_sintered
Kcu_solid
300
250
200
0
5
10
15
20
25
q" [W/cm 2]
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
30
35
Sintering process development cont.
 A sintering temperature of 1030 ºC in a gas mixture consisting of 75%
Argon and 25% Hydrogen for two hours was found to provide the
optimal contact conditions between the sintered mesh and the solid
copper heating bar
 sintering temperature at 950 ºC
July 18, 2005
 sintering temperature at 1030 ºC
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Sintered copper mesh
Top view
Side view
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Two-Phase Heat Transfer Lab @ RPI
Sample design
single layer copper mesh
multi-layer copper mesh
30 µm copper foil
center line of bar
TC1
TC2
copper bar
TC3
q’’
July 18, 2005
q’’
Two-Phase Heat Transfer Lab @ RPI
Sample fabrication
Fabrication of the test articles consisted of three steps:
 First, the required number of
layers of isotropic copper mesh
was sintered together to obtain
the required porosity and
thickness;
 Second, the sintered wick
structure was then carefully cut
into 8 mm by 8mm piece;
 Third, the sintered copper mesh
strips were sintered directly onto
the copper heating block.
July 18, 2005
0.03mm
copper foil
Two-Phase Heat Transfer Lab @ RPI
sintered
copper mesh
heater
Experimental study of thickness effects
Sample # Thickness(mm)
Porosity
Wire diameter(μm)
E145-2
0.21
0.737
56
E145-4
0.37
0.693
56
E145-6
0.57
0.701
56
E145-8
0.74
0.698
56
E145-9
0.82
0.696
56
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
Experimental Test Facility
Vapor
Ambient
Pyrex glass
Y
Vapor
Sintering copper mesh
x
Evaporation Zone
Outlet
Inlet
TC1
TC4
TC2
Thermal insulation layer
TC3
Distilled water
q”
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
TC5
Picture of test facility
Voltage meter
Water reservoir
Power supply
Inlet
Pyrex glass cover
Heater
Guarding heaters
Data acquisition system
Aluminum chamber
Outlet
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
System calibration
Capillary length
1000
Increaseing [Auracher et.al]
1/ 2





 g  l   g  
Decreasing [Auracher et.al]
Present data#1
Present data#2
100
Taylor critical wave length
Zuber [1959]
Heat Flux [W/cm 2]
 2.505mm
Moissis and Berenson [1962]
1/ 2



0  2 

 g  l   g  
Lienhard and Dhir [1973]
10
 15.738mm
Auracher et al.
139 watt/cm2
Zuber
110.8 watt/cm2
Moissis & Berenson
152.4 watt/cm2
Lienhard and Dhir
126.9 watt/cm2
Present data
149. 7 watt/cm2
1
0.1
1
10
100
Tw all-Tsat [K]
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
Data reduction and uncertainty
Tw  Tsat  TTC1  (TTC 4  TTC 5  TTC 6 ) / 3  q '' tSTC1 / Kcu
q "  K cu
heff 
TTC 2  TTC1
thole
(1)
(2)
q"
Tw  Tsat
(3)
The uncertainty of the temperature measurements, the length (or width) and
the mass are 0.5C, 0.01mm and 0.1mg, respectively. A Monte Carlo error
of propagation simulation indicates the following 95% confidence level
tolerance of the computed results: the heat flux is less than 5.5 watt/cm2;
the heat transfer coefficient is less than  20%; the superheat (Twall-Tsat) is
less than 1.3 C and the porosity, ε, is less than 1.5%.
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
Contact conditions
400
E145-8
Heat flux [W/cm2]
350
Plain surface pool boiling
300
E145-9 Non-sintered
250
200
150
100
50
0
0
50
100
150
TW-Tsat [K]
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200
250
Heat Transfer coefficient [W/m2K]
Contact conditions cont.
250000
200000
150000
E145-8
100000
Plain surface pool boiling
E145-9 Non-sintered
50000
0
0
100
200
Heat Flux [W/cm2]
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
300
400
Thickness Effects
400
350
E145-2
E145-4
E145-6
E145-8
Pool boiling on plain surface
2
Heat flux [W/cm ]
300
250
200
150
100
50
0
0
July 18, 2005
5
10
15
TW-Tsat [K]
20
Two-Phase Heat Transfer Lab @ RPI
25
30
Thickness Effects cont.
Heat Transfer coefficient [W/m2K]
300000
250000
200000
150000
E145-2
E145-4
E145-6
E145-8
Plain surface pool boiling
100000
50000
0
0
July 18, 2005
50
100
150
200
250
2
Heat Flux [W/cm ]
Two-Phase Heat Transfer Lab @ RPI
300
350
400
Heat transfer curve
300
E
Thin film liquid evaporation
2
Heat flux [W/cm ]
250
200
150
Nucleate boiling
D
100
C
50
Nucleate boiling onset point
B
A
Convection
0
0
2
4
6
8
Tw -Ts [K]
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
10
12
Heat transfer curve cont.
2
Heat transfer coefficient [W/m K]
300000
Thin film liquid evaporation
F
E
250000
D
200000
Partial dry-out
C
150000
Nucleate boiling
100000
Nucleate boiling onset point
B
50000
Convection
A
0
0
50
100
150
200
2
Heat flux [W/cm ]
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
250
300
Evaporation/boiling process on
sintered copper mesh coated surface
Evaporation
B
A
C
Boiling
R
Partial dry-out
D
E
q”, applied heat flux
R, meniscus radius
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
Bubbles on thin sintered copper mesh
coated surface
 No bubble departs
 Bubbles grow from heated wall and broke up at the top liquid-vapor
interface
 Size of dominated bubble decreases and number of bubbles increase
with increase heat flux applied from heated wall
B
A
D
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Two-Phase Heat Transfer Lab @ RPI
C
E
What will happen when heat flux
reaches CHF?
Temperature increases 20 to100 °C
or more in one second
July 18, 2005
Dying-out area is amplified from
about ½ heating area to the whole
heating area in just a second
Two-Phase Heat Transfer Lab @ RPI
CHF as a function of thickness
400
350
2
Heat flux [W/cm ]
300
250
200
150
100
50
0
0
0.2
0.4
0.6
Wick thickness [mm]
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
0.8
1
Main conclusions






The test results demonstrate that a porous surface comprised of sintered
isotropic copper mesh can dramatically enhance both the
evaporation/boiling heat transfer coefficient and the CHF. The maximum
heat transfer coefficients for the multiple layers of sintered copper mesh
evaluated here were shown to be as high as 245.4 KW/m2K and 360.4
W/cm2 respectively;
The interface thermal contact resistance between the heated wall and the
porous surface plays a critical role in the determination of the CHF and the
evaporation/boiling heat transfer coefficient.
Heat transfer regimes of evaporation/boiling phenomenon on this kind of
wick structure have been proposed and discussed based on the visual
observations of the phase-change phenomena and the heat flux-super heat
relationship.
For evaporation/boiling from the porous wick surface with a thickness
ranging from 0.37mm to the bubble departure diameter, Db, the ideal heat
transfer performance can be achieved and CHF is improved dramatically.
The wick still works during partial dry-out and the capillary induced pumping
functions effectively.
Exposed area determines the heat transfer performance when other key
parameters are held constant.
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
Acknowledgments
 The authors would like to acknowledge the
support of the National Science Foundation
under award CTS-0312848;
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI
 Thanks!!
 Suggestions and Questions?
July 18, 2005
Two-Phase Heat Transfer Lab @ RPI