Evaporation/boiling Phenomena on Thin Capillary Wick

Evaporation/boiling Phenomena on Thin Capillary Wick

Yaxiong WangFoxconn Thermal Technology Inc., Austin, TX 78758

Chen Li G. P. Peterson

Rensselaer Polytechnic Institute Department of Mechanical, Aerospace & Nuclear Engineering, Troy, NY 12180

July 18, 2005 Two-Phase Heat Transfer Lab @ RPI

How good is the performance of the evaporation/boiling on the thin capillary wick?

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

3

1510

50

50003000

15000

25

100200

10000

50000100000

250000

1

10

100

1000

10000

100000

1000000

Free conv(air) Free conv(water) Forced conv(air) Forcedconv(water)

condensing steam boiling water evaporation/boilingon sintered-

copper-mesh

Hea

t tra

nsfe

r coe

ffcie

nt [W

/m2 K

]


The porous media coating dramatically improves the Critical Heat Flux

All data are from our experiments

Comparisons among plate-surface pooling boiling, copper-mesh-coating surface pooling boiling and

copper-mesh-coating surface evaporation

0

50000

100000

150000

200000

250000

0 100 200 300 400

Heat Flux [W/cm2]

He

at

tra

ns

fer

co

eff

icie

nt

[W/m

2 K]

Pool boiling on plain surfacePB145-8E145-8

Pool boiling on planesurface Evaporation/boiling on

capillary wick

112

152

217

367

0

100

200

300

400

500

600

He

at

Flu

x [

W/m

2 ]


Why use a THIN capillary wick?

Dd(mm)

Fritz’s model [1935] 2.884

Cole and Rohsensow’s model [1969] 2.426

Bubble departure diameter Infinite fin length

0 1 104

2 104

3 104

4 104

5 104

0

5

10

15

20

25

h [ W/m^2 K]

Infi

nite

Fin

e T

hick

ness

[m

m]

2


Objective

Experimental study

εt

Geometric & thermal properties

Properties of fluid

and flowContact conditions

ε

pore size or dwire

σ, hfg, f, etc.

Locate positions of bubble

&meniscus

Heat transfer regime

Heat Transfer Coefficient and CHF of Evaporation/boiling on thin capillary wick

theoretical study

βKeff

Parametric study Visual Study

Predict heff and CHFObtain physical understanding of

this phenomena


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.


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 block

200

250

300

350

400

450

0 5 10 15 20 25 30 35

q" [W/cm2]

K [

W/m

K]

Kcu_sintered

Kcu_solid


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 1030 ºC sintering temperature at 950 ºC


Sintered copper mesh

Side view

Top view


Sample design

single layer copper mesh

30 µm copper foil

copper barTC2

TC1

TC3

q’’q’’

center line of bar

multi-layer copper mesh


Sample fabrication

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.

Fabrication of the test articles consisted of three steps:

heater

sintered copper mesh

0.03mm copper foil


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


Experimental Test Facility

Outlet

Vapor

q”

Thermal insulation layer Distilled water

x

Y

Evaporation ZoneSintering copper mesh

TC1

TC2

TC3

Inlet

Pyrex glassAmbientVapor

TC4

TC1

TC5


Aluminum chamber

Data acquisition system

Power supply

Guarding heaters

Outlet

Water reservoir

Inlet

Voltage meter

Pyrex glass cover Heater

Picture of test facility


System calibration

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

0.1

1

10

100

1000

1 10 100

Twall-Tsat [K]

He

at

Flu

x [

W/c

m2 ]

Increaseing [Auracher et.al]

Decreasing [Auracher et.al]

Present data#1

Present data#2

Zuber [1959]

Moissis and Berenson [1962]

Lienhard and Dhir [1973]

Capillary length

Taylor critical wave length

1/ 2

2.505l g

mmg

1/ 2

0 2 15.738l g

mmg


Data reduction and uncertainty

1 4 5 6 1( ) / 3 '' /w sat TC TC TC TC STC cuT T T T T T q t K

2 1" TC TCcu

hole

T Tq K

t

"eff

w sat

qh

T T

(1)

(2)

(3)

The uncertainty of the temperature measurements, the length (or width) and the mass are 0.5C, 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%.


Contact conditions

0

50

100

150

200

250

300

350

400

0 50 100 150 200 250

TW-Tsat [K]

He

at

flu

x [

W/c

m2 ]

E145-8

Plain surface pool boiling

E145-9 Non-sintered


Contact conditions cont.

0

50000

100000

150000

200000

250000

0 100 200 300 400

Heat Flux [W/cm2]

He

at

Tra

ns

fer

co

eff

icie

nt

[W/m

2 K]

E145-8

Plain surface pool boiling

E145-9 Non-sintered


Thickness Effects

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25 30TW-Tsat [K]

He

at

flu

x [

W/c

m2 ]

E145-2E145-4E145-6E145-8Pool boiling on plain surface


Thickness Effects cont.

0

50000

100000

150000

200000

250000

300000

0 50 100 150 200 250 300 350 400Heat Flux [W/cm2]

He

at

Tra

ns

fer

co

eff

icie

nt

[W/m

2 K]

E145-2E145-4E145-6

E145-8Plain surface pool boiling


Heat transfer curve

0

50

100

150

200

250

300

0 2 4 6 8 10 12

Tw-Ts [K]

He

at

flu

x [

W/c

m2 ]

Convection

Nucleate boiling

Thin film liquid evaporation

Nucleate boiling onset point

A B

C

D

E


Heat transfer curve cont.

0

50000

100000

150000

200000

250000

300000

0 50 100 150 200 250 300

Heat flux [W/cm2]

He

at

tra

ns

fer

co

eff

icie

nt

[W/m2 K

]

Convection

Nucleate boiling

Thin film liquid evaporation

Partial dry-out

Nucleate boiling onset point

A

B

C

D

E

F


Evaporation/boiling process on sintered copper mesh coated surface

Evaporation

BoilingR

A B C

D E

R, meniscus radius

q”, applied heat flux

Partial dry-out


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

A B C

D E


What will happen when heat flux reaches CHF?

Temperature increases 20 to100 °C or more in one second

Dying-out area is amplified from about ½ heating area to the whole heating area in just a second


CHF as a function of thickness

0

50

100

150

200

250

300

350

400

0 0.2 0.4 0.6 0.8 1

Wick thickness [mm]

He

at

flu

x [

W/c

m2 ]


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.


Acknowledgments

The authors would like to acknowledge the support of the National Science Foundation under award CTS-0312848;


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Evaporation/boiling Phenomena on Thin Capillary Wick