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Evaporation/boiling Phenomena on Thin Capillary Wick

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Evaporation/boiling Phenomena on Thin Capillary Wick . Yaxiong Wang Foxconn Thermal Technology Inc., Austin, TX 78758. How good is the performance of the evaporation/boiling on the thin capillary wick?.

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Evaporation/boiling Phenomena on Thin Capillary Wick

Yaxiong Wang

Foxconn Thermal Technology Inc., Austin, TX 78758

how good is the performance of the evaporation boiling on the thin capillary wick
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

Two-Phase Heat Transfer Lab @ RPI

the porous media coating dramatically improves the critical heat flux
The porous media coating dramatically improves the Critical Heat Flux
  • All data are from our experiments

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why use a thin capillary wick
Why use a THIN capillary wick?
  • Bubble departure diameter
  • Infinite fin length

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objective

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

Experimental study

theoretical study

Visual Study

Geometric & thermal properties

Parametric study

Properties of fluid and flow

pore size or dwire

Contact conditions

t

ε

Locate positions of bubble &meniscus

Heat transfer regime

Keff

ε

β

σ, hfg, f, etc.

Obtain physical understanding of this phenomena

Predict heff and CHF

Objective

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what we could gain from perfect contact conditions
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.

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sintering process development
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

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sintering process development cont
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
  • sintering temperature at 1030 ºC

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sintered copper mesh
Sintered copper mesh

Top view

Side view

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sample design

single layer copper mesh

multi-layer copper mesh

30 µm copper foil

center line of bar

TC1

TC2

copper bar

TC3

q’’

q’’

Sample design

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sample fabrication

0.03mm copper foil

sintered copper mesh

heater

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:

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experimental study of thickness effects
Experimental study of thickness effects

Two-Phase Heat Transfer Lab @ RPI

experimental test facility

Y

x

Vapor

Ambient

Pyrex glass

Vapor

Sintering copper mesh

Evaporation Zone

Outlet

Inlet

TC1

TC1

TC2

TC4

Thermal insulation layer

Distilled water

TC5

TC3

q”

Experimental Test Facility

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picture of test facility

Water reservoir

Voltage meter

Power supply

Inlet

Pyrex glass cover

Heater

Guarding heaters

Data acquisition system

Aluminumchamber

Outlet

Picture of test facility

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system calibration
System calibration

Capillary length

Taylor critical wave length

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data reduction and uncertainty
Data reduction and uncertainty

(1)

(2)

(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%.

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contact conditions
Contact conditions

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contact conditions cont
Contact conditions cont.

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thickness effects
Thickness Effects

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thickness effects cont
Thickness Effects cont.

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heat transfer curve

E

Thin film liquid evaporation

Nucleate boiling

D

C

Nucleate boiling onset point

B

A

Convection

Heat transfer curve

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heat transfer curve cont

Thin film liquid evaporation

F

E

D

Partial dry-out

C

Nucleate boiling

Nucleate boiling onset point

B

Convection

A

Heat transfer curve cont.

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evaporation boiling process on sintered copper mesh coated surface

Evaporation

C

B

A

Boiling

R

Partial dry-out

D

E

q”, applied heat flux

R, meniscus radius

Evaporation/boiling process on sintered copper mesh coated surface

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bubbles on thin sintered copper mesh coated surface

B

C

A

D

E

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

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what will happen when heat flux reaches chf
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

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chf as a function of thickness
CHF as a function of thickness

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main conclusions
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.

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

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slide29
Thanks!!
  • Suggestions and Questions?

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