Evaporation/boiling Phenomena on Thin Capillary Wick - PowerPoint PPT Presentation

Evaporation/boiling Phenomena on Thin Capillary Wick

Yaxiong Wang

Foxconn Thermal Technology Inc., Austin, TX 78758

• 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

• All data are from our experiments

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• Bubble departure diameter
• Infinite fin length

Two-Phase Heat Transfer Lab @ RPI

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

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• 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.

Two-Phase Heat Transfer Lab @ RPI

• 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

Two-Phase Heat Transfer Lab @ RPI

• 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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Top view

Side view

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single layer copper mesh

multi-layer copper mesh

30 µm copper foil

center line of bar

TC1

TC2

copper bar

TC3

q’’

q’’

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0.03mm copper foil

sintered copper mesh

heater

• 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:

Two-Phase Heat Transfer Lab @ RPI

Two-Phase Heat Transfer Lab @ RPI

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”

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Water reservoir

Voltage meter

Power supply

Inlet

Pyrex glass cover

Heater

Guarding heaters

Data acquisition system

Aluminumchamber

Outlet

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Capillary length

Taylor critical wave length

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(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%.

Two-Phase Heat Transfer Lab @ RPI

Two-Phase Heat Transfer Lab @ RPI

Two-Phase Heat Transfer Lab @ RPI

Two-Phase Heat Transfer Lab @ RPI

Two-Phase Heat Transfer Lab @ RPI

E

Thin film liquid evaporation

Nucleate boiling

D

C

Nucleate boiling onset point

B

A

Convection

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Thin film liquid evaporation

F

E

D

Partial dry-out

C

Nucleate boiling

Nucleate boiling onset point

B

Convection

A

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Evaporation

C

B

A

Boiling

R

Partial dry-out

D

E

q”, applied heat flux

R, meniscus radius

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B

C

A

D

E

• 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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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

Two-Phase Heat Transfer Lab @ RPI

Two-Phase Heat Transfer Lab @ RPI

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

Two-Phase Heat Transfer Lab @ RPI

• Suggestions and Questions?

Two-Phase Heat Transfer Lab @ RPI