Wicking Study

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1 Copyright 2007 by ASME

Proceedings of IPACK2009InterPACK'09

July 19-23, 2009, San Francisco, California, USA

IPACK2009-89173

NOVEL FLUORESCENT VISUALIZATION METHOD TO CHARACTERIZETRANSPORT PROPERTIES IN MICRO/NANO HEAT PIPE WICK STRUCTURES

Pramod Chamarthy, H. Peter J. de Bock, Boris Russ, Shakti Chauhan,Brian Rush, Stanton E. Weaver, Tao Deng,

GE Global ResearchNiskayuna, NY, USA

Kripa VaranasiMassachusetts Institute of Technology

Cambridge, MA, USA

ABSTRACTHeat pipes have been gaining a lot of popularity in

electronics cooling applications due to their ease of operation,

reliability, and high effective thermal conductivity. An important

component of a heat pipe is the wick structure, which transports

the condensate from condenser to evaporator. The design ofwick structures is complicated by competing requirements to

create high capillary driving forces and maintain high

permeability. While generating large pore sizes will help

achieve high permeability, it will significantly reduce the wicks

capillary performance. This study presents a novel experimental

method to simultaneously measure capillary and permeability

characteristics of the wick structures using fluorescent

visualization. This technique will be used to study the effects of

pore size and gravitational force on the flow-related properties

of the wick structures. Initial results are presented on wick

samples visually characterized from zero to nine g acceleration

on a centrifuge. These results will provide a tool to understand

the physics involved in transport through porous structures andhelp in the design of high performance heat pipes.

INTRODUCTIONRecently there has been an increasing demand for faster

and smaller chips. Since the invention of the microprocessor,

the number of transistors per chip has increased by five orders

of magnitude [1]. As a result, the heat generated by some high

performance chips has already exceeded 100 W/cm2 [2]. The

junction temperature of these chips should be maintained below

a certain value (typically 125 C) to ensure reliable operation.

With this constraint, considerable effort is being made to

develop cooling technologies with the capability to remove heat

from the device while maintaining acceptable component

operating temperatures [3,4].

In typical electronics cooling solutions the chip is attached

to a heat sink with a thermal interface in between. The thermalinterface resistance, spreading resistance, and the convection

resistance at the heat sink are the common thermal bottlenecks

that need to be minimized to achieve better cooling. The

performance of the heat sink can increase by increasing the

convective heat transfer coefficient or by increasing the surface

area. It is often challenging to increase heat transfer coefficients

due to fluid selection or pumping power limitations. Even with

a very low convection resistance, the conduction (spreading)

resistance can account for a high overall thermal resistance.

Several high performance heat sinks have heat pipes embedded

in them to facilitate heat transport with very low spreading

resistance.

liquid

Transportadiabatic)Section

vapor

EvaporatorSection

Heat

In

CondenserSection

Heat

Out

liquidevaporation condensation

Heat pipe casing Wicking structure Vapor channel

Figure 1: Heat pipe configuration


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Heat pipes are passive devices that utilize two-phase heat

transfer to transport heat. In general, a heat pipe, as shown in

Figure 1, consists of a hollow casing with an internal wickstructure and a vapor channel. The wick structure is saturated

with a working fluid, which is in equilibrium with the saturated

vapor. When heat is added to the system, the liquid inside the

wick evaporates and flows to condenser region. At the

condenser, heat is rejected as the vapor condenses back to a

liquid. The capillary forces in the wick structure act as a pump

to transport the liquid to the evaporator region. The effective

pore size of the wick determines the capillary force available to

pump the liquid. Hence, a smaller pore size can generate the

necessary force required to pump against g forces. However,

as the pore size becomes smaller, the resistance to the flow

increases, limiting the mass transport, and, as a result, the

amount of heat that can be transported. Hence, the design of thewick structure is critical both for the mass transport and heat

transfer characteristics of a heat pipe [5,6].

NOMENCLATURE

g gravitational acceleration constant [-]

h height [m]

P pressure [Pa]

cP capillary pressure difference [Pa]

gP hydrostatic pressure difference [Pa]

N mesh number [-]

cr effective capillary radius [m]

T temperature [K]

density [kg/m3]

surface tension [N/m]

contact angle [-]

MEASUREMENT METHODSeveral techniques have been proposed to study the

capillary performance of wick structures [7,8]. The rising

meniscus method provides a simple and effective way to test thewicks [9]. In this method a portion of the wick is submerged in

the liquid and the height to which the liquid rises is measured.

The height (h) to which the liquid column rises is a balance

between the capillary pressure acting at the liquid interface

( cP ) and the hydrostatic pressure and can be obtained by the

equation,

cgrh

cos2= Eq. 1

where, is the contact angle, cr is the effective pore radius,

is the surface tension of the liquid and is the density.

From Eq. 1 it can be seen that the height to which the liquid

rises is inversely proportional to the pore radius. For wick

structures with micro- or nano-sized pores the liquid columncan be a few meters in height For example, the water column

height for a 1 m pore radius will be greater than 10 m, making

the method impractical for laboratory measurement.

This limitation can be overcome by measuring the rate of

rise of the liquid column. The theoretical expression for the rise

time is given by the Lucas-Washburn [10,11](Eq. 2);

,)/1ln(// refrefref HHHHtt = Eq. 2

which takes the following form in the asymptotic short-time

limit

./2~/ refref ttHH Eq. 3

Here H=H(t) is the rise height and Href= 4/ gDpand tref=

32Href/ gD2p are, respectively, reference heights and times.

Adkins et al. [12,13] presented a different method to

characterize wicks. In this method, the wick is saturated with

the fluid and a pressure difference is applied across the sample.

If the wick is flooded on one side, the capillary force at the

downstream section will hold the fluid against the pressure

gradient. This pressure gradient is steadily increased until themeniscus is not able to withstand the pressure and a leak

develops. When the gas leaks through the wick, it is visible

through the bubbles, which are formed in the fluid, and hence

the test is called bubble point measurement technique. By

knowing the pressure at which the gas leaks through the wick,

the effective pore radius can be estimated using Eq. 4.

c

cP

r

= cos2

Eq. 4

The bubble point test is able to identify the minimum

pressure at which the gas leaks through the wick. This

corresponds to the maximum pore size on the wick surface.Hence, if the wick has non-uniform pore sizes or imperfections

on the surface, the bubble point measurement will be

substantially different from the mean pore size of the wick [7].

The purpose of this study is to demonstrate a technique that

can be used to characterize the capillary performance of wick

structures from 0-9 g accelerations. As described in the previous

section, bubble point measurement is only able to measure the

maximum pore size of the wick, which might under-estimate the

actual pore size of the wick. Hence, a visualization-based


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meniscus tracking method was developed to characterize the

wicks.

Several methods have been described in the literature tovisualize flow through porous media. Khalili et al. [14]

demonstrated a non-intrusive method to make quantitative

measurements in opaque wicks using positive emission

tomography. Gupta et al. and Bichenkov et al. [15,16] proposed

the use of X-Rays to study single and multi-phase flows through

porous media. Sederman and Gladden [17] used three-

dimensional magnetic resonance imaging and flow visualization

data to study the flow characteristics through packed bed of

glass spheres. Howle et al. [18] used a variation of the

shadowgraph technique to study the formation of patterns at the

onset of convection in fluid saturated porous media. Wang and

Khalili [19] used particle image velocimetry technique to study

the flow of a refractive-index-matched fluid through a packedbed of glass spheres.

All the flow visualization methods described above either

require that the wicks are transparent or need an elaborate setup

to make the measurements. Since the high g acceleration tests

were to be conducted on a centrifuge, the visualization method

needed to be portable.

In the present study, a novel fluorescent dye visualization

method is developed to characterize the capillary performance

of wick structures with micro- and nano-sized pores. The

method is tested and calibrated against bubble point

measurement. The fluorescent dye visualization method is usedto demonstrate the wick performance from zero - nine g

accelerations on a centrifuge.

FLUORESCENT DYE VISUALIZATION AT 1GThe visualization method had to satisfy two objectives. 1)

The method should provide a clear contrast between saturated

and unsaturated regions of the wick structure. 2) The setup

should be light and be portable onto a spin table.

After evaluation of several visualization techniques, best

results were obtained using UV visualization. A schematic of

this method is sketched in Figure 2. A UV light source shines on

a wick structure that is saturated with water/UV dye solution.The dye absorbs the incident UV light and emits in the visible

wavelength. A filter between the sample and the camera is used

to block reflected UV light and only allow light in the visible

spectrum to reach the camera. Since the experiment is

performed in a dark room, the saturated region in the wick will

be visible in the image while the unsaturated region will remain

dark.

UV light source 254 nm

Wick saturated with water/UV dye

Filter sheet that blocks UVbut passes visual light

Regular video camera

Wick

Vapor Chamber

Figure 2: UV Visualization setup

Given the complexity of the challenge, the following parameters

were identified to be crucial for performance and addressed in

detail: Dye selection: analyze excitation and emission

characteristics

Dye concentration: find optimal concentration level

Camera settings: optimum exposure times for best

contrast

Filter selection: filter to block UV light to camera

Baseline: UV light onunsaturated sample

est: UV light onsaturated sample

est: UV light &hood light onsaturated sample

Figure 3: UV Baseline results

Optimization of these parameters resulted in a method where a

clear contrast between saturated and unsaturated regions is

achieved (see Figure 3). IFWB-C7 (Risk Reactor) was chosen

as the fluorescent dye and a UV pen ray lamp (UVP, LLC) was

chosen as the light source. Excess liquid pools at the sides and

corners between the package and wick structure and can be

observed as bright spots in the images. The package was able to

retain the fluid in vertical (1g) position and the fluid was able to

wick against the gravity vector.


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t=0s t=14s t=28s

t=42s t=56s t=70s

Figure 4: Visualization of wick saturation in copper

sintered wick of 75 m using fluorescent dye

(circular wick sample in vertical orientation)

Figure 4 shows the progression of the saturation front through a

circular wick structure in vertical orientation captured with a

video camera. Image analysis was used to track the height of the

wicking front as a function of time (Figure 5).

Figure 5: Comparison of the experimental and theoreticalmeniscus rise rates.

A theoretical model for the rise time of the meniscus has

been made by modeling the wick as a bundle of cylindrical

capillaries whose pore diameter is equal to the average pore

diameter of the porous medium comprising the wick. In the

model, zero contact angle and textbook values for the surface

tension, density, and viscosity of methanol were assumed.

Calculated values for reference times in Eq. 2 were ~106

seconds so a fit to the experimental wick rise time data was

made by varying only the assumed pore diameter in Eq. 3, as

shown in Figure 5. The theoretical average pore diameter found

to give the best fit for short times was 1.6 microns. The

experimental maximum pore diameter for the wick, obtained

through bubble point measurements, was 8 m. As explainedabove, the bubble point measurement only gives the maximum

pore size of the wick and the mean pore diameter is expected to

be less than 8 m. The evaporation of the working fluid and

non-uniform porosities are expected to be the causes for the

mismatch of the curves at longer time scales. This effect is

unaccounted for in the theoretical model.

This method can be used to characterize both capillary and

permeability performance of wick structures. The team hopes to

use this method to further study wicks and compare these results

to analytical models.

EXPERIMENTAL SETUP FOR HIGH-GACCELERATION TESTS

Test fixtures were designed and fabricated to hold the

camera, light sources and wick samples. The sample holder,

presented in Figure 6, was designed in order to fix the location

of the light sources with respect to the sample and provide a

repetitive method with consistent illumination levels. This

fixture was fabricated out of Polycarbonate using rapid

prototyping methods.

Camera window

2xUVP 254 nm light source

Edge to clamp on TGPTGP

A-A

Camera window

2xUVP 254 nm light source

Pockets for flexible strip toclamp on penlight source

Edge to clamp

A-A

A

A

Wick substrate

Edge to clamp on substrate

Figure 6: Wick sample and light source holder fixture

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0 10 20 30 40 50 60

Time (s)

Heightofthedyefront

(mm).

D_pore = 1.6e-7 m

H_ref = 7.3 m

t_ref = 6.9e6 s

Theory


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Figure 7: Wick sample high g text fixture

An aluminum camera fixture was designed and fabricated for

mounting on the AFRL centrifuge (Figure 7). The frame

provides fixed positioning of the camera with respect to the

sample. The structure was designed out of aluminum profile for

its favorable strength/weight ratio.

The high-g acceleration experiments were conducted at the

AFRL test facility. The setup was mounted and balanced on the

AFRL centrifuge and an accelerometer was added to the setup.

The acceleration of the sample was calculated by correcting for

the difference in radius between the accelerometer and the

sample. The mean sample radius was found to be at 44 inches.A live video feed of the sample was available in the control

room as well as live data feed from the accelerometer. The

testing plan consisted of:

- acceleration 0 g to 2.5 g 30 seconds.

- hold at 2.5 g for 60 seconds.

- deceleration 2.5 g to 0 g 30 seconds.

HIGH-G ACCELERATION TEST RESULTS

Figure 8 presents images of the wick sample as recorded by

the camera at different acceleration conditions. A graph presents

the acceleration profile. A slight excess pool of liquid can be

observed on the sides of the sample at 0.0gs. At higher

acceleration, the excess liquid pools at the top.

Saturated wick at 2.6g, excess liquid pools on top

Sample is decelerated, excess liquid returns

Figure 8: Wick saturation in accelerated environment

(g-force direction up)

The sample was accelerated to 2.6gs and held at that

acceleration for 60 seconds. It is important to note that the main

square area of the wick remains visible. This indicates that the

UV dye solution is present inside the square wick throughout

the acceleration to 2.6 gs. After a deceleration to 0.0 gs theliquid pool from the top re-floods the edges of the wick sample.

This acceleration profile was repeated. During the test

confidence was gained in the strength of the setup and an

additional acceleration profile was added. In this profile the

sample is accelerated to around 2.5 gs in 30 seconds after

which the sample is accelerated an additional 1g every 30

seconds. This was repeated until the accelerometer reading was

at 10 gs. As the accelerometer was at larger radius than the

sample the effective corresponding sample acceleration was

found to be 8.9 gs. Interestingly the camera position adjusted

itself when the acceleration exceeded 5 gs. This is attributed to

the internal suspension of the camera and did not affect the

measurements.


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Saturated wick at 8.9g, excess liquid pools on top

Sample is decelerated, excess liquid returns

Figure 9: Wick saturation in accelerated environment (g-

force direction up)

At the high g-force acceleration, a reduction in brightness

is observed at the bottom edge of the sample. This gradient in

intensity was observed to be a strong function of the g force

acting on the wick sample. Figure 10 shows the intensity profile

along the centerline (along the g-force direction) at 4 different

accelerations. This is attributed to the meniscus shape change at

the accelerated condition and will be a subject of further study

in subsequent phases of the program. The fact that there is

measurable emission at the bottom edge gives confidence that

fluid is present at this location (compared to the zero emission

from unsaturated regions as seen in the Figure 3 baseline).

0.4

0.5

0.60.7

0.8

0.9

1.0

1.1

0 2 4 6 8 10 12 14 16 18 20

Length along the cross section (mm)

Normaliz

edintensity

(A

.U.)

0 g

2.5 g5.0 g

8.9 g

Figure 10: The intensity profile along the centerline (along

the g-force direction) at 4 different accelerations.

CONCLUSIONS

Experimental validation of capillary performance of wick

structures is crucial in the development of high performanceheat pipes. A discussion is presented in which the challenges of

measuring such performance for wicks with micro- to nano-

sized wick structures is highlighted. The advantages and

disadvantages of using a bubble point test method for

evaluation of such performance are presented. A disadvantage

of the bubble point method is that it only gives the maximum

pore size in the wick. If the sample has a large variation in pore

sizes, this might not be a good representation. However, if the

spread in pore size is reasonable, the bubble point method

proves to be a fast and simple method to get an initial estimate

for sample performance.

A novel method for saturation visualization is presented inwhich a sample is saturated using a fluid with a fluorescent dye.

An ultraviolet light source and a camera with UV-filter are used

to track the fluorescent dye front through the sample. Using a

post-processing algorithm, the dye front rise as a function of

time was established. The rate of rise method was used to

measure the mean pore diameter of the wick and compared with

the value obtained from bubble point measurement. After

validating the method, the technique was used to characterize

the performance of wick samples in high-g accelerated

environments. The wick visualization technique is found to be a

valuable method for obtaining information on the wicks global

and local capillary and permeability performance.

ACKNOWLEDGMENTSThis paper is based upon work supported by DARPA under

SSC SD Contract No. N66001-08-C-2008. Any opinions,

findings and conclusions or recommendations expressed in this

material are those of the author(s) and do not necessarily reflect

the views of the SSC San Diego.

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