Heat Pipe Engineering eBook Datasheet by Advanced Thermal Solutions Inc.

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ATS Engineering eBook

Collection of Technical Articles on Heat Pipes and

Their Roles in the Thermal Management of Electronics

89-27 Access Road, Norwood, MA 02062 | T: 781.769. 2800 F: 781.769.9979 | www.qats.com

Table of Contents

Heat Pipes: Heat Super Conductors 2

Fundamentals: Effective Thermal

Conductivity Of A Heat Pipe 5

How Wicks and Orientation Affect

Heat Pipe Performance 6

Integrated Heat Pipe Technology

for Thermal Management of Electronics 10

Nanofluids in Heat Pipes 15

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Heat Pipes: Heat Super Conductors

Figure 1. Schematic View of a Heat Pipe [1].

Heat pipes are transport mechanisms that can carry heat

fluxes ranging from 10 W/cm2 to 20 KW/cm2 at a very fast

speed. Essentially, they can be considered as heat super

conductors. Heat pipes can be used either as a means to

transport heat from one location to another, or as a means

to isothermalize the temperature distribution.

The first heat pipe was tested at Los Alamos National

Laboratory in 1963. Since then, heat pipes have been used

in such diverse applications as laptop computers, spacecraft,

plastic injection molders, medical devices, and lighting

systems. The operation of a heat pipe is described in Figure 1.

A heat pipe has three sections: the evaporator, adiabatic,

and condenser. The interior of the pipe is covered with a

wick, and the pipe is partially filled with a liquid such as

water. When the evaporator section (Le) is exposed to a

heat source, the liquid inside vaporizes and the pressure in

that section increases. The increased pressure causes the

vapor to flow at a fast speed toward the condenser section

of the heat pipe (Lc). The vapor in the condenser section

loses heat to the integral heat sink and is converted back

to liquid by the transfer of the latent heat of vaporization

to the condenser. The liquid is then pumped back to the

evaporator through the wick capillary action. The middle

section of the heat pipe (La), the adiabatic portion, has a

very small temperature difference.

Figure 2. Pressure Drop Distribution in a Heat Pipe [1].

Figure 2 shows the pressure drop distribution inside a

heat pipe. In order for the capillary force to drive the vapor,

the capillary pressure of the wick should exceed the

pressure difference between the vapor and the liquid at the

evaporator. The graph also shows that if the heat pipe is

operated against the force of gravity, the liquid undergoes a

larger pressure drop. The result is less pumping of the wick

with reduced heat transfer. The amount of heat transfer

decrease depends on the particular heat pipe. A typical

heat pipe is made of the following:

1. Metallic pipe The metal can be

aluminum, copper or stainless

steel. It must be compatible

with the working fluid to prevent

chemical reactions, such as

oxidation.

2. Working fluid Several types of

fluids have been used to date.

These include methane, water,

ammonia, and sodium. Choice

of fluid also depends on the

operating temperature range.

3. Wick The wick structure comes in

different shapes and materials.

Figure 3 shows the profiles of

common wick types: axial groove, Figure 3. Different Wick

Structures

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3. Wick (continued) fine fiber, screen mesh, and

sintering. Each wick has its own characteristics.

For example, the axial groove has good conductivity

poor flow against gravity, and low thermal resistance.

Conversely, a sintering wick has excellent flow

in the opposite direction of gravity, but has high

thermal resistance.

Table 1 shows experimental data for the operating

temperature and heat transfer for three different types of

heat pipes [1].

Working

Fluid

Operating

Temp (°C)

Wick

Design

Wall

Material

Axial Heat

Transport,

(W)

Methane -140 Circumferential

mesh

Stainless

steel 12

Water 100 Axial grooves

Copper with

rectangular

cross

section

Sodium 430-790

Circumferential

stainless steel

screen

Stainless

steel 1309

Table 1. Heat Pipes with Different Structures and Operating

Conditions [1]

Certain factors can limit the maximum heat transfer rate

from a heat pipe. These are classified as follows:

1. Capillary limit Heat transfer is limited by the pumping

action of the wick

2. Sonic limit When the vapor reaches the speed of sound,

further increase in the heat transfer rate can only be

achieved when the evaporator temperature increases

3. Boiling limit High heat fluxes can cause dry out.

4. Entrainment limit High speed vapor can impede the

return of the liquid to the condenser

A heat pipe has an effective thermal conductivity much

larger than that of a very good metal conductor, such

as copper. Figure 4 shows a copper-water heat pipe

and a copper pipe dipped into an 80°C water bath. Both

pipes were initially at 204°C temperature. The heat pipe

temperature reaches the water temperature in about 25

seconds, while the copper rod reaches just 30°C after 200

seconds. However, in an actual application when a heat

pipe is soldered or epoxied to the base of a heat sink,

the effective thermal conductivity of the heat pipe may be

drastically reduced due to the extra thermal resistances

added by the bonding. A rule of thumb for the effective

thermal conductivity of a heat pipe is 4000 W/mK.

Heat pipe manufacturers generally provide data sheets

showing the relationship between the temperature

difference and the heat input. Figure 5 shows the

temperature difference between the two ends of a heat

pipe as a function of power [2].

Figure 4. Experiment Comparing Speed of Heat Transfer Between

a Heat Pipe and a Copper Pipe [1].

Figure 5. Temperature Difference Between the Evaporator and the

Condenser in a Heat Pipe [2].

There are many heat pipe shapes in the market, but the

most common are either round or flat. Round heat pipes can

be used for transferring heat from one point to another. They

can be applied in tightly spaced electronic components, such

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as in a laptop. Heat is transferred to a different location that

provides enough space to use a proper heat sink or other

cooling solution. Figure 6 shows some of the common round

heat pipes available in the market.

Figure 6. Typical Round Heat Pipes in the Market.

Flat heat pipes (vapor chambers) work conceptually the

same as round heat pipes. Figure 7 shows a flat pipe

design, they can be used as heat spreaders. When the

heat source is much smaller than the heat sink base, a flat

heat pipe can be embedded in the base of the heat sink,

or it can be attached to the base to spread the heat more

uniformly on the base of the heat sink. Figure 8 shows

some common flat heat pipes.

Figure 7. Conceptual Design Schematic of a Flat Heat Pipe [1].

Figure 8. Commonly-used Flat Heat Pipes.

Figure 9. Thermal Spreading Resistances for Different Materials.

Although a vapor chamber might be helpful in minimizing

spreading resistance, it may not perform as well as a plate

made from a very high conductor, such as diamond. A

determining factor is the thickness of the base plate. Figure

9 shows the spreading resistance for 80 x 80 x 5 mm base

plate of different materials with a 10 x 10 mm heat source.

The vapor chamber has a spreading resistance that is better

than copper, but worse than diamond. However the price of

the diamond might not justify its application. Figure 9 also

includes the spreading resistance from the ATS Forced

Thermal Spreader (FTS), which is equal to that of diamond

at a much lower cost. The FTS uses a combination of mini

and micro channels to minimize the spreading resistance by

circulating the liquid inside the spreader.

Heat pipes have a very important role in the thermal

management arena. With projected lifespans of 129,000-

260,000 hours (as claimed by their manufacturers), they will

continue to be an integral part of some new thermal systems.

However, with such problems as dry out, acceleration, leakage,

vapor lock and reliable performance in ETSI or NEBS types

of environments, heat pipes should be tested prior to use and

after unsatisfactory examination of other cooling methods.

References:

1. Faghri, A. Heat Pipe Science and Technology Taylor &

Francis, 1995.

2.Thermacore Internation, Inc., www.thermacore.com.

3. Xiong, D., Azar, K., Tavossoli, B., Experimental Study on

a Hybrid Liquid/Air Cooling System, IEEE, Semiconductor

Thermal Measurement and Management Symposium 2006.

Fundamentals:

Effective Thermal Conductivity Of A Heat Pipe

Introduction

The question often arises that what is the effective thermal conductivity of a heat pipe. The correct answer

depends on the construction and the wick material inside the heat pipe. To understand this better, consider figure 1

which shows all the resistances from the hot source on the evaporator side to the cold side which is the condenser.

Figure 1. Thermal Resistances of a Heat Pipe [1]

The various thermal resistances are defined as follows:

Rext,e = Contact resistance between hot source

and the heat pipe

Rpe, Rpc = Conduction resistance of the heat pipe wall

in the radial direction

Rwe,Rwc = Resistance of the wick liquid structure

in the radial direction

Rpa = Conduction resistance of the heat pipe wall

in the axial direction

Rwa = Resistance of the wick liquid structure in

the axial direction

Rie,Ric = Resistance of the liquid vapor interface

Rva = Resistance of the vapor phase

The total resistance of the heat pipe is a combination

of series and parallel resistances and can be calculated

as follows:

(1)

Considering that the thermal resistance of the vapor space

is extremely small in the range of 10-8 °C/W, equation 1

can be simplified to:

tot = 2(Rpe + Rwe) (2)

To gain a better understanding a simple calculation can

reveal some insight. Assume a 6cm long heat pipe with

inner and outer diameters of 5mm and 6mm respectively

having two layers of #500-mesh copper screens with wire

diameters of 0.0215mm. Calculations show that:

Rpe = 3.618X10-3 °C/W

Rwe = 1.474°C/W

Substituting these values into equation 2 results in:

Rtotal = 2.95°C/W

When you compare it to a solid copper with the same sizes

results in R = 5.3°C/W, almost a factor of two better, with

the added advantage that the heat pipe is much lighter.

This results in an effective thermal conductivity of this heat

pipe to be 730 W/m∙K

Equation 2 shows the importance of the wick structure

resistance and is clear that it is one of the dominant factors

in the overall thermal performance of the heat pipe.

The above mentioned arguments show that the common

assumption of a very high conductivity around 40,000 W/

m∙K for the heat pipe is not correct. The correct way is

to accurately calculate the different thermal resistances

based on the geometry of the heat pipe, wall material, wick

structure and the liquid to find the total thermal resistance

and hence the effective thermal conductivity. If the

information is not readily available, the best option is to test

the heat pipe to calculate its thermal resistance.

Reference:

Peterson, G.P, “An introduction to Heat pipes –

Modeling, Testing and Applications”, New York:

John Wiley and sons, 1994

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How Wicks and Orientation Affect

Heat Pipe Performance

Introduction

A heat pipe is a device with very high thermal conductance that can transport large quantities of heat with small

temperature difference between its hot and cold ends. It is normally used to transport heat from one area to

another or to smooth the temperature distribution on a solid surface. Heat pipes are widely used in aerospace

applications, military devices, temperature control systems, and now in personal computers.

A heat pipe is a self-driven, two-phase device. A

schematic view is shown in Figure 1. At its hot end

(evaporator) the liquid evaporates and turns to vapor. This

vapor flows to the cold end (condenser) where it liquefies.

The liquid is driven back from the cold end to the hot end

by capillary forces within the heat pipe’s wick structure.

Figure 1. Typical Heat Pipe [1].

The heat transfer ability of a heat pipe is determined by its

diameter, fluid type, wick structure, and orientation. The

heat flux limitations of a heat pipe are governed by the

following factors (Figure 2):

1) Viscous Limit. At low temperature, the vapor pressure

difference between the evaporator and the condenser

may not be enough to overcome viscous forces.

2) Sonic Limit. This occurs when the vapor velocity

reaches sonic speed at the evaporator and any increase

in the pressure difference will not speed up the flow.

3) Entrainment Limit. At high vapor velocities, droplets

of liquid are torn from the wick and entrained in vapor.

The droplets flow to the condenser with the vapor, which

results in drying out on the evaporator.

4) Capillary Limit. This is reached when the capillary

ΔPcapillary ≥ ΔPv + ΔPl + ΔPg

If this condition is not met, the wick on the evaporator

will dry out and the heat pipe will overheat. The maximum

allowable heat flux ΔPcapillary_max is referred to as the

capillary limit. In typical operating conditions, the capillary

limit determines the maximum heat transfer rate of the

heat pipe.

For a heat pipe, the pumping power ΔPcapillary occurs on

the gas and liquid interface of the wick structure due to

surface tension differences. Pore radius and permeability

are the two most important characteristics of a wick

structure. The pore radius determines the pumping

pressure that the wick can develop. The smaller the pore

radius, the larger the pumping power. The permeability

determines the fractional pressure losses of the working

fluid ΔPl. The pressure drop ΔPV is directly related to

the rate of vapor traveling from the evaporator to the

condenser.

The heat transfer rate is also affected by the diameter and

the length of the heat pipe. In a large diameter heat pipe,

the cross sectional area will allow higher vapor volume

Figure 2. Limits to Heat Transfer in a Heat Pipe [2].

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to be transported from the evaporator to the condenser

than in a small diameter pipe. The cross sectional area

of a heat pipe affects the sonic limit and entrainment

limit, as well. In general, heat pipes with larger diameters

transport more heat. The gravitational pressure head ΔPg

is determined by the relative positions of the evaporator

and condenser.

If the angle between a straight heat pipe and horizontal

is f (f is positive when the evaporator is lower than

condenser), the gravitational pressure head ΔPg can

be calculated as follows:

ΔPg = - ρIgIsinø

Where ρl is liquid density and l is heat pipe length.

The most commonly used wick structures for heat pipes

are simple and homogeneous, such as grooves, wire

mesh, sintered metal powders, and fiber. Other composite

wick structures are included in Figure 3 [2]. Each wick

structure has its advantages and disadvantages.

Figure 3. Heat Pipe Wick Structures [2].

The HP-1 is a series of high performance, sintered wick

structure heat pipes produced by Thermacore. These

pipes are available in diameters of 6.4 mm (1/4"), 9.5 mm

(3/8"), 12.7 mm (1/2") and 15.9 mm (5/8"). Thermacore

Corporation tested these 304.8 mm (12") long heat pipes

with 76.2 mm (3") evaporator and 76.2 mm (3") condenser

sections at a 100˚C operating temperature. The results

are presented in Figures 4 and 5 [3].

Figure 4 shows the temperature difference between

evaporator and condenser at different power levels when

the heat pipes are vertical. Compared to small diameter

heat pipes, large diameter heat pipes transport more heat

with the same ΔT. Figure 5 illustrates the relationship

between power and the inclination angle for different

heat pipes. Clearly the sintered wick structure pipes are

better with the help of gravity. When the inclination angle

is larger than 10˚, the heat flux that the heat pipes can

transport does not vary much. As the inclination angle

gradually decreases from 10˚C, the heat flux decreases

as well. At a -90˚ angle, the heat flux is less than half of

that at 10˚.

Figure 4. HP-1 Delt-T vs. Power [3].

Loh et al [4] experimentally studied the effects of wick

structure and orientation on heat pipe performance. The

bench they used for the tests is shown in Figure 6. The

heat pipes they tested were 4, 5, and 6 mm in diameter.

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The pipes were 200 mm long with a 35 mm evaporator

and a 35 mm condenser. Each test started with an

inclination angle f of 90°, the vertical position at which

the evaporator block was located at the bottom. The

tests ran through a 180° rotation that paused at each of

the following inclination angles: 60°, 30°, 0° (horizontal),

-30°, -60° and –90°.

Figures 7, 8 and 9 show the temperature differences

between evaporator and condenser for mesh, groove

and sintered metal powder heat pipes, respectively.

Figure 6. Photo of Heat Pipe Test Bench [4].

A heat pipe with a mesh wick structure has the largest

thermal impendence. The orientation has a large

effect on its heat transfer, but it manages to work at

low and moderate heat flux even at a -90° angle. A

heat pipe with a groove wick has the smallest thermal

impendence among three wick structures when the

inclination angle is positive. However, its temperature

difference increases dramatically when the inclination

angle changes to negative, even at low heat flux.

It fails at a negative inclination angle when the heat

flux is larger than 15 W.

The performance of a heat pipe with sintered

metal powder is not affected much by its orientation

when the heat flux is less than 15 W. At moderate

heat flux, the sintered metal powder heat pipe can

still work against gravity with an increased temperature

difference. At high heat flux (>25 W), it can only work

with help of gravity.

Figure 7. ΔT of a 6mm Mesh Heat Pipe at Different Inclination

Angles [4].

Figure 8. ΔT of a 6mm Groove Heat Pipe at Different Inclination

Angles [4].

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DESIGN CORNER

VISIT COOLINGZONE’S DESIGN CORNER

FOR CALCULATION TOOLS ON:

• Impingement Heat Transfer

• Jet Impingement (Circular Nozzle)

• Radiation Shielding

• Effect of Radiation on Temperature Measurement

• Heat Transfer from a PCB

• Flat Plate Inside a Duct

• Heat Sink Optimum Spacing

• Radiation from a Heat Sink

• Heat Transfer Through a Composite Slab

WWW.COOLINGZONE.COM

Re = V

TIRED OF SPENDING TIME

CALCULATING EQUATIONS?

Nu = h∙L

Rtotal = Rconv,1 + Rwall,1 + Rwall, 2 + Rwall,3 + Rconv,4

Q = h∙A(T

- T

∞

)

∙

h(Tf - Tt ) = εtσ(Tt - Tw)

It is important to select the proper wick structure for

heat pipes based on their real application. If a heat pipe

works in conditions with favorable gravitational force

and a few bends, the grooved wick heat pipe is a good

choice because of its superior thermal performance.

If a heat pipe has complex geometry and works at a

small or negative tilting angle, sintered powder metal

is the optimum wick structure. For cooling electronic

components in telecommunications devices and computer

products, the sintered powder metal wick is the best

choice because such applications require a

compact heat sink size with many turns and bends. The

high capillary pumping pressure achieved by using a

sintered powder metal wick due to its small pore size,

allows a heat pipe to operate in any orientation. Other

wick structures do not work as well in non-vertical

orientations because they cannot lift the returning working

fluid along the length of the heat pipe against gravity.

References:

1. http://www.lightstreamphotonics.com/technology.htm

2. Reay, D. and Kew, P., Heat Pipes: Theory, Design and

Applications, 5th Edition, Butterworth-Heinemann, 2006.

3. HP-1 Heat pipe specification, www.thermacore.com.

4. Loh, C., Harris, E. and Chou, D., Comparative Study

of Heat Pipes Performances in Different Orientations,

Semiconductor Thermal Measurement and

Management Symposium, 2005.

Figure 9. ΔT of a 6mm Sintered Metal Powder Heat Pipe at

Different Inclination Angles [4].

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Integrated Heat Pipe Technology

for Thermal Management of Electronics

Introduction

Increasingly, consumers and industry are demanding more functionality and better performance, along

with increased miniaturization, from electronics products. The decrease in size of the new generation of

electronic devices imposes a severe constraint on their incorporated thermal management devices. Since

the cooling hardware has become large in size relative to the components to be cooled, serious challenges

are ahead for the design of future products.

The vast majority of cooling solutions have been

developed to be either attached on existing electronic

products or managed through packaging techniques. As

no integrative effort has been made, all these devices

remain essentially adds on (Figure 1 [1]). The devices

presented in Figure 1 are well embraced by the industry,

especially from an assembly and cost point of view.

However, such solutions prevent significant progress in

the design of electronic products. A radical new way of

thermal management can be achieved by integrating

thermal design criteria early into the electronic design

process. This approach will allow the design of a

printed circuit board with full integration of the thermal

management hardware, with the aim of reducing thermal

gradients inside electronic products.

Background

Before addressing the specifics of the heat pipe PCB

integration let us briefly present basic background

information about heat pipes and PCB technologies. A

heat pipe is a heat transfer device consisting of a sealed

vessel containing a small amount of working fluid. It has

three sections: evaporator, adiabatic transport section

and condenser. In order for the device to function, the

condensate must return to the evaporator in a timely

manner. This function is served by a capillary or wick

structure that must be placed on the walls of the enclosure.

The wick enables the liquid return from the condenser to

the evaporator through the capillary pressure caused by

the difference in curvature of the liquid menisci. In order

for a heat pipe to operate, the following condition must be

satisfied:

ΔPcap ≥ ΔPv + ΔPl + ΔPg (1)

where,

ΔPcap = capillary pressure (Pa)

ΔPv = pressure drop of vapor flow (Pa)

ΔPl = pressure drop of liquid flow (Pa)

ΔPg = pressure drop due to gravity (Pa)

So, the capillary pressure provided by the wick must

overcome the pressure drops due to vapor and liquid flows.

The capillary limit refers to the maximum heat that a heat

pipe can transfer and this can be expressed as:

a) ATS maxiFLOW™ heat sink

c) ThermoShuttle3 SVH001

vapor chamber

b) Corsair2 A50 heat sink

with heat pipes

d) On board heat pipes

by McGlen et al.4

15w XII

where,

σ = surface tension of working liquid (N/m)

reff = effective pore radius of the wick (m)

θ = mesh wall - working liquid contact angle (degrees)

g = gravitational constant (m/s2)

L = heat pipe length (m)

γ = heat pipe’s inclination angle (degrees)

φ = porosity of the wick

fRe

= Reynolds based friction factor

Av = vapor space cross-sectional area (m2)

μl = viscosity of the liquid (Pa•s)

μv = viscosity of the vapor (Pa•s)

Leff = effective length (m)

K = permeability of the wick (m2)

Aw = wick cross-sectional area (m2)

ρl = density of the liquid (kg/m3)

ρv = density of the vapor (kg/m3)

Hfg = latent heat of vaporization of the working fluid (J/Kg)

Since the heat pipes presented in this paper have a

rectangular cross-section, hydraulic properties are used in

equation (2).

In general, a PCB serves as a carrier, providing mechanical

support and enabling electrical connections between all

mounted electronic components. A typical PCB consists of

several polymeric layers (such as FR4) laminated together.

Conductive patterns (traces) need to be etched in order to

interconnect the electronic components. Usually, several

layers are laminated together, forming a multilayer PCB.

The process of manufacturing a PCB can be presented

sequentially as follows:

a) individual layers are patterned

b) layers are stacked with intermediate bonding layers

c) the entire stack is laminated in a special press at an

elevated temperature

Advances in Technology Integration

In recent years, significant advances in integrating the heat

pipe technology and PCB technology have been made.

Jones et al. [2] proposed embedded micro heat pipes in

laminate substrates with the wick microgrooves placed

vertically in a staggered lay-up (Figure 2).

The staggered lay-up introduces an additional constraint on

the board design. As heat pipe performance is a function

of the number of PCB layers, additional, electronically

non-functional layers must be introduced. Water was

used as working fluid and the cavity was made of copper.

To reduce the contact angle and, therefore, increase the

capillary pressure, a cleaning method was used. A reduction

in contact angle from 104 degrees to 56 degrees was

achieved. In terms of thermal performance, heat pipe failure

occurred around 10W, due to delamination of the PCB.

a) staggered lay-up (schematically)

b) microgrove detail

Figure 2. Integrated Heat Pipe [2]

Wits et al. [3] proposed another way of integrating a heat

pipe into the PCB. In their design, the microgroove wick

was placed on the top and bottom layers of the internal

cavity (Figure 3). The grooves are manufactured using a

metallic plating technique based on conventional dry film

lithography, commonly used in electronics manufacturing.

The working fluid was also water and the cavity was made

of copper. Figure 3 b) shows the technology demonstrator.

The heat transfer in and out of the heat pipe was

accomplished with thermal vias.

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The heat pipe pictured in Figure 3 b) was tested using

the experimental set up illustrated in Figure 4. The heat

pipe was insulated and clamped in a hinge to allow testing

in various orientations. The power input was raised in

steps. After reaching steady state, readings were made

and the next step was implemented. The experimental

run was terminated when the heat pipe would experience

a sudden rise in temperature, signaling a dry out condition.

a) integrated heat pipe structure

b) technology demonstrator

Figure 3. Copper Mesh Used for Ultra-Thin Heat Pipes [3]

Figure 4. Experimental Setup [3]

Figure 5. Temperature Distribution Along The Heat Pipe [3]

Figure 5 presents some of the test results, for two power

input levels (2.5 W and 10 W), and for two orientations

(horizontal and vertical). The vertical orientation refers

to the condition whereby the working fluid returns to the

evaporator assisted by gravity. For 2.5 W input power, the

thermal performance for both orientations is similar. At

the evaporator end, the temperature profile is fairly Figure

5. Temperature Distribution Along The Heat Pipe [3] flat.

However, just over the mid-way point, the temperature

starts to drop, indicating the presence of non-condensable

gases (air). Vertically, the heat pipe was able to transport a

maximum of 12 W of heat.

When tested with the working fluid ascending against

gravity, the prototype did not function at all. The capillary

microgrooves were not able to pump the working fluid back

up to the evaporator. However, the PCB did not delaminate

as in the Jones et al. [2] case.

Integration Challenges

Based on a thorough analysis of the previously proposed

designs, the following three challenges have been identified

for the design of a PCB with an integrated heat pipe:

a) operation against gravity: microgroove structure must be

improved

b) lower thermal resistance from the top to the bottom of the

PCB in the normal direction

c) minimize the amount of dead volume (bonding layers in

contact with heat pipe cavity)

To address the first challenge, it is useful to take a look

at Equation (2). In the numerator, the first term, indicating

Thermal V1115 Sealmg groove Re<>

capillary pressure, must be enlarged in comparison with

the second term, corresponding to pressure losses due

to gravity. The capillary microgrooves of both presented

designs failed to transport the working fluid against gravity;

therefore, a different wick structure, with a smaller pore

radius, should be chosen. Both microgroove designs had an

effective pore radius of about 50-100 μm. A screen mesh,

sintered media or metal foam will yield lower pore radii.

Also from Equation (2), it is clear that for a chosen mode

of operation, geometry and working fluid, only the internal

distribution of vapor and liquid areas and the wick structure

remain at the designer’s discretion.

Based on this discussion, a new PCB prototype was built

by Wits et al. [1] (Figure 6). The PCB features an insertable

wick structure with a smaller pore diameter, to address the

first challenge. The wick is positioned inside the cavity with

the aid of copper inserts, which are also lowering the top to

bottom thermal resistance. Therefore, the second challenge

is addressed as well.

Figure 6. Prototype Heat Pipe Design [1]

The prototype is constructed from three layers of double

plated polymeric material (FR 4+). The top layer has two

machined openings for the pressure sensor and filling

tube. Also, multiple small thermal vias are provided.

The vias are plated and filled with a thermally enhanced

epoxy (k = 3.5 W/m∙K).

The middle layer features a rectangular slot that forms

the heat pipe cavity. The third (bottom) layer is a regular

flat laminate. The second and third layer will be laminated

together and subsequently metalized. Before assembling

the multilayer board, each layer is given a surface finish to

enhance the bonding strength. Since the surface finish also

enhances the wettability, the wick structure is also treated

with this surface finish. The wick structure is placed inside

the heat pipe cavity and the copper inserts are placed in the

evaporator and condenser areas.

The third challenge can be overcome by introducing an

intermediate process step during the construction of the

PCB. For instance, the bottom and intermediate layers can

be assembled first to allow overplating and the sealing of the

intermediate permeable zones from the inside. The second

process step will then include stacking and laminating the

bottom half of the board with the wick structure, copper insert

and top layer. Finally, a seal groove needs to be machined

and plated Figure 6. Prototype Heat Pipe Design [1] to seal

the remaining top intermediate permeable zone.

The theoretical maximum amount of heat that can be

transported with a PCB such as this one, presented in

Figure 6 with a sintered wick structure, is 11 W, according

to Equation (2). This value was obtained assuming an

operational temperature of 80°C. Also, this value is for

the worst case scenario, with the heat pipe working fluid

ascending against gravity.

Conclusions

In conclusion, two existing concepts for integrating heat

pipes into PCBs have been presented. Several challenges

have been identified and a new prototype for a PCB-

integrated heat pipe design has been proposed.

_ ADVANCED \ AT THERMAL SOIUTIONS, INC. Innovations In Thelmnl Mnnngement‘D

The heat pipe is integrated within the laminated structure of

the PCB. The wick structure of the heat pipe is positioned

with the help of two copper inserts that also minimize the

top-bottom thermal resistance. Additional plating steps were

introduced in the manufacturing process with the result of

considerably reducing the dead volume. The new prototype

should transport 11 W of power independent of orientation.

This is the first step towards further integration, with the

designers of the future PCBs also designing the thermal

management solution. Obviously, validation of the concept

needs to be done, with short and long term tests for reliability

as a part of it. For volume production, reliability testing

(including shock and vibration) is absolutely necessary.

Even though it looks like a very promising new integrative

approach, the end user should always be cautious when

deciding to use it in a real application. First of all, an analysis

of the risks versus rewards of this technology should be

performed. Why should one use such a method versus the

traditional heat pipe, what are the benefits and at what risks?

Last but not least, what is the cost of a PCB integrating a

heat pipe and is it justifiable?

References:

1. Wits, W., and Riele, G., “Advanced in Integrated Heat Pipe

Technology for Printed Circuit Boards”, 40th Conference on

Environmental Systems, 2010.

2. Jones, K., Cao Y., and Cao M., ”Development of micro

heat pipes embedded in laminate substrates for enhanced

thermal management for printed wiring boards”, Technical

Report Number AFRLPR-WP-TR-2003-2011, Florida

International University, Miami, USA, 2002.

3. Wits, W., Legtenberg, R., Mannak, J., and van Zalk,

B., “Thermal management through in-board heat pipes

manufactured using printed board multilayer technology”,

Proceedings of the 31st International Conference on

Electronic Manufacturing and Technology (IEMT), Petaling

Jaya, Malaysia, 2006, pp. 55-61.

HAND

HELD

ANEMOMETER

THERMOELECTRIC MODULES

CHILLERS THERMOELECTRIC ASSEMBLIES

HEAT

EXCHANGERS

LEAK

DETECTORS

FLOW METERS

HEAT PIPES

._I

Nanofluids in Heat Pipes

Introduction

Heat pipes are simple heat transfer devices that are ubiquitous in electronics cooling applications. They

transfer heat with a very low thermal resistance but are limited in the maximum load by various factors. The

performance of the heat pipe is defined by both the maximum heat load and the thermal resistance when

operating within that limit.

Figure 1 shows the schematic of a heat pipe and the

different component thermal resistances. One of the

variables that determine the performance of a heat pipe

is the thermophysical properties of the working fluid.

Because of the temperature range of electronics, the most

cost effective working fluid is water. Although water is the

best heat transfer fluid, its properties can be enhanced by

introduction nano-particles.

Figure 1. A Heat Pipe and its Thermal Resistance Network [1]

Nanofluids

The mixture of nanoparticles and a base fluid is known

as a nanofluid. Nanofluids have been studied extensively

over the past few decades because of their use in a wide

Figure 2. CuO Nanoparticles in an 80:20 Water-Glycerin

Base Fluid at Concentrations of 0.1, 0.3, 0.5, 0.8 and 1%

by Weight [3] range of applications [2]. The techniques for

creating such colloidal suspensions vary depending on the

base fluid and the nanoparticle itself. The most common

method is to disperse the nanoparticle powder in water

through the use of an ultrasonic bath. Harikrishnan et al. [3]

studied CuO nanoparticles in an 80:20 water-glycerin base

fluid at different concentrations as seen in Figure 2.

Figure 2. CuO Nanoparticles in an 80:20 Water-Glycerin Base

Fluid at Concentrations of 0.1, 0.3, 0.5, 0.8 and 1% by Weight [3]

Heat Pipe Performance

Sureshkumar et al. [4] conducted an extensive review of

the considerable research on nanofluids and heat pipes

over the past two decades. As an aggregate, the results

showed a positive change in the performance of heat

pipes by decreasing the thermal resistance and sometimes

through maximum allowable heat load.

Shafahi et al. [5] used experimental and theoretical

analysis and found CuO to be the most effective nanofluid

for use within heat pipes. As Figure 3 shows, the maximum

heat transfer from a heat pipe increases 15-20% with

a CuO concentration of 0.15. The nanofluid made from

Alumina (Al2O3) and Titanium Oxide (TiO2) have little

overall effect on the maximum heat transfer rate. However,

all three nanofluids reduced the overall thermal resistance

of the heat pipe. Figure 4 shows the relative decrease

in thermal resistance for the three nanofluids at a 4%

concentration.

In addition to the common nanofluids listed above, other

researchers have explored more exotic nanofluids. Tsai et

al. [6] used gold nanoparticles and found that the thermal

i 5 w I 55°0____—r——._ G T3112! ”“0 0.01 0.“ I.“ ”I o 1 on [as r ---------------------- _ O 0.1 A‘2 1 ---CuO ”0"" . .1102 my,——ﬂ—l§ 1-:mm 5 mo ’m—l—ﬂ—— 500' new 1’ run o 0.09 0.1 ofu a: 0.15 o ‘50. ﬁlm, 7m 0'500 3m M we 600 m Holt Inpqu) transfer charameristics of the nanoﬂuids [7]. 0,85 0,80 0.75 0.70 065 060 0.55 Thermal Condncﬁvity (WI-1K) 050 0.0 02 0‘ 06 Mu: rncllon NW.) 03 1.0 Figure 5. Thermal Conductivity vs Concentration of CuO l Maximum kginnl I Rrxion II I lit-ﬂu! IV | I (:rhienllhnll m". l Nuclwc ‘ M "m I Fi|m l boiling 1 mm". I mum | l M“; I E msV Naumﬂ | l | g common I H 1 | ‘ ‘ 3; boiling I l Mmmmm .. ml _ | | 5mm, l "I hm ﬂuxﬂ"... A ,J m; lmlu l qumlll I Imus' I | m. «I l l I s m 30 I00 :20 mo AT=T.. - T“ ('C)

resistance of the heat pipe dropped from 0.215 to 0.17°C/W

with a specific cocktail of HAuCl4, Na3 Citrate and Tannic

Acid. This is almost the same fluid as Alumina as used in [5]

but with added complexity and more expensive gold. Kang et

al. [7] found success with 35 nano-meter silver particles at a

concentration level of 10 parts-permillion in pure water. The

thermal resistance reduced from a range of 0.004-0.005°C/

W to 0.001-0.002°C/W for different power loads.

Figure 3. Maximum Heat Transfer Limit of Heat Pipes

with Various Nanofluids [5]

Figure 4. Thermal Resistance of Various Nanofluids Shown

as a Ratio of the Thermal Resistance with Pure Water [5]

Explanation For The Effect Of Nanofluids

Most of the researchers in this field have concluded that the

largest effect of nanofluid on the performance of heat pipe is

due to an increase in thermal conductivity over the base fluid

[4]. As an example, Figure 5 shows an increase in thermal

conductivity by using Copper Oxide (CuO) nanoparticles

in an 80:20 water-glycerin base fluid, measured by using

differential scanning calorimetry [3]. Curiously, although

attempts have been made in understanding the mechanism

for the increase in thermal conductivity, there is no clear

explanation. Wang et. al summarized the plethora of

research work that has gone into understanding the heat

transfer characteristics of the nanofluids [7].

Figure 5. Thermal Conductivity vs Concentration of CuO

Nanoparticles in an 80:20 Water-Glycerin Base Fluid [3]

In addition to an increase in thermal conductivity, nanofluids

also change the density and viscosity of the fluid. The increase

in density allows for a more efficient mass transfer per volume

but the increased viscosity leads to a higher pressure drop [5].

The reason for the optimal performance at the middle of the

concentration range in Figure 3 can be partially explained due

to an increase in pressure loss that is much higher than the

increase in mass transfer per volume. In other words, after a

certain increase in nanoparticle concentration, the resulting

fluid becomes so viscous that it negates any other gains. In

practice, higher concentration of nanoparticles also leads to

sedimentation and blocking of the wicking material.

Figure 6. The Pool Boiling Curve. Heat Pipe Evaporators Should

Stay in Regions I and II [7]

The increase in maximum allowable heat and the thermal

conductance can also be attributed to the shift in the

boiling curve. Figure 6 shows the pool boiling curve and its

associated regimes [7]. Beyond the critical heat flux (CHF),

the heat pipe evaporator requires a higher wall temperature

for the same amount of heat flux. Additionally, the larger

nucleation bubbles close to and beyond the CHF can also

block the liquid flow in the wicking and disrupt the capillary

pumping. The general consensus in the operation of heat

pipes and thermosyphons (a wickless heat pipe) is to remain

to the left, regions I and II in Figure 6, of the CHF [8].

The addition of nanofluids increases the CHF [4]. This

phenomenon raises the overall boiling limit of the heat pipe.

As an example, Yang and Liu [10] found a 14% increase in

allowable maximum heat flux for a looped thermosyphon

by using a 1.5% concentration of Al2O3 by weight. The

researchers attributed that performance increase to an

increase in CHF.

Conclusions

The addition of nanoparticles to a base working fluid can

increase the thermal performance of a heat pipe. The

performance increase comes as a decrease in thermal

resistance and an increase in total allowable heat load.

The two most common nanofluids with water are Alumina

(Al2O3) and Copper Oxide (CuO); although positive effects

have also been seen with Titanium Oxide (TiO), Gold

and Silver. While many of these nano-particles come in

commercially available powder form, manufacturers should

evaluate the increased complexity of creating such a heat

pipe vs the benefits outlined in this article.

References:

1. Prasher, R., “A Simplified Conduction Based Modeling

Scheme for Design Sensitivity Study of Thermal Solution

Utilizing Heat Pipe and Vapor Chamber Technology”,

J. Electron Packaging, 2003

2. www.hindawi.com/journals/ame/2010/519659/

3. Harikrishnan, S., Roseline, A., Kalaiselvam, S.,

“Preparation and Thermophysical Properties of Water-

Glycerol Mixture-Based CuO Nanofluids as PCM for Cooling

Applications”, IEEE Transactions on Nanotechnology, 2013.

4. Sureshkumar, R., Mohideen, S., “Heat Transfer

Characteristics of nanofluids in heat pipes: A review”,

Renewable and Sustainable Energy Reviews, 2013

5. Shafahi, M., Bianco, V., Vafai, K., Manca, O., “An

Investigation of the thermal performance of cylindrical

heat pipes using nanofluids”, International Journal of

Heat and Mass Transfer, 2009.

6. Wang, X., Mujamdar, A., “Heat Transfer characteristics

of nanofluids: a review”, International Journal of Thermal

Sciences, 2007.

7. Ghiaasiaan “Two-Phase Flow, Boiling, and

Condensation”, 2008.

8. Reay, D. and Kew, P., “Heat Pipes: Theory,

Design and Applications”, 2006.

9. Yang, X., Liue, Z., “Flow Boiling heat transfer

in the evaporator of a loop thermosyphon operating

with CuO based aqueous nanofluid”. International

Journal of Heat and Mass Transfer, 2012.

pedia For additional Heat Pipe Articles,

as well as our complete Qpedia Library of articles, visit

www.qats.com/Qpedia-Thermal-eMagazine

The Online Publication and Book Series Dedicated

to the Thermal Management of Electronics

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