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
© Advanced Thermal Solutions, Inc. 1
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Heat Pipes: Heat Super Conductors
© Advanced Thermal Solutions, Inc. 2
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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© Advanced Thermal Solutions, Inc. 3
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
70
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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© Advanced Thermal Solutions, Inc. 4
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.
© Advanced Thermal Solutions, Inc. 5
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:
R
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.
© Advanced Thermal Solutions, Inc. 6
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
ΔPcapillaryΔPv + 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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© Advanced Thermal Solutions, Inc. 7
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].
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.
© Advanced Thermal Solutions, Inc. 8
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
e
D
h
v
TIRED OF SPENDING TIME
CALCULATING EQUATIONS?
Nu = hL
k
Rtotal = Rconv,1 + Rwall,1 + Rwall, 2 + Rwall,3 + Rconv,4
Q = h∙A(T
s
- T
)
h(Tf - Tt ) = εtσ(Tt - Tw)
44
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.
© Advanced Thermal Solutions, Inc. 9
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.
© Advanced Thermal Solutions, Inc. 10
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:
ΔPcapΔPv + 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
© Advanced Thermal Solutions, Inc. 11
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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© Advanced Thermal Solutions, Inc. 12
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<>
© Advanced Thermal Solutions, Inc. 13
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
© Advanced Thermal Solutions, Inc. 13
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.
© Advanced Thermal Solutions, Inc. 15
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)
© Advanced Thermal Solutions, Inc. 16
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]
© Advanced Thermal Solutions, Inc. 17
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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