Design of Practical Liquid Metal
Cooling Device for Heat
Dissipation of High Performance
CPUs
Yueguang Deng
Jing Liu1
e-mail: jliu@mail.ipc.ac.cn
Key Laboratory of Cryogenics,
Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences,
Beijing 100190, P. R. China
Broad societal needs have focused attention on technologies that can effectively dissipate
huge amount of heat from high power density electronic devices. Liquid metal cooling,
which has been proposed in recent years, is fast emerging as a novel and promising
solution to meet the requirements of high heat flux optoelectronic devices. In this paper,
a design and implementation of a practical liquid metal cooling device for heat dissipation of high performance CPUs was demonstrated. GaInSn alloy with the melting point
around 10° C was adopted as the coolant and a tower structure was implemented so that
the lowest coolant amount was used. In order to better understand the design procedure
and cooling capability, several crucial design principles and related fundamental theories were demonstrated and discussed. In the experimental study, two typical prototypes
have been fabricated to evaluate the cooling performance of this liquid metal cooling
device. The compared results with typical water cooling and commercially available heat
pipes show that the present device could achieve excellent cooling capability. The thermal resistance could be as low as 0.13° C / W, which is competitive with most of the latest
advanced CPU cooling devices in the market. Although the cost (about 70 dollars) is still
relatively high, it could be significantly reduced to less than 30 dollars with the optimization of flow channel. Considering its advantages of low thermal resistance, capability
to cope with extremely high heat flux, stability, durability, and energy saving characteristic when compared with heat pipe and water cooling, this liquid metal cooling device is
quite practical for future application. 关DOI: 10.1115/1.4002012兴
Keywords: liquid metal cooling, electromagnetic pump, high performance CPUs, heat
transfer enhancement, thermal management, chip cooling
1
Introduction
The last five decades have witnessed great prosperity and development of very large scale integrated circuits and personal
computers in the microelectronic industry. Meanwhile, the ever
increasing power density of electronic component and more compact package technology lead to the tough issue that thermal management becomes rather hard to solve 关1兴.
So far, lots of cooling methods have been proposed and investigated to meet the needs of various kinds of microelectronic devices. As for the typical applications, which have the heat flux
density below 100 W / cm2, such as LED lamp and advanced
CPUs, conventional fan heat sink, and heat pipe, are most widely
used because these cooling methods can effectively meet the heat
dissipation requirements and own the evident predominance of
low cost 关2,3兴. However, if the heat flux density goes beyond
100 W / cm2 or even reaches 1000 W / cm2, typical liquid based
cooling methods such as microchannel and jet impingement are
necessary and become the mainstream in industry 关4,5兴. Apart
from those mature technologies, some novel and advanced cooling methods such as electrohydrodynamic approach, thermoelectric method, nanofluid or phase change microcapsule fluid cooling, and piezoelectric fan also have been proposed and widely
investigated 关6–10兴. But these innovative cooling methods often
involve complex fabrication process, high cost, reliability issues,
1
Corresponding author.
Contributed by the Electronic and Photonic Packaging Division of ASME for
publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received February
28, 2010; final manuscript received May 26, 2010; published online September 9,
2010. Assoc. Editor: Giulio Lorenzini.
Journal of Electronic Packaging
or yet, insufficient cooling capability, which are the main obstacles for their large scope commercialization and utilization.
In recent years, liquid metal cooling has been proposed and fast
emerged as a novel and effective heat dissipation method due to
the superior thermal physical properties and the unique
electromagnetic-driven characteristic of liquid metal 关11兴. A lot of
researches have been performed to investigate the cooling capability and the typical applications of liquid metal such as the heatdriven liquid metal cooling device, nanoliquid metal fluid, liquid
metal injection, and so on 关12–16兴. However, commercially practical liquid metal cooling devices for heat dissipation of high performance CPUs are rarely reported in scientific literatures. Moreover, the fundamental design theory and the economic feasibility,
which are of great concern to guide the device, design, and determine the commercial prospect of liquid metal cooling products,
are hard to find in existing literatures.
In the present work, a comprehensive description of practical
liquid metal cooling system design theory, device fabrication, performance evaluation, and economic feasibility has been demonstrated and discussed. Several critical fundamental design principles were given. Two typical liquid metal cooling prototypes
were fabricated and cooling capability comparison with typical
cooling devices in the market was performed. The compared result and economical feasibility were presented and discussed in
the end and some important conclusions were drawn.
2
System Design Principles
2.1 Thermal Resistance Evaluation Theory. Generally
speaking, system thermal resistance is the most effective physical
quantity to evaluate the heat dissipation performance of a cooling
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Fig. 1 „a… Schematic view of liquid cooling and „b… the corresponding thermal resistance network
device operating under steady state condition. The smaller system
thermal resistance one has, the better cooling capability could be
achieved. Generally, the thermal resistance of a cooling system
can be defined as
Rsys =
T j − Ta
q
共1兲
where Rsys is the system thermal resistance, q is the heat flux, and
T j and Ta are the heat source and ambient temperature, respectively. Therefore, no matter which kind of cooling method is
implemented, the system cooling capability can be conveniently
evaluated when the heat flux, heat source, and ambient temperature are measured.
The typical schematic view of a liquid cooling system can be
depicted as Fig. 1共a兲. From which it can be seen that the heat
would first be transferred from the heat source to the cold plate
and then to the flowing coolant through the liquid convection, and
finally, would be dissipated to the ambient air when the hot fluid
flows through the remote radiator.
Therefore, based on the fundamental heat transfer theory and
considering the coolant temperature rise effect 关17兴, a typical thermal resistance evaluation theory for liquid metal cooling system
can be presented as follows 共Fig. 1共b兲兲:
Rsys = RTIM + Rconv + Rc + Rradiator
RTIM =
Rconv =
T j − Tcold
q
Tcold
Rc =
plate
plate −
Tf
q
T f − Tin
q
Rradiator =
Tin − Ta
q
共2兲
共3兲
共4兲
共5兲
共6兲
where RTIM, Rconv, Rc, and Rradiator are the contact thermal resistance, convective thermal resistance in the cold plate, heat capacity thermal resistance, and radiator thermal resistance, respectively, Tcold plate is the temperature of cold plate, T f is the fluid
mean temperature in the cold plate, and Tin is the fluid inlet temperature in the cold plate. In this theory, the system thermal resistance of liquid metal cooling system would consist of the contact
thermal resistance existing between heat source and cold plate,
convective thermal resistance in the cold plate, heat capacity ther031009-2 / Vol. 132, SEPTEMBER 2010
mal resistance due to the temperature rise of liquid metal, as well
as the remote radiator thermal resistance. Among them, the contact thermal resistance is closely related to the contact interface
condition, heat-conducting oil thermal properties, and the installation pressure. The convective thermal resistance mainly depends
on the convective heat transfer coefficient of liquid metal and the
cold plate heat transfer area. Because the liquid metal has very
high convective heat transfer coefficient, generally, this thermal
resistance is relatively small. The heat capacity thermal resistance
results from the temperature rise when liquid metal flows through
the cold plate. It is introduced to evaluate the negative impact of
fluid temperature rise on system cooling performance. In a general
water cooling system, the heat capacity thermal resistance usually
can be neglected due to the very small temperature difference
between the inlet and outlet. But as for a liquid metal cooling
system, the heat capacity thermal resistance could be very evident
and should not be neglected because liquid metal owns a much
lower heat capacity. Lastly, the radiator thermal resistance is used
to evaluate the cooling capability of the remote radiator. The
closer the cold plate inlet temperature to the ambient temperature,
the smaller radiator thermal resistance would be. Ideally, the cold
plate inlet temperature could equal to the ambient temperature
under the condition of super strong fan and large fins, thus, “zero”
radiator thermal resistance could be achieved. However, most of
the actual radiators could not achieve that performance due to the
restriction of the system volume and cost. Moreover, the radiator
thermal resistance always accounts for a considerable proportion
of the whole system thermal resistance.
The foregoing theory can be used very conveniently to evaluate
the heat dissipation performance and optimize the liquid metal
cooling system. By actually measuring the temperature data and
system heat flux, these four thermal resistances could be easily
calculated and compared. Thus, the bottleneck of the cooling system can show up and tell which section should be improved. A
qualified cooling system should ensure that the four thermal resistances are of relatively average level and no obvious heat transfer
bottleneck appears. The mean temperature of the fluid cannot be
measured experimentally, thus, the following equation can be
used:
T f = Tcold
plate −
Tout − Tin
Tcold plate − Tin
ln
Tcold plate − Tout
共7兲
where Tout is the fluid outlet temperature of cold plate. Both Tin
and Tout could be obtained with thermocouples placed in the inlet
and outlet pipelines of the cold plate. Because the cold plate generally has thick substrate and is commonly made of copper alloy
with high thermal conductivity, its temperature can be assumed to
be uniform in the substrate. Similar to the derivation process of
logarithmic mean temperature difference, the detailed derivation
of Eq. 共7兲 is not described here.
2.2 Electromagnetic Pump Design Principles. Because the
liquid metal owns very high conductivity, it can be driven using a
high efficient electromagnetic pump with no moving parts. The
basic principle of the electromagnetic pump lies in that under the
combined effect of electric and magnetic fields, the Lorenz force
could be generated and then drives the liquid metal to flow, thus,
achieves the energy conversion from electricity to the fluid mechanical energy. According to Bernoulli’s equation, the driving
pressure generated by an electromagnetic pump would be directly
used to overcome the flow resistance of pump itself as well as the
outside flow loop, which can be described as
Hdriv = Hpump + Hloop
共8兲
where Hdriv is the driving pressure generated by electromagnetic
pump and Hpump and Hloop are the flow resistance of pump and the
outside flow loop, respectively. Therefore, the key design principle of electromagnetic pump is to maximize the pump driving
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pressure while reducing the flow resistance of pump itself, so as to
enable the outside loop to get the highest driving pressure and
achieve the largest volume flow.
As for a typical electromagnetic pump designed for heat dissipation of high performance CPUs, the pump body is generally
made of insulating materials and the magnetic field would cover
all the current flow area. Therefore, the electricity loss of wall
current and spreading current could be neglected. Then, the driving pressure of electromagnetic pump can be expressed as
Hdriv =
BIL
␳gAcs
共9兲
where B is the magnetic induction intensity, I is the current, L is
the flowing distance of current in the pump, ␳ is the fluid density,
and Acs is the cross section area of pump flow channel. A typical
pump cross section may be rectangular or circular and the corresponding driving pressure can be calculated as
Hdriv_rect =
BI
␳ga
共10兲
Hdriv_circ =
2BI
␳g␲r
共11兲
where a is the height of flow channel with rectangular cross section and r is the radius of flow channel with circular cross section.
As can be seen from Eq. 共10兲 and 共11兲, the driving pressure of
electromagnetic pump with rectangular cross section has no relationship with the channel width along the current direction but
only depends on the channel height. The lower channel height it
is, the higher pump driving pressure can be generated. Similarly,
for the electromagnetic pump with circular cross section, the
smaller the diameter of flow channel would lead to the higher
pump driving pressure. As for the flow resistance, the local resistance plays a decisive role in the total flow resistance of pump,
which, thus, can be expressed as
Hpump = SQ2
共12兲
where S is the flow resistance coefficient of pump and Q is the
volumetric flow while the flow resistance of outside loop consists
of both one-way resistance and local resistance and can be described as
Hloop = ␭
l v2
v2
+␨
d 2g
2g
共13兲
where ␭ is the frictional resistant coefficient, l is the flow distance,
d is the diameter of flow channel, v is the velocity, and ␨ is the
local resistance coefficient. In general, due to the diversity of
pump structure and the influence of fabrication process, the theoretical calculated flow resistance coefficient has evident deviation
with actual flow resistance condition. Therefore, once the volume
flow is determined according to the heat flux, generally, experiments are needed to measure the actual flow resistance drop of
pump and outside loop. Then, the required current for the electromagnetic pump could be determined.
In fact, the key design principle for electromagnetic pump is to
achieve the balance between the driving pressure and the pump
flow resistance. Under a certain current, the height of flow channel
needs to be minimized to achieve a high driving pressure but that
will inevitably bring out a substantial increase in pump flow resistance. Therefore, a suitable channel size is of great importance
and needs to be determined based on the aforementioned theories,
then, an optimized electromagnetic pump with high driving capability can be achieved.
2.3 Radiator Design Principles. A qualified design of radiator should not only have small thermal resistance but also own the
small size, light weight, and beautiful appearance. Due to the
diversity of fans and the complexity of air side convective paramJournal of Electronic Packaging
eters, the current optimization of the radiator is generally achieved
using numerical simulation and experimental justification 关18,19兴.
The typical radiator optimization parameters include: heat dissipation area, fin length and width, thickness, spacing, size and
location of copper tubes, etc. Among them, the most important
parameter is the fin heat dissipation area, which directly determines the thermal resistance and the radiator size. The typical fin
heat dissipation area can be estimated using the following formula:
A = 1/共hRradiator兲
共14兲
where A is the fin heat dissipation area, h is the air side convective
coefficient, and Rradiator is the designed air side thermal resistance
of radiator. The air side convective coefficient can refer to the
current design parameters of commercialized heat pipes and cooling fans. In fact, the remote radiator is similar to the conventional
heat pipe and air conditioning condenser. Therefore, the design
theory of this part can draw on the traditional fin heat exchanger
design theory, as well as the typical design parameters of commercial excellent heat pipe and will not be detailed here.
3
System Fabrication and Characterization
3.1 Electromagnetic Pump Design. The basic structure of
electromagnetic pump comprises two longitudinally symmetrically arranged magnets, a pair of electrodes located on both sides
of the pump body and the liquid metal flow channel in the middle.
Therefore, the actual implementation of electromagnetic pump
can be very flexible and would only be restricted by the geometrical dimension of the radiator. The pump body can be made of any
nonconductive material, which is compatible with liquid metal. A
lot of engineering plastics, such as polyethylene, polypropylene,
polyvinyl chloride, etc., can be adopted as the suitable structure
material. In view of the electrical conductivity and chemical compatibility, copper substrate with surface nickel-plate is appropriate
to fabricate electrodes. As for the magnet, both permanent magnet
and electromagnet magnet are suitable. But taking into account
the pump size, weight, and driving force factors, the adoption of
NdFeB permanent magnet is more appropriate since NdFeB owns
the largest magnetic induction intensity in all the magnetic materials and the temperature range and price are also acceptable. In
order to avoid the electromagnetic interference on the electronic
components, a soft magnetic material, which has high saturation
magnetic induction, should be implemented to shield the magnetic
field. Typical soft magnetic material can be silicon steel, 2Cr13
stainless steel, and so on. The basic principle of magnetic field
shielding lies in that the magnetic flux leakage should be very
small while the shielded magnetic ring should not be too thick to
increase system weight. In general, Eq. 共15兲 can be used to determine the cross sectional area of the shielding magnetic ring
Scross =
BsurSsur
Bs
共15兲
where Scross is the cross section area of the shielding magnetic
ring, Bsur is the surface magnetic induction of magnet, Ssur is the
surface area of magnet, and Bs is the saturation magnetic induction of shielding soft magnetic material. In addition to shielding
the inner magnetic field, the magnetic shield ring can also reduce
the outside reluctance and strengthen the magnetic induction in
the liquid metal flow channel.
Because the tower structure of the liquid metal cooling device
in this work was designed to have three independent flow channels, therefore, the system needs three electromagnetic pumps to
drive the flow of liquid metal. Considering the large current requirement, the three electromagnetic pumps were designed to be
electrically connected in series. Two kinds of electromagnetic
pumps with integrated and separated structure were designed in
this work of which the integrated pump had internal fluid of three
pumps connected, therefore, the pump was smaller and easy to fill
SEPTEMBER 2010, Vol. 132 / 031009-3
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Fig. 2 Two liquid metal cooling prototypes with „a… square and
„b… flat structure
the coolant while the separated pump appeared more complicated
although the coolant amount could be relatively smaller.
3.2 Cooling System Design. Comprehensively considering
the safety, physical and chemical stabilities, and thermal properties, Ga66In20.5Sn13.5 was adopted as the coolant of these liquid
metal cooling prototypes because it owned the lowest melting
point in the gallium based alloy as far as we knew. Based on our
experiments, its melting point can be as low as about 10° C and
the biggest advantage lies in the much higher thermal conductivity
compared with that of water 关20兴. Because the price of liquid
metal is still relatively high, the coolant amount is the key factor
to determine the cost of the whole system. In order to minimize
the connecting pipes so as to make full use of liquid metal, the
tower structure was implemented in this experiment.
Taking into account the fabrication process, liquid metal compatibility, system weight, and appearance, the remote radiator was
implemented with the conventional copper tube and aluminum fin
structure. The fin thickness of 0.5 mm with spacing of 2 mm was
adopted when learning from the typical design data of commercial
heat pipes. Two prototypes, namely, square radiator and flat radiator with different geometry size, matching fan, and coolant
amount were designed. In the experiment, the separated electromagnetic pump was installed on the square radiator while the
integrated pump was installed on the flat one. The schematic view
of these two system prototypes are shown in Fig. 2 from which we
can see that the flat one owns more reasonable structure and beautiful appearance.
The square radiator has a smaller volume and weight with the
total heat dissipation area of 0.36 m2 and the matched 8cm fan
has the speed of 1900 RPM and maximum air flow of 23CFM
while the flat radiator owns larger volume with the heat dissipation area of 0.58 m2 and the matched 12cm fan has the speed of
1300 RPM and maximum air flow of 53CFM. Therefore, it can be
known that the flat radiator owns higher air flow but with lower
fan speed and noise. In order to ensure a higher air side convective coefficient and fin efficiency, staggered copper tubes were
arranged so the heat accumulation effect could be effectively
eliminated.
3.3 System Cooling Capability Evaluation. Based on the
current thermal design power of typical high performance CPUs, a
100 W simulating heat source was adopted to evaluate the cooling
performance of these two liquid metal prototypes. Figure 3 presents the comparison result with commercial excellent heat pipes
and water cooling on the same experimental platform. Due to the
great uncertainty of contact thermal resistance, which is closely
related to the surface conditions, the ordinate of Fig. 3 only shows
the substrate temperature of these cooling devices, therefore, the
deviation induced by contact thermal resistance can be eliminated.
In the experiment, the ambient temperature is 21° C. The other
four heat pipes are CoolerMaster Hyper Z200, Thermaltake Mini
Tower, Thermalright U120E, and PCCooler HP-1216V, which are
in the medium top grade of commercially available heat pipes and
031009-4 / Vol. 132, SEPTEMBER 2010
Fig. 3 The substrate temperature comparison of testing cooling devices under heat flux of 100 W
the latter two even have been awarded as the king of CPU heat
pipes in international market. The experiment of water cooling
was performed using the same square radiator with the same configurations as the liquid metal prototype but only substituting the
coolant with water and the fluid circulation was driven using a
peristaltic pump with the same volume flow as liquid metal prototype. As for the equipped fans, two medium grade fans with
performance parameters shown above were adopted to equip these
two liquid metal prototypes while the remaining products used
inborn fans so as to ensure the results authenticity of original
products. However, because PCCooler HP-1216V and Thermalright U120E had not inborn fans, the same fan as the flat liquid
metal prototype was equipped on these two products.
As can be seen from Fig. 3 under the same ambient temperature
of 21° C, both the liquid metal prototypes can achieve the medium
top grade cooling capability in the market and the flat prototype
can compete with most of the CPU cooling devices and even
reach the top cooling performance of commercial heat pipes. The
current inferiorities of flat prototype lie in the smaller heat dissipation area and less coolant pipes, which would greatly reduce the
fin efficiency and increase the thermal resistance when compared
with HP-1216V or U120E. Moreover, the nonmold fabrication
process also has some negative effect on the overall cooling performance, especially for the air side performance of radiator.
Therefore, more elaborate optimization on geometric structure
needs to be conducted so the liquid metal cooling devices can
compete with the commercially matured top CPU heat pipes.
When compared with water cooling, it can be found that the liquid
metal cooling could always have much better cooling performance
under the same structure form and experimental condition. This is
mainly because the liquid metal owns very high convective heat
transfer coefficient in the cold plate, therefore, results in a very
small convective thermal resistance.
The thermal resistance distributions of water cooling and liquid
metal cooling with square and flat structure, respectively, are
shown as Fig. 4. Because the installation pressure and contact
surface conditions are quite different for different devices, the
contact thermal resistance is not discussed here.
When comparing water cooling with the square liquid metal
prototype, it can be found that the greatest advantage of liquid
metal lies in its much larger convective coefficient so the convective thermal resistance in the cold plate could be much smaller.
Then, comparing the square prototype with the flat one, it can be
found that the more excellent cooling performance of flat prototype lies in its smaller radiator thermal resistance, which is attributed to that the flat structure has a larger heat dissipation area,
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Fig. 4 Thermal resistance distributions of water cooling and
liquid metal cooling „the “TR” in the figure means “thermal
resistance”…
smaller air flow resistance, and greater air flow. Additionally, the
higher convective thermal resistance of flat prototype compared
with square one is due to the relatively smaller area of cold plate.
Therefore, as for the liquid metal cooling system, the convective
thermal resistance in the cold plate could be effectively reduced
but air side thermal resistance of remote radiator would become
the bottleneck of the whole system, thus, optimized design of
radiator is of crucial importance to achieve the highest performance of liquid metal cooling system.
4
Economic Analysis and Other Practical Issues
The cooling performance and system cost are two most important factors to determine whether a liquid metal cooling device
can be large scope commercialized or not. As for the cooling
performance, the liquid metal cooling prototypes we designed can
compete with most of the medium top grade commercial cooling
devices. Although it lost the competition with the top heat pipes
such as the U120E, which was awarded as the king of fan radiator,
but if one increases the heat dissipation area, increasing the number of flow channel while reducing the channel diameter, as well
as to improve the quality of fabrication process, the liquid metal
cooling device can be effectively improved and go beyond the top
heat pipes in the market. That is mainly because the convective
coefficient of liquid metal could be very high; it even reaches
105 W / m2 ° C 关12,15兴, which is much higher than the boiling
heat transfer coefficient of heat pipes. However, due to the cost
restriction, liquid metal cooling devices cannot have so many flow
channels as heat pipes, thus, the heat transfer area is smaller and
the cooling capability would be reduced. The high cost of liquid
metal is the most critical bottleneck to preclude the large scope
commercialization of liquid metal cooling devices. Therefore, optimizing the structure and reducing the coolant amount are of
great importance. The current flat prototype has a coolant amount
of 160 g, which means the cost of liquid metal is about 60 dollars.
A series of additional experiments have been performed to reduce
the system cost, mainly by reducing the number of flow channel,
reducing the channel inner diameter while increasing the outer
diameter and increasing the electromagnetic pump drive current,
etc. The experimental results showed that while ensuring the appropriate heat transfer area, flow resistance, as well as the medium
cooling capability 共typical thermal resistance of 0.17° C / W兲, the
current cost of coolant could be reduced to 20 dollars. Therefore,
the total cost of the system can be controlled at around 30 dollars.
When compared with the market price of high-end commercial
CPU radiators such as the Thermalright U120E with the price of
60 dollars and the water cooling Corsair Hydro Series H50 with
Journal of Electronic Packaging
the price of 80 dollars, the liquid metal cooling device, as a brandnew product in the CPU cooling field, is really economically feasible and competitive.
In fact, liquid metal cooling as a new solution of liquid cooling,
owns a series of other unique advantages. On the one hand, liquid
metal has very stable thermal physical properties and the boiling
point can be as high as 2000° C, therefore, the single-phase flow
of liquid metal can be very stable and reliable under the condition
of instantaneous thermal shock and extremely high heat flux 共typically 100 W / cm2兲. The operation failures such as being burned
out or burst occurred in the heat pipes, could be effectively
avoided in the liquid metal cooling system. One the other hand,
when compared with water cooling, liquid metal solution not only
owns excellent cooling performance but can also be very energyefficient. That is mainly because the electromagnetic pump has no
mechanical moving part, therefore, the mechanical efficiency loss
and flow resistance loss in the pump itself can be significantly
reduced. For example, in this experiment, the pump input power
of the flat prototype is only 1.5 W while the typical water cooling
system in the market generally consumes electric power of more
than 10 W. Therefore, the energy saving characteristic of electromagnetic pump is rather evident.
Although the liquid metal cooling method cannot be considered
as the best solution and to replace all the CPU cooling devices in
the market, it provides a new practical option, which can achieve
the most excellent cooling performance for rapidly developing
CPUs. In fact, in the case of small heat flux, liquid cooling cannot
show very evident predominance compared with heat pipes, for
instance, the water cooling in Fig. 3 has the highest system thermal resistance and consumes the highest pump power. However,
liquid cooling is still a very important and practical heat dissipation solution. The main reason lies in its excellent stability and
capability for solving high heat flux applications and the convenient implementation flexibility. As for the liquid metal cooling
system except for the merits of liquid cooling presented above, it
also has the advantages of being not easy to evaporate and leak,
absolute silent operation, and energy saving characteristic. Therefore, the liquid metal cooling could serve as a very broad application technology, which should not be limited to the CPU cooling area. Typical applications include large power lasers, high
brightness LED, and so on. In these extremely high heat flux
applications, the liquid metal cooling technology could show
more apparent advantages and, thus, is more practical.
5
Conclusion
In this paper, a design and implementation of a practical liquid
metal cooling device for heat dissipation of high performance
CPUs was demonstrated. A comprehensive description of liquid
metal cooling system design theory, device fabrication, performance evaluation, and economic feasibility analysis has been presented. In the experimental study, two typical liquid metal cooling
prototypes with different geometry structures were fabricated and
cooling capability comparison with typical cooling devices in the
market was performed. The compared results indicated that this
liquid metal cooling device could achieve excellent cooling capability. The thermal resistance could be as low as 0.13° C / W,
which is competitive with most of the water cooling and commercially available heat pipes. As for the economic feasibility, the
total cost of 30 dollars shows a promising prospect of liquid metal
cooling devices for high performance CPUs. Taking consideration
of the advantages of excellent thermal shock resistance, capability
to cope with extremely high heat flux, and stability and energy
saving characteristic when compared with heat pipe and water
cooling, the present liquid metal cooling device is quite practical
and useful.
Acknowledgment
The authors would like to thank the assistance of Mr. Rui Guo
during the process for the device fabrication. This work is parSEPTEMBER 2010, Vol. 132 / 031009-5
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tially supported by the National Natural Science Foundation of
China.
Nomenclature
a ⫽ height of flow channel with rectangular cross
section, m
A ⫽ fin heat dissipation area, m2
Acs ⫽ cross section area of pump flow channel, m2
B ⫽ magnetic induction intensity, T
Bsur ⫽ surface magnetic induction of magnet, T
Bs ⫽ saturation magnetic induction of shielding soft
magnetic material, T
d ⫽ diameter of flow channel, m
h ⫽ convective coefficient, W / m2 ° C
Hdriv ⫽ driving pressure generated by electromagnetic
pump, m
Hdriv_rect ⫽ driving pressure of electromagnetic pump with
rectangular cross section, m
Hdriv_circ ⫽ driving pressure of electromagnetic pump with
circular cross section, m
Hpump ⫽ flow resistance of pump, m
Hloop ⫽ flow resistance of outside flow loop, m
I ⫽ current, A
l ⫽ flow distance, m
L ⫽ flowing distance of current in the pump, m
q ⫽ heat flux, W
Q ⫽ volumetric flow, m3 / s
r ⫽ radius of flow channel with circular cross section, m
Rsys ⫽ system thermal resistance, °C / W
RTIM ⫽ contact thermal resistance, °C / W
Rconv ⫽ convective thermal resistance in the cold plate,
°C / W
Rc ⫽ heat capacity thermal resistance, °C / W
Rradiator ⫽ radiator thermal resistance, °C / W
S ⫽ flow resistance coefficient of pump, s2 / m5
Ssur ⫽ surface area of magnet, m2
Scross ⫽ cross section area of the shielding magnetic
ring, m2
T j ⫽ temperature of heat source, °C
Tcold plate ⫽ temperature of cold plate, °C
T f ⫽ fluid mean temperature in the cold plate, °C
Tin ⫽ fluid inlet temperature in the cold plate, °C
Tout ⫽ fluid outlet temperature in the cold plate, °C
Ta ⫽ ambient temperature, °C
v ⫽ velocity, m/s
␳ ⫽ fluid density, kg/ m3
␭ ⫽ frictional resistant coefficient
␨ ⫽ local resistance coefficient
031009-6 / Vol. 132, SEPTEMBER 2010
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