2. 32 Bell Labs Technical Journal DOI: 10.1002/bltj
ensure reliable equipment operation over a given time
period. There are a number of reasons for the emerg-
ing importance of thermal management such as
increased power densities and loads resulting from
massively-enhanced functionality placed within
smaller footprints, increased electricity and fuel costs,
and recent environmental awareness resulting in
widespread promotion of “green” credentials across
all industries. Telecommunication equipment
providers are coming under greater pressure to design
energy efficient equipment that consumes less power
and is environmentally friendly from a recycling per-
spective. This paper focuses on novel air-cooled ther-
mal solutions that extend the current limits of
conventional air cooling. Recently, liquid cooling solu-
tions have received significant attention in the litera-
ture owing to the ability of liquids to remove vast
quantities of heat; however, the majority of data cen-
ter operators are concerned over the introduction of
liquid cooling for cost (as the existing infrastructure is
predominantly air cooled) and reliability constraints
(owing to the destructive nature that most fluids have
on electronic components). It is for this reason that
extending the limits of air cooling in the short term
can have a positive impact until the general accep-
tance of liquid cooling in commercial electronic appli-
cations is achieved.
The following sections provide an overview of the
importance of thermal management from reliability
and environmental perspectives. Table I provides a
reference to the nomenclature used throughout the
paper.
Importance of Thermal Management
The subject of thermal management is intrinsi-
cally linked to the science of heat transfer. Heat trans-
fer is the transfer of thermal energy from a hot object
to a cold object. There are three modes of heat trans-
fer: conduction, convection, and radiation.
1. Conduction is the transfer of heat via the direct
contact of particles. This mode of heat transfer is
employed when moving the heat generated by
the hot component to the heat sink via layers of
thermal interface material (TIM) and heat spread-
ers that constitute the component package.
2. Convection is the transfer of thermal energy from a
solid to a gas or liquid. There are a number of
convection modes that can be employed by the
thermal engineer:
• Natural convection is the mode of convection
heat transfer where the fluid/gas develops
momentum due to the buoyancy forces
induced by density (caused by temperature)
changes in the fluid.
• Forced convection is the process that is most
evident in modern electronics cooling and
involves the forced movement of fluid parti-
cles by a mechanical device such as a fan.
Panel 1. Abbreviations, Acronyms, and Terms
3D—Three dimensional
AoA—Angle of attack
ATCA—Advanced Telecommunications
Computing Architecture
BCC—Body-centered cubic
CAD—Computer aided design
CFD—Computational fluid dynamics
EPA—Environmental Protection Agency
ETSI—European Telecommunications Standards
Institute
FCC—Face-centered cubic
FFHS—Fin foam heat sink
GPS—Global Positioning System
HCHS—Honeycomb heat sink
IC—Integrated circuit
ICT—Information and communications
technology
L/D—Length-to-diameter
LFHS—Longitudinally-finned heat sink
METI—Japanese Ministry of Economy, Trade
and Industry
NEBS—Network Equipment-Building System
OPEX—Operation expenditure
PIV—Particle image velocimetry
RFID—Radio frequency identification
RTD—Resistance temperature detector
SHS—Schwartz heat sink
TIM—Thermal interface material
UV—Ultraviolet
VG—Vortex generator
3. DOI: 10.1002/bltj Bell Labs Technical Journal 33
In telecommunications equipment, forced
convection is typically achieved by forcing air-
flow over a longitudinally finned heat sink
(LFHS), also referred to as a parallel fin heat
sink. There are other types of forced convec-
tion processes such as direct spray cooling that
have not been introduced into telecommuni-
cation equipment design owing to cost and
reliability constraints.
3. Radiation heat transfer occurs when thermal
energy is emitted via electromagnetic waves con-
centrated in the ultraviolet (UV) and infrared
spectrum [9]. This mode of heat transfer in
telecommunications equipment is typically small.
The importance of heat transfer within thermal
management is evident in all facets of life as heat
transfer is dominant in nearly all energy conversion
and production devices. Find below three examples
that elucidate the importance of heat transfer in pro-
viding novel thermal management architectures:
• In modern jet engines, the turbine blades extract
energy from the upstream combusted flow. The
gas temperatures observed by the turbine blades
are well above the melting temperatures of the
metal blades. In order to prevent the blades from
melting, a number of novel thermal management
techniques are employed. For example, jets of cool
air are ejected from the surface of the blade to act
as an insulting layer between the hot gas and the
metal surface. In addition, the blade surface can
be treated with a low-conductivity ceramic surface
and internal cooling passages are employed within
the blade structure to enhance heat transfer.
• Temperature control is important in biology
where temperature regulates and triggers biologi-
cal responses. Detailed knowledge of heat transfer
is required when treating cancerous legions via
hyperthermal treatments and when using
cryosurgery for localized freezing [9].
• Integrated circuit (IC) technology has grown
exponentially following the prediction of Moore’s
Law, which states that the number of transistors
on a chip will double every 18 months. Thermal
management of ICs is becoming one of the key
restrictions to future growth in this area, as many
more transistors are now packed into the same
footprint, which implies that thermal densities are
increasing considerably.
According to the U.S. Environmental Protection
Agency (EPA) [19], a typical rack of 2’ ϫ 3.5’ ϫ 6’
volume populated with blade servers requires approxi-
mately 20KW to 25KW of power to operate. This is
the equivalent of the peak electricity demand of 25
standard California homes. This figure highlights the
thermal challenge facing telecommunication equip-
ment providers—the majority of this power is con-
verted to heat, and is concentrated in such a small
volume. In order to remove all of this heat from the
blade servers, an equivalent amount of energy
(20 KW to 25 KW) will be required to maintain the
components at or below their critical junction tem-
perature.
Table I. Nomenclature.
A Area (m2
)
AoA Angle of attack (°)
D Diameter of probe (m)
g Acceleration due to gravity (m/s2
H Heat transfer coefficient (W/m2
K)
k k—Thermal conductivity (W/mk)
L L—Length of hole in heat sink base (m)
Nu Nu—Nusslet number (-)
P P—Static pressure (Pa)
Q Q—Power input to base of heat sink (W)
R R—Thermal resistance (°C /W)
Ra Rayleigh number (-)
Re Reynolds number (-)
T Temperature (°C)
u’ Streamwise fluctuating velocity
component (m/s)
u_ Uncertainty in quantity (-)
X Characteristics length (m)
Greek
a Thermal diffusivity (m2
/s)
b Thermal expansion coefficient (1/K)
⌬ Difference between two states
(temperature)
e Emissivity (-)
k Thermal conductivity (w/mk)
V Kinematic viscosity (m2
/s)
s The Stefan-Boltzmann constant (W/m2
K4
)
Subscripts
Amb Ambient
Base Heat sink base measurement
Ins Insulation
Max Maximum
4. 34 Bell Labs Technical Journal DOI: 10.1002/bltj
In order to reduce the cost of cooling it is becom-
ing standard practice that data center operators are
increasing data center set-point temperatures so that
energy can be saved due to increased efficiencies in
the chiller system. The energy savings stem mainly
from the fact that chiller power consumption can be
reduced with increased operating efficiencies under
higher chiller set-point temperatures. According to
[21], for every 1°C increase in chiller set-point tem-
perature, about 3.5 percent of chiller power can be
saved. Increasing the ambient temperature in the data
center reduces equipment reliability, and this trend
further highlights the importance of improved ther-
mal management architectures.
Eco-Sustainability
As stated previously, the key drivers highlighting
the importance of thermal management are the cur-
rent economic and climate concerns. Energy costs and
the potential for regulations mandating carbon emis-
sion reductions are driving telecommunication ser-
vice providers to seek new approaches for reducing
their energy usage. For example, the U.K.’s Climate
Change Act seeks to reduce carbon dioxide emissions
by at least 26 percent by 2020 and 80 percent by 2050
relative to a 1990 baseline [18]. In the context of the
telecommunications industry, global energy usage was
552 terawatt hours (TWh) in 2007 and accounted for
303 MtonsCO2e (equivalent to 63 ϫ 1 GigaWatt
power plants or €48.5 billion in electricity costs)
and is expected to increase at a 5 percent com-
pounded annual growth rate under current business-
as-usual conditions [3]. The Japanese Ministry of
Economy, Trade, and Industry (METI) forecasts elec-
trical energy usage by telecommunications will
increase from 47 TWh in 2006 (almost 5 percent of
the total annual electricity consumption in Japan) to
240 TWh in 2025 [1]. In 2006, data centers in the
U.S. consumed 61 billion kilowatt hours (BkWh) of
electricity and the EPA predicts that by 2012 energy
consumption in data centers will double from 2007
levels [19].
There are a number of reasons for the unprece-
dented growth in data center operations, and hence
growth in thermal densities. These drivers for growth,
detailed in [19], include:
• Migration of banking from paper based to online
systems,
• Health care moving more towards electronic
databases,
• Retail moving towards real time inventory and
supply chain management, and
• Transportation shifting towards Global Positioning
System (GPS) navigation and radio frequency
identification (RFID) tracking.
This growth has led to a significant shift in mind-
set regarding eco-sustainability. For example, a recent
survey found that almost 75 percent of global enter-
prises, governments, and individuals were expecting
moderate-to-strong demand for green products within
the next five years [16]. This shift in mindset is exem-
plified by the fact that many industries are reporting
greenhouse gas emissions as part of their corporate
responsibility.
Today, information and communications tech-
nology (ICT) contributes approximately 2 percent of
the total global greenhouse gas emissions, which
amounts to almost the same contribution as the avia-
tion industry. It is projected that ICT’s contribution to
greenhouse gas emissions will double by 2020 [20].
Therefore it can be seen that novel thermal manage-
ment solutions will contribute significantly to reduc-
ing the contribution of ICT (2 percent of emissions)
towards climate change. Moreover, novel thermal
management solutions have the potential to impact
industries external to ICT (the other 98 percent of
emissions), considering the ubiquitous use of elec-
tronics in modern day society.
This paper presents two novel air-cooled thermal
management architectures that provide enhanced
heat transfer while aiding in the reduction of energy
usage within the electronics cooling environment.
One class of technology discussed in detail is the 3D or
so-called three-dimensional heat sink design, owing to
its geometric complexity over standard LFHS heat sink
designs. 3D heat sinks enhance thermal performance
by increasing the heat transfer surface area and by
manipulating the airflow within the heat sink in vari-
ous ways. The decision to investigate heat sink design
stemmed from the well-known fact that the standard
LFHS has reached its limit of cooling performance in
modern high power electronics. A further advantage
5. DOI: 10.1002/bltj Bell Labs Technical Journal 35
of improving the design of heat sinks is that they are
employed ubiquitously in all electronics cooling, and
the heat sink itself can contribute up to 60 percent
of the overall resistance to heat flow between the die
and the ambient air, thus elucidating that improve-
ments in heat sink performance can have a positive
environmental impact. The other technology detailed
in this paper is the vortex generator, which manipu-
lates airflow to improve heat transfer. The key to this
technology is that it can be placed almost anywhere in
telecommunications equipment to improve heat
transfer. One example is that vortex generators can be
placed upstream of standard LFHS resulting in
improved performance of the heat sink. The impor-
tant fact to note regarding both the 3D heat sink and
vortex generator technologies is that they enable
reductions in pumping the power required to provide
a given amount of cooling. Examples and explana-
tions on how novel air-cooled thermal designs can
improve energy efficiency will be given in the fol-
lowing sections.
Experimental Arrangement and Measurement
Procedure
In advance of presenting the performance of the
novel air-cooled architectures it is first necessary to
describe the experimental arrangement and mea-
surement procedures that enable high-fidelity ther-
mal measurements.
Experimental Arrangement
The wind tunnel used to characterize the heat
sinks consists of honeycomb, contraction, and screen
sections upstream of the test section inlet to reduce
the background turbulence intensity of the flow and to
produce a uniform velocity profile in the test section.
The test section is made from plexiglass of internal
dimensions 610mm ϫ 406mm ϫ 77mm. The LFHS is
placed in a fully ducted arrangement within a wind
tunnel test section. The internal duct cross sectional
area is of the same dimensions as the heat sink (32mm
ϫ 15mm); the external duct dimensions are 40mm
long by 77mm deep, and the unit is made from plas-
tic. Extra ducting at the test section inlet is provided by
foam in order to force all of the flow through the heat
sink. The wind tunnel is powered by two 12W fans
that are placed downstream of the diffuser section.
The inlet turbulence intensity of the wind tunnel at
the test section entrance was measured at 0.4 percent
using a TSI IFA300 hotwire anemometer system.
The LFHS dimensions are 32 mm ϫ 32 mm ϫ
15mm and the base thickness is 2mm. These dimen-
sions were chosen to match the form factor of typical
heat generating components in telecommunications
circuit packs. The LFHS consists of 11 fins with 0.5mm
fin thickness and fin spacing of 2.65mm. The fin thick-
ness was limited to 0.5mm as this was taken as the
lower limit of the conventional extrusion manufactur-
ing process, which is commonly used in the production
of heat sinks for telecommunications equipment. These
dimensions provide an optimally low thermal resis-
tance at a pressure drop of 1 Pa. The heat sink was
made from an investment cast copper alloy with 90
percent pure copper and 10 percent pure silver. This
alloy composition was chosen to accommodate more
complicated designs where poor flow during the cast-
ing process can cause defects. The heat sink has exter-
nal “leg” regions that allow the heat sink to be mounted
to the wind tunnel wall and the duct. Figure 1 provides
an illustration of the LFHS dimensions.
Wall mounted static pressure taps are located
20 mm upstream and downstream of the heat sink
leading and trailing edges and are connected to a digi-
tal differential micro manometer (Furness Controls
FC0150). The pressure taps are located in the center
plane of the duct. The duct wall is sealed to the wind
tunnel wall with silicone to ensure that there are no
adverse flow leakage effects. The ambient tempera-
ture is measured with a type-T metal-sheathed ther-
mocouple (Omega TMQSS-062U-6) placed 50 mm
downstream of the test section inlet or equivalently
250 mm upstream of the heat sink inlet. The mea-
surement of the maximum heat sink base tempera-
ture is achieved by drilling a 0.6 mm diameter hole
to a depth of 5mm into the center of the heat sink
base and a metal-sheathed type-T (Omega SCPSS-
020G-36) thermocouple is placed within the hole
with Omega OT-201-2 thermal paste. This gives a
length-to-diameter (L/D) ratio of 10 for improved
accuracy. The temperatures are acquired via a
National Instruments data acquisition system (SCXI-
1000). The thermal resistance (R) of a heat sink is
given by
6. 36 Bell Labs Technical Journal DOI: 10.1002/bltj
(1)
The significance of the thermal resistance parame-
ter can be understood if one considers an example
where a heat sink has a thermal resistance of 10°C/W
and dissipates 10 W of power resulting in a 100°C
increase in the heat sink temperature over the ambi-
ent temperature. This implies a significant increase in
the operating temperature of the component due
to the establishment of thermal equilibrium between
the component and the heat sink via intermediate
layers of TIM and heat-spreading material.
The power input to the base of the heater is sup-
plied via a Kapton* pressure-sensitive adhesive heater
(MINCO HK5163R157L12B). The heater is powered
by a Hewlett-Packard 6655A DC power supply. For
the majority of tests presented in this investigation,
the heater power is 10.3W unless stated otherwise. To
mitigate against heat loss to the environment a foam
insert is placed directly on the heater in the heat sink
base cavity and two layers of Aspen Aerogels insula-
tion with a thickness of approximately 3mm each and
a thermal conductivity of approximately 0.014W/mk
are attached external to the foam insert and the
R ϭ
Tmax Ϫ Tamb
Q
mounting legs of the heat sink. Furthermore, foam
inserts are also placed on the back of the ducting to
hinder any heat loss in the region where the metal
mounting screws are exposed to the air.
Velocities in the duct are measured using a United
Sensor PCA-8-KL pitot-static probe, which is placed
approximately 30mm upstream of the heat sink lead-
ing edge and in the center of the duct flow. Therefore,
the velocity measured in this investigation is the maxi-
mum attainable in the duct centerline. The maximum
velocity measured in the duct centerline during the
current experiments was approximately 5 m/s. The
pitot-static probe is connected to an Alnor EW-05949-
10 digital manometer.
Two different types of vortex generator (VG),
illustrated in Figure 2, were used in the current
investigation and descriptions are provided below. In
the first example, delta winglet VGs are placed
upstream of an LFHS within a fully ducted geometry
similar to that described above for the heat sink tests
and shown in Figure 2a. The VGs are of the delta
winglet type and a picture of the plastic VGs is shown
in Figure 2b. The delta winglets were mounted to the
wall of the wind tunnel with double-sided tape. The
angle of attack (AoA) is kept constant in the current
2 mm
13 mm
0.5 mm
Base
Fin
Mounting holes
2.65 mm
External mounting
legs
Test
section
wall
LFHS—Longitudinally-finned heat sink
32 mm
Heat source
Figure 1.
Illustration of horizontal cut through the LFHS where the airflow is into the page. Drawing not to scale.
7. DOI: 10.1002/bltj Bell Labs Technical Journal 37
investigation at 21.5 degrees, the height of the VG is
15mm (same height as the heat sink), and the walls
are 1mm thick. The constant AoA is achieved by hav-
ing VG leading and trailing edge separations of 2mm
and 24mm, respectively. Note that this is the maxi-
mum AoA possible within the duct geometry and this
implies that the heat transfer measured in this inves-
tigation is not the maximum possible with the VGs.
The second example is shown in Figure 2c. In this
example, the VGs are of the delta wing design and
form part of the metal board guide rail which is used
to guide circuit packs into position within a shelf of
equipment. The tests for the board guide rail investi-
gation were preformed on an actual product under
the Advanced Telecommunications Computing
Architecture version 2 (ATCA v2) where the tempera-
tures recorded are those measured on the chip.
Measurement Procedure
The accuracy of the thermocouples was checked
in order to ascertain the uncertainties in the tempera-
ture measurement. The thermocouples were placed
around the circumference of a resistance temperature
detector (RTD) probe. The RTD probe is placed within
a temperature controlled water tank of a Julabo F33
circulator that can maintain the water temperature
to within 0.01°C. The variation in thermocouple tem-
peratures was recorded over a range of water set-
point temperatures from 20 to 60°C. The variation
between all of the thermocouples is approximately
0.2°C at 30°C set point and 0.5°C at 60°C set point.
The heater is applied to the base of the heat sink
with a pressure sensitive adhesive. The quality of the
bond between the heater and the base of the heat
sink is validated by powering the heater and probing
it with the tip of a sheathed thermocouple, as voiding
will be reflected by a marked increase in the surface
temperature on the backside of the heater. No signs of
voiding were found in the current tests as the maxi-
mum difference in temperatures recorded on any two
points on the heater was approximately 2°C. A simu-
lation of the copper heat sink with a non-uniform
heating on the base, similar to that measured, was
carried out using FLUENT*. It was demonstrated that
small differences in temperature were spread evenly
across the heat sink base due to the high thermal con-
ductivity of the copper alloy.
Following this, the thermocouples are inserted
into the 0.6 mm diameter (5 mm deep hole) in the
base of the heat sink on the upstream and downstream
locations. The first 6mm of the sheathed thermocou-
ple is placed in Omega OT-201-2 thermal grease and
Inflow
Heat sink
Delta
wing VG
Board guide
rail
(a) Test setup with delta winglet
VGs placed upstream of LFHS in
fully ducted flow.
(b) Delta winglet VGs. (c) Delta wing VGs placed
on the board guide rail.
LFHS—Longitudinally-finned heat sink
VG—Vortex generator
VG
Duct
Inflow
Figure 2.
Different types of vortex generators. Drawings not to scale.
8. 38 Bell Labs Technical Journal DOI: 10.1002/bltj
the thermocouples are then pushed fully into the hole
in the base of the heat sink. Any excess thermal paste
was removed. The sheathed thermocouples are bent
around the base of the heat sink and are strain-
relieved with Kapton tape. In order to prevent any
damage to the probes, the bend radius of the sheathed
thermocouple is not less than two times the diameter
of the probe, as per the manufacturer’s instructions.
The temperatures measured in the base of the
heat sink were deemed to reach steady state when
the temperature fluctuations varied by no more than
Ϯ0.05°C for three minutes. This typically took 30
minutes depending on the operating conditions. The
temperatures were obtained at set pressure drops
across the heat sink. The pressure drops were set by
varying the fan speed until a desired pressure drop
was measured across the heat sink. The upstream and
downstream temperatures measured in the base of
the heat sink were found to be equal to within 0.1°C
thereby experimentally verifying computational fluid
dynamics (CFD) simulations which demonstrated that
the temperature rise across the predominantly cop-
per heat sink was insignificant.
The determination of maximum velocity was
achieved by moving the tip of the pitot-static probe to
different depths within the duct passage until a maxi-
mum velocity was recorded on the manometer.
Repeatability of Results and Uncertainty Analysis
The repeatability of the thermal resistance versus
pressure drop and velocity data is detailed in Figure 3.
Tests were carried out over two power settings, and
the degree of repeatability is shown in Figure 3a,
where the maximum deviation between measure-
ments is Ͻ2%. In Figure 3a, two power settings were
tested, 10.3 W and 16 W. All measurements were
taken at 10 W unless otherwise stated. For the
repeatability tests, the test section side wall, the heat
sink, the thermocouple probes in the heat sink base,
the pitot-static probe, and pressure tap tubing were
removed and subsequently reinstalled.
Using equations 2, 3, and 4 [13], we calculated that
the heat loss to the environment is approximately 0.07
percent on the portion of the heat sink incorporating
the heater covered with the Aspen Aerogels insulation.
Equation 2 is the standard relationship between Nusslet
number (Nu is a dimensionless number representing
the relationship between convection and conduction
heat transfer processes) and the Rayleigh number (Ra is
a dimensionless number associated with buoyancy
driven flow) for flat plates. Equation 3 is an expansion
of equation 2 showing explicitly the terms that make up
each dimensionless number, and equation 4 is the heat
loss equation used in calculating the heat lost to the
environment due to natural convection and radiation
processes. The ⌬T term in equation 4 was measured to
be 2°C with a metal-sheathed thermocouple where Tamb
is the ambient temperature and Tins was the tempera-
ture on the airside of the insulation. Therefore, it can be
estimated that the total heat loss to the environment is
less than 1 percent owing to the insulation properties of
the plastic ducting encasing the heat sink and the vari-
ous foam inserts employed around the test section.
Nu ϭ 0.5Ra0.25
(2)
(3)
Q ϭ hA⌬T 1 Ase(T4
ins Ϫ T4
amb) (4)
Using the method of propagation of uncertainties
(equation 5) it is possible to calculate the absolute
uncertainties in the thermal resistance measurements
(given by equation 1) based on the individual uncer-
tainties of each measurement parameter that con-
tributes to the thermal resistance. As demonstrated
in Figure 3, the uncertainty in ⌬T (u_⌬T) is a maxi-
mum of 0.5°C. From equation 4, the uncertainty in Q
(u_Q) is 1 percent. By substituting the measured val-
ues for the LFHS at 24.7 Pa and 10 W with a ⌬T of
15°C, the uncertainty in the thermal resistance mea-
surements is Ϯ3 percent. At 2 Pa, with a higher ⌬T
value of 37°C, the uncertainty is Ϯ3.5 percent.
(5)
To keep velocity measurement error at a mini-
mum, the pitot-static probe must be placed at least 5
probe diameters away from the wall. In the rectan-
gular duct geometry, the distance between the wall
ϩ
G
a
0
0Q
a
¢T
Q
bb
2
(uϪQ)2
uϪR ϭ
G
a
0
0¢T
a
¢T
Q
bb
2
(uϪ ¢T)2
hX
Kair
ϭ 0.5c
gbX3
¢T
na
d
0.25
9. DOI: 10.1002/bltj Bell Labs Technical Journal 39
and the probe is 4.5 D which gives an error of 1 per-
cent. There are two boundaries in the duct arrange-
ment (upper and lower walls), therefore the total
error is 2 percent due to wall boundary effects. The
error due to the manometer reading is Ϯ3 percent
over the measurement range. Therefore, the total
error associated with the velocity measurements using
the pitot-static probe are of order Ϯ5 percent. The
error in pressure drop measurement is approximately
Ϯ3 percent of the reading. The pressure drop mea-
surements were compared with two different
manometers and negligible difference in the average
results was observed.
Shown in Figure 3b are some examples of the
repeatability in the pressure drop versus velocity data.
It can be seen that the repeatability is relatively good.
At the high velocity range for the LFHS there is a dif-
ference of approximately 5 percent in velocity read-
ings. Note, however, that the repeat result shown in
Figure 3b was the worst out of four tests obtained.
In the following results sections it is worthwhile
to note that the uncertainty in the thermal resistance,
pressure drop, and velocity values at 10 W are Ϯ3
percent, Ϯ3 percent, and a maximum of Ϯ5 percent,
respectively.
Description of Two Novel Air Cooled Thermal
Architectures
This section provides an overview of the main
physical phenomena employed in enhancing heat
transfer and describes the application of these phe-
nomena to the design of 3D heat sinks and vortex
generators.
Description of Methods to Enhance Heat Transfer
As stated previously, the most common heat sink
design used in telecommunications is the LFHS shown
in Figure 4. The main concept behind any heat sink
design is to have the maximum heat transfer surface
area (dependent on required thermal resistance and
geometric constraints) while at the same time main-
taining a manageable pressure drop across the heat
sink. When the heat transfer surface area of a heat sink
is increased, so too is the pressure drop associated
(a) Thermal resistance (R) versus pressure drop
results for the LFHS.
0
10
20
30
40
0 1 2 3 4 5
Velocity (m/s)
Pressuredrop(Pa)
LFHS
LFHS
HCHS
HCHS
1.5
2
2.5
3
3.5
0 5 10 15 20 25
Pressure drop (Pa)
R(°C/W)
LFHS
LFHS
LFHS 16 W
(b) Pressure drop and velocity data for
a number of different heat sinks.
HCHS—Honeycomb heat sink
LFHS—Longitudinally-finned heat sink
Figure 3.
Examples of result repeatability.
10. 40 Bell Labs Technical Journal DOI: 10.1002/bltj
with pumping a given flow rate of air through the
heat sink. This increased pressure drop is due to the
increased frictional drag and the larger flow blockage
induced by increasing the heat sink frontal area. The
latter is an unwanted effect in typical telecommuni-
cations systems owing to the fact that if the pressure
drop across the heat sink is too large, some of the
incoming cool air from the fans will bypass the heat
sink thereby reducing cooling capacity. In this
instance, in order to supply more cool air, the fan
power may have to be increased. This may not be pos-
sible due to fan reliability, operational expenditure
(OPEX) cost, and fan noise constraints. Therefore, the
ideal thermal solution is to enhance the heat sink heat
transfer without incurring a significant pressure drop
penalty. Of course, the overall thermal design of the
circuit pack must be optimized given all of the known
constraints.
In the standard operation of the LFHS cool air-
flow from upstream of the heat sink is passed through
the heat sink fin passages. The fins are attached to a
base, which is in turn attached to the component
package via one layer of TIM. The heat is conducted
through the base and up to the tips of the fins.
Boundary layers are formed on the fins and if the fin
length is long enough (for a particular the fin spac-
ing), the boundary layers will merge and eventually
form a fully developed flow. Fully developed flow hin-
ders heat transfer since the velocity and thermal gra-
dients at the fin wall will be reduced significantly.
Boundary layers are regions of flow adjacent to a solid
boundary that contain temperature and velocity gra-
dients and act as a thermal insulator. The gradients
are set up due to the fact that the velocity at the wall
is zero; this condition is referred to as the no-slip con-
dition. Well away from the boundary, i.e., outside the
boundary layer, the flow has a uniform (so-called
freestream) velocity profile in which there are no
velocity gradients. Therefore, the flow must go from
zero velocity at the wall to the freestream velocity
away from the wall within the boundary layer thick-
ness. The boundary layer and its development are criti-
cal in determining the heat transfer from a solid
surface such as the fins in an LFHS. A thin boundary
layer provides better heat transfer rates but also
increased skin friction drag. Therefore, there is always
a tradeoff between increased heat transfer and
increased drag (pressure drop).
In fluid mechanics, there are many fluid flow
phenomena that can be utilized to increase heat trans-
fer. One technique, which has been studied exten-
sively in the literature, is the concept of boundary
layer restarting. The key concept in this design is to
stop the growth of the boundary layer at certain
streamwise positions and then “restart” the bound-
ary layer growth at fixed streamwise increments,
thereby achieving increased heat transfer rates due
to thinner boundary layers encountered on the fins.
In this design the increase in heat transfer can out-
weigh the increase in pressure drop. Another method
of enhancing heat transfer is to generate unsteadiness
in the flow. Unsteadiness in the flow causes the gen-
eration of secondary flows that may thin the bound-
ary layers, thereby increasing heat transfer. Unsteady
flow also has the benefit that fast moving and cooler
air located well away from the heated surface can be
brought closer to the relatively slow moving hot air
near a heated surface thus providing enhanced heat
transfer.
Unsteady flow can be generated by a number of
techniques. One technique is to use vortex genera-
tors. In this technique, triangular or rectangular
shaped structures are placed in the flow path. The
flow separates on these surfaces thereby generating
streamwise vortices that rotate about the streamwise
flow direction. Another method of generating local
unsteadiness is to place cylinders (or any other shape)
Figure 4.
Picture of a standard longitudinally-finned heat sink.
11. DOI: 10.1002/bltj Bell Labs Technical Journal 41
perpendicular to the flow direction between the fin
spaces or upstream of the heat sink. The flow sepa-
rates downstream of the object, and under certain
flow conditions, the downstream flow pattern
becomes unsteady and eventually turbulent, thereby
increasing the local mixing, and concomitantly, the
heat transfer on any downstream surface. Flow
unsteadiness can also be generated due to local flow
instabilities such as Kelvin-Helmholtz or Tollmien-
Schlichting instabilities and these instabilities may
trigger transition to turbulence [15]. However, tur-
bulent flow is generally unwanted due to the signifi-
cant pressure drop penalty associated with it. Some of
these flow instabilities, when coupled with flow sepa-
rations, can be used to generate self-oscillating flows
which can provide high heat transfer rates without
significant increase in the pressure drop that is asso-
ciated with turbulent flow.
Noteworthy effort has been invested in heat sink
design over the past number of years and there are
various designs available depending on the applica-
tion. A good review of standard air cooling methods
and their limitations is available in [14]. One com-
mon heat sink design, the pin fin, is comprised of
cylindrical posts separated by some distance. There is
increased heat transfer around the pin fins due to
local flow separations that create flow unsteadiness;
however, the pin fin heat sink typically does not per-
form as well as the LFHS owing to the reduction in
heat transfer surface area. The main advantage that
the pin fin has over the LFHS is that the incoming
flow can originate from any direction. In the LFHS,
the flow must be aligned with the direction of the fins
for best performance. Therefore, pin fins are the heat
sink of choice when used in a fan-mounted heat sink
assembly due to the omnidirectional properties of the
air, e.g., in a computer cooling application where
the fan is directly attached to the heat sink. In recent
years the strip fin design has been incorporated with
elliptically-shaped fins that reduce the overall drag of
the heat sink allowing a reduction in pressure drop
and flow bypass effects. This design would typically be
employed in a densely populated circuit pack where
there may be many heats sinks. Little improvement
has been gained with these new designs over the
LFHS.
What follows is a description of new heat sink
designs and a fabrication process that enables the reali-
zation of novel prototype 3D heat sinks.
Proposed Novel 3D Heat Sink Designs and Fabrication
Technique
Three proposed novel 3D heat sink designs are
discussed here, namely the fin foam heat sink (FFHS),
honeycomb heat sink (HCHS), and Schwartz heat sink
(SHS) illustrated in Figure 5. All the designs discussed
below increase the heat transfer surface area com-
pared to a standard LFHS of the same form factor and
use some or all of the above listed flow phenomena to
enhance heat transfer.
Figure 5a represents the FFHS structure. One can
immediately see the difference between the LFHS and
FFHS designs, where the cross-sectional area of the
3D heat sink periodically varies throughout the length
over which the flow travels. The FFHS has greater
heat transfer surface area compared to an LFHS of the
same form factor, and each of the ligaments acts simi-
lar to a cylinder in cross flow generating local
unsteadiness. Off-the-shelf foams have been investi-
gated [2] when placed between the fins of an LFHS;
however, a shortcoming with this approach is that the
foam must be attached to the fins of the heat sink via
thermal grease or epoxy, which forms a significant
thermal barrier. In our approach, the foam structure
and the fins are one monolithic structure due to the
casting process (discussed at the end of this section).
Another key difference between the traditional foams
and our proposed designs is that we can generate both
structured and unstructured (random) foam cells
whereas in the traditional approach the foams are
inherently stochastic due to the manufacturing pro-
cess. The proposed novel 3D ordered foam structures
can be generated with body-centered cubic (BCC),
face-centered cubic (FCC), and the area minimizing
A15 lattice arrangements.
Another example of a 3D heat sink is shown in
Figure 5b and is referred to as a honeycomb struc-
ture, a type of cellular structure in which fluid flows
through hexagonal channels with or without various
types of openings called slots. Honeycomb structures
have been reported in the literature [12] in heat
exchanger applications where they are brazed or
12. 42 Bell Labs Technical Journal DOI: 10.1002/bltj
attached via thermal grease to the upper and lower
heat transfer surfaces. As stated previously, this cre-
ates an additional thermal interface that reduces the
effectiveness of the design. Once again it can be seen
that the heat transfer surface area has increased sub-
stantially over an LFHS with the same volume. The
honeycomb channels can be straight channels, or as
shown in Figure 5b, the honeycomb can incorporate
openings of any design in both the horizontal and in
the vertical directions. (The vertical slot orientation
is shown in Figure 5b). The reason for the openings is
to disrupt the boundary layer development and gen-
erate local unsteady flow. We decided to investigate
vertically orientated slots (rather than horizontal slots
which could be generated by simply sawing across the
HCHS) since this type of design is not known in
the literature and because it tested the ability of our
investment casting process to generate complex
designs.
Another example of a 3D heat sink design is shown
in Figure 5c. This design is called a Schwartz structure
and it is constructed based on the principal of zero-
mean curvature. The Schwartz structures are of interest
as they are conducive to self-sustaining flow oscillations
in the laminar flow regime that may be used to enhance
heat transfer without severe increases in flow resistance
associated with turbulent flow. The unit cell does not
have to be an area minimizing structure, but can be of
any arbitrary shape such that its disrupts the internal
flow within the cell by creating unsteadiness due to
flow separations.
As is evident from Figure 5, the new 3D heat sink
designs are geometrically complex. We discussed pre-
viously how the 3D heat sinks are monolithic in struc-
ture, which implies they cannot be manufactured
using conventional machining or extrusion processes.
For this reason, a new heat sink fabrication process
was developed whereby the heat sinks can be fabri-
cated as one monolithic structure in high thermal
conductivity material giving enhanced thermal per-
formance benefits over existing technologies.
The first step in the manufacturing process is to
generate a computer aided design (CAD) file of the
heat sink. This CAD model is then exported to a high-
resolution 3D printer (3D Systems’ InVision* HR)
which prints the part in an exact plastic form with
minimum resolution of approximately 40 mm. The
void of the plastic model (where the air will flow) is a
wax which is used for structural stability during the
printing process. It is subsequently removed by melt-
ing the wax out in an oven at 70°C. The plastic part is
employed as a sacrificial pattern for the investment
casting process. The sacrificial pattern is embedded in
a slurry, or investment, which hardens to form an
outer shell over the complete plastic mold. Following
this process, the entire piece (plastic pattern and
(a) Fin foam heat sink (FFHS) (b) Honeycomb heat sink (HCHS) (c) Schwartz heat sink
Figure 5.
Casts of 3D heat sink designs.
13. DOI: 10.1002/bltj Bell Labs Technical Journal 43
investment) is placed in an oven and the plastic pat-
tern is burned away. At this stage what remains is a
mold of the hardened investment, which can be filled
with a molten metal. The best casting results are
achieved by evacuating the investment mold to
remove trapped air, and then forcing the molten
metal into the mold using centrifugal force. After the
metal cools, the investment is removed by using a
pressurized water jet. The technique can support the
use of a range of metals such as aluminum alloys,
bronzes, stainless steels, copper, and precious metals
such as gold and silver. Combinations of the above
metals also can be cast depending on the characteris-
tics of the end product needed.
Although the investment casting process can pro-
duce complex 3D monolithic designs that are not pos-
sible to produce using conventional techniques, it
must be noted that the process is not without its limi-
tations. For example, the prototypes are costly and
limited in overall dimensions, and more importantly,
the process is not scalable for mass production. We
are currently investigating different processes that are
more conducive towards mass production and lower
costs; however, it must be stated that the investment
casting process has provided a reasonable means of
evaluating the new heat sink prototype designs.
Results and Discussion on 3D Heat Sinks
Results were presented in [8] comparing all three
of the novel monolithic 3D heat sink designs against
velocity. The current paper will concentrate on the
performance of the one design that is most likely to
have a positive impact on telecommunications equip-
ment due to its superior thermal and hydrodynamic
performance. The following paragraphs detail the per-
formance of the HCHS design.
Figure 6 shows the thermal resistance measure-
ments of two honeycomb structures compared to the
LFHS at constant pressure drop. It can be seen that
the continuous channel HCHS performs less optimally
than the LFHS. At 5 Pa, the HCHS performs 15 percent
worse than the LFHS and at 25 Pa it performs 8.5 per-
cent worse. It is evident from Figure 6 that introduc-
ing slots enhances heat transfer as demonstrated by
1.3
1.8
2.3
2.8
3.3
3.8
0 10 20 30 40
Pressure drop (Pa)
R(°C/W)
LFHS HCHS straight HCHS 6 mm slot
LFHS—Longitudinally-finned heat sink
HCHS—Honeycomb heat sink
Figure 6.
Plot of thermal resistance (R) versus pressure drop for the honeycomb structures compared to the LFHS. All
measurements at 10.3 W.
14. 44 Bell Labs Technical Journal DOI: 10.1002/bltj
the significant improvement between the continuous
channel HCHS and the 6 mm slot HCHS results. At
low pressure drop, the 6mm slot performs 6 percent
worse than the LFHS; however, at 25 Pa the 6mm slot
HCHS outperforms the LFHS by 4 percent. It is also
evident from Figure 6 that higher pressure drops are
measured across the honeycomb, heat sinks compared
to the LFHS. For example, at maximum fan power,
the pressure drop across the LFHS heat sink is approxi-
mately 24 Pa and for the continuous channel honey-
comb the maximum pressure drop was measured at
36 Pa; however, introducing the slots provides a reduc-
tion in pressure drop.
Figure 7 summarizes the thermal and hydrody-
namic performance of the 6 mm slot HCHS compared
to the LFHS. It can be seen from Figure 7 that the
6 mm slot outperforms the LFHS against thermal resis-
tance but incurs a greater pressure drop penalty.
Figure 7 also highlights the velocity range of telecom-
munication equipment from legacy to next genera-
tion. It can be seen that the best performance is
achieved when the 6 mm slot HSHC is exposed to
high velocity flow. It can also be seen from Figure 7
that beyond 1.5m/s the slot HCHS outperforms the
LFHS. Although not shown here, this trend was also
observed in all of the other slotted designs tested. The
literature [15] reports that flow becomes unsteady at
Reϳ60, where Re is based on the width of the inter
slot metal components of 1.2 mm. In Figure 7, at
1.5m/s, where the profiles change slope, the Re value
is 120. This could be an indication that significant flow
separation and unsteady effects are occurring. Further
insight into this is needed through flow visualization
and detailed measurements.
From Figure 7 it can be seen that at 4 m/s there is
a reduction in the 6 mm slot HCHS heat sink base
temperature of 3.5°C compared to the LFHS. This may
not seem like a noteworthy result, however, a margin
of 3.5°C can provide significant thermal benefits. For
example, such a margin could enable the realization
of a next generation product that is at the very limit of
thermal compliance. From Figure 7 it is also evident
Legacy Current generation
Tbase ϭ 38.5°C
Tbase ϭ 35°C
Next generation
LFHS—Longitudinally-finned heat sink
HCHS—Honeycomb heat sink
HCHS
R(°C/W)
4
3.5
35
25
15
5
541 2 3
3
2.5
2
1.5
1
Velocity (m/s)
⌬P(Pa)
Figure 7.
Plot of thermal resistance (R) and pressure drop versus velocity for the 6 mm slots HCHS compared to the LFHS.
The HCHS is represented by the circles.
15. DOI: 10.1002/bltj Bell Labs Technical Journal 45
that improved thermal performance is accompanied
by an attendant increase in pressure drop. This is typi-
cally encountered with improved thermal perfor-
mance and is a consequence of Reynolds analogy that
relates hydrodynamic and thermal relationships. This
increase in pressure drop is relatively small and the
heat sink design can be optimized based on the com-
plete design of the circuit pack in order to achieve
optimum performance.
However, pressure drop is not the only critical
parameter when designing thermal systems. One of
the other key parameters with respect to energy effi-
ciency is the pumping power which is defined as the
product of the volumetric flow rate times the pressure
drop and it provides a measure of the amount of
power required to pump a given volume of fluid
through the heat sink. To explain how the slotted hon-
eycomb designs can provide enhanced energy effi-
ciency, let us consider the reduction in pumping power
achieved to provide the same thermal resistance as
illustrated in Figure 8. It can be seen from Figure 8
that the 6 mm slot HCHS provides approximately a
35 percent reduction in pumping power compared to
the LFHS in order to achieve the same thermal resis-
tance of 1.86°C/W.
Vortex Generators
It was stated previously that LFHS are ubiquitous
in electronics cooling in general and particularly so
in telecommunications equipment. Alcatel-Lucent
alone incorporates thousands of heat sinks in its
product base and the majority of these designs are of
the LFHS type. Another possible means of improving
the energy efficiency of telecommunication systems,
rather than redesigning the heat sink, is to try and
improve the performance of the LFHS designs that
we currently employ. Vortex generators are a tech-
nology that offers enhanced heat transfer by creating
unsteady flow and by thinning boundary layers. VGs
have been employed in a range of different disciplines
such as chemical mixing, drag reduction on cars,
maintaining flow attachment on aircraft wings and
enhancing heat transfer in heat exchangers. Extensive
reviews may be found in [10] and [11]. Some of the
1.3
2.3
3.3
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07
Pumping power (W)
R(°C/W)
LFHS HCHS 6 mm slot
LFHS—Longitudinally-finned heat sink
HCHS—Honeycomb heat sink
Figure 8.
Plot of thermal resistance (R) versus pumping power for the 6 mm slot HCHS compared to the LFHS.
All measurements at 10.3 W.
16. 46 Bell Labs Technical Journal DOI: 10.1002/bltj
earlier attempts at creating VGs were based on placing
cubes and rectangular obstructions in the flow path.
Edwards and Alker [4] provide an early example of
research into cubes and delta wings. The authors
found that the flow disturbances generated by the
delta wings persisted over greater flow lengths com-
pared to those generated by the cube. Following from
this, delta winglets and delta wings have been inves-
tigated extensively and they were shown to exhibit
additional benefits over other types of VG design.
Different types of popular VG are illustrated in Figure 9.
VGs in the form of delta wings or winglets are
studied extensively in the literature owing to the bene-
fits that such devices have shown in reducing the air
side thermal resistance of heat exchangers while not
incurring very large pressure drop penalties. VGs
increase heat transfer by a number of mechanisms:
• Enhanced mixing due to the swirling motion of
the vortices.
• Secondary flows are set up normal to the main
streamwise flow which causes local thinning of
the boundary layer when the secondary flow is
directed towards the surface.
• Unsteady separation of the flow from the VG
causes an unsteady flow downstream of the VG.
Figure 10 provides an illustration depicting the flow
characteristics around and downstream of the delta
winglet pair [5]. VGs are examined extensively in heat
exchanger applications due to their use in varied indus-
tries such as automotive, air conditioning, process plant
and geothermal [17].
Two examples of instances where VGs can be used
to improve the thermal performance of electronic sys-
tems while at the same time saving energy and main-
taining component reliability are described below.
Results and Discussion on Two Types of VG Design
A brief description of one VG embodiment in
telecommunications equipment was given in [8] and
covered more extensively in [6]. The papers demon-
strated that reasonable improvement in heat transfer per-
formance can be achieved if one incorporates small
Reprinted from Exp. Therm. Fluid Sci., 11, A.M. Jacobi and R.K. Shah, “Heat Transfer Surface
Enhancement Through the Use of Longitudinal Vortices: A Review of Recent Progress,” 295–309,
Copyright Elsevier 1995.
Delta wing.
⌳ϭ2b/c
y
z
Air Flow
x
Rectangular
wing, ⌳ϭb/c
Rectangular
winglet, ⌳ϭb/c
␣ Angle of attack
⌳ Aspect ratio
Delta
winglet,
⌳ϭ2b/c
b/2
b/2
b
bc
c
c
c
␣
␣
␣
␣
Figure 9.
Examples of vortex generators.
17. DOI: 10.1002/bltj Bell Labs Technical Journal 47
plastic delta winglet VGs upstream of an LFHS. Figure 11
illustrates the performance gained with the intro-
duction of the VG upstream of the LFHS. The main
advantage of this design is that the small plastic parts
are cheap to fabricate, very light, and can be placed
almost anywhere on the circuit pack to provide
enhanced heat transfer. Figure 11 shows that a
10 percent reduction in thermal resistance is achieved
with the introduction of the plastic VG upstream of the
heat sink. This reduction in thermal resistance equates
to a 2°C reduction in the heat sink base temperature.
In order to design more efficient systems for future
generations of equipment it is essential to fully under-
stand the underlying flow physics. For this reason, we
are employing high-fidelity measurement techniques
such as hotwire anemometry and particle image
velocimetry (PIV) to explore in detail the hydrody-
namic and thermal characteristics of the flow down-
stream of the vortex generators. A detailed description
of the operation of both measurement techniques is
given in [7]. An example of the level of detail possible
with these two measurement techniques is highlighted
in Figure 12. In this figure, one can see a time-averaged
PIV image of the flow downstream of the VGs on the
left. The counter-rotating vortex pair is evident from
the PIV image where the flow in between each of the
Reprinted from Appl. Therm. Eng., 26, S. Ferrouillat, P. Tochon, C. Garnier and
H. Peerhossaini “Intensification of Heat-Transfer and Mixing in Multifunctional
Heat Exchangers by Artificially Generated Streamwise Vorticity,” 1820–1829,
Copyright Elsevier 2006.
Vortex
generator
Flow
direction
Vortex
Z
Figure 10.
Computational fluid dynamics simulation of the flow field surround a delta winglet pair of vortex generators.
LFHS—Longitudinally-finned heat sink
VG—Vortex generator
Tbase ϭ 42.3°C
Tbase ϭ
40.3°C
5 10 15 20 25
3
2.5
2
1.5
Pressure drop (Pa)
R(°C/W)
No VG VG#3 AoAϭ21.5 Lϭ50 mm
Figure 11.
Experimental results showing the reduction in thermal
resistance of a LFHS with the introduction of upstream
VGs.
18. 48 Bell Labs Technical Journal DOI: 10.1002/bltj
vortices is directed towards the lower wall and thin-
ning the boundary layer in the process. Time traces of
the fluctuating velocity from the hotwire are shown
on the right and further highlight the complexity of
the flow field. The large spike at the upper right of the
figure indicates a region of slow moving fluid that has
been lifted from the wall to the freestream region. All
of these complex flow phenomena provide insight into
the mechanisms of enhanced heat transfer.
Since the publication of [8], a prototype has
been built and tested on the ATCA v2 platform
where the VGs are placed on the board guide rail as
previously illustrated in Figure 2c. In this instance,
because of design constraints, the VGs are of the
delta wing design and they are located directly on
the metal board guide rail. As the name suggests, a
board guide rail is simply a piece of sheet metal that
provides a path for the circuit pack to be guided into
position. In the past, the board guide rails have not
been utilized for improved heat transfer. Tests were
performed on the ATCA v2 product at 27°C inlet air
temperature and it was demonstrated that with
the introduction of the VGs on the board guide rail,
the processor temperature was reduced by 3°C with
an attendant reduction in fan power of 5 percent.
Reducing the processor temperature improves relia-
bility, improves thermal margins, and also may
enable the realization of future products that incor-
porate significantly increased thermal densities.
Furthermore, energy savings are achieved by reduc-
ing the fan speed. Since fan power consumption
increases with the cube of fan speed (which is related
to airflow rate), this implies that significant power
savings can be achieved even with small reductions
in fan speed [21]. The reduction in fan speed also
enables a 3 dB reduction in emitted noise. As stated
previously thermal engineers are finding it increas-
ingly difficult to provide an adequate thermal solu-
tion while adhering to noise levels as set out in the
Network Equipment Building System (NEBS) and
European Tele-communications Standards Institute
(ETSI) standards.
From the preceding discussions it can be seen that
there are significant performance gains to be achieved
by incorporating VG technology into electronics
equipment for cooling purposes.
0 0.1 0.2 0.3 0.4 0.5
Ϫ0.4
Ϫ0.2
0
0.2
0.4
0.6
Time (s)
u’(m/s)
Ϫ0.5
Ϫ0.4
Ϫ0.3
Ϫ0.2
Ϫ0.1
0
0.1
Ϫ0.6
u’(m/s)
Z mm
ymm
40 30 20 10 0 10 20 30 40
Ϫ0.2
Ϫ0.15
Ϫ0.1
Ϫ0.05
0
0.05
0.1
0.15
0.2
5
10
15
20
25
30
35
(a) Time-averaged PIV measurement flow
downstream of a delta winglet pair.
PIV—Particle Image velocimetry
(b) Instantaneous fluctuating velocity
traces from a hotwire.
Figure 12.
Level of detail possible with the measurement techniques.
19. DOI: 10.1002/bltj Bell Labs Technical Journal 49
Conclusions
Energy efficiency is becoming one of the key driv-
ing parameters in equipment design considering
recent increased environmental awareness and gen-
eral promotion of eco-sustainable solutions. This
transformation in attitude has promoted the thermal
engineer as one of the key assets in the product design
cycle. The Thermal Management Research Group at
Bell Laboratories has developed a number of novel
thermal technologies that enable energy efficiency
while maintaining component reliability. This paper
focuses on two examples of novel air-cooled archi-
tectures, specifically 3D monolithic heat sinks and
vortex generators.
The decision to investigate improved heat sink
designs was based on the fact that current longitudi-
nally finned heat sinks have reached their limit of
cooling ability in high-power telecommunication
equipment. Considering the ubiquitous use of longi-
tudinally finned heat sinks in all electronics cooling,
and also the fact that the heat sink represents up to
50 percent of the resistance to heat flow from the die
to the ambient air, it was felt that performance gains
in this technology could permeate across many dif-
ferent industries. In order to allow the realization of
complex 3D monolithic heat sinks, we first had to
develop a new fabrication process for the prototypes.
Investment casting of high thermal conductivity alloys
has enabled the fabrication of complex heat sink
designs that have not been possible in the past.
Experimental results for the initial prototype 3D heat
sink designs are promising, considering the demon-
stration of reduced pumping power to achieve the
same cooling performance as the longitudinally finned
heat sink. The next stage in developing this technol-
ogy is the pursuit of low cost mass production meth-
ods, considering the current investment casting
approach is not scalable for mass production.
The other novel technology presented is the vortex
generator, which improves heat transfer by creating
unsteady vortical flow that impinges on hot surfaces and
mixes out hot and cold airstreams. The primary advan-
tage of the vortex generator is that it is small, light, cheap
and can be placed almost anywhere on electronic equip-
ment. The paper presents two different embodiments:
1. By placing small plastic vortex generators
upstream of a longitudinally finned heat sink, we
demonstrated considerable improvement in heat
sink performance, which in turn lead to a 2°C
reduction in the heat sink temperature.
2. By incorporating the vortex generators on the
equipment board guide rail, we demonstrated
better overall performance of a complete circuit
pack, e.g., the processor temperature was reduced
by 3°C while the fan speed was reduced from
38 percent to 33 percent, saving energy and
reducing noise.
We also demonstrated that improvements over
the current state-of-the-art air-cooled architectures
are possible by fully understanding the underlying
thermo-physical fluid flow phenomena. Fully under-
standing the underlying physics involves high-fidelity
analytical, numerical, and experimental research pro-
grams.
Acknowledgements
The author would like to thank Christian
Joncourt and Robin Odabachian for obtaining the per-
formance measurements of the vortex generators
placed on the board guide rail. I would like to thank
John Mullins, Liam McGarry, Shankar Krishnan,
Marc Hodes, and Alan Lyons for their contributions to
the 3D heat sink program. The author would also like
to acknowledge the continued financial support from
the Industrial Development Agency (IDA) Ireland.
*Trademarks
InVision is a registered trademark of 3D Systems, Inc.
FLUENT is a registered trademark of ANSYS, Inc.
Kapton is a registered trademark of E.I. DuPont
DeNemours and Company.
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(Manuscript approved March 2010)
21. DOI: 10.1002/bltj Bell Labs Technical Journal 51
DOMHNAILL HERNON is a member of technical staff in
the Thermal Management Research Group
at Alcatel-Lucent Bell Labs in
Blanchardstown, Ireland. He earned a
B.Eng. in aeronautical engineering and
received his Ph.D. titled “Experimental
Investigation into the Routes to Bypass Transition,”
from the University of Limerick, Ireland. He joined the
thermal management research group at Bell Labs
Ireland in 2006. His current research focus is on projects
that extend the current limits of air cooling, and
additional research interests include high-fidelity
measurements in the complex flow field downstream
of vortex generators, and intelligent airflow system
design. He has authored 15 technical papers and has six
patents pending. ◆
