Bltj Energy Effy

◆ Enhanced Energy Efficiency and Reliability ofTelecommunication Equipment with theIntroduction of Novel Air Cooled ThermalArchitecturesDomhnaill Hernon

In the past, thermal management was an afterthought in the design process ofa product owing to the fact that heat dissipation loads and densities wereminute and did not adversely affect component reliability. In fact, it may bestated that, historically, the sole purpose of thermal management was toensure component operation below a critical temperature thereby providingreliable equipment operation for a given time period. However, this mindsethas evolved in recent years given current economic and energy concerns.Climate change concern owing to vast green house gas emissions, increasingfuel and electricity costs, and a general trend towards energy-efficiencyawareness has promoted thermal management to the forefront of “green”innovation within the information and communications technology (ICT) sector.If one considers the fact that up to 50 percent of the energy budget of a datacenter is spent on cooling equipment and that two percent of the UnitedStates’ annual electricity is consumed by telecommunications equipment, itbecomes obvious that thermal management has a key role to play in thedevelopment of eco-sustainable solutions. This paper will provide an overviewof the importance of thermal management for reliable component operationand highlight the research areas where improved energy efficiency can beachieved. Novel air-cooled thermal solutions demonstrating significant energysavings and improved reliability over existing technology will be presentedincluding three dimensional (3D) monolithic heat sinks and vortex generators.© 2010 Alcatel-Lucent.

equipment providers. Traditionally, thermal manage-

ment was the last step in the design process and func-

tioned solely to maintain component junction

temperatures below their threshold limit so as to

IntroductionThermal management has recently been pro-

moted to the highest levels within the critical path in

a product’s design cycle and it is now one of the key

enablers, and differentiators, for telecommunications

Bell Labs Technical Journal 15(2), 31–52 (2010) © 2010 Alcatel-Lucent. Published by Wiley Periodicals, Inc.Published online in Wiley Online Library (wileyonlinelibrary.com) • DOI: 10.1002/bltj.20439

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 ManagementThe 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 dimensionalAoA—Angle of attackATCA—Advanced Telecommunications

Computing ArchitectureBCC—Body-centered cubicCAD—Computer aided designCFD—Computational fluid dynamicsEPA—Environmental Protection AgencyETSI—European Telecommunications Standards

InstituteFCC—Face-centered cubicFFHS—Fin foam heat sinkGPS—Global Positioning SystemHCHS—Honeycomb heat sinkIC—Integrated circuit

ICT—Information and communicationstechnology

L/D—Length-to-diameterLFHS—Longitudinally-finned heat sinkMETI—Japanese Ministry of Economy, Trade

and IndustryNEBS—Network Equipment-Building SystemOPEX—Operation expenditurePIV—Particle image velocimetryRFID—Radio frequency identificationRTD—Resistance temperature detectorSHS—Schwartz heat sinkTIM—Thermal interface materialUV—UltravioletVG—Vortex generator

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’ � 3.5’ � 6’

volume populated with blade servers requires approxi-

mately 20 KW to 25 KW 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/m2K)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)

Greeka 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/m2K4)

SubscriptsAmb AmbientBase Heat sink base measurementIns InsulationMax Maximum


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-SustainabilityAs 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 � 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


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 MeasurementProcedure

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 ArrangementThe 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 � 406mm � 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

� 15 mm); the external duct dimensions are 40 mm

long by 77 mm 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 12 W 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 � 32 mm �

15 mm and the base thickness is 2 mm. 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 0.5 mm 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 5 mm 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


(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 0.014 W/mk

are attached external to the foam insert and the

R �Tmax � 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 30 mm 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 mountinglegs

Testsection

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.


investigation at 21.5 degrees, the height of the VG is

15 mm (same height as the heat sink), and the walls

are 1 mm thick. The constant AoA is achieved by hav-

ing VG leading and trailing edge separations of 2 mm

and 24 mm, 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 ProcedureThe 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 6 mm of the sheathed thermocou-

ple is placed in Omega OT-201-2 thermal grease and

Inflow

Heat sink

Deltawing VG

Board guiderail

(a) Test setup with delta wingletVGs placed upstream of LFHS in

fully ducted flow.

(b) Delta winglet VGs. (c) Delta wing VGs placedon the board guide rail.

LFHS—Longitudinally-finned heat sinkVG—Vortex generator

VG

Duct

Inflow

Figure 2.Different types of vortex generators. Drawings not to scale.


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

�0.05°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 AnalysisThe 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 �2%. 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 � 0.5Ra0.25 (2)

(3)

Q � hA�T 1 Ase(T4ins � 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 �3 percent. At 2 Pa, with a higher �T

value of 37°C, the uncertainty is �3.5 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

�Ga00Qa¢T

Qbb2

(u�Q)2

u�R �Ga00¢T

a¢T

Qbb2

(u�¢T)2

hX

Kair

� 0.5 c gbX3¢T

nad 0.25


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 �3 percent

over the measurement range. Therefore, the total

error associated with the velocity measurements using

the pitot-static probe are of order �5 percent. The

error in pressure drop measurement is approximately

�3 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 �3

percent, �3 percent, and a maximum of �5 percent,

respectively.

Description of Two Novel Air Cooled ThermalArchitectures

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 TransferAs 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 dropresults for the LFHS.

0

10

20

30

40

0 1 2 3 4 5

Velocity (m/s)

Pres

sure

dro

p (

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 fora number of different heat sinks.

HCHS—Honeycomb heat sinkLFHS—Longitudinally-finned heat sink

Figure 3.Examples of result repeatability.


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.


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 FabricationTechnique

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


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.


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 SinksResults 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 sinkHCHS—Honeycomb heat sink

Figure 6.Plot of thermal resistance (R) versus pressure drop for the honeycomb structures compared to the LFHS. Allmeasurements at 10.3 W.


the significant improvement between the continuous

channel HCHS and the 6 mm slot HCHS results. At

low pressure drop, the 6 mm 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 1.5 m/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

Re�60, where Re is based on the width of the inter

slot metal components of 1.2 mm. In Figure 7, at

1.5 m/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 � 38.5°C

Tbase � 35°C

Next generation


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.


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 GeneratorsIt 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


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.


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 DesignA 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 SurfaceEnhancement Through the Use of Longitudinal Vortices: A Review of Recent Progress,” 295–309,Copyright Elsevier 1995.

Delta wing.��2b/c

y

z

Air Flow

x

Rectangularwing, ��b/c

Rectangularwinglet, ��b/c

Angle of attack� Aspect ratio

Deltawinglet,��2b/c

b/2

b/2

b

bc

c

c

c

Figure 9.Examples of vortex generators.


plastic delta winglet VGs upstream of an LFHS. Figure 11illustrates 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 andH. Peerhossaini “Intensification of Heat-Transfer and Mixing in MultifunctionalHeat Exchangers by Artificially Generated Streamwise Vorticity,” 1820–1829,Copyright Elsevier 2006.

Vortexgenerator

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 sinkVG—Vortex generator

Tbase � 42.3°C

Tbase �40.3°C

5 10 15 20 25

3

2.5

2

1.5

Pressure drop (Pa)

R (

°C/W

)

No VG VG#3 AoA�21.5 L�50 mm

Figure 11.Experimental results showing the reduction in thermalresistance of a LFHS with the introduction of upstreamVGs.


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

�0.4

�0.2

0

0.2

0.4

0.6

Time (s)

u’ (

m/s

)

�0.5�0.4�0.3�0.2�0.1

00.1

�0.6

u’ (

m/s

)

Z mm

y m

m

40 30 20 10 0 10 20 30 40�0.2

�0.15

�0.1

�0.05

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.


ConclusionsEnergy 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.

AcknowledgementsThe 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.

*TrademarksInVision 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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[7] D. Hernon, M. G. Hyde, and N. Patten,“Comparison Between Time Averaged andInstantaneous PIV and Hotwire MeasurementsDownstream of a Delta Winglet Pair,” Proc. 7thWorld Conf. on Exp. Heat Transfer, FluidMechanics and Thermodynamics (Krakow,Pol., 2009).

[8] D. Hernon, T. Salamon, R. Kempers, S.Krishnan, A. Lyons, M. Hodes, P. Kolodner, J.Mullins, and L. McGarry, “ThermalManagement: Enabling EnhancedFunctionality and Reduced Carbon Footprint,”Bell Labs Tech. J., 14:3 (2009), 7–19.

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[18] United Kingdom, Department forEnvironment, Food and Rural Affairs, Office ofPublic Sector Information, Climate Change Act2008, Chapter 27, 2008, �http://www.defra.gov.uk/environment/climatechange/uk/legislation.

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(Manuscript approved March 2010)


DOMHNAILL HERNON is a member of technical staff inthe Thermal Management Research Groupat Alcatel-Lucent Bell Labs inBlanchardstown, Ireland. He earned aB.Eng. in aeronautical engineering andreceived his Ph.D. titled “Experimental

Investigation into the Routes to Bypass Transition,”from the University of Limerick, Ireland. He joined thethermal management research group at Bell LabsIreland in 2006. His current research focus is on projectsthat extend the current limits of air cooling, andadditional research interests include high-fidelitymeasurements in the complex flow field downstreamof vortex generators, and intelligent airflow systemdesign. He has authored 15 technical papers and has sixpatents pending. ◆

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