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Bltj Energy Effy

  1. 1. ◆ Enhanced Energy Efficiency and Reliability of Telecommunication Equipment with the Introduction of Novel Air Cooled Thermal Architectures Domhnaill Hernon In the past, thermal management was an afterthought in the design process of a product owing to the fact that heat dissipation loads and densities were minute and did not adversely affect component reliability. In fact, it may be stated that, historically, the sole purpose of thermal management was to ensure component operation below a critical temperature thereby providing reliable equipment operation for a given time period. However, this mindset has evolved in recent years given current economic and energy concerns. Climate change concern owing to vast green house gas emissions, increasing fuel and electricity costs, and a general trend towards energy-efficiency awareness 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 data center is spent on cooling equipment and that two percent of the United States’ annual electricity is consumed by telecommunications equipment, it becomes obvious that thermal management has a key role to play in the development of eco-sustainable solutions. This paper will provide an overview of the importance of thermal management for reliable component operation and highlight the research areas where improved energy efficiency can be achieved. Novel air-cooled thermal solutions demonstrating significant energy savings and improved reliability over existing technology will be presented including 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 Introduction Thermal 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 ( • DOI: 10.1002/bltj.20439
  2. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. References [1] Y. Abe, M. Fukagaya, T. Kitagawa, H. Ohta, Y. Shinmoto, M. Sato, and K. Iimura, “Advanced Integrated Cooling Systems for Thermal Management in Data Centers,” Proc. ASME/ Pacific Rim Tech. Conf. and Exhibition on Packaging and Integration of Electron. and Photonic Syst., MEMS, and NEMS (InterPACK ‘09) (San Francisco, CA, 2009). [2] A. Bhattacharya and R. L. Mahajan, “Finned Metal Foam Heat Sinks for Electronics Cooling in Forced Convection,” J. Electron. Packaging, 124:3 (2002), 155–163.
  20. 20. 50 Bell Labs Technical Journal DOI: 10.1002/bltj [3] The Climate Group and Global e-Sustainability Initiative (GeSI), SMART 2020: Enabling the Low Carbon Economy in the Information Age, 2008, Ͻhttp://www.smart2020.orgϾ. [4] F. J. Edwards and C. J. R. Alker, “The Improvement of Forced Convection Surface Heat Transfer Using Surface Protrusions in the Form of (A) Cubes and (B) Vortex Generators,” Proc. 5th Internat. Heat Transfer Conf. (Heat Transfer ‘74) (Tokyo, Jpn., 1974), pp. 244–248. [5] S. Ferrouillat, P. Tochon, C. Garnier, and H. Peerhossaini, “Intensification of Heat-Transfer and Mixing in Multifunctional Heat Exchangers by Artificially Generated Streamwise Vorticity,” Appl. Thermal Eng., 26:16 (2006), 1820–1829. [6] D. Hernon, “Effect of Upstream Vortex Generators on a Longitudinally-Finned Heat Sink,” Proc. 11th Intersoc. Conf. on Thermal and Thermomechanical Phenomena in Electron. Syst. (ITherm ‘08) (Orlando, FL, 2008), pp. 480–488. [7] D. Hernon, M. G. Hyde, and N. Patten, “Comparison Between Time Averaged and Instantaneous PIV and Hotwire Measurements Downstream of a Delta Winglet Pair,” Proc. 7th World Conf. on Exp. Heat Transfer, Fluid Mechanics 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, “Thermal Management: Enabling Enhanced Functionality and Reduced Carbon Footprint,” Bell Labs Tech. J., 14:3 (2009), 7–19. [9] F. P. Incropera, D. P. Dewitt, T. L. Bergman, and A. S. Lavine, Introduction to Heat Transfer, 5th ed., John Wiley & Sons, Hoboken, NJ, 2007. [10] A. M. Jacobi and R. K. Shah, “Heat Transfer Surface Enhancement Through the Use of Longitudinal Vortices: A Review of Recent Progress,” Exp. Thermal and Fluid Sci., 11:3 (1995), 295–309. [11] A. Joardar and A. M. Jacobi, “Impact of Leading Edge Delta-Wing Vortex Generators on the Thermal Performance of a Flat Tube, Louvered-Fin Compact Heat Exchanger,” Internat. J. Heat and Mass Transfer, 48:8 (2005), 1480–1493. [12] T. J. Lu, “Heat Transfer Efficiency of Metal Honeycombs,” Internat. J. Heat and Mass Transfer, 42:11 (1999), 2031–2040. [13] A. F. Mills, Heat Transfer, Irwin, Homewood, IL, 1992. [14] A. Ortega, “The Evolution of Air-Cooling in Electronic Systems and Observations About Its Limits,” Proc. 18th National and 7th ISHMT- ASME Heat and Mass Transfer Conf. (Guwahati, Ind., 2006). [15] H. Schlichting and K. Gersten, Boundary-Layer Theory, 8th ed., Springer, New York, 2004. [16] A. J. Shah, C. Patel, and C. E. Bash, “Designing Environmentally Sustainable Computer Systems Using Networks of Exergo-Thermo- Volume Building Blocks,” Proc. ASME/Pacific Rim Tech. Conf. and Exhibition on Packaging and Integration of Electron. and Photonic Syst., MEMS, and NEMS (InterPACK ‘09) (San Francisco, CA, 2009). [17] K. Torii, J. I. Yanagihara, and Y. Nagai, “Heat Transfer Enhancement by Vortex Generators,” Proc. 3rd ASME/JSME Thermal Eng. Joint Conf. (Reno, NV, 1991), pp. 77–83. [18] United Kingdom, Department for Environment, Food and Rural Affairs, Office of Public Sector Information, Climate Change Act 2008, Chapter 27, 2008, Ͻhttp://www.defra legislationϾ. [19] United States, Environmental Protection Agency, Energy Star Program, Report to Congress on Server and Data Center Energy Efficiency – Public Law 109-431, Aug. 2, 2007. [20] B. J. Watson, A. J. Shah, M. Marwah, C. E. Bash, R. K. Sharma, C. E. Hoover, T. W. Christian, and C. D. Patel “Integrated Design and Management of a Sustainable Data Center,” Proc. ASME/Pacific Rim Tech. Conf. and Exhibition on Packaging and Integration of Electron. and Photonic Syst., MEMS, and NEMS (InterPACK ‘09) (San Francisco, CA, 2009). [21] X. Zhang, J. W. VanGilder, and C. M. Healey, “A Real-Time Data Center Airflow and Energy Assessment Tool,” Proc. ASME/Pacific Rim Tech. Conf. and Exhibition on Packaging and Integration of Electron. and Photonic Syst., MEMS, and NEMS (InterPACK ‘09) (San Francisco, CA, 2009). (Manuscript approved March 2010)
  21. 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. ◆