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DISCOVER BETTER DESIGNS, FASTER. 2016

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DISCOVER BETTER DESIGNS, FASTER.

cd-adapco.cominfo@cd-adapco.com

FLOW − THERMAL − STRESS − EMAG − ELECTROCHEMISTRY − CASTING − DESIGN EXPLORATION

LI-ION CELL DESIGN − REACTING CHEMISTRY − VIBRO-ACOUSTICS − MULTIDISCIPLINARY CO-SIMULATION

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CONTENTSINTRODUCTION Ruben Bons - CD-adapco

PUSHING THE BOUNDARIES OF ELECTRONIC DESIGN Stephen Ferguson - CD-adapco

IS YOUR ELECTRONICS COOLING SOFTWARE FIT FOR PURPOSE? Stephen Ferguson - CD-adapco

DR MESH ON HEAT TRANSFER IN NARROW ENCLOSURES Stephen Ferguson - CD-adapco

MODELING OF ULTRA-HIGH PRESSURE LAMPS (UHP) Sergei Shulepov - Philips

IMPLEMENTING CFD AT OVERVIEW Alex Pope - Overview Ltd

OPTIMIZATION IN ELECTRONICS COOLING Titus Sgro - CD-adapco

LED PERFORMANCE Pier Angelo Favarolo & Lukas Osl - Zumtobel Group Sabine Goodwin & Ruben Bons - CD-adapco

WORLD'S LARGEST TELESCOPE Stephen Ferguson - CD-adapco

IR LAMPS DESIGN Larisa Von Riewel - Heraeus Noblelight

HIGH POWER DENSITY ELECTRONICS PACKAGES Kevin R. Anderson - California State Polytechnic University

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CD-adapco customers in the electronics industry use our simulation tools to investigate & design:

• Temperature predictions of ruggedized surveillance systems

• Performance and reliability of consumer electronic products

• Battery life (including charge & discharge cycles) for portable devices

• Headlamp thermal design with evaporation & condensation

• Semiconductor manufacturing equipment

• Power conversion equipment• Thermal management devices such as

fans, heat sinks, and heat pipes• Remotely-installed telecommunications

systems• Power semiconductor devices...and

many more (in electronics, not counting dozens of other industries)

This report will highlight a wide sampling of these applications, addressing a range of physical phenomena important to the different designs, spanning component to system design. We hope these articles spark some thoughts on what could be done to aid your design process, and we look forward to speaking with you in more depth about your specific applications.

INTRODUCTION

RUBEN BONS INTRODUCTION

The end goal of any design is a reliable, well-performing product, yet achieving these characteristics in the design of electronics systems is a challenging endeavor. This is true whether you are designing a data center, an engine control unit, a luminaire, or an avionics chassis. A range of factors contribute to the challenge including the constant need for ever-increasing capabilities (no device is ever powerful enough), compressed design schedules (no design can be completed fast enough), and ever-decreasing sizes (no device is ever small enough) - all while avoiding a host of failure modes. One of the most common causes of mechanical failure is overheating, which can lead to performance degradation, failure of individual components, temperature cycling failure of solder joints, and reduced system lifetime. Fortunately there are a number of tools at the design engineer’s disposal to help mitigate these challenges.

Analytical calculations are particularly well-suited for broad system performance modeling and early design envelope evaluation. Physical testing is the standard for final design validation & qualification. But between these two ends of the design cycle simulation is typically the most effective tool, allowing the engineer to efficiently explore the performance of a particular design, virtually test design variants, predict system robustness & reliability.

Our expertise at CD-adapco is detailed simulation of systems - detailed in terms of geometry, physical phenomena, and time scales.

RUBEN BONSDirector, ElectronicsCD-adapco

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PUSHING THE BOUNDARIES OF ELECTRONICS DESIGNSTEPHEN FERGUSONCD-adapco

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Whereas previous generations of engineers were able to rely on a combination of engineering intuition and experience to design the cooling of their electronic systems, the ever-increasing, consumer demand for greater performance in a smaller package, means this type of speculative prototyping is no-longer effective. Reliance on intuition to predict the cooling of complex systems is an almost certain guarantee of bad results. Not only is trial-and-error prototyping ill suited for today’s demanding markets, it also fails to derive the full value of design processes. The time and cost associated with this approach limits an engineering organization’s opportunity to optimize designs, by limiting the resources available to explore all the ideas designers may have. This affects products by decreasing profit margins, delaying product time to market, and increasing product cost. Perhaps most importantly, it limits the innovation that can be designed into the product itself.

The simple truth is that, in an increasingly competitive market place, only simulation can provide the necessary insight into the performance of a new device: “If I do not use simulation and the physical test

does not give me the right information the first time, I don’t know how to correct that situation,” said Andrew Slater, Director of Flight Sciences at Gulfstream Aerospace Corp. “If I have to fix it, I am very constrained, or I’m into a very expensive project to figure out how to fix it. The benefit of having simulation is that I get an indication of how to change the environment and fix the particular problem.” APPLICATIONS Electronics simulation can take place on many levels; two of the most important are at the component level and the system level. Components, such as dies and heat spreaders, are made of a wide assortment of materials, from ceramics and silicon to metal and hybrid materials. Each material reacts differently to changes in temperature, expanding and contracting at disparate rates. The interactions among the different materials can be critical. If you have a component made of conflicting materials, where one material’s volume grows significantly as it expands with heat and the other’s changes little, the component can fail as a result of the conflict. At the component level, therefore, design engineers must

understand the thermal expansion and thermal stress characteristics of the materials making up the parts they use.

At the system level, design engineers pay some attention to structural considerations, but in thermal management, these concerns tend to take a backseat to the prediction of the likely flow path. By directing a stream of air past heat generating components or sensitive components, convection can be used to directly remove thermal energy from the enclosure. Maintaining a thermal environment that allows the components to remain in a safe temperature operating range is the dominant focus.

The analysis at this level usually does not consider all possible conditions. Instead, attention is focused on worst-case scenarios. “We don’t look at all possible scenarios,” said Gary Schwartz, Engineering Fellow at Raytheon Network Centric Systems. “We just look at the worst scenario. If the electronics can survive that, they can survive mundane conditions.”Unfortunately, the difficulties encountered in electronics design are ramping up as systems, interactions, and operating factors become increasingly complex.

The control of component and system temperatures remains one the most significant challenges in the design of electronic systems. From chip to chassis and beyond, excessive thermal loading limits the maximum performance of an electronic device and significantly increases the energy footprint of the system.

One solution to these problems is the STAR-CCM+ surface wrapper: a tool that creates a geometric representation by shrink-wrapping a high-resolution surface on the complex aspects of the geometry. It allows the user to ignore many of the inadequacies of the 3D model and create a geometric representation that is ready for simulation.

PUSHING THE BOUNDARIES OF ELECTRONIC DESIGN FEATURE ARTICLE

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FEATURE ARTICLE PUSHING THE BOUNDARIES OF ELECTRONIC DESIGN

“Previously, we would have looked at individual parts, by themselves,” says Schwartz. “Individually, the parts may be all right, but the interaction of all the parts may not. The key now is to look at how all the parts work together.”

This situation is exacerbated by the fact that modern electronic systems need to be designed to strict energy usage guidelines. This means that engineers can also no longer rely on the “brute-force” approach of flooding an electronics package with the largest possible amount of cold air. Again, where a more subtle approach is required, the insight gained through simulation is key in determining an energy efficient, yet effective cooling system.

CHALLENGES The traditional physics involved in the simulation of electronic systems, such as heat conduction and convection, are well understood. On paper, many simulation tools have the ability to solve problems involving these physics. However, solving industrial strength problems, within the constraints of a product development program, is often much tougher in reality than it seems on paper. Today, many traditional simulation tools struggle to keep up with the fast-paced development schedules that engineering teams are confronted with. In a world where customers need to know performance characteristics in order to keep development moving forward, simulation has some inherent inefficiencies that are keeping design teams from delivering their results quickly enough to have the maximum impact on a program. The inefficiencies are in two main areas: the simulation process itself and the limited number of physical environments that can be represented virtually. One of the biggest stumbling blocks is geometry creation and geometry capture. THE STARTING POINT Geometry is the starting point of any simulation. It is the virtual representation of a system and its components. The geometry typically comes from CAD tools, in either a 2D or 3D format. CAD geometry may come in a simplified, conceptual form, quickly created without adequate attention to errors or, more likely, is overly detailed, containing more definition than the simulation requires. Both production geometry and simplified geometry can contain a lot of errors and problems that must be addressed before the geometry can efficiently be used as

a basis for an engineering simulation. Usually, a design engineer creates the geometry. There are times, however, where no CAD data exists, which forces the simulation engineer to create it directly within the simulation tool. “The most difficult part of the process is dealing with geometrically complex parts—taking those parts from a design model, such as Pro/Engineer or NX, and putting them into some kind of simulation code so that you can use that geometry,” said Raytheon’s Schwartz. “There is potentially a lot of clean-up that has to be done. Sometimes you have to massage the geometry of the parts to create a model.” Using the wrong simulation tool, engineers are forced to spend weeks attempting to make the geometry simulation-ready. They have to de-feature it, remodel or completely remove aspects, destroying in the process the link to the original CAD model. This reduces the impact that simulation can have on product development because the time required to create the geometry and prepare it for simulation is sometimes longer that it would take an engineer to prototype and test a configuration.

“There are times when it may take hours or days just to work on one part to get it to the point where you can actually include it in your simulation,” said Schwartz. “That is the bottleneck. Time is a real factor. You don’t want to spend three months building a model because by that point it’s of no relevance. You

need to do things fairly rapidly to have an impact on the design.” THE FIX One solution to these problems is the STAR-CCM+® surface wrapper: a tool that creates a geometric representation by shrink-wrapping a high-resolution surface on the complex aspects of the geometry. It allows the user to ignore many of the inadequacies of the 3D model and create a geometric representation that is ready for simulation. For those involved in electronics design, the most common use of the surface wrapper is for the enclosures that house large numbers of components. The surface wrapper creates one solid surface over these areas. If the enclosure is made of sheet metal, the surface wrapper can quickly help the user address the gaps inherent due to bend reliefs, assembly tolerances, and overlapping tabs. If the enclosure is made of molded parts, the complex geometry of the parts themselves as well as the interlocking connection between parts can be quickly prepped for analysis by the surface wrapper.

The corners of enclosures often have gaps that present significant problems. “The designer may have built a rack in CATIA that shows gaps around the shelves for manufacturing tolerance,” said Gulfstream’s Slater. “The gaps become flow paths that in reality should not be there, and they won’t close the boundary properly from a modeling perspective.”

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ABOVE: Simulation results showing flow through a cooling fan and temperature profile on a heat sink inside an electronics enclosure

PUSHING THE BOUNDARIES OF ELECTRONIC DESIGN FEATURE ARTICLE

One of the surface wrapper’s settings, called gap closure, lets you specify the size of the holes in the assembly that are closed automatically. Another setting called contact prevention, allows the simulation engineer to maintain spacing between components that should not come in contact with each other. With the surface wrapper, you can control the replica’s resolution either globally or on the part, surface, and edge level and automatically take care of problematic areas. The surface wrapper not only provides a way of transforming bad or difficult geometry into a form ready for efficient simulation; it also helps to dramatically shorten what has been a cumbersome and time-consuming process that prevented designers from being more productive and reaping the full benefits of simulation. “The speed of being able to put these models together and start using them is quite important,” said Slater. “The power with which we can handle the geometries and meshing within the simulation code is quite important.” Once a high quality closed surface is available, STAR-CCM+ can automatically fill the enclosure with a computational mesh of trimmed hexahedral or polyhedral cells, allowing the simulation to proceed.

NEW PROBLEMS, BROADER HORIZONSUntil recently, electronics designers have been fully occupied with the solution of traditional problems, such as conduction and convection. However, advances in simulation technology (such as those described above) are freeing designers to tackle more unusual problems such as contamination resistance, water intrusion, and condensation. For example, military electronic systems need to be able to operate in the desert,

where sand ingression is a challenge. Portable electronic devices such as cell phones are increasingly required to withstand water intrusion. Even cooling problems are now increasingly relying upon “non-standard technology”. To meet the increased cooling requirements of the latest generation of electronic equipment, engineers are expanding from simple air and gas cooling systems to liquid and spray cooling approaches. To develop these new systems, they must use heat exchanger models that allow them to interact with multiple fluids and spray cooling, as well as represent multiphase environments where liquid droplets interact with air, and evaporation and condensation come into play. The problem is that many mainstream simulation tools capable of solving simple conduction and convection problems cannot accurately represent the physics required to simulate these scenarios virtually. As a full-spectrum simulation tool that is used to solve fluid and structural mechanics problems over a wide range of industries, STAR-CCM+ is uniquely able to address the most difficult physics problems that electronics engineers encounter.

This gives them the confidence to develop and optimize cooling technologies for this new class of electronics problems, limited only by what is physically possible, and not by shortcomings of their simulation tool.

BENEFITS OF BEST-IN-CLASS SIMULATIONWhen you look at the bottom line of the balance sheet, the question isn’t whether you should use simulation to design electronic systems. Simulation enables you to visualize what is happening in the

component or system you are designing and why. It allows you to make informed design decisions, optimize product performance, manage risks, and pursue innovation. “Once you start to use simulation and you build your confidence with it, you can push the boundaries of your design and make sure that you achieve the maximum value of the product,” said Gulfstream’s Slater. “After we see the simulation results, the light bulb goes on,” said Raytheon’s Schwartz. “Until we do the simulation, it’s difficult to know what’s really going to be the behavior or response. We are just guessing unless we do some kind of simulation because things have gotten so complex that you really don’t know what the behavior is going to be like until you build the model, run the simulation, and look at the results. It shows us what we have to change to get what we want.” The real question is what features you should require in the simulation tool that you use. Find the tool that includes the most efficient surface wrapper, and the process of converting CAD data into a simulation-ready geometry is no longer prohibitive in terms of time and resources. Complex surfaces do not preclude accurate representation. Select the software that offers advanced meshing, and no project is too big. With the right mesh, you can get the optimum benefit from your computing resources. Choose the simulation tool that has the greatest variety of physics models, and you can design the new class of electronics that have captured the market’s attention. Also make sure that you choose the simulation tool that provides the best support organization to allow you to most effectively leverage the software in the design cycle.

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STEPHEN FERGUSONCD-adapco

IS YOUR ELECTRONICS COOLING SOFTWARE FIT FOR PURPOSE?

For electronic devices, temperature is a limiting factor. Packing technology, driven by constant consumer demand and competitive pressure, allows higher power density than current cooling technology can handle. Sustained elevated temperatures act to not only reduce component efficiency, but also to reduce product life. Effectively controlling the temperature of electronic systems, in an intelligent and sustainable manner, is therefore the key to producing smaller, more powerful and more resilient electronic devices.

FEATURE ARTICLE IS YOUR ELECTRONICS COOLING SOFTWARE FIT FOR PURPOSE?

ABOVE: Trimmed mesh and pressure distribution

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Did You Know?

You could fit more than 700

million transistors on the head

of a pin!

Air-cooling, while effective for low- to medium-power applications (where space and noise are not a concern), is generally neither practical nor cost-effective for high-powered systems. Put simply, the “brute force” approach, in which high-temperature components are strapped onto a large aluminum heat sink and blasted with cold air is no longer an option. SO, WHAT IS THE BEST METHOD FOR COOLING HIGH-POWER ELECTRONICS WHEN AIR SOLUTIONS ARE NOT PRACTICAL OR POSSIBLE? The future of electronics cooling involves the implementation of cooling strategies that leverage multiple modes of heat transfer. The problem for engineers developing electronics cooling solutions is that many of the simulation tools were developed entirely for analyzing simple “bread-and-butter” scenarios. These tools, although adequate for obtaining a “quick and dirty” fan-assisted air-cooled solutions, are generally not fit for simulating the more advanced physics required to represent more recent and sophisticated cooling strategies.

In this article, we look at some of the problems that simulation software will have to face in the electronics industry, and ask the question: “Is your electronics cooling software fit for purpose?” A QUESTION OF SCALE The length scales represented in electronics cooling problems can span 11 orders of magnitude: from individual transistors that are measured in nanometers (of order 10-9 m) to entire datacenters (of order 102 m).

Now obviously, no tool can account for every single electronic component in a datacenter cooling simulation. Even if it were possible to do so, it is doubtful that such a simulation would provide additional useful information. Out of necessity, engineers use a combination of simplifying assumptions and imposed boundary conditions to focus the simulation on those length scales that are most important for the simulation (using generous amounts of “engineering judgment” in the process). However, care must be taken not to over simplify things: if the assumptions are too great, or the imposed boundary conditions are too unrepresentative, then the results predicted by the simulation begin to diverge from those that would occur in reality. When this happens, no amount of judgment (engineering or otherwise) can rescue useful data from the simulation. Worse still, wrong or inaccurate results can mislead the design process, potentially sending the product up a non-optimal design branch. So, ideally your simulation tool will allow you to solve problems across multiple length scales. Instead of just simulating flow across a single circuit board, you want to be able to model a whole blade server, or better still, how several blade servers interact with each other and their environment.

MULTI-CORE PROCESSING

STAR-CCM+ includes parallel view factor calculation, which allows users to exploit the processing power of multiple computer cores when performing radiation view factor calculations.

IS YOUR ELECTRONICS COOLING SOFTWARE FIT FOR PURPOSE? FEATURE ARTICLE

STAR-CCM+STAR-CCM+® is designed to handle complex problems and large model sizes. It has been used by our industrial partners for calculations numbering billions of computational cells far beyond the size of any electronics-cooling problem. No matter how big, or how complex your design scenario is, STAR-CCM+ allows you to solve it without compromise, using a model size that fits your problem and ultimately utilizes less assumption.

ABOVE: Team Lotus Renault CFD Centre: a $50m underground bunker for computers

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More than just a CFD code, STAR-CCM+ is a complete multi-physics toolbox, able to solve flow, thermal and stress problems involving multiple phases. From liquid jets, to water ingression, STAR-CCM+ allows you to simulate any cooling strategy that you can define, and even the effect of what happens when those strategies go wrong.

THE ALL-IN-ONE MUTI-PHYSICS TOOLBOX

FEATURE ARTICLE IS YOUR ELECTRONICS COOLING SOFTWARE FIT FOR PURPOSE?

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Fit for purpose?

My Electronics Cooling Software:

Can handle complex geometries without gross simplification

Solve problems that span multiple length scales

Solve problems that involve natural convection and radiation

Allows me to simulate problems involving liquids as well as gasses

NATURAL CONVECTION AND THERMAL RADIATION In traditional forced convection “air-cooled” systems, thermal radiation plays a relatively minor role in the overall heat transfer, typically accounting for less than 5% of the thermal energy rejected by the system (with the rest split evenly between convection and conduction).However, in “no flow” situations, whether by design or in unintentionally “dead” regions of the compartment, radiation plays a much more important role accounting for between 30-50% of heat transfer. Simply neglected in many simulations, for the reasons described below, including radiation heat transfer in a simulation will generally act to decrease maximum temperature in the system, spread out the temperature distribution and reduce the exterior surface temperature (touch temperature).

However, including radiation heat transfer can significantly increase the computational overhead for a simulation, as view factors (basically lines of sight) must be calculated for every computational face of every component in the system. Although these view factors need only be calculated once per geometry, this process can be computationally expensive even for a large uncluttered enclosure. In a typical crowded electronics enclosure, consisting of hundreds, if not thousands of components, calculating these view factors is beyond the capability of a single processor machine (both in terms of memory requirement and physical time needed to complete the calculation).

By including radiation, you can reveal the effects of an additional heat flow path – this is critical in low flow or no flow situations and can be important when trying to squeeze every degree of cooling from a forced convection system. LIQUID COOLING: CHILLING, DUNKING AND SPRAYING While air-cooling continues to be the most widely employed method of thermal management, its ultimate effectiveness is always limited by the fact that air has relatively poor thermal capacity compared to other fluids. For serious cooling impact for high-power density systems, designers are increasingly turning to different types of liquid cooling.

A common feature of liquid-based cooling systems is that they exploit the additional heat transfer of phase change to increase the cooling effect of the liquid. Because of the higher thermal capacity of the coolant liquid, they benefit from greater sensible heat transfer (which raises the temperature of the coolant) and latent heat transfer (which changes the phase of the coolant, through boiling or evaporation).

The simplest way of doing this is by “indirect” liquid cooling, where the coolant never comes into direct contact with the electronic component being cooled, usually accomplished through the attachment of a liquid cooled “cold plate” which is attached to the chip.A more effective (although less practical) solution is to submerge the chip directly into a (non electrically conductive) coolant. If the temperature of the component increases beyond a critical level (the boiling point of the liquid), then nucleate boiling will commence, greatly increasing the heat flux from the chip to the fluid. At high temperature, this approach is around 5 times more effective than indirect liquid cooling, and about 25 times more effective than direct air-cooling. However, this approach makes routine maintenance much more difficult (as the components must be removed from the liquid bath and cleaned prior to inspection). Care must be taken so that the boiling regime does not progress to “film boiling” at which point the component

becomes surrounded by a film of vapor, and heat transfer is suddenly reduced, resulting in a sudden rise in component temperature, followed by rapid failure. Most effective of all are direct spray systems, in which a fine mist of non-corrosive, non-conductive coolant is sprayed directly on the surface of the component, forming a liquid film that rapidly evaporates. The coolant vapor is extracted from the enclosure and condensed, rejecting heat to the surroundings. THE PROBLEM? As we discussed above, many simulation tools are specifically designed to handle single-phase air-cooling and, at a push, simplified indirect liquid cooling (modeled using a source term or a boundary condition). If you want to explore any of the more advanced liquid cooling technologies, your simulation tool needs to be able to model multi-phase calculations, which include the interaction between air, liquid and various gasses, as well as boiling and phase change. Without this functionality, your simulations and your designs will be limited to simple, ineffective air-cooling.

OTHER ADVANCED PHYSICS Of course, it’s not all about cooling. Engineers in the electronics industry have a whole multitude of problems to deal with, to name but a few: • fan performance and acoustics (if you’ve ever been inside a

datacenter,then you’ll know how important that is);• water intrusion; • dust build ingress and accumulation; • hydrogen build up (from battery decay).

The benefit of employing a fully featured simulation tool is that, no matter how rarely these problems occur, your engineering software will allow you to solve them when they do.

IS YOUR ELECTRONICS COOLING SOFTWARE FIT FOR PURPOSE? FEATURE ARTICLE

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Small gaps occur frequently in electronic devices, typical examples include: the narrow space between a housing and a PCB, the gap between two circuit cards or the space between a metal divider and a PCB. In long thin unventilated enclosures, and to a lesser extent tall narrow unventilated enclosures, it is possible that the temperature differences across the gap, are not big enough to generate flow due to natural convection, and instead the heat transfer is dominated by conduction. In this situation it is a waste of time and effort to solve the full flow equations in an enclosure full of motionless air, when you can simply solve the energy equation for conduction.

WHAT IS A SMALL ENOUGH GAP? The first thing we need to determine is the critical size at which natural convection will begin to drive flow in the gap.

In fluid mechanics the transition between conduction and convection heat transfer is characterized by the Rayleigh number, which measures the ratio between viscous and buoyant numbers:

WHERE: Thot = hot side temperature Tcold = cold side temperature X = the size of the gap g = acceleration due to gravity = 9.81 m2/s β = thermal expansion coefficient of the fluid = 1/Tbulk = 0.003 K-1 @ 45oC ν = kinematic viscosity = 17.45e-6 m2/s @ 45oC α = thermal diffusivity = 2.46e-5 m2/s s @ 45oC When the Rayleigh number is below the critical value for that fluid, heat transfer is primarily in the form of conduction; when it exceeds the critical value, heat transfer is primarily in the form of convection. For a horizontal gap the critical Rayleigh number is 1708. For a vertical gap the critical Rayleigh number is about 1000. If you plug the numbers into the equation, you see that, for a 10K temperature difference, the critical gap width is about 13mm for a horizontal cavity, and about 11mm for a vertical enclosure. If your enclosure size is smaller than that, there will be no flow in the cavity, and you can assume that it is filled with solid ‘block’ of conducting air.

STAR-CCM+® is a “no-compromise” solution for modeling problems in electronic cooling, allowing you to account for the influence of multiple length scales when performing cooling scenarios. However, with length scales in a typical problem scaling 11 orders of magnitude (from transistors a 10-9m to datacenters at 102m), it is neither sensible nor prudent to include them all in a single model.

DON’T MIND THE GAP: DR MESH ON HEAT TRANSFER IN NARROW ENCLOSURESSTEPHEN FERGUSONCD-adapco

DR. MESH NATURAL CONVECTION ION SMALL GAPS

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WHY BOTHER? There are several advantages of this approach:

1. Since we are now solving a single equation, instead of 5, conduction simulations are much less computationally expensive than full 3D flow simulations, and therefore run much more quickly.

2. It is much easier to get a mesh independent solution for a conduction only simulation than for a full 3D flow simulation.

3. If used appropriately, this approach can deliver accurate results (this can be verified by simply reverting the “solid air” continuum to a regular “fluid continuum” and re-running the solution.

4. Even if you aren’t convinced about the accuracy of this approach, it is very good for getting a “quick and dirty” solutions that can then be used as the initial condition for a more complete flow simulation (again by simply switching continuum properties and restarting the solution).

5. If need be you can model the “solid air” as a transparent

solid and include the influence of radiation within the enclosure.

6. It’s elegant and it’s clever (which is my book is reason enough in itself).

The combination of all these things can lead to quick and accurate answers, that hopefully translate into better-designed cooling systems.

MIND YOUR STEP! However, as with all modeling assumptions, you need to make sure that you apply this technique carefully:

1. It can only be used for appropriate gap sizes and temperature differences (as defined above)

2. Make sure that you adjust the air properties to reflect the bulk air temperature in your enclosure.

3. It can only be used an unventilated sealed enclosure, or a fully enclosed part of a larger simulation.

The ‘Solid Air’ Approach in STAR-CCM+

NATURAL CONVECTION ION SMALL GAPS DR. MESH

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SERGEI SHULEPOVPhilips, Netherlands

PHILIPS STAYS COOL WITH CD-ADAPCO

Philips Applied Technologies assists the product divisions within Philips in development of new, and optimization of existing industrial processes and products. A number of competences at Philips Applied Technologies heavily rely upon high-end, multi-physics computational software. In problems related to thermo-fluid behavior, (micro) fluidics, phase transitions, and multiphysics in general, CD-adapco software is used. In this article, as an example of application of CD-adapco solutions within Philips, modeling of ultra-high pressure lamps (UHP) is considered.

MODEL DESCRIPTIONA UHP lamp consists of the burner, reflector and closing glass. The closed UHP lamp has two electrical contacts (front and back), and the burner is fixed within the reflector, using cement of a special composition. Typically, lamps of this type have the power input ranging from 120 through 150 W and are manufactured with either parabolic or elliptic reflectors. However, for special applications, lamps of deviating reflector shape and power can be produced. This type of lamp is used, for example, in

projection televisions and rear projectors.

Currently, there is a clear trend towards miniaturization. In order to improve the lifetime and safety of smaller products, thermal housekeeping of special lamps has to be well understood, and thermal behavior of the lamp in an application must be well controlled. The UHP burner physics is described in [1], and in our modeling, results of developments within Philips Research/Aachen have been used. We have not modeled the electrical discharge in the burner, but rather used, as an input, the power/frequency distribution of radiation resulting from such modeling. The UHP burner is made of quartz which is semi-transparent to the plasma radiation. Quartz starts to absorb typically above 4mm wavelength. This means that quartz is also semi-transparent to the infrared radiation. As a consequence, the phonon thermal conductivity will be enhanced at higher temperatures (typically, higher than about 250OC) by the “photon” conductivity. For optically “thick” materials, a well-known Roseland approach can

be used to describe this phenomenon. In optically “thin” materials, this effect may be neglected. Quartz burners are, however, neither optically “thick” nor optically “thin”. We have, therefore, used an “in-house” semi-phenomenological model to describe this phenomenon, developed in collaboration with Philips/Central Development of Lamps (CDL).

THERMAL PLUMES AROUND AN UHP LAMP At the inner surface of reflector, an optically reflective coating is applied. This coating reflects the visible portion of irradiation coming out of the burner. However, the reflector is semi-transparent to the rest of irradiation spectrum. Semitransparency of the reflector and burner can be expressed in terms of the Lambert-Beer law, i.e. a fraction of the total power absorbed in materials is proportional to e-ax, where a is an effective attenuation constant, and x is the optical length of radiation beam passing through the material. This absorption has been implemented in CD-adapco’s CFD code, based on the geometry given. Furthermore, it is not

Royal Philips Electronics of the Netherlands is a diversified Health and Well-being company, focused on improving people’s lives through timely innovations. As a world leader in healthcare, lifestyle and lighting, Philips integrates technologies and design into people-centric solutions, based on fundamental customer insights and the brand promise of “sense and simplicity”.

Headquartered in the Netherlands, Philips employs over 120,000 employees with sales and services in more than 100 countries worldwide. With sales of EUR 22.3 billion in 2010, the company is a market leader in cardiac care, acute care and home healthcare, energy efficient lighting solutions and new lighting applications, as well as lifestyle products for personal well-being and pleasure with strong leadership positions in male shaving and grooming, portable entertainment and oral healthcare.www.philips.com ABOVE: Rear projection TV utilising UHP technology

FEATURE ARTICLE MODELING OF ULTRA-HIGH PRESSURE LAMPS (UHP)

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Air re-circulation inside the enclosed UHP lamp Comparason of the temperature distribution obtained from the simulations and measured experimentally

Direction of the re-circulation.

possible at the moment to model specular reflections within the code. Therefore, an optical analysis software ASAP (Breault research [2]) has been used for ray-tracing to identify possible hot spots due to specular reflections. The results have been translated into volumetric sources, and introduced into the CFD model for refinement. Thermal properties of materials are all non-linear functions of temperature. Air has been modeled as an ideal gas, with all properties depending on the temperature. NUMERICAL IMPLEMENTATION Temperature in closed UHP lamps can be rather high. The outer burner surface can reach about 1000OC, whereas typical temperature at the outer reflector surface can be about 300-350OC. The coldest part of reflector can have temperatures about 180-200OC. This means that inside the closed UHP lamp, an intensive air recirculation takes place as seen in the image opposite. On the other hand, the air plume(s) around the outer reflector surface may be unstable, especially around the reflector neck, where the temperature gradient is the largest. All together, this means that the modeling of the lamp operation at “steady” conditions can be quite difficult.Because of the lamp dimensions, a low- Reynolds number, turbulence model has been utilized in our computations. We found that the k-ε, low-Reynolds number model yielded rather good results. Finally, the

standard version of this model has been used, which delivered best accuracy/speed performance. In the image, temperature distribution around a closed UHP lamp is given. One can see that there are typically two plumes: one rising along the front glass of the closed lamp, and another around the lamp neck. In these figures, a snapshot of an iso-thermal surface (at 80OC) is given to indicate the temperature distribution in the plume. RESULTS In the accompanying image, comparison of the temperature distribution obtained from the simulations and measured experimentally is given for the outer surface of a reflector. Experiments have been carried out using AGEMA900 infrared camera equipped with an infrared filter cutting off frequencies lower than 4.7 mm. This setup assures that the temperature of the surface of a semi-transparent material is measured, and it is not affected by the thermal radiation from bulk. Measurements have been verified using conventional thermocouples. At Philips UHP/Turnhout, a special testing program has been developed to control the most critical (from the thermal point of view) features of closed UHP lamps. This testing program allows for an evaluation of the product lifetime. Comparison of the model results with data from these tests showed that the model developed is capable of predicting the temperature distribution in a closed UHP lamp with accuracy of about 5-7%. Using

this model approach, behavior of UHP lamps in different applications has been analyzed. In this way, an optimal cooling concept can be designed within a very short time for a given application.

CONCLUSIONS CFD models, combined with experimental validation and testing under factory conditions, dramatically reduce time required to develop an optimized product for new applications. CD-adapco’s code offers the flexibility for geometrical modeling required in industrial environment, combined with the state-of-art physical models in the area of CFD simulations. This combination allows the simulation of very tiny details which is necessary for an accurate prediction of the behavior of critical features of products. Using CD-adapco solutions, we were able to predict thermal behavior of a complex product such as closed UHP lamps in different applications with accuracy better than about 7%. This accuracy allows for virtual prototyping of new generation of products. Optimal cooling of lamps in different applications can readily be designed in this way.

REFERENCES:[1] H. Moench, Optical Modeling of UHP

lamps, Modeling and Characterization of Light Sources, Proceedings of SPIE Vol. #4775, 2002.

[2] www.breault.com

Thermal plumes around an UHP lamp

MODELING OF ULTRA-HIGH PRESSURE LAMPS (UHP) FEATURE ARTICLE

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IMPLEMENTING CFD AT OVERVIEWALEX POPEOverview Ltd.

Overview Limited is a British Company, located in Wandsworth, South West London,

specializing in the design and manufacture of CCTV dome mechanisms and associated

electronics for OEM supply.www.overview.co.uk

FEATURE ARTICLE IMPLEMENTING CFD AT OVERVIEW

19SPECIAL REPORT

Overview Limited is a London based British company that specializes in the design and manufacture of CCTV (closed circuit television) camera dome mechanisms and associated technology. The story of CAE integration at Overview has followed a fairly typical route: mirroring the development of computational techniques for design, test and manufacture using CAD and FEA structural and thermal analysis, and resulted in the first implementation of CFD technology within Overview’s design process.

From a thermal perspective, the analysis of CCTV domes presents a significant challenge. A large number of power hungry components are enclosed in a small space, which is sealed in order to avoid water ingress. This small sealed space is then subjected to extreme environmental conditions from long hot summer days to cold winter weather with high winds, snow and rain.

As with any electrical component, temperature is a limiting factor and without proper ventilation and cooling, failure may occur. This presents a problem. Because the unit is sealed, ambient air may not be drawn through the enclosure. Traditionally, this would not have been an issue for Overview enclosures, but an expanding product line, which incorporates larger cameras and more powerful motors means that the enclosures are subjected to increasingly large thermal loads.

FIRST STEPS ALONG THE ROAD TO CFD In common with many companies that have taken the first step towards integrating CFD simulation into their design process, the management and design engineers at Overview needed to be convinced of the technology’s usefulness and accuracy. Although the benefits of CFD may be apparent

to experienced users in larger OEMs, for smaller companies the (apparently) relatively high cost of the software (compared to CAD programs for example) must be justified.

After an extensive evaluation process, STAR-CCM+ was chosen for our applications. We successfully validated the software using a range of simple components, which gave us confidence in the accuracy of the code and helped us to establish a set of best practices for the types of simulations that would be carried out in future. The idea of these initial tests was to start simple then steadily increase complexity, to the point of modeling and validating a full system. These initial simulations included: • Modeling a heat sink to validate

pressure drop and heat transfer. • Measurement of single component

heating and cooling within a simplified enclosure.

The increased accessibility of CFD, provided in part by innovative products such as STAR-CCM+®, has led to the adoption of simulation technology at many companies that have little previous experience of Computational Fluid Dynamics. In this article, Alex Pope, of Overview Limited, explains the challenges and rewards of introducing Computational Fluid Dynamics into an organization for the first time.

CFD simulation was able to save the company between £1000 - £1500 (US $2000 - $3000) per day.

ABOVE: A traditional CCTV Dome produced by Overview Ltd

ABOVE: The internal electronics of the Sparkle dome

SMILE - YOU’RE ON CCTV!Orleans, New York, was the first US city to install CCTV along its main shopping street in 1968. The first CCTV camera in Times Square wasn’t installed until 1973.The UK has more CCTV cameras per head of population than ANY other country. About one camera for every 14 people…The largest CCTV system in the World is in Australia, the New South Wales City Rail system has over 7,000 fully networked cameras! Almost every bank cash machine now uses CCTV to record customers making transactions

IMPLEMENTING CFD AT OVERVIEW FEATURE ARTICLE

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ABOVE: External heating of the CCTV enclosure caused by incident Solar radiation

LEFT: Motor temperatures inside the new dome

FEATURE ARTICLE IMPLEMENTING CFD AT OVERVIEW

21SPECIAL REPORT

For each of the initial tests, experimental results were gathered and compared to the computational data, with good agreement shown. Once the results were presented to fellow engineers and management and confidence in the accuracy and applicability of CFD simulation had been established, the range of simulations carried out could be extended to more complex situations.

INCREASING THE COMPLEXITY With the basic tests carried out, the CFD analysis was extended to a full analysis of the newest product in the Overview range - the Sparkle dome. This newly developed CCTV enclosure is Overview’s most technically sophisticated product, and so the implementation of CFD into the design process would be key to a successful deployment of the final product. Within the relatively small enclosed space of the dome there are stepper motors for camera control, as well as the camera itself and all of the control electronics, which combined produce significant amounts of heat: potentially posing problems in component performance and ultimately reducing product lifespan. Early designs of the Sparkle dome had shown that glue used within the assembly was failing and causing the system to malfunction. The glue itself located on the central shaft was rated only to a specific temperature and so the first analyses concentrated on discovering the conditions in the area of the shaft that were responsible for causing this failure.The first step in the analysis process was to build a suitable geometry for study. This involved taking the original CAD configuration and simplifying it

to a level whereby run times would be reduced to reasonable timescales without overly compromising accuracy. Once complete, the CAD model was imported and meshed in STAR-CCM+ using polyhedral cells, which we considered to offer the best compromise between accuracy and simulation time. Results from the simulation identified a “hot spot” in the enclosure in which the design temperature of the glue was exceeded. Immediately, CFD had illustrated its value in the engineering process by helping to identify a critical mechanism for failure that was easily resolved by adopting a new adhesive rated above the identified peak temperature, ensuring that future system malfunctions will be prevented.

RADIATION MODELING A key additional step when looking at the modeling of Overview’s CCTV enclosures was understanding the effect of the ambient environment on conditions within the dome and how these conditions can affect (adversely or otherwise) the electronic systems inside. One of the key advantages of STAR-CCM+ over the other codes evaluated was the solar radiation modeling. As the domes are usually placed outside and may be exposed to direct sunlight in the height of summer, this can lead to significant heating of the system as a whole. SOLAR RADIATION The aim of this solar radiation modeling was to both analyze the effect of high intensity direct sunlight on the dome and the system within it, as well as to investigate possible ways of “shielding”

the enclosure from the sun. This solar shielding could be accomplished either by constructing a physical barrier to shade the dome from the sun, or by using reflective coatings. Both these scenarios, as well as the un-shielded case, were easily run in STAR-CCM+, with simple modifications to solar properties able to provide quick “what-if” studies of the different configurations, and again provide previously unavailable insight. CONCLUSIONS The implementation of CFD simulation technology at Overview Ltd has, over the course of the past year, proven to be of increasing benefit to the design and analysis process across a range of products at both a system and component level. The first step along this road to full integration was for CFD to “prove itself” to the company at large, and to justify the investment in the software technology. Indeed with the implementation of solar radiation modeling, a tangible benefit, both financially and in engineering terms, was gained as CFD simulation was able to save the company between £1000-£1500 (US $2000-$3000) per day in solar testing, as well as cutting down on turnaround time and allowing the study of many different configurations, quickly and easily. The technology has since been used across a wider range of applications throughout Overview and more engineers have been exposed to its benefits. The software is now being used further upfront in the design process allowing analysis results to drive the design and not the other way round.

CCTV HISTORY...AND BROKEN TV’SThe first CCTV system was installed by Siemens AG at Test Stand VII in Peenemünde, Germany in 1942, for observing the launch of the liquid propellant ballistic V-2 rockets, the progenitor of all modern rockets. WW2 German engineer, Walter Bruch (March 2, 1908 - May 5, 1990), was responsible for the design and installation of the system. Walter is also, more notably, credited with inventing the PAL color television system at Telefunken in 1963, which has been adopted by more than 100 countries. When interviewed by German talk show host Hans Rosenthal on why he had named it the “PAL system”, Bruch replied that certainly no German would be willing to buy a “Bruch-System” (which literally translates to “broken system” in German).

ABOVE: Incident solar radiation on the internal of the CCTV dome

IMPLEMENTING CFD AT OVERVIEW FEATURE ARTICLE

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FEATURE ARTICLE OPTIMIZATION IN ELECTRONICS COOLING

23SPECIAL REPORT

While every engineer strives to create the new best thing since sliced bread, often his or her work must be focused on the more mundane: taking a product or idea already on the market and making it better, faster, smaller, lighter, cheaper or somehow more improved. This optimization, while not as exciting as groundbreaking new designs, is still vitally important for technological advancement. Compare the latest cell phones to the very first cell phone models ever made, or a laptop to the ENIAC, and you can see what optimization can do for the world, much less for customers, who are almost universally less curmudgeonly when they own a better product, even if it is only incrementally better.

CASE STUDY: A SIMPLE HEAT SINKWhile this is easily said, most engineers know that even optimizing a product with a very few number of variables can be expensive, time-consuming and painfully difficult. In the past, millions could be spent on trying to optimize a product and only being partly successful. CD-adapco™ has the tools to turn that paradigm on its head.

As an example of applying optimization to the electronics industry, a heat sink was optimized using CD-adapco’s Optimate+™ in conjunction with STAR-CCM+®. The guidelines of the heat sink were laid out as follows:

• A 40mm square base• A total height of 10mm• 3-8 fins that have a sharp kink at a

variable height, the angle of which must grow from the middle to the end, and no kink can have more than a 70 degree turn

• Fins will be set equal distances apart and must have a reasonable gap between them FIGURE 1: Examples of heat sink geometry created by Optimate+

THE KEY TO THE FUTURE:DESIGN EXPLORATIONTITUS SGROCD-adapco

OPTIMIZATION IN ELECTRONICS COOLING FEATURE ARTICLE

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FIGURE 4: Air velocity profile through heat sink fins

FIGURE 2: Far view of air velocity profile around heat sink

FIGURE 3: Temperature profile of heatsink and surrounding air

FEATURE ARTICLE OPTIMIZATION IN ELECTRONICS COOLING

25SPECIAL REPORT

• The base thickness may vary to a thickness of up to 8mm

• The solution should be examined for several airflow speeds (1, 3, 5, 8 and 10 m/s)

With these requirements, it was a simple matter to create and build a heat sink within the STAR-CCM+ 3D-CAD tool. Using a series of constraints to ensure a robust and logical design, CD-adapco set up a simulation that could test literally thousands of different heat sink designs using hundreds more potential operating conditions quickly and easily. Figure 1 shows three different possible configurations generated from this.

Once this was properly designed, Optimate+ was tied in, with the software given instructions on what was the acceptable range for the variables of base thickness, “kink-height” (the height at which the sharp kink occurred on the fin), the angle of the kink, and the number and width of the fins, while a small Java code was written to control other case specific functions. The ability to code specific functions like this was essential for making the optimization of the heat sink easy to set up. Yet the code itself was trivial; even beginner coders can quickly learn how to write Java scripts for STAR-CCM+.

With all this set up, Optimate+ took everything the rest of the way. Optimate+ was built in conjunction with Red Cedar Technology, a subsidiary of CD-adapco, to bring its HEEDS multi-disciplinary optimization tool more directly into STAR-CCM+. It ran through hundreds of possible potential designs using the exclusive “Simultaneous Hybrid Exploration that is Robust, Progressive and Adaptive” (or SHERPA) optimization strategy. Using SHERPA, Optimate+ was able to determine the best possible design quickly and easily (a full optimization routine of 100+ runs took only two days on 80 total processors) and this was completely automatic.

Optimate+ did all the fine tuning and calculations without any user input besides what the variables were and how many runs per routine were to be done. Even the post processing data for every run was automatically generated, removing the painfully tedious work and allowing detailed examination of the solutions the moment Optimate+ was done optimizing.

RESULTSThe targeted results were to minimize the thermal resistance and the mass of the heat sink. While these can conflict with one another, SHERPA’s Pareto search

worked to minimize both at the same time. Table 1 shows the optimum heat sink for each of the various flow speeds. The ‘Performance’ term indicates SHERPA’s evaluation of how good a design is, and equally weighs ‘Thermal Resistance’ with mass, along with its special optimization calculations. As is readily apparent from table 1, a large drop (a 90% decrease) in ‘Thermal Resistance’ increases the performance drastically, despite the mass increasing by just over 15%.

The first thing that can easily be noticed about the results of the optimization is that, for each speed, the minimum base thickness turned out to be the most efficient, as well as the thinnest possible fins. This makes sense, as this reduces the mass and creates the most area possible for heat transfer. In addition, the maximum angle of the kink proved to be the most desirable for each of the most optimized results, though this was not nearly as universal. Table 2 shows the ten best results for a flow speed of 3 m/s.

A more important design result highlighted by Optimate+ was that the number of fins that was optimal went up as the flow speed increased. For an ideal cooling situation, the boundary layers for each of the fins should just touch at the downstream end of the fins. At low flow speed, the boundary layers between the fins are thicker, thus a fewer number of fins produce an optimum cooling

situation, while at higher speeds, the thinner boundary layers call for a thinner gap between fins, pushing up the number of fins on the same size sink. The optimization results reflect this, showing the greater number of fins as the air speed increases, as well as the drastically lower thermal resistance.

Figures 2 to 4 show a sampling of temperature and airflow results from the optimization routines.

FUTURE WORKAs the reader can imagine, the potential application for this would be nearly limitless. Already, many companies are using STAR-CCM+ to examine their designs in the digital world and perfect them before they ever build a physical prototype, and most often only one physical prototype is ever needed to be built. With Optimate+ and SHERPA, even more of this work can be automated.

Imagine inputting variables into a program and designing an entire passenger aircraft carrier with the click of a button. Picture an architect building the perfect comfort system for his building, ensuring every room is at a perfect temperature while avoiding “cold spots” in a room. Consider the design of an entire super computer, perfected, compacted and as energy efficient as possible being designed in days. Where will your imagination take you?

Flow Speed (m/s)

Evaluation # (out of

100)Performance

Thermal Resistance

Mass of Heatsink

Base Thickness

Kink Angle Fin WidthNumber of

fins

1 43 -0.952 4.2101 0.0040 1 35 0.2 3

3 51 -0.8964 1.2999 0.0045 1 35 0.2 5

5 79 -0.7878 0.5584 0.0047 1 35 0.2 6

8 41 -0.6195 0.1550 0.0047 1 35 0.2 6

Evaluation # (out of

100)Performance

Thermal Resistance

Mass of Heatsink

Base Thickness

Kink Angle Fin WidthNumber of

fins

29 -0.9674 1.6990 0.0039 1 35 0.2 3

49 -0.9643 1.4688 0.0046 1 25 0.2 6

80 -0.9631 1.3324 0.0051 1 35 0.2 7

78 -0.9500 1.4148 0.0047 1 30 0.2 6

36 -0.9399 1.3735 0.0047 1 35 0.2 6

54 -0.9364 1.5431 0.0041 1 25 0.2 4

46 -0.9209 1.4164 0.0044 1 25 0.2 5

81 -0.9201 1.4605 0.0042 1 35 0.2 4

43 -0.9022 1.3441 0.0044 1 30 0.2 5

51 -0.8964 1.2999 0.0045 1 35 0.2 5

TABLE 1: Specifications of optimized heat sinks

TABLE 2: The ten best designs for flow speed of 3 m/s

OPTIMIZATION IN ELECTRONICS COOLING FEATURE ARTICLE

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Light Emitting Diode (LED) manufacturers sort their products into ‘bins’ based on forward voltage, with the purpose of delivering the most consistent light possible. Despite the tight grouping of forward voltages in these bins, manufacturing tolerances continue to lead to significant variations in both current draw and temperatures inside the LEDs, resulting in a inhomogeneous light distribution, even within the same batch. These discrepancies also undermine the most noteworthy selling point of the LED : its long operational life. Zumtobel, a leading supplier of integral lighting solutions for professional lighting applications, has addressed this problem by investigating the highly interconnected process between the thermal and electrical characteristics of LEDs using coupled simulations with STAR-CCM+ and NGSPICE.

FEATURE ARTICLE LED PERFORMANCE

27SPECIAL REPORT

COUPLED THERMAL-ELECTRICAL SIMULATIONS SHED LIGHT ON LED PERFORMANCEPIER ANGELO FAVAROLO & LUKAS OSLZumtobel Group

SABINE GOODWIN & RUBEN BONSCD-adapco

THE LED EXPLAINEDFigure 1 shows the forward current vs. forward voltage (I-V) characteristics of a typical diode. When placing a forward voltage across an LED, as is the case with any diode, the current initially does not flow until the forward voltage is increased to a level sufficiently high to pass a certain threshold, after which it flows freely in the normal conducting direction. For LEDs, the I-V characteristics at which point this occurs affects the color and intensity of the light emitted; in other words, although the principal behavior is the same for all LEDs, the light produced highly depends on each individual current voltage characteristics.

THE LED THERMO-ELECTRICAL CHALLENGEAlthough the working principle of a diode is rather simple, in reality, designing luminaires that produce consistent light (both intensity and color) can be particularly challenging due to a number of unique operational characteristics of LEDs. Inherent manufacturing variations (both in materials and processes) often cause unexpected variations in the electrical response of an LED. As discussed above, its optical output (both total amount and spectrum) is highly dependent on the electrical energy (voltage and current) driving it and thus these manufacturing variations can lead to a deterioration of light quality. A typical disparity of the

I-V characteristics due to manufacturing variations is shown in figure 1. Binning LEDs after assembly to group them in batches that have similar responses narrows these variations, but it does not eliminate them.

The choice of driver circuit topology – whether the LEDs are electrically in series or parallel – also makes a significant difference in its sensitivity to variations (Figure 2). To make sure that failure of one LED does not cause

an entire circuit to break down, they are often placed in parallel on the circuit driver. This means that each light in the circuit operates at the same voltage (as opposed to when they are placed in series where they see the same current). Thus, they are typically run on the steep part of their characteristic curve, resulting in a higher sensitivity in response to variations in manufacturing within even a single bin.

Much like a computer chip, an LED is also very sensitive to temperature changes.

FIGURE 1: Typical forward and reverse voltage characteristics of a diode/LED

LED PERFORMANCE FEATURE ARTICLE

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The operating temperature not only affects its lifetime, it determines the optical light output (how the eye perceives it) and thermal characteristics (amount of dissipated heat) of the LED-powered luminaire. Problems such as color shift (change in color over time) and luminous flux depreciation (loss in light amount) resulting from temperature variations can quickly become daunting for a manufacturer. One of the main selling points of LEDs is that they can run for 8 hours a day for 15 to 30 years, but if the thermal design is not right, this full potential will never be reached.

Some of the variations described above can be mitigated with electrical control circuitry, however, the proper design of an LED-powered luminaire that produces both consistent light color and intensity calls for a coupled electrical and thermal approach that can address the interdependencies between the electrical circuit response, temperature, heat dissipation and cooling approach.

COUPLING THE THERMAL AND ELECTRICAL RESPONSESZumtobel has coupled STAR-CCM+® (a computational tool for simulating flow/thermal behavior) to NGSPICE (an open-source circuit simulation software) to enable the accurate prediction of the interplaying effects between electrical and thermal behaviors of LEDs. Communication between the codes is established using an interactive JAVA macro.

Figure 3 illustrates the approach taken to solve this closely coupled electro-thermal problem. When NGSPICE is executed from the macro, the circuit is solved including the forward voltage and forward current of the LEDs. From this, using a proprietary method developed by Zumtobel, the electrical power is determined, and using the temperature supplied by STAR-CCM+, the optical power (how much of the energy goes out as visible light) is computed. The heat rate (representing the portion of the power that is dissipated as heat) can subsequently be calculated by subtracting the radiant power from the electrical power. This heat rate is then fed to STAR-CCM+ which in turn computes all the system temperatures to be passed back to NGSPICE for the next step in the simulation. As discussed above, the temperatures have a significant impact on the electrical characteristics of the LED, thus this cycle is repeated until the simulation converges to an equilibrium state.

FIGURE 2: LEDs in parallel have a high sensitivity to variations within a single bin

FIGURE 3: Electro-thermal simulation using STAR-CCM+ and NGSPICE

FIGURE 4: Test luminaire consisting of two LEDs connected in parallel

This process was demonstrated and validated on a test luminaire consisting of two LEDs connected electrically in parallel, mounted on an aluminum channel and placed on a wood table (Figure 4). This model has been extensively validated in the laboratory as it is one of Zumtobel’s standard experimental test configurations. A four terminal sensing method was used to measure current and voltage of each LED. The temperature was obtained through thermocouples (type T) on specific points of the copper pad, Printed Circuit Boards (PCB) and heat-sinks and these locations were used as reference points for the thermal simulations. In addition, a parametric study was performed during testing to better understand the impact of manufacturing tolerances (e.g. thickness of the traces, thermal conductivity) on the thermal behavior of the system. One significant detail of the set-up that must also be noted is that there is a current meter connected in series with one of the LEDs which increases the resistance in that branch of the circuit. As

FIGURE 5: The STAR-CCM+ luminaire model includes the PCB, LED pad and semi-conductor die

a result, each of the lights in the circuit is expected to have a unique electrical response and operating temperature.

For the simulation, geometrical and material properties of the LEDs were provided by the manufacturer and

FEATURE ARTICLE LED PERFORMANCE

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FIGURE 6: Conformal mesh showing the details of the model in STAR-CCM+

FIGURE 7 : Interface of NGSPICE and STAR-CCM+ after 20 updates (200 STAR-CCM+ iterations)

FIGURE 8: Final solution showing solid temperatures and circulation around each of the LEDs

half symmetry was used to keep the simulation time to a minimum. To include the effects of the current meter from the experiment in the simulation, a current-limiting resistor was modeled in series with one of the LEDs on the circuit. As shown in figure 5, significant attention was paid to modeling the important details of the system, including the Metal-Core Printed Circuit Board (MCPCB) with copper traces, the LED pad (with electrical connections) all the way down to the semiconductor die. For the simulation in STAR-CCM+, accurate capturing of the physics, including natural convection cooling and limiting temperature of the semiconductor die, was key. Furthermore, unique meshing capabilities available in STAR-CCM+ were applied to ensure accuracy of the simulations. As shown in figure 6, an all-conformal polyhedral mesh was generated and extrusions were used to allow for efficiently meshing the surrounding air while at the same time capturing the small physical features in the core area of the LEDs themselves. The flow and thermal behavior of the system were obtained by performing steady-state simulations using the segregated flow and energy solvers.

Figure 7 shows a screenshot of the coupled interface which facilitates real-time tracking of the various system parameters, including the forward current through each LED and the die temperatures as the solution unfolds. At start-up, an initialization (first guess) of current, temperature and optical power was made and during the simulation, for every one step of NGSPICE, ten steps of STAR-CCM+ were performed. After approximately

200 iterations of STAR-CCM+ (which required only about 20 minutes of simulation time on a laptop), the forward current started to converge and the die temperatures settled. A fully converged solution was obtained after approximately 50 updates between NGSPICE and STAR-CCM+.

Figure 8 depicts the surface temperatures of the solids of the system. As discussed above, the simulation shows exactly what is expected: the increased resistance due to the presence of a current meter (modeled with a current-limiting resistor) in the experiment results in a significant difference in the final temperatures of each of the LEDs. A cut through one of the LEDs also shows the velocity field at convergence, displaying the expected natural convective air flow that cools the system.

CONCLUSIONLEDs have gained a tremendous amount of popularity in recent years due to their small size, efficiency and long life. In order to meet these expectations, the interdependencies between the electrical circuit response, temperature, heat dissipation and cooling must be taken into account during the design phase of LEDs. For companies like Zumtobel, delivering consistent light with the right intensity and color is crucial to the success of their product portfolio, and performing electro-thermal simulations early in the development process facilitates the prediction of the luminaire performance in lieu of physical prototyping and testing.

ABOUT ZUMTOBELZumtobel, a company of the Zumtobel Group, is an internationally leading supplier of integral lighting solutions for professional indoor and outdoor building lighting applications. For more than 50 years, Zumtobel has been developing innovative, custom lighting solutions that meet extremely exacting requirements in terms of ergonomics, economic efficiency and environmental compatibility as well as delivering aesthetic added value. Besides the very latest technology advances and research developments, the company’s many years of experience in project business with leading international architects, lighting designers and artists provides valuable impetus that stimulates the ongoing development of the company’s already comprehensive product portfolio.

LED PERFORMANCE FEATURE ARTICLE

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FEATURE ARTICLE WORLD'S LARGEST TELESCOPE

31SPECIAL REPORT

Currently under construction on the Pacific Island of Maui, the 41.5 m tall Daniel K. Inouye Solar Telescope (DKIST) will be the world’s largest solar telescope. Once operational, the DKIST will be able to provide the sharpest views ever taken of the solar surface, which will allow scientists to learn even more about the sun and solar-terrestrial interactions. The DKIST will allow astronomers to resolve the extremely small, violently active, magnetic fields that control the temperature of the corona and the solar wind that produce flares and x-ray emissions, and help to improve prediction of the way these “space weather” phenomena influence the earth.

The telescope is currently being built atop the Haleakala volcano on the Pacific island of Maui, which was chosen from a list of 72 possible global locations after two years of monitoring daytime seeing conditions. Haleakala has the darkest, clearest skies, and its tropical location and elevation mean that the telescope sits above the turbulent inversion layer so there is little turbulence to blur its view or moisture to block the infrared spectrum.

At the heart of the telescope is a huge 13ft (4m) primary mirror which, when combined with adaptive optics technology that reduces the amount of blurring from earth’s atmosphere, produces images 33 times sharper than those of common telescopes. The resolution of the DKIST is comparable with space telescopes, but at a much lower cost and with the

benefit of greater accessibility. Unlike a space telescope, it will be relatively easy to upgrade the technology of the DKIST throughout its lifetime.

A solar telescope-specific problem is the heat generated by the tightly-focused sunlight. Unlike most large ground-based telescopes, which are used at nighttime to capture a small number of photons from distant astronomical bodies, the DKIST will spend its working life pointed directly at the sun, absorbing large quantities of focused light and heat energy.

A heat stop is an integral part of the design of solar telescopes, and represents one of its larger engineering challenges. It performs the role of what is called a “field stop” in a conventional telescope, limiting the field of vision to the area with minimal distortion. Located at the prime focus, the heat stop prevents unwanted solar disc light from heating and scattering on subsequent optics. In a solar telescope such as the DKIST, in addition to blocking light, the heat stop must also dissipate huge amounts of thermal energy.

For the upcoming DKIST, the heat load is 2.5 MW/m2, reducing the heat load on subsequent optics from an enormous12 kW to a minuscule 300 W (a reduction factor of 40). Designed by Thermacore, the heat stop assembly is actively cooled by an internal system of porous metal heat exchangers that

BLASTED BY THE SUN: THERMACORE COOLS THE WORLD’S LARGEST SOLAR TELESCOPE

STEPHEN FERGUSONCD-adapco

WORLD'S LARGEST TELESCOPE FEATURE ARTICLE

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Solar filament as captured by NASA’s Solar Dynamics Observatory (SDO)Credit: NASA/SDO/AIA/GSFC

resulted in the telescope being aligned outside of its design range.

The surface temperatures (and generated flow around the heat stop) depend on a number of interacting physical phenomena. In simulating the heat stop assembly, the Thermacore engineers had to take into account multiphase flow within the porous metal heat exchangers, conjugate heat transfer through the heat stop assembly and the interior wick structure, and both natural convection and radiation heat transfer around the heat stop.

Although some of these configurations provided lower direct thermal loading, Thermacore engineers needed to demonstrate that asymmetries in thermal loading would not lead to local “hot spots” that might generate additional buoyancy-driven flow patterns (the potential source of a “self-induced seeing” problem). Using STAR-CCM+®, the Thermacore engineers were not only able to fulfill the design criteria demonstrating that the surface temperature of the reflector could be kept below 10°C above ambient, but they also demonstrated that the criteria was sensible by visualizing the flow patterns generated within the enclosure as a result of natural convection.

The Thermacore team also examined the influence of various atmospheric wind loading scenarios, predicting flow patterns and heat transfer across a range of possible flow orientations, demonstrating that surface temperatures and turbulence levels would not exceed design criteria.

Located at the prime focus, the heat stop limits the field of vision of the telescope and absorbs large amounts of solar energy, preventing it from reaching subsequent optics.

The heat stop must dissipate 1,700 W at peak operating load, without ever allowing thesurface temperature to rise by more than 10˚C over ambient.

A polyhedral mesh (including fine prism layers) of the fluid space around the heat stop

Limiting air disturbance, whether thermally generated or as a result of the wind, plays animportant role in reducing “selfinduced seeing”.

FEATURE ARTICLE WORLD'S LARGEST TELESCOPE

dissipate approximately 1,700 W at peak operating load (see side box for a discussion of porous metal heat exchangers).

The heat stop must not only be able to survive this heat load (without cooling, the heat stop reflector would last only about 30 seconds before catastrophic failure), but also must remain cool enough not to induce any additional turbulence inside the telescope’s dome.

One of the obstacles of ground-based astronomical observatories is a phenomenon known as “self-induced seeing”. It consists of the degradation of image quality, mostly resulting in an increased blurring of objects and a reduction of contrast in long exposure images. This occurs when thermal and wind disturbances create fluctuating layers of refractive indices within the optical beam path. With this in mind, a small hot object in close proximity to the secondary mirror could have potentially disastrous consequences for the accuracy of the telescope. A key requirement for the system is that the surface temperature of the heat stop must never be more that 10°C higher than the temperature of the ambient air so as to prevent buoyancy-induced flows from creating turbulent disturbances that would result in “self-induced seeing”.

As part of the design process, hermacore was required to demonstrate the efficacy and robustness of their heat stop cooling system across the full range of potential operating conditions, as well as in some “failure mode” scenarios in which the failure of some other component had

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Heat

CoolantInlet

CoolantOutlet

CoolantInlet

Porous MetalParticle

ABOUT THERMACOREFounded in 1970, Thermacore specializes in the custom design, development, and manufacturing of innovative, high performance thermal management and material solutions. Thermacore’s thermal management solutions can be found at both the system and component levels for a variety of OEM applications in the Military/Aerospace, Computer, Communications, Industrial, Government, and Medical/Test Equipment markets. With intellectual property (over 100 patents), trade secrets and professional staff (over 50 engineers), Thermacore applies this know-how to solve complex thermal problems for their customers and help to enable their customers’ products.

Thermacore has the industry’s broadest collection of thermal management technologies, products, and services, which include: k-Core encapsulated Annealed Pyrolytic Graphite (APG) based solid conduction heat spreader assemblies and thermal straps (all metal and k-Core based); passive two-phase devices such as heat pipe assemblies, vapor chamber assemblies, thermal ground planes, loop heat pipes, extreme temperature heat pipes, CCHP and VCHP Spacecraft heat pipes, and more; liquid cooled cold plates; aluminum vacuum brazed assemblies, cold plates, heat exchangers, chassis; pumped single and two-phase liquid systems; Intelligent Thermal Management Systems (iTMS); rugged liquid cooling systems (rLCS™); and enclosure heat exchangers. Other unique capabilities include the ability to develop custom refractory metal alloys, cryomilled aluminum and magnesium, and material characterization and testing services.

Thermacore brings unparalleled engineering design expertise and thermal management solution performance, quality, and reliability to help enable our customer’s products and services. Thermacore employs more than 175 employees at six facilities located in the United States (Lancaster, Pennsylvania; Langhorne, PA; Ronkonkoma, NY; Pittsburgh, PA) and the United Kingdom (Ashington, Northumberland). Thermacore facilities are certified to the AS9100, ISO 9001 and ISO 14001 standards.

For information about Thermacore, visitwww.thermacore.com.

The past decade has witnessed a significant growth in the number of applications for a new category of heat transfer device known as “porous metal heat exchangers”. These devices, when used in conjunction with a pumped single-phase coolant or a pumped gas, employ a porous layer of a thermally conductive medium beneath the heat transfer surface to effect efficient heat transfer. In this device, convective heat transfer to the selected coolant combines with the “fin effect” produced by the large surface area of the conductive porous structure to produce efficient heat transfer. A porous media heat exchanger can be used to dissipate very large heat fluxes, such as those encountered in this solar telescope, or can be used to provide very efficient heat transfer at much lower heat fluxes.

The increased surface area, however, is obtained at the expense of increased flow resistance or pressure drop. To overcome the constricted flow paths, multiple closely-spaced inlets and outlets are used. For example, a section through the wall of a porous metal heat exchanger is illustrated schematically above. In a scale drawing, the circles, which represent grains of metal powder, would be many times smaller than shown. The pore sizes are sufficiently small to prevent a flow “short-circuit” near the walls. The thickness of the porous structure depicted below is typically on the order of 0.020-0.050mm.

A significant effort has been made in recent years to improve the fundamental understanding of convective forced flow heat transfer in porous metal heat exchangers, which has resulted in improved understanding of governing principles, and has thus opened new applications for these devices such as cooling the DKIST solar telescope.

ALL ABOUT POROUS METAL HEAT EXCHANGERS

WORLD'S LARGEST TELESCOPE FEATURE ARTICLE

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FEATURE ARTICLE IR LAMPS DESIGN

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IR LAMPS DESIGN FEAFEATURE ARTICLE

LARISA VON RIEWELHeraeus Noblelight

INTRODUCTIONHeraeus Noblelight is a global business unit of the technology company Heraeus in Hanau, Germany, and counts itself among the market and technology leaders worldwide for special lamps with wavelengths from ultraviolet to infrared for industrial, scientific, and medical applications. With locations in Germany, the United Kingdom, China, and the USA, the segment manufactures lamps for analytical measurement technology and the printing industry, infrared emitters for industrial heating processes, arc and flash lamps, and products for water disinfection and air treatment, as well as sun simulation and photochemistry with a high level of vertical integration.

In order to meet rising expectations of customers and out-perform both predecessors and competitors, Heraeus Noblelight Industrial Processing (HNG-IP) is constantly developing not just standard but also individual infrared emitters and infrared heating systems. An extensive range of products including round or twin tube emitters in linear or in individual 3D geometries with specular or diffuse reflectors is manufactured and incorporated into production cells or modules. In terms of effectiveness, different wavelengths or Infrared (IR) spectral domains – short- or middle-wave range – are suited for different applications.

CAE REPLACES ‘TRIAL AND ERROR’Complementary to the traditional ‘trial

NUMERICAL ANALYSIS OF IR HEAT TRANSFER PROCESSES

and error’ methods, Computer-Aided Engineering (CAE) tools based on 3D design (Autodesk), ray tracing (Zemax) and Computational Fluid Dynamics (CFD) (STAR-CCM+®) are now extensively used to achieve a better understanding of IR heat processes like wafer cleaning, paint/lacquer drying or plastics welding and to predict solutions for several technical failures. In all applications, the main technological challenge is to obtain a well-controlled uniform temperature on the substrate surface. In this respect, the perfect knowledge of radiative heat emitted by the infrared lamps and their

behavior under special environmental conditions is mandatory.

The expensive "IR Thermal Process" laboratory verification tests have been augmented or even replaced with more efficient and advanced computational methods. The aim is to investigate the temperature distribution in substrates with complicated structures, to analyze the robustness and performance of components and assemblies from the thermal perspective as well as the interaction of those with the environment. Furthermore, CAE can accurately predict

FIGURE 1: Short-wave infrared lamp (source: Heraeus Noblelight)

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FEATURE ARTICLE IR LAMPS DESIGN

stress in the lamp ends, with the lamp end eventually cracking and letting air in. As discussed above, all penetration by air needs to be avoided. The lamp ends must therefore be cooled so as to maintain a temperature below 350 °C.

The geometry of the lamp is automatically imported from 3D mechanical CAD design software as step file and represents a very small part of the entire lamp. A polyhedral grid with prism layers for capturing the boundary layers accurately, and thin mesher for molybdenum spacers was created as shown in Figure 3. Energy conservation is applied to each component of the heater, and non-grey radiation heat transfer between the various components and the black surroundings that are taken into account. The ideal gas model is applied to the argon in the bulb and to the surrounding air. Thermal properties are defined as temperature-dependent functions and wavelength dependence of optical parameters is

FIGURE 2: Micro-model - Geometrical set-up (source: Heraeus Noblelight / Dr. Lotta Gaab)

FIGURE 3: Micro-model - Meshing (source: Heraeus Noblelight / Dr. Lotta Gaab)

FIGURE 4: Micro-model - Temperature distribution on the quartz tube: measurement (top) and CFD (bottom) (source: Heraeus Noblelight / Dr. Lotta Gaab & Thomas Piela)

and visualize the fluid flow pattern within systems with cooling devices like blowers and fans and the influence of temperature gradients or pressure drops or flow’s behavior. The main technological challenge is to obtain a well-controlled uniform temperature at the wafer surface, so accurately predicting the radiative heat emitted by the infrared lamps is essential.

In this regard, and using CD-adapco’s numerical simulation tools, we developed complex three-dimensional models (micro-models) of infrared (IR) lamps accurately representing a lamp portion and including the complete lamp geometry, as well as large size models (macro-models) based on phenomenological assumptions for industrial applications.

MICRO-MODEL – SIMULATING THE MICROPHYSICSThe micro-model focuses on the thermal boundary conditions and analyzes the entire physics in the lamp. This model is based on detailed HNG-IP emitter geometries and configurations. A regular or high intensity short wave infrared lamp consists of a tungsten filament in the middle of a quartz bulb. The resistive element is the spiral tungsten coil, operating at 1800-2500 °C and surrounded by a quartz glass envelope, employed by its optical and thermo mechanical properties. Although tungsten has a melting point well above 3200 °C, it would oxidize rapidly and be burned out in an atmosphere containing oxygen. Thus, the quartz envelope needs to be tight to exclude any contact between the tungsten filament and oxygen. Such lamps contain either a vacuum or, more frequently, an inert atmosphere of nitrogen or of a noble gas such as neon or argon. The lamp glass is made of quartz, which has high transmissivity in the infrared range. Thus, most of the radiation emitted is transmitted by the lamp glass. Still, some radiation is absorbed by the glass, which heats up to about 700 °C. Since quartz has high temperature stability, is insensitive to thermal shocks, and has low thermal conductivity and low thermal expansion, the elevation in temperature is not an issue. Since tungsten shows considerable thermal expansion, however, it is difficult to maintain perfect tightening when the lamp is in operation. In practice, the conductor is fitted into the tube by use of a small piece of molybdenum, as indicated in Figure 1. Above about 350 °C, the molybdenum combines with oxygen in the air to form molybdenum oxide, which has a greater volume than molybdenum. This results in

considered (the simulation is performed using surface-to-surface radiation and multiband modeling capabilities). Convective cooling of the reflector, the lamp glass and the protective glass, as well as conduction in the opaque reflector were also taken into account. Because the model includes a small part of the lamp only, making the actual physical domain very narrow, periodical boundary conditions were implemented.

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IR LAMPS DESIGN FEATURE ARTICLE

Theoretical values were compared with experimental data in order to validate the models. Quartz glass and filament temperatures were measured with a thermo graphic camera or with thermocouples. Since the temperature variations of the filament are very rapid in transient state, the time response uncertainty is large. Therefore, validating CFD results against experimental data is more reliably done in steady state. STAR-CCM+ results for the filament and quartz glass temperatures in steady state compared very well with experimental measurements for the same power density, as shown in Figure 4. For example, the temperature measured on the filament was Tfilament - measured = 1837 °C, and the simulated value Tfilament - CFD = 1821 °C. Similarly the temperature measured on the quartz glass, Tquartz glass - measured = 625 °C is very close to the predicted value Tquartz glass - CFD = 643 °C. The difference between experimental and computed data was found to be always less than 5%. This validated model allows for an accurate estimation of the heat flowing towards the glass of the tubes and of the temperatures of different emitter components.

The main drawback of the micro-model of IR emitters is the large number of cells for a small physical domain. Furthermore, the numerical infrastructure is usually limited. On the other hand, the industrial applications of our customers are becoming larger in terms of design and number of built-in emitters, and more complex and demanding. Companies are progressively seeking industrial solutions through the extensive use of CAE for the optimization of product development and processes in order to predict the performance of new designs before they are even implemented or manufactured. MACRO-MODEL – A PHENOMENOLOGICAL MODEL FOR COMPLEX APPLICATIONSTo meet the needs of our customers, we derived and validated a simpler model of emitter (macro-model) which still contains all the physics information. The macro-model focuses on the interaction process between IR lamps and systems or environment; it is consistent with experimental results and requires fewer cells for CFD calculations. The geometry of quartz glass is kept the same but the filament is replaced with a hollow cylinder. As the lamp filament consists of a large number of loops that are very close to one another, its representation as a hollow cylinder in three dimensions is an educated approximation of the original design. However, this representation

A manufacturer of steel cylinders sought a more efficient method to powder-coat the product. The previous method, a gas-heated air oven, was 30 meters long, taking up valuable production space. The company replaced it with an infrared oven from Heraeus Noblelight. The size was reduced to less than one-fifth of the size of the old oven. The infrared lamps are contoured to match the shape of the cylinder, allowing even heat application and curing. Such a system can be designed and optimized by means of CAE. (Source: Heraeus Noblelight)

Two purpose-designed infrared heating systems from Heraeus Noblelight are helping to ensure a perfect fit and to increase longevity of the headliner interior leather trim on the Bentley Continental’s four-door and two-door car models. Positions, lamp types and energy density can be optimized using CAE. (Source: Heraeus Noblelight)

APPLICATION 1

APPLICATION 2

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FEATURE ARTICLE IR LAMPS DESIGN

involves in principle a greater tungsten volume than the real helix-shaped filament and for a given amount of supplied electrical power, the filament temperature is overestimated. To solve this issue, two conditions are imposed on the filament geometry:1. The outer surface of the hollow cylinder

has to be equal to the surface of the helix filament (Stefan-Boltzmann law for grey radiation).

2. The hollow cylinder and the actual wire must have the same mass.

From these two constraints, the inner and outer diameters of the filament can be estimated and the geometry defined. The actual computational domain is large; periodical boundary conditions are not needed, but symmetries are defined where necessary. As with the previous model, energy conservation and non-grey radiation heat transfer between the various components are taken into account. For verification and validation purposes,

the simulated filament and quartz glass temperatures were compared to experimental values (see Figures 7 to 10). A very good agreement between CFD and experimental results was found. The quartz temperature on the lower part of the emitter was measured at 620 °C (+/- 3%) for an energy density of 96 W/cm, and predicted at 607 °C (+/- 3%) by the numerical simulation. Another very important parameter for the stability of thermal processes is the reflector temperature. The IR emitters with Quartz Reflective Coating (QRC) reflectors are often integrated in vacuum chambers to assist Rapid Thermal Processing (RTP). The environment temperature in those applications is usually reaching 1000 °C, therefore a stable IR system, including an emitter and a reflector resistant to elevated temperatures, is necessary. Measurements of QRC temperatures in vacuum systems are very tedious, making modeling the only practical

solution available. With the self-developed macro-model, such complex configurations can be analyzed; the reliability of the model was confirmed by the comparison between the computed reflector temperatures at the top of the emitter, 590 °C (+/- 5%), and the experimental measurement, 600 °C (+/- 5%), performed either with a pyrometer or a pilot tube. The last significant parameter that needed to be checked against experimental results is the filament temperature, which sets the spectral window. The filament temperature depends on the energy density and the filament material. For 96 W/cm, it was measured at 1837 °C (+/- 3%), which was once again in very good agreement with its numerical counterpart, at 1880 °C (+/- 5%), still within the range of acceptable accuracy.

SUMMARYUsing CD-adapco’s software STAR-CCM+ and mechanical design tools, we

FIGURE 5: Macro-model - Geometrical set-up (source: Heraeus Noblelight / Dr. Larisa von Riewel)

FIGURE 6: Macro-model - Meshing (source: Heraeus Noblelight / Dr. Larisa von Riewel)

FIGURE 7: Quartz glass temperatures computed with the macro-model for an energy density of 96 W/cm (source: Heraeus Noblelight / Dr. Larisa von Riewel)

Applied from the concept definition to the product release, already experimentally validated CAE methods have become a powerful tool to decrease costs, shorten the design cycle, increase the quality of the end product, allow for flexibility in the development process, and finally, strategically drive innovation.

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IR LAMPS DESIGN FEATURE ARTICLE

developed comprehensive 3D models (micro-models) of IR lamps, as well as large-size models (macro-models) based on phenomenological assumptions for more complex applications. The macro-model is the default model used for CFD analyses of HNG-IP infrared modules for industrial processes. For example, the numerical analysis of an IR heating system with nine emitters (100 cm length) and external cooling can predict the temperature homogeneity on a substrate. Edge effects are always an issue for nonconductive continuous large surfaces (plastics) or for substrates containing metal-plastics components. Additionally, the convective cooling of the heated surface may have a considerable contribution to the total heat management, making the qualitative evaluation of hot and cold spots in the pre-development phase a necessity to optimize the industrial process.

CONCLUSIONThis article introduced the various types of IR heaters developed by Heraeus Noblelight, and described their design characteristics and operating principles. Special attention was given to the self-developed models of electric IR heater. The models include non-grey radiative heat transfer between the different parts of the heater, as well as conduction in the reflector material and convective cooling of the surfaces. Using electrical power as the only input, the models’ predictions of temperatures were found to agree well with experimental data at steady-state conditions. The validated models are used either for investigations and improvements of emitters (micro-model) or for large scale simulations of industrial processes (macro-models).

In the last few years, it became increasingly important to not only be able to supply heat, but also to supply it in a controlled

and efficient manner. Production speed and product quality demanded precise amounts of heat. Energy costs are often an important part of production costs. Better understanding the heat transfer process has long since become an important area of engineering research. More recently, energy conservation has also become an environmental issue in itself, independent from the economic incentive involved.

From this perspective, numerical simulation (CAE) has become for HNG-IP a valuable feature, a key enabling factor in the virtual analysis and optimization of highly competitive and advanced systems. Applied from the concept definition to the product release, already experimentally validated CAE methods have become a powerful tool to decrease costs, shorten the design cycle, increase the quality of the end product, allow for flexibility in the development process, and finally, strategically drive innovation.

FIGURE 11: Temperature distribution and edge effects on a substrate (theoretical prediction and measurements) (source: Heraeus Noblelight / Dr. Larisa von Riewel)

FIGURE 8: Quartz glass (blue line) and QRC (red line) temperatures vs energy density: The measurements were performed on the lower part of the emitter and on the reflector respectively. (Source: Heraeus Noblelight / Dr. Larisa von Riewel)

FIGURE 9: QRC reflector temperature for an energy density of 96 W/cm (source: Heraeus Noblelight / Dr. Larisa von Riewel)

FIGURE 10: Computed temperature on the filaments for an energy density of 96 W/cm (source: Heraeus Noblelight / Dr. Larisa von Riewel)

Measurements of Quartz Glass and QRC Reflector Temperature

Tem

pera

ture

[˚C

]

Energy density in emitter [W/cm]

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FEATURE ARTICLE HIGH POWER DENSITY ELECTRONICS PACKAGES

As telecommunication and Radio-Frequency (RF) power electronics applications continue to push the envelope of waste heat dissipation, we increasingly see a need for active thermal control employing forced air electronics cooling fans in unison with pumped fluid loops to successfully meet temperature and performance requirements. A system-level approach using simulation tools enables examination of “cause-and-effect” scenarios for such novel electronics cooling solutions and helps to address the ever-changing requirements of affording more power in a smaller packaging level.

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HIGH POWER DENSITY ELECTRONICS PACKAGES FEATURE ARTICLE

KEVIN R. ANDERSONCalifornia State Polytechnic University

FIGURE 1: Thermacore k-Core technology [2]

MORE POWER TO YOU !AFFORDING MORE POWER IN SMALLER PACKAGES USING NOVEL ELECTRONICS COOLING SOLUTIONS

INTRODUCTIONIn this article, STAR-CCM+® is used for heat transfer and fluid flow simulations of a novel heat exchanger/cold plate fabricated from k-Core high thermal conductivity material in order to realize a thermal control system hardware design for intermediate compact electronics packaging scenarios with large power densities. K-Core thermal spreader cold plates provide high-efficiency heat transfer in the absence of moving parts and are invaluable in applications where space, volume, seamless hardware integration and weight constraints dictate thermal design solutions. In this work [1], trade studies involving different heat exchanger/cold plate materials as well as various fault scenarios within a typical electronic system are investigated to illustrate the upper bounds placed on the convective heat transfer coefficient.

HEAT SPREADER THERMAL TECHNOLOGYPhysically, k-Core-based thermal spreader cold plates absorb heat from sources such as high power dissipating electronic components, rejecting it to ambient air or liquid coolants. These types of cold plates are well suited for operating in harsh environments. For cooling high-power density applications (e.g. semiconductors, lasers, power generation, medical equipment, transportation, military electronics, etc.) and other demanding applications where air cooling is insufficient, liquid cooling is essential. In this work, the heat transfer of k-Core thermal conduction plates available from Thermacore, Inc. [2] is studied in conjunction with a pumped fluid loop.

The physical concept of Thermacore’s k-Core heat transfer technology is illustrated in Figure 1. Using encapsulated Annealed Pyrolytic Graphite (APG) allows one to manufacture a device such as a heat spreader with heat sink integrated on one face while a labyrinth of tubing for a pumped fluid loop can be fabricated on the opposing face. Since APG is anisotropic in thermal conductivity, with the x- and y-components of thermal conductivity being very large with respect to the z-component, the APG heat sink/heat spreader greatly enhances overall heat transfer via preferential heat spreading. Figure 2 illustrates the temperature dependency of the thermal conductivity of APG.

SYSTEM HARDWARE DESCRIPTIONThe configuration of Figure 3 was selected to represent a medium-sized high power density electronics application. density electronics application. The overall system consists of a very large housing with eight identical sub-modules arranged in order to minimize overall system volume. The sub-modules were each populated with various components, totalling twelve each per sub-module. The thermal control system has three major components: the heat sink, the cold plate and the forced convection fans/housing enclosure. The enclosure is roughly the size of a footlocker (W x L x H = 0.5 m x 1.0 m x 0.5 m).

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FEATURE ARTICLE HIGH POWER DENSITY ELECTRONICS PACKAGES

density electronics application. The overall system consists of a very large housing with eight identical sub-modules arranged in order to minimize overall system volume. The sub-modules were each populated with various components, totalling twelve each per sub-module. The thermal control system has three major components: the heat sink, the cold plate and the forced convection fans/housing enclosure. The enclosure is roughly the size of a footlocker (W x L x H = 0.5 m x 1.0 m x 0.5 m).

CFD MODEL AND GRID STUDYThe mesh for the simulations with STAR-CCM+ consisted of a polyhedral unstructured mesh with five prismatic layers to resolve the boundary layer at all solid-fluid interfaces (Figure 4).Typical cell count for the system-level CFD model was on the order of 2.5 million cells; the particular mesh size was selected from runs to determine the optimum mesh size giving accurate answers as quickly as possible. CPU run-time on a 64-bit quad-core workstation was on the order of six hours.

The simulations included fully 3D conjugate heat transfer, forced external air convection, forced internal liquid cooling loop, solid conduction, and surface-to-surface gray body radiation. To mimic typical Commercial Off-The-Shelf (COTS) cooling fans, the fan curve (for a fan rated at 30 CFM with 50 Pa static pressure) of Figure 5 was used in the simulations.

BASELINE RESULTSThe baseline simulation results showing temperature distribution and streamlines are illustrated in Figure 6. The parameters for each of the hardware subcomponents

in the simulation were set up so the power density at the liquid cold plate is 1.6 kW/m2, which is in the category of a very high power density application. Once the baseline results were obtained, trade studies were performed to determine system sensitivities.

EFFECT OF CONDUCTIVITY ON HEAT TRANSFERThe primary objective of this work is to demonstrate the heat spreading characteristics of APG k-Core material in comparison to traditional heat sink materials. To accomplish this, the thermal conductivity of the heat sink/cold plate was varied from a very low (bare aluminum) to a very high value (APG k-Core). The results of this parametric study are shown in Figure 7 showing the expected linear variation of the system

temperature as a function of the heat sink/cold plate thermal conductivity.

EFFECT OF INLET FAN AIRSPEEDThe thermal performance of a system will significantly vary with the flow rate provided by the fan and a key way to measure the variance is with the convective heat transfer coefficient. Figure 8 shows values of convective heat transfer coefficient for a typical CFD simulation. These were in agreement with the heat transfer literature [3] for systems of this category.

To quantify the effect of inlet fan air speed on the heat transfer coefficient, a series of simulations was performed with varying flow rates of air, resulting in a different average velocity emanating from the fan. Figure 9 shows the results of the

FIGURE 2: Thermal conductivity for APG material [2] FIGURE 3: Electronic system components

FIGURE 4: CFD mesh used for simulations

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HIGH POWER DENSITY ELECTRONICS PACKAGES FEATURE ARTICLE

simulations compared with Ellison [4]The correlation of Ellison is conservative, as expected, and corresponds to standard textbook-based correlations with typical uncertainties of at least 25%. The data from our CFD study are also found to be in qualitative agreement with the simulations

and correlations offered by the research of [5] and [6]. Next, the heat transfer coefficient of the system is plotted against temperature delta of the system as shown in Figure 10. The CFD data points are assigned a power-law curve fit and compared to the correlation offered by [7].

FIGURE 5: Fan curve specifications for thermal control hardware simulations

FIGURE 6: Streamlines colored by air stream speed and electronic components colored by temperature

FIGURE 7: Thermal conductivity effect on heat spreading in CFD simulations

FIGURE 8: Convective heat transfer coefficient contours

FIGURE 9: Heat transfer coefficient versus fan air speed

FIGURE 10: Heat transfer coefficient comparison

The handbook correlation is found to once again over-predict the heat transfer coefficient. As mentioned above, the heat transfer correlations are expected to carry an uncertainty of at least 25%. The agreement of the CFD results in Figures 9 and 10 are within the realm of this uncertainty range.

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FEATURE ARTICLE HIGH POWER DENSITY ELECTRONICS PACKAGES

FIGURE 1: Thermacore k-Core technology [2]

STAR-CCM+ has enabled our team with a turn-key solution to complex CFD problems. By using STAR-CCM+, our research team has been able to address critical path design related issues. The user friendly interface and technical support offered by CD-adapco help put STAR-CCM+ at the forefront of today's CFD and multi-physics CAE simulation tools in both industry and academia.

EFFECT OF COOLANT FLOW RATENext, a series of CFD simulations was carried out varying the inlet flow rate of the water in the cold plate. The heat transfer coefficient of the entire system is plotted as a function of the flow rate in Figure 11.

EFFECT OF CONTACT RESISTANCEA trade study was performed on the thermal interface material providing thermal conductance between the various components in the system. Simulations were done for a range of contact heat transfer coefficients for typical thermal interface materials in the commercial electronics cooling industry (ranging from a “very good” thermal interface to a “very poor” thermal interface). Figure 13 plots maximum temperature in the system versus thermal contact resistivity. As theory predicts, the trend is linear. Figure 14 shows isothermal contours for a sub-module which has a very poor thermal contact interface. The image

shows a wide range of temperatures, with the lowest temperature where the PCB card contacts the heat sink/cold plate assembly, to a highest value where the warmest component on the PCB is near the air flow region. Here, the impact of a poor thermal contact interface is profound, leading to a large temperature gradient across this particular sub-model.

THERMAL RUNAWAY AND FAN OUTAGEThermal runaway due to faulty components in the system is readily predicted using this simulation approach. Figure 15 shows the result of a bad component (e.g. due to a manufacturing flaw or an internal short) on a particular PCB. The component experiencing thermal runaway is immediately identified as the outlier with very high temperature in Figure 15. One common cause of failure in thermally-controlled electronic systems is fan failure. This forces the thermal designer to use a redundant system, i.e. back-up fans. To understand the

effects of fan failure, various CFD simulations were performed at different airflow rates. Figure 16 is a plot of the system level maximum component/chip temperature versus fan air speed. The trend depicted is as expected, i.e. as the fan speed approaches zero, the various components and PC cards within the sub-modules will witness a large temperature. The results are in agreement with the trends in the study of [5] and [6].

CONCLUSIONSTAR-CCM+ simulations of heat and fluid flow behavior in a moderately sized package of electronics undergoing very large power dissipations enabled the study of novel thermal control strategies at the system level. Results were in agreement with previous studies, and the approach used demonstrated quick and cost-effective evaluation of “cause-and-effect” scenarios for new electronics cooling solutions. This helps address the ever-changing requirements of delivering more power in smaller packages.

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HIGH POWER DENSITY ELECTRONICS PACKAGES FEATURE ARTICLE

The approach used with STAR-CCM+ demonstrated a quick and cost-effective evaluation of cause-and-effect scenarios for new electronics cooling solutions. This helps address the ever-changing requirements for delivering more power in smaller packages.

REFERENCES[1] K.R. Anderson, M. Devost, W. Pakdee, N. Krishnamoorthy: “STAR-CCM+ CFD Simulations of Enhanced Heat Transfer in High-Power Density Electronics Using Forced Air Heat Exchanger and Pumped Fluid Loop Cold Plate Fabricated from High Thermal Conductivity Materials,“ Journal of Electronics Cooling and Thermal Control, 2013, 3, 144-154[2] “Thermacore k-Core Data Sheet,” 2013: http://www.thermacore.com/products/kcore.aspx [3] F. P. Incropera & D. P. Dewitt: “Heat Transfer,” Mc- Graw-Hill, New York, 1991[4] G. Ellison: “Thermal Computations for Electronics - Conductive, Radiative, and Convective Air Cooling,” CRC Press, Boca Raton, 2011[5] I. Tari & Y. Fidan-Seza: “CFD Analyses of a Notebook Computer Thermal Management System and a Proposed Passive Cooling Alternative,” IEEE Transactions on Components and Packaging Technologies, Vol. 33, No. 2, 2010, pp. 443-452[6] M. A. Ismail, M. Z. Abdullah & M. A. Mujeebu: “A CFD Based Analysis on the Effect of Free Stream Cooling on the Performance of Micro Processor Heat Sinks,” International Communications in Heat and Mass Transfer, Vol. 35, No. 6, 2008, pp. 771-778 [7] Y. A. Cengel: “Heat Transfer - A Practical Approach,” McGraw-Hill, New York, 2010

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FLUID DYNAMICS − SOLID MECHANICS − HEAT TRANSFER − PARTICLE DYNAMICS − ELECTROCHEMISTRY REACTING FLOW − ACOUSTICS − RHEOLOGY − MULTIDISCIPLINARY CO-SIMULATION − DESIGN EXPLORATION