Heat Transfer Presentation

8/3/2019 Heat Transfer Presentation

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HEAT TRANSFER FROM EXTENDEDSURFACES - FINS

By Lt Cdr Saifullah Khan


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WHY FINS?

When an available surface (Prime Surface) is

found inadequate to transfer the required

quantity of heat, we need to

q=hAS (tst)

A. Increase the convective heat transfer coefficient

(h)

B. Increase the surface area (Fins)


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NEED FORFINS


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USES OF FINS

RadiatorsofAutomobiles

Electric MotorBodies

Transformer & Electronic Equipment CoolingCoils & CondenserCoilsinRef& A/Cs

AirCooled EnginesCylinder Heads


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The thin plate fins of a car

radiator

greatly increase the rate ofheat transfer to the air


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NeedforFins


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COMMON TYPES OF FIN

CONFIGURATION


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HEAT FLOW THROUGH RECTANGULAR

FINS


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Assumptions

1. Theheat

conduction

in

the

fin

is

steady

and

one-dimensional.

2.Thefinmaterialishomogeneousandisotropic.

3.Thereisnoenergygenerationinthefin.

4.Theconvectiveenvironmentischaracterizedbyauniformandconstantheat

transfercoefficientandtemperature.

5.Thefinhasaconstantthermalconductivity.

6.Thecontactbetweenthebaseofthefinandtheprimarysurfaceisperfect.

7.Thefinhasaconstantbasetemperature.


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Examples


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Fin Efficiency


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Table


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HEAT FLOW THROUGH RECTANGULAR

FINS


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FIN OPTIMIZATION

Fin Rating

Predict Heat Transfer from a

given Fin Configuration .

Material

Configuration(Geometry)

Cost

Findensity

Finlength

High K

Smallh

Optimalthermalgains


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FIN OPTIMIZATION

Materials of Construction

Aluminum

Carbon Steel Alloys

Copper

Cupro Nickel

In conel

Nickel Alloys

Stainless Steel

Design Configuration

CustomFlat Crested

Lanced and Offset

Louvered

Multi-Louvered

Round Crested

Wavy or Ruffled

Secondary Operations Available

Anodizing

Cleaning

EDM (Electric Discharge Machining)

Typical Unit Size Range

Fin Densities from 4 36 fins/inch

Heights from 0.030 in. (0.76mm) to 2.00 in.

(50.8mm)

Thicknesses from 0.001 in. (0.025mm) to 0.040

in. (1.016mm)

Widths up to 36.0 in. (914.4mm)


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FIN OPTIMIZATION


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FUTURE DESIGN

FUTURE DESIGN CONSTRATION

In todays electronics equipment,total system dissipated powerlevels are increasing with every

new design. Increases in powerlevels combined with the marketexpectation of reduced packagesizes lead to heat problems that,if uncontrolled, can significantlyshorten the life of the electronics.Although this increased power

decreased size scenario hasbeen prevalent for many decades,the industrys ability to makesmaller electronic componentsmandates reduced size coolingcomponents.

CROSS-CUT EXTRUSIONS


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FUTURE DESIGN

Cross-cut extrusions improve overall thermal performance in forcedconvection. Market demands have shifted attention to decreasingthe volume and increasing thermal efficiency. For some electronics(typically uninterruptible power supplies and telecommunicationssystems) the market also demands reduced floor space and places

severe limitations on acoustic noise. These limitations force designstoward more expensive heat removal techniques.

Total power level is not the only problem. Heat density, or Wattsper square centimeter, at the semiconductor is a growing problem,too. This is the result of denser electronics and increased clock

frequencies. High heat flux densities combined with high overallpower levels are the main problems for thermal management.These thermal challenges can cause equipment failure even if theheat sink surface area and airflow are sized correctly.


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FUTURE DESIGN

Trends WhichWay Is Electronics Cooling Going?

Professor Avram Bar-Cohen from the University ofMinnesota, known for its electronics cooling program,addresses the question What does the future of thermal

management hold for electronics? in the following way: Idont know. One things for sure; pretty soon plain old air-cooling isnt going to cut it !. This message sums up boththe problem and the answer. The problem is that currentair-cooling technologies are not adequate to deal with the

demands of the electronics. The potential answer is also inthis statement. Liquid-cooling offers many benefitsincluding significantly higher heat removal capacity inreduced volume.


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FUTURE DESIGN

Air-Cooling in the Past Air-cooling is the traditional method of cooling

electronics. In the past, thermal design was only

considered when lab tests failed or a componentturned the PC board brown. As a heat transfer agent,air is a poor media with the following limitations:

Low density and low specific heat, resulting in low heatcarrying capacity

Low thermal conductivity However, due to its availability, low cost, ease of

maintenance and no problem if theres a spillcharacteristics, air-cooling is still the preferred choice.


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FUTURE DESIGN

Potential Enhancements in Air-Cooled Heat Sinks Improved heat transfer is achieved in either of two ways:

Increase the heat transfer coefficient, h.

Add more exposed surface.

The heat transfer coefficient is a measurement of how effectively heat is

removed from a surface. It is expressed in terms of Watts dissipated persquare meter per degree C of temperature of rise above the coolingmedium. Newtons law of cooling states that heat dissipated is equal tothe heat transfer coefficient (h) times area times the temperature risebetween the heat sink and the cooling air or: q = h A (Theat sink Tair)(1)

A designer can control only A, the number of square centimeters ofsurface area of the fins, and h, the effectiveness of the heat removalfrom each square centimeter of surface. Both of these parameters arebounded by the limits of cost and marketability.

The heat transfer coefficient h can be increased by a number ofmethods:


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FUTURE DESIGN

Effect of Heat Transfer Co-efficient Increasing airflow velocity is most common. Increased

airflow past the heat sink fins generally increases h. Italso increases the mass flow rate, reducing the overalltemperature of the exit air. Increasing air velocities past 8to 10 m/s adds significantly to the backpressure and theacoustic noise.

Crosscutting of flat fins into multiple short sections. Thisimproves the heat transfer coefficient at the fin surfacewhile slightly reducing the surface area exposed to the airstream. The drawback of this method is the resultantadditional pressure drop. Typically, crosscutting is bestemployed where airflow is from an unpredictabledirection

Augmentation of fins is similar to cross cutting but adds aseparate twist in the leading and trailing edge of a fin.

This curvature of the metal into the air flow reduces theh by scrubbing the dead air molecules away from thesurface. Augmentation works best in air velocities of 3m/s or higher

Impingement (jet) cooling of heat sinks is achieved usinga high-speed airflow directed at the fin tip toward thebase. This cooling layout can increase heat removal butmust be designed with an eye toward the system as awhole and tested in situ to be effective.


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FUTURE DESIGN

Augmented flat fins add air flow turbulenceat increased cooling air velocities. Increasedheat removal from any of these heat transferenhancement schemes comes at the price ofincreased pressure drop. The heat sink, fanand system as a whole must be examined in

conjugate analysis to determine if any ofthese techniques will produce lower overall

thermal resistance than just increasedsurface area through increased numbers ofthin, flat fins. A real limitation in this area isthe ability to control contamination in the airstream. Closely spaced fins can causeclogging of the fins and a reduction inthermal performance.


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FUTURE DESIGN

Higher Performance Fans The technology of fans is also under scrutiny in the electronics cooling

industry. If a fan with higher performance (significantly increasedairflow at equivalent pressure drop) becomes available, many designswill be built around that fan. The difficulty in achieving this higherperformance comes back to fan blade technology and motor design,as well as acoustic noise limitations. Future developments areexpected in this market.

Increase Exposed Cooling Surface Area Inexpensively In convective heat transfer, the increase in heat removal is based on

available cooling surface area. In many cases, the use of an increasednumber of tall, thin, flat fins will give better performance and less

pressure drop than any modification of the fin shape. Based on airflowresistance, the flat fins maintain the lowest pressure drop and hencethe greatest mass flow of air. A great deal of research is being put intolow cost manufacturing methods of producing extended surfaces thatincorporate more fins in smaller heat sink package sizes.


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FUTURE DESIGN Aluminum Extrusions

Aluminum has many characteristics that make it an excellent heat sinkmaterial. It is easily shaped in fine detail with inexpensive tooling. But theheight and number of fins, or extrusion ratio, limit surface area. The extrusionor aspect ratio of the fins is the limit that the extrusion tool can producewithout failure. The aspect ratio of fin height to spacing has increased over thepast decade from 6:1 up to as much as 15:1 depending on size and extrusionpress operator. The objective of increasing those ratios is to create more fins in

a smaller volume.

Serrations on extruded fins have for years been thought to increase surfacearea and hence heat transfer. In most cases they have no added thermaleffect. Serrations can increase the heat removal rate but, as typically seen, arenot tall enough to provide any increase in cooling over flat fins.

Impact Extrusions

Three-dimensional extrusions formed by impact extrusion offer an alternativeto the standard two-dimensional parts. Using the same high-conductivity alloy,impact extrusions are commonly seen as pin fin or elliptical fin parts. Extrusionor draw ratios exceed the capability of linear extrusions, offering an increasein the amount of surface area per unit volume.


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FUTURE DESIGN

Bonded Fin Heat Sinks

Bonded fin heat sinks use separate base and finsassembled to form a heat sink. For over a decadethese parts have offered an alternative to thelimitations of the extrusion process. Increases incooling surface area of 200 to 300% allow significantdecreases in heat sink volume. Many different

assembly styles are available on the market. These are: Epoxy bonding of fins into a heat-spreading base Brazed assemblies

Cold formed or swaged

Welded ultrasonic or resistance

Stacked fins (fin and base extruded individually andformed together)

All of these styles offer different advantages. Emergingtechnology, involving diffusion bonding and othertechniques new to the market, will increase availablealternatives. The lowest cost per square inch ofexposed cooling surface will be the ultimate yardstick.


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FUTURE DESIGNFolded Fin

Folded fins were originally developed for the military and aerospace market to provide largeamounts of surface area with very little weight. This method of making cooling surface area usingcorrugated metal sheet has been adopted by the commercial cooling market and is now used inevery application from personal computers to radio transmitters. Bonded, brazed or soldered toheat spreading surfaces, these folded lengths of metal come in many styles and sizes.Manufacturing capability offers fin heights up to 4.0 inches with densities of 8 to 10 fins per inch ofwidth. Flat fins are the most common style.

Castings

Castings are commonly used as low cost heat sinks integrated into enclosures. Due to cost, castingsare limited to high volume production programs that use tens of thousands of parts. Castingstypically provide near net-shaped parts that require little or no machining prior to assembly.Traditionally, the thermal conductivity of aluminum casting alloys has been significantly lower thanextruded alloys. This, coupled with the potential for air pockets and inclusions in some castingstyles, can yield poor thermal designs.

Newly developed aluminum casting materials offer conductivities as high as 170 W/m-K. Thesematerials offer conductivity close to extruded aluminum but still are limited by fin height to spacingratios noted here. Also, die life using this new cast material is reduced to 1/3 of normal.

Most cast fin sections are limited in height and density to an aspect ratio of ~4:1. This limits heatdissipation levels or requires additional surfaces be added to the casting; i.e., bonded fin or foldedfins.


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FUTURE DESIGNFuture Developments

Leading-edge alternatives in air-cooled heat removal are increasing. Sintered heat sinks, metalinjection molding, 5X performance fans, aluminum extrusions @ 25:1 ratio and self-fanningpiezoelectric fin heat sinks are visions that have not yet come to market. Many companies areworking to develop these next generation products.

Improving Thermal Conductivity

Thermal conduction comes into play in two areas, heat spreading in the base, and fin efficiency.Conduction is the transfer of heat within a material due to intimate contact between molecules.

Pure aluminum has a conductivity of 230 W/m-K. Aluminums ability to conduct heat in thinsections is not sufficient to cool many of todays high heat flux components.

To overcome this conduction limitation many alternative materials and techniques are beingexplored. These include:

Pure copper as a base material Heat pipes and thermosiphons

Increased conductivity of heat spreaders Copper or carbon based materials

Vapor chambers

Each of these materials or systems will increase heat conduction compared to an aluminumextrusion. However, they will all be more costly and maybe heavier than aluminum.

FUTURE DESIGN


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FUTURE DESIGNCopper

At 390 W/m-K copper offers a 70% increase in conduction over

aluminum. Thats the good news. The down side of copper is that itweighs three times more than aluminum, costs the same on a perpound basis and is more difficult to machine. Due to limited hightemperature formability, a copper extrusion will not yield the samedetail as aluminum. Also, machining copper takes more time andwears cutters at a much higher rate. However, when an applicationis limited in conduction, copper is a commonly used alternative.

Heat Pipes

A heat pipe is a closed-loop, phase-change heat transfer systemthat, by itself, does not dissipate heat. It does move heat from onepoint to another with a very low temperature rise. An average heatpipe has an equivalent thermal conductivity of up to 1000 W/m-K.Although it is not strictly correct to quote thermal conductivity of aheat pipe, it is used here as a comparison to solid materials.

The heat transport mechanism of a heat pipe is latent heattransport via vapor flow and not thermal conduction. When avalue of thermal conductivity is given for a heat pipe, it is usuallythe equivalent thermal conductivity that a solid conductor of thesame cross-sectional area and length would require to conduct thesame amount of heat from end-to-end with the same temperaturerise. The addition of heat pipes to an aluminum heat spreader canimprove the conductivity and reduce spreading resistance with aminimum of weight increase .


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FUTURE DESIGN

Example of an electrically isolated heat pipe assembly. Heat

pipes come in various sizes and styles, typically round butalso in square or rectangular cross sections. Most heatpipes used in electronic cooling are copper with water oralcohol as the working fluid. Heat pipes with sintered wicksare the current state of the art and provide the highest heat

flux

density control. They also are best at returning liquidagainst gravity by capillary flow.

Thermosiphons use buoyancy effects caused by atemperature difference in the working fluid to move heatinside a closed system. These systems can be either singlephase (liquid only) or two phase (evaporative andcondensing).

Loop heat pipes are similar to conventional heat pipes inthat they use a phase change liquid to move heat. They aredifferent in that the system forms a circulating loop withhot vapor and cooled liquid in separate lines to and from

the condenser.


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FUTURE DESIGNVapor Chambers

Vapor chambers, or flat plate heat pipes, are currently receiving a lot of attention.These parts also use a change of phase liquid to move heat. The vapor chamberprovides a uniform temperature across the heat dissipation surface with low mass.This technology allows direct attachment of heat dissipation fins and can be usedfor air-cooling of applications including microprocessors, RF frequencytransmitters, thermoelectric devices and power modules.

Advances in vapor chamber technology will revolve around improved wickstructures and working fluids. However, water still appears to have the bestcharacteristics for moving heat.

Improved Conductivity Materials

High thermal conductivity polymers, carbon-based composites, sintered metallicpowders, phase-change compounds, synthetic diamonds and strand-orientedgraphite materials are all part of leading edge thermal conductivity materials.What are the most desirable properties? These are controlled levels ofconductivity, high machinability, low weight, low coefficient of thermal expansion,low toxicity and above all a per pound cost that will be lower than aluminum. Canthis be done? So far it hasnt happened. Many new materials do offer better

physical properties than aluminum, but the cost is many times higher.


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FUTURE DESIGN

Aluminum Silicon Carbide

AlSiC stands out as one of the mostpromising new materials. Mixed withvarious aluminum alloys it can be tailoredto specific physical properties. Controlled

thermal expansion, high conductivity andsignificant strength make AlSiC attractive.Due to cost, this material is typically usedin base sections and as substrates fordirect die attachment in power modulebases. Some applications use thismaterial to form the entire heat sinkalthough due to cost an aluminumextrusion is typically used.

Table offers a comparison of various heatsink materials.

Comparison of Heat Sinks

CTE k

ppm W/m-K

Aluminum 23 209

Copper 17 390

Copper-Molybdenum 7.2 195

Cu 20% Tungston 80% 7 250

Copper Graphite 2 ~350

AISiC 6.5 8.0 180 210

Silicon 3.3 4.2 150


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FUTURE DESIGN

Liquid-Cooling

Liquid-cooling, typically a waterand glycol mixture, overcomesmost of the heat removal andtransport limitations of air-cooled

systems. Smaller heat sinkvolumes, elimination of audiblenoise, the ability to moverejected heat away easily fromthe user area, and significantlyincreased system reliability are all

liquid-cooling characteristics. Theproblem is that only a fewmarkets currently accept liquid-cooled electronics.


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FUTURE DESIGN Liquid cooled heat sinks reduce volume over air-cooled parts.The primary resistance is customer

acceptance. The concept of a liquid-cooled personal computer has yet to gain general market

acceptance. However, as heat fluxes exceed 100 Watts per square inch, the need for phase changeheat transfer or liquid cooling are the only alternatives.

System Solutions Start with Thermal in Mind

Over the last few decades heat dissipation has become an increasing problem for electronicsdesigners. Today the thermal engineer needs to be an integral part of the design team and not anafterthought. Typically the electronic designs are done and PC boards complete before heat sinks

are considered. Thermal performance can limit a products marketability due to the amount ofpower output and the features that can be incorporated.

To ensure reduced time to market and success, heat dissipation must be one of the top designconsiderations. If engineering teams design with thermal considerations in mind, the resultingsystems will be quieter, smaller and less expensive. To do this, every level of a companysmanagement must be aware of the importance of including the thermal engineer in the designconcept team.

The following precepts are key to success:

The initial design effort must include a thermal engineer with experience enough to spot trouble

sites before they are locked in. The increased usage of computer design programs such as Icepak, Maya and FLOTHERM will allow

problems to be addressed before electrical designs are finished.

The best defense is education of electrical engineers, while they are still in college, about thenature and consequences of thermal problems and their solution sets.


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Q& A


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ANK Y

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Heat Transfer Presentation

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