Fundamentals of Liquid Cooling Thermal Management of Electronics San José State University...

Fundamentals of Fundamentals of Liquid CoolingLiquid Cooling

Thermal Management of ElectronicsThermal Management of Electronics

San JosSan Joséé State University State UniversityMechanical Engineering DepartmentMechanical Engineering Department

Air as a CoolantAir as a Coolant

PROS:PROS: SimplicitySimplicity Low CostLow Cost Easy to MaintainEasy to Maintain ReliableReliable

CONS:CONS: Inefficient at heat Inefficient at heat

removal removal

(low (low kk and and PrPr)) Low thermal Low thermal

capacitance (low capacitance (low ρρ and and CCpp))

Large thermal Large thermal resistanceresistance

Using Alternate CoolantsUsing Alternate Coolants

As electronic components get smaller and As electronic components get smaller and heat transfer requirements increase air heat transfer requirements increase air becomes a less efficient coolantbecomes a less efficient coolant

Liquid cooling provides a means in which Liquid cooling provides a means in which thermal resistance can be reduced thermal resistance can be reduced dramaticallydramatically

Types of Liquid CoolingTypes of Liquid Cooling

Indirect – Indirect –

The coolant does not come into The coolant does not come into contact with the electronics.contact with the electronics.

Direct (Immersion) – Direct (Immersion) –

The coolant is in direct contact with The coolant is in direct contact with the the electronics.electronics.

Fluid SelectionFluid Selection

Is the fluid in direct contact with the Is the fluid in direct contact with the electronics?electronics?

No. Water will normally be used due to the No. Water will normally be used due to the fact that it is cheap and has superior thermal fact that it is cheap and has superior thermal properties.properties.

Yes. A dielectric must be used. Consideration Yes. A dielectric must be used. Consideration must be given to the thermal properties of must be given to the thermal properties of different dielectric fluids.different dielectric fluids.

MicrochannelsMicrochannels

Microchannels are most commonly used Microchannels are most commonly used for indirect liquid cooling of IC’s and may for indirect liquid cooling of IC’s and may be:be:

Machined into the chip itself.Machined into the chip itself. Machined into a substrate or a heat sink and Machined into a substrate or a heat sink and

then attached to a chip or array of chips.then attached to a chip or array of chips.


Example:Example: Thermal Thermal Conduction Module Conduction Module used on IBM used on IBM 3080X/3090 series3080X/3090 series

Heat is transmitted Heat is transmitted through an through an intermediate structure intermediate structure to a cold plate to a cold plate through which a through which a coolant is pumpedcoolant is pumped Incropera, pg. 3


RRth,hth,h – Conduction Resistance – Conduction Resistance through the chipthrough the chip

RRth,cth,c – Contact Resistance at – Contact Resistance at the Chip/Substrate Interfacethe Chip/Substrate Interface

RRth,subth,sub – 3-D Conduction – 3-D Conduction Resistance in the substrate Resistance in the substrate (spreading resistance)(spreading resistance)

RRth,cnvth,cnv – Convection Resistance – Convection Resistance from the substrate to the from the substrate to the coolantcoolant

Incropera, pg. 155

Note that this network ends with the mean fluid temperature. If we use the inlet fluid temperature, we also need to include Rcaloric

Motivating ExampleMotivating ExampleLaminar flow through a rectangular channel:Laminar flow through a rectangular channel:

Kandlikar and Grande, pg. 7 Kandlikar and Grande, pg. 8

Pressure Drop in MicrochannelsPressure Drop in Microchannels The pressure drop due to forcing a fluid The pressure drop due to forcing a fluid

through a small channel may produce through a small channel may produce design limitations.design limitations.

V is the mean flow velocityL is the flow lengthρ is the fluid densityf is the friction factor, depends on the aspect ratio.

Limitations may include:

1)Pumping Power

2)Mechanical Stress Limitation of the Chip Material

Pressure Drop ExamplePressure Drop Example

If chip power If chip power increases mass flow increases mass flow rate must increaserate must increase

If mass flow rate If mass flow rate increases pressure increases pressure drop increasesdrop increases Kandlikar and Grande, pg. 9

Optimization of MicrochannelsOptimization of Microchannels

How should the channels in the silicon substrate How should the channels in the silicon substrate be designed for optimal heat transfer? Should be designed for optimal heat transfer? Should the channel be deep or shallow? Make sure to the channel be deep or shallow? Make sure to give a valid reason.give a valid reason.

Kandlikar and Grande, pg. 9

The channels should be deep so that the The channels should be deep so that the hydraulic diameter is small but the channel hydraulic diameter is small but the channel surface area is large.surface area is large.

Caution: Caution: Making the channels too small Making the channels too small may result in unreasonable pressure drop. may result in unreasonable pressure drop.

Optimization of MicrochannelsOptimization of Microchannels


Microchannel IssuesMicrochannel Issues

Liquids + ElectronicsLiquids + Electronics Self-explanatorySelf-explanatory

Fouling Leading to CloggingFouling Leading to Clogging Clogging prevents flow of liquids through a Clogging prevents flow of liquids through a

channelchannel Local areas where heat is not pulled away Local areas where heat is not pulled away

from components at a high enough rate are from components at a high enough rate are developeddeveloped

Microchannel IssuesMicrochannel Issues Mini-PumpsMini-Pumps

Able to move liquid through the channel at a Able to move liquid through the channel at a required rate required rate

Able to produce large pressure heads to Able to produce large pressure heads to overcome the large pressure drop associated overcome the large pressure drop associated with the small channelswith the small channels

Tradition rotary pumps can not be used due to Tradition rotary pumps can not be used due to their large size and power consumptiontheir large size and power consumption

For information on some current solutions For information on some current solutions refer to refer to http://www.electronics-cooling.com/html/2006_may_a3.htmlhttp://www.electronics-cooling.com/html/2006_may_a3.html

Current Research for Single Phase Current Research for Single Phase Convection in MicrochannelsConvection in Microchannels

Surface AreaSurface Area Adding protrusions to the channels to Adding protrusions to the channels to

increase surface area.increase surface area. Adding and arranging fins in a manner that is Adding and arranging fins in a manner that is

similar to a compact heat exchanger.similar to a compact heat exchanger.Microstructures

Silicon Substrate

Examples of different geometries:

• Staggered Fins

•Posts

•T-Shaped Fins



Manufacturing TechnologyManufacturing Technology

Reducing cost of manufacturingReducing cost of manufacturing

Producing enhanced geometriesProducing enhanced geometries

For further information refer to article by For further information refer to article by Kandlikar and GrangeKandlikar and Grange

Justifying deviation from classical theory Justifying deviation from classical theory for friction and heat transfer coefficients for friction and heat transfer coefficients when microchannel diameters become when microchannel diameters become smallsmall Lack of a good analytical modelLack of a good analytical model Surface RoughnessSurface Roughness Accurate measurements of system Accurate measurements of system

parametersparameters Ect.Ect.

***If you are interested in this take a look at:***If you are interested in this take a look at:Palm, B. “Palm, B. “Heat Transfer in Microchannels”Heat Transfer in Microchannels”. . Microscale Thermophysical Engineering 5:155-175, Microscale Thermophysical Engineering 5:155-175,

2001. 2001. Taylor Francis, 2001.Taylor Francis, 2001.


Jet ImpingementJet Impingement

Benefits of using a jet in Benefits of using a jet in thermal management of thermal management of a surface:a surface:

A thin hydrodynamic A thin hydrodynamic boundary layer is formedboundary layer is formed

A thin thermal boundary A thin thermal boundary layer is formedlayer is formed

Incropera, pg. 56

Classifying Impinging JetsClassifying Impinging Jets

Jets can be:Jets can be: Free-Surface – Free-Surface –

discharged into an discharged into an ambient gasambient gas

Submerged – Submerged – discharged into a liquid discharged into a liquid of the same typeof the same type

Cross Sections:Cross Sections: CircularCircular RectangularRectangular

Confinement:Confinement: Confined – Flow is Confined – Flow is

confined to a region confined to a region after impingementafter impingement

Unconfined – Flow is Unconfined – Flow is unconfined after unconfined after impingementimpingement

Classify the Following JetsClassify the Following Jets

Liquid jet released Liquid jet released into ambient gasinto ambient gas

Liquid release into Liquid release into liquid of the same liquid of the same typetype

Incropera, pg. 56

Incropera, pg. 65

Classify the Following JetsClassify the Following Jets

Unconfined, circular, Unconfined, circular, free-surface jetfree-surface jet

Unconfined, circular, Unconfined, circular, submerged jetsubmerged jet

Incropera, pg. 56

Incropera, pg. 65

Nozzle DesignNozzle Design

Nozzles are designed to create different jet Nozzles are designed to create different jet characteristicscharacteristics Example: Sufficiently long nozzles will produce both Example: Sufficiently long nozzles will produce both

fully developed laminar or turbulent jets (Shown in b)fully developed laminar or turbulent jets (Shown in b)

Incropera, pg. 58

Flow RegionsFlow Regions Stagnation Region – Jet Stagnation Region – Jet

flow is decelerated normal flow is decelerated normal to the impingement to the impingement surface and accelerated surface and accelerated parallel to it. parallel to it. Hydrodynamic and Hydrodynamic and thermal boundary layers thermal boundary layers are uniform.are uniform.

Wall Jet Region – Wall Jet Region – Boundary layers begin to Boundary layers begin to growgrow

Incropera, pg. 62

Degradation of Heat Transfer Degradation of Heat Transfer During Jet ImpingementDuring Jet Impingement

Splattering – Droplets are eject from the Splattering – Droplets are eject from the wall jet region due to the distance the wall jet region due to the distance the nozzle is from the heat source and the nozzle is from the heat source and the surface tension of the jet fluidsurface tension of the jet fluid

Hydraulic Jump – An abrupt increase in Hydraulic Jump – An abrupt increase in film thickness and reduction in film velocity film thickness and reduction in film velocity occurring in the wall jet regionoccurring in the wall jet region

Confining Fluid FlowConfining Fluid Flow

Adding a confining Adding a confining wall:wall: Adds low and high Adds low and high

pressure regions pressure regions Sometimes adds Sometimes adds

secondary stagnation secondary stagnation regionsregions

Degrades convection Degrades convection heat transferheat transfer

Decreases space Decreases space needed to use jet needed to use jet impingementimpingement

Incropera, pg. 69

Two-Phase Boiling in Two-Phase Boiling in MicrochannelsMicrochannels

Fluid entering microchannels is heated to Fluid entering microchannels is heated to the point where it boilsthe point where it boils

Flow in microchannels is highly Flow in microchannels is highly unpredictable and can produce large voids unpredictable and can produce large voids and multiple flow regimes inside of tubesand multiple flow regimes inside of tubes

No accurate analytical models currently No accurate analytical models currently exist; many analytical models have errors exist; many analytical models have errors ranging from 10% to well over 100%ranging from 10% to well over 100%

Flow Regimes in Two-Phase Flow Regimes in Two-Phase ApplicationsApplications

Garimella, pg. 107

Immersion (Direct) CoolingImmersion (Direct) Cooling

In direct cooling electronics are immersed In direct cooling electronics are immersed into a dielectric liquidinto a dielectric liquid

Closed loop systems are normally used Closed loop systems are normally used due to both the cost of the liquids used due to both the cost of the liquids used and the environmental issues associated and the environmental issues associated with the liquids escaping into the with the liquids escaping into the atmosphereatmosphere

Typical Liquids Used in ImmersionTypical Liquids Used in Immersion

Cengel, pg. 920

Boiling Used in ImmersionBoiling Used in Immersion

Electronics expel heat into the liquidElectronics expel heat into the liquid Vapor bubbles are formed in the liquidVapor bubbles are formed in the liquid The vapor is collected at the top of the enclosure where The vapor is collected at the top of the enclosure where

it comes in contact with some sort of heat exchangerit comes in contact with some sort of heat exchanger The vapor condenses and returns to the liquid portion of The vapor condenses and returns to the liquid portion of

the reservoirthe reservoir

Cengel, pg. 918

Boiling Used in ImmersionBoiling Used in Immersion

Electronics dissipate heat through the liquidElectronics dissipate heat through the liquid Vapor bubbles are generatedVapor bubbles are generated As vapor bubbles rise they come in contact with the As vapor bubbles rise they come in contact with the

cooler liquid produced by an immersed heat exchange cooler liquid produced by an immersed heat exchange and they implodeand they implode

*The prior example is more efficient due to the heat transfer *The prior example is more efficient due to the heat transfer coefficient associated with condensationcoefficient associated with condensation

Cengel, pg. 919

Cray-2 SupercomputerCray-2 Supercomputer

Cold fluid enters between Cold fluid enters between the circuit modulesthe circuit modules

Convection occurs, pulling Convection occurs, pulling heat from the electronics to heat from the electronics to the liquidthe liquid

The heated fluid is pumped The heated fluid is pumped to a heat exchangerto a heat exchanger

Heat is transfer from the Heat is transfer from the immersion liquid to chilled immersion liquid to chilled water in the heat exchangerwater in the heat exchanger Incropera, pg. 6

Concerns with ImmersionConcerns with Immersion

Introduction of incompressible gasses into Introduction of incompressible gasses into a vapor spacea vapor space This will limit the amount of condensation that This will limit the amount of condensation that

is allowed to occur and degrade heat transferis allowed to occur and degrade heat transfer

LeakageLeakage Environmental ConcernsEnvironmental Concerns ReliabilityReliability

SourcesSources Cengel, Yunus A. Cengel, Yunus A. Heat Transfer: A Practical ApproachHeat Transfer: A Practical Approach. 1. 1stst edition. New York, NY: edition. New York, NY:

McGraw Hill. 1998McGraw Hill. 1998

Incropera, Frank P. Incropera, Frank P. Liquid Cooling of Electronic Devices by Single Phase ConvectionLiquid Cooling of Electronic Devices by Single Phase Convection. . Danvers, MA: John Wiley & Son. 1999Danvers, MA: John Wiley & Son. 1999

Kandlikar, Satish G. and Grande, William JKandlikar, Satish G. and Grande, William J. “Evaluation of Single Phase Flow in . “Evaluation of Single Phase Flow in Microchannels for High Heat Flux Chip Cooling – Thermohydrolic Performance Microchannels for High Heat Flux Chip Cooling – Thermohydrolic Performance Enhancement and Fabrication Technology”. Heat Transfer Engineering.Enhancement and Fabrication Technology”. Heat Transfer Engineering. Taylor Taylor Francis Inc. 25(8). 2004Francis Inc. 25(8). 2004

Palm, Bjorn. “Palm, Bjorn. “Heat Transfer in Microchannels”Heat Transfer in Microchannels”. . Microscale Thermophysical Microscale Thermophysical Engineering 5:155-175, 2001. Engineering 5:155-175, 2001. Taylor Francis, 2001.Taylor Francis, 2001.

Kandlikar, Satish G. and Grande, William JKandlikar, Satish G. and Grande, William J. “Condensation Flow Mechanisms in . “Condensation Flow Mechanisms in Microchannels: Basis for Pressure Drop and Heat Transfer Models”. Heat Transfer Microchannels: Basis for Pressure Drop and Heat Transfer Models”. Heat Transfer Engineering.Engineering. Taylor Francis Inc. 25(3). 2004 Taylor Francis Inc. 25(3). 2004

Fundamentals of Liquid Cooling Thermal Management of Electronics San José State University...

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