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LOOP HEAT PIPESLOOP HEAT PIPES

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OUTLINEOUTLINE IntroductionIntroduction What are heat pipes?What are heat pipes? What are loop heat pipes?What are loop heat pipes? Basic components of loop heat pipesBasic components of loop heat pipes Operating principles of LHPOperating principles of LHP Conditions for the operation of LHPConditions for the operation of LHP LHP designLHP design Types of LHP’sTypes of LHP’s Limitations of loop heat pipesLimitations of loop heat pipes Applications of LHPApplications of LHP ConclusionsConclusions ReferencesReferences

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INTRODUCTIONINTRODUCTION The loop heat pipe (LHP) was invented in Russia in The loop heat pipe (LHP) was invented in Russia in

the early 1980’sthe early 1980’s Thermal management is an important factor .All Thermal management is an important factor .All

applications generate high concentrated heat so applications generate high concentrated heat so need to control this heatneed to control this heat

The LHP is known for its high pumping capability and The LHP is known for its high pumping capability and robust operation because it uses fine-pored metal robust operation because it uses fine-pored metal wicks and the integral evaporator/hydro-accumulator wicks and the integral evaporator/hydro-accumulator design design

two-phase heat transfer devices that utilize the two-phase heat transfer devices that utilize the evaporation and condensation of working fluid to evaporation and condensation of working fluid to transfer heat transfer heat

capillary forces developed in fine porous wicks to capillary forces developed in fine porous wicks to circulate the fluid circulate the fluid

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HEAT PIPES ?HEAT PIPES ? Offer 100 to a 1000 times more effective thermal Offer 100 to a 1000 times more effective thermal

conductivity than that offered by a solid copper rod.conductivity than that offered by a solid copper rod. Hollow cylindrical channel lined with a wick Hollow cylindrical channel lined with a wick

structure.structure. Channel evacuated and a working fluid is injected.Channel evacuated and a working fluid is injected. One end heated so phase change of the working fluid One end heated so phase change of the working fluid

occurs.occurs. Vapor flows through the hollow middle section. Vapor flows through the hollow middle section. Fluid is wicked back Fluid is wicked back

through the wick structure.through the wick structure. Used as an efficient Used as an efficient

heat path between a heatheat path between a heat source and a heat sink.source and a heat sink.

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LOOP HEAT PIPES??LOOP HEAT PIPES?? serious constraint on conventional serious constraint on conventional

heat pipes is the reduction of heat pipes is the reduction of transport capabilities when condenser transport capabilities when condenser is located below evaporator section in is located below evaporator section in a gravitational field. a gravitational field.

Loop heat pipes are the solutionLoop heat pipes are the solution Can perform at any orientation in a Can perform at any orientation in a

gravitational field over long distances gravitational field over long distances Phase change from liquid to vapor Phase change from liquid to vapor

state by absorbing latent heatstate by absorbing latent heat vapor is transported to the cooling sink vapor is transported to the cooling sink

where it cools down and change phase where it cools down and change phase to the liquid form to the liquid form

utilizes the thermodynamic pressure utilizes the thermodynamic pressure difference developed between the difference developed between the evaporator and condenser to circulate evaporator and condenser to circulate a working fluid through a closed loop a working fluid through a closed loop

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BASIC COMPONENTS OF A BASIC COMPONENTS OF A LOOP HEAT PIPELOOP HEAT PIPE

The evaporatorThe evaporator The working The working

fluidfluid The wick or The wick or

capillary capillary structurestructure

condensercondenser

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EVAPORATOREVAPORATOR Three main layersThree main layers

Top layerTop layer “ vapor chamber ”“ vapor chamber ” Middle layerMiddle layer “ Wick material ““ Wick material “ Bottom layerBottom layer “compensation “compensation

chamber “chamber “Three modes of Heat Three modes of Heat transfer in evaporatortransfer in evaporator•conducted through the conducted through the walls of the pores in axial walls of the pores in axial and lateral direction.and lateral direction.•heat lost in compensation heat lost in compensation chamber by convectionchamber by convection•Evaporation in meniscus of Evaporation in meniscus of pores.pores.

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WORKING FLUIDWORKING FLUID

Requirements Requirements Compatibility with wick and wall materialsCompatibility with wick and wall materials Good thermal stabilityGood thermal stability Wettability of wick and wall materialsWettability of wick and wall materials High latent heatHigh latent heat High thermal conductivityHigh thermal conductivity Low liquid and vapor viscositiesLow liquid and vapor viscosities High surface tensionHigh surface tensionSome working fluidsSome working fluids Liquid AmmoniaLiquid Ammonia Liquid Nitrogen Liquid Nitrogen Water Water

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function of the wicking material•Hold the meniscus using capillary pressures•Surface meniscus to withstand the back pressure effects of the gas•Provide liquid return from condenser to evaporator

WICKWICK

Types of wicks •Primary wick

primary wick will not be totally wetted

• Secondary wickThe primary function of secondary wick is to wet the primary CPS wick

There are sintered ceramic or metal wicks (nickel, titanium, stainless steel and copper)working fluid initially is pumped through the secondary wick into the primary wick where evaporation occurs.

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CONDENSERCONDENSER

dual core condenser was designed dual core condenser was designed and developed and developed

inner core with working fluidinner core with working fluid outer core with cooling water outer core with cooling water opposite directions for efficient heat opposite directions for efficient heat

transfer transfer

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OPERATING PRINCIPLES OPERATING PRINCIPLES OF LHPOF LHP

Heat applied into evaporatorHeat applied into evaporator liquid is vaporized and the menisci formed at the liquid/vapor interface in the liquid is vaporized and the menisci formed at the liquid/vapor interface in the

evaporator wick develop capillary forces to push the vapor through the vapor evaporator wick develop capillary forces to push the vapor through the vapor line to the condenser line to the condenser

Vapor condenses in the condenserVapor condenses in the condenser the capillary forces continue to push liquid back to the evaporator the capillary forces continue to push liquid back to the evaporator The waste heat from the heat source provides the driving force for the The waste heat from the heat source provides the driving force for the

circulation of the working fluid and no external pumping power is required circulation of the working fluid and no external pumping power is required The two-phase compensation chamber stores excess liquid and controls the The two-phase compensation chamber stores excess liquid and controls the

operating temperature of the loop operating temperature of the loop In order for the loop to continue to function, the wick in the evaporator must In order for the loop to continue to function, the wick in the evaporator must

develop a capillary pressure to overcome the total pressure drop in the loop. develop a capillary pressure to overcome the total pressure drop in the loop.

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CONDITIONS OF CONDITIONS OF OPERATION OPERATION

Total pressure drop =frictional pressure Total pressure drop =frictional pressure of(evaporator grooves +vapor line of(evaporator grooves +vapor line +condenser +the liquid line evaporator wick) +condenser +the liquid line evaporator wick) +any static pressure drop due to gravity +any static pressure drop due to gravity

Capillary pressure rise= 2Capillary pressure rise= 2σσ cos cos αα /R /R σσ is the surface tension of the working fluid, is the surface tension of the working fluid, R is the radius of curvature of the meniscus in the R is the radius of curvature of the meniscus in the

wick,wick, αα is the contact angle between the liquid and the wick is the contact angle between the liquid and the wick

The radius of curvature will continue to The radius of curvature will continue to decrease with increasing heat loads until it is decrease with increasing heat loads until it is equal to the pore radius of the wick, Rp. equal to the pore radius of the wick, Rp. Under this condition, the wick has reached Under this condition, the wick has reached its maximum capillary pumping capability its maximum capillary pumping capability

Total pressure drop ≤Capillary pressure riseTotal pressure drop ≤Capillary pressure rise

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LHP DESIGNLHP DESIGN the LHP consists of sealed tubes (liquid line and evaporator line) the LHP consists of sealed tubes (liquid line and evaporator line)

connecting an evaporator (heat source) with a condenser (heat sink) connecting an evaporator (heat source) with a condenser (heat sink) The evaporator consists of a top cap, coherent porous silicon wick The evaporator consists of a top cap, coherent porous silicon wick

(CPS) and the compensation chamber which acts as a reservoir for the (CPS) and the compensation chamber which acts as a reservoir for the working fluid.working fluid.

The CPS wick as shown is an array of micron-range silicon dioxide The CPS wick as shown is an array of micron-range silicon dioxide capillaries capillaries

micro machined using KOH through ordinary (100) electronic quality micro machined using KOH through ordinary (100) electronic quality silicon wafers. silicon wafers.

main concerns in LHP is heat leak from the vapor side to the liquid main concerns in LHP is heat leak from the vapor side to the liquid sideside

Ideally the pump core is always fully primed with liquid and the only Ideally the pump core is always fully primed with liquid and the only heat leak is through the wick materialheat leak is through the wick material

presence of two phases can arise in the core section due to heat leak.presence of two phases can arise in the core section due to heat leak. extra volume is provided in the form of a reservoirextra volume is provided in the form of a reservoir Major components of the evaporator package like top cap, CPS wick Major components of the evaporator package like top cap, CPS wick

and compensation chamber gets heated up before the water in the and compensation chamber gets heated up before the water in the wick due to their thermal capacitance.wick due to their thermal capacitance.

The CPS wick as shown an array of micron-range silicon dioxide The CPS wick as shown an array of micron-range silicon dioxide capillariescapillaries

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TYPES OF LHPSTYPES OF LHPS 00THTH generation LHP generation LHP

top-cap CPS wick and the bottom top-cap CPS wick and the bottom reservoir were epoxied togetherreservoir were epoxied together

condenser was not designedcondenser was not designed

11STST generation LHP generation LHP A new compensation chamber was A new compensation chamber was

included in this design replacing the included in this design replacing the bottom chamber.bottom chamber.

allowing the wick not to dry outallowing the wick not to dry out condenser was also designedcondenser was also designed Quartz wool was chosen to have good Quartz wool was chosen to have good

wetting properties to be a secondary wetting properties to be a secondary wickwick

secondary wick in a LHP will consist of secondary wick in a LHP will consist of a reticulated structure similar to the a reticulated structure similar to the primary wick. The main difference will primary wick. The main difference will be the larger size pores to allow for a be the larger size pores to allow for a low-pressure drop and sufficient low-pressure drop and sufficient capillary force to bring the liquid back capillary force to bring the liquid back to wet the primary wick.to wet the primary wick.

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22NDND generation LHP generation LHP To avoid the thermal mismatch between the top-cap and To avoid the thermal mismatch between the top-cap and

the compensation chamber, the top-cap was machined in the compensation chamber, the top-cap was machined in Pyrex glassPyrex glass

A condenser was built but was never tested as it was A condenser was built but was never tested as it was leaking significantly to make any calorimetric leaking significantly to make any calorimetric measurements. measurements.

Without insulation around the evaporator package the heat Without insulation around the evaporator package the heat lost to convection dominated the heat transferlost to convection dominated the heat transfer

33RDRD generation LHP generation LHP out-gassing of the Lexan® used for the evaporator out-gassing of the Lexan® used for the evaporator

package. Thus it was decided to use a thick piece of package. Thus it was decided to use a thick piece of borosilicate 7740 (Pyrex®) as of the back plate. borosilicate 7740 (Pyrex®) as of the back plate.

44THTH generation LHP generation LHP A new compensation chamber was designed to use gravity A new compensation chamber was designed to use gravity

to feed the liquid back to the primary wick through the to feed the liquid back to the primary wick through the secondary wick.secondary wick.

condenser which was capable to perform calorimetric condenser which was capable to perform calorimetric calculations was built in this generation devicecalculations was built in this generation device

dual core condenser was designed and developeddual core condenser was designed and developed

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LIMITATIONS OF LOOP LIMITATIONS OF LOOP HEAT PIPESHEAT PIPES

The capillary limit for a silicon wick is the pressure that will break the The capillary limit for a silicon wick is the pressure that will break the interface and force the interface to leave the wick. So one bad or large interface and force the interface to leave the wick. So one bad or large pore in the wick will dominate the capillary pressure effect and will be pore in the wick will dominate the capillary pressure effect and will be responsible for the vapor-liquid interface to burst throughresponsible for the vapor-liquid interface to burst through

The sonic limit is the maximum allowable mass flow rate or heat transfer The sonic limit is the maximum allowable mass flow rate or heat transfer that could choke the loop heat pipes. The vapor flow rate will choke, that could choke the loop heat pipes. The vapor flow rate will choke, when the vapor reaches the sonic speed. This could happen if the duct when the vapor reaches the sonic speed. This could happen if the duct cross-sectional area decreases while the working fluid is flowing in the cross-sectional area decreases while the working fluid is flowing in the pipe.pipe.

The entrainment limit is the maximum allowable mass flow rate or heat The entrainment limit is the maximum allowable mass flow rate or heat transfer rate that can be used before causing the evaporator to dry out. transfer rate that can be used before causing the evaporator to dry out. In general, this could happen in a conventional heat pipe when the vapor In general, this could happen in a conventional heat pipe when the vapor shear is able to carry water droplets from the liquid stream flowing back shear is able to carry water droplets from the liquid stream flowing back to the condenser. This will cause less flow to go to the evaporator and dry to the condenser. This will cause less flow to go to the evaporator and dry out the evaporator.out the evaporator.

Superheated liquid limitSuperheated liquid limit - -Underneath the vapor-liquid interface a Underneath the vapor-liquid interface a superheated liquid exists, since the interface is separates a high-pressure superheated liquid exists, since the interface is separates a high-pressure vapor and a low-pressure liquid with heat transfer across the interface. vapor and a low-pressure liquid with heat transfer across the interface. Most interfacial studies support the assumption of no temperature jump Most interfacial studies support the assumption of no temperature jump across the interface. Assuming no temperature jump across the interface, across the interface. Assuming no temperature jump across the interface, one can deduct that the liquid underneath the interface has to be one can deduct that the liquid underneath the interface has to be superheated liquid and the evaporation process is a non-equilibrium one.superheated liquid and the evaporation process is a non-equilibrium one.

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APPLICATIONSAPPLICATIONS LOOP HEAT PIPES FOR AVIONICSLOOP HEAT PIPES FOR AVIONICS

Aircraft thermal control applications Aircraft thermal control applications Acturator-mounted electronics coolingActurator-mounted electronics cooling Wing and cowl anti-icing using engine waste heatWing and cowl anti-icing using engine waste heat Avionics CoolingAvionics Cooling

SATELLITE THERMAL CONTROL TRENDS waste heat dissipation

LOOP HEAT PIPES FOR AEROSPACE APPLICATIONS

More electronic packages have to be accommodated

LOOP THERMOSYPHON TECHNOLOGYLOOP THERMOSYPHON TECHNOLOGYUsed when length from Used when length from

evaporator to condenser is smallevaporator to condenser is small

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CONCLUSIONCONCLUSION to have an operating LHP, a buildup pressure should to have an operating LHP, a buildup pressure should

exist between the top side of the evaporator and the exist between the top side of the evaporator and the bottom side of the evaporator circulating working fluid bottom side of the evaporator circulating working fluid from the evaporator to condenser back to evaporator from the evaporator to condenser back to evaporator

buildup pressure depends on the operating temperature buildup pressure depends on the operating temperature LHP design, CPS wick, and the working fluid LHP design, CPS wick, and the working fluid

pressure-temperature gradient depends on operating pressure-temperature gradient depends on operating temperature and the working fluid temperature and the working fluid

less circulated mass in the LHP means either the total less circulated mass in the LHP means either the total heat removed decrease or the vapor temperature heat removed decrease or the vapor temperature decrease. decrease.

LHP with short pumping distance and large tube size is LHP with short pumping distance and large tube size is betterbetter

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REFERENCESREFERENCES http://www.thermacore.com/Technologies/loop-heat-pipes.aspx http://www.1-act.com/lhptech.htmlhttp://www.1-act.com/lhptech.html http://www.nlr.nl/smartsite.dws?id=3029http://www.nlr.nl/smartsite.dws?id=3029 http://altmine.mie.uc.edu/fgerner/public_html/lhp5.pdfhttp://altmine.mie.uc.edu/fgerner/public_html/lhp5.pdf http://www.patentstorm.us/patents/pdfs/patent_id/6892799.htmlhttp://www.patentstorm.us/patents/pdfs/patent_id/6892799.html http://www.patentstorm.us/patents/7347250/fulltext.htmlhttp://www.patentstorm.us/patents/7347250/fulltext.html http://altmine.mie.uc.edu/fgerner/public_html/lhp2.pdfhttp://altmine.mie.uc.edu/fgerner/public_html/lhp2.pdf http://encyclopedia.thefreedictionary.com/Loop+Heat+Pipehttp://encyclopedia.thefreedictionary.com/Loop+Heat+Pipe http://www2.tku.edu.tw/~tkjse/8-2/8-2-5.pdfhttp://www2.tku.edu.tw/~tkjse/8-2/8-2-5.pdf http://etd.ohiolink.edu/send-pdf.cgi/HAMDAN%20MOHAMMAD%20OMAR.pdf?http://etd.ohiolink.edu/send-pdf.cgi/HAMDAN%20MOHAMMAD%20OMAR.pdf?

acc_num=ucin1049987207acc_num=ucin1049987207 http://etd.ohiolink.edu/send-pdf.cgi/SHARMA%20MONIKA.pdf?acc_num=ucin1132344889http://etd.ohiolink.edu/send-pdf.cgi/SHARMA%20MONIKA.pdf?acc_num=ucin1132344889 http://etd.ohiolink.edu/send-pdf.cgi/Medis%20Praveen%20S.pdf?http://etd.ohiolink.edu/send-pdf.cgi/Medis%20Praveen%20S.pdf?

acc_num=ucin1131996727acc_num=ucin1131996727 http://etd.ohiolink.edu/send-pdf.cgi/Suh%20Junwoo.pdf?acc_num=ucin1131033062http://etd.ohiolink.edu/send-pdf.cgi/Suh%20Junwoo.pdf?acc_num=ucin1131033062 http://etd.ohiolink.edu/send-pdf.cgi/Shuja%20Ahmed%20A.pdf?http://etd.ohiolink.edu/send-pdf.cgi/Shuja%20Ahmed%20A.pdf?

acc_num=ucin1179501051acc_num=ucin1179501051 http://www2.dem.inpe.br/rriehl/lhp.htmhttp://www2.dem.inpe.br/rriehl/lhp.htm

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QUESTIONS ???QUESTIONS ???

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THANK YOU !!THANK YOU !!