Loop Heat Pipes - Development and Application

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Loop Heat Pipes - Development and Application

Yu. F. Maydanik

Ural Branch / Institute of Thermal Physics (ITP)

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Contents

• Identifications of a Loop Heat Pipe

• Historical background

• Theoretical foundations of the LHP operation

• Materials and working fluids

• Classification of LHPs

• Different types of LHPs and the main results of their investigations

• Application of LHPs

• Conclusion

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Brief historical background

• The LHP creation was a response to the challenge to develop a heat-transfer

device operating on the principle of a heat pipe and possessing all its

advantages, but at the same time capable of transferring heat for distances

up to 1 m and more at different orientations in the gravity field.

• Such a device was first invented in 1972 by Yu. Gerasimov and Yu. Maydanik

at the Ural Politechnical Institute.

• The first name of the device was “a heat pipe”. Later the names “a heat pipe

with separate channels” and “an antigravitational heat pipe” were used.

• In 1989, when these devices came into use in space engineering, there

appeared a new name “a Loop Heat Pipe”, which is now generally recognized.

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The first LHP scheme

evaporator

main wick

vapor removalchannels

secondarywick

compensation chamber

liquid line

vapor line

condenser

Total length, mm 1000

Evaporator diameter,mm 30

Active zone length, mm 60

Body material ss

Wick material nickel

Working fluid water

Capacity, W 500

Year of development 1972

USSR certification 449 213 1974

Specification


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Identifications of the Loop Heat Pipe (LHP)

1. By the principle of operationA loop heat pipe is a hermetic heat-transfer device operating on aclosed evaporation-condensation cycle, in which the circulation of vapor andliquid flows in the transportation section is realized along separatesmooth-walled tubing, and the capillary structure (wick), localized in the heat-supply zone, acts simultaneously as a capillary pump, a thermal and ahydraulic gate.

2. By designA loop heat pipe is a hermetic heat-transfer device made in theform of a closed loop filled with a working fluid in the vapor and in theliquid phase containing an evaporator with a capillary structure (wick) combined with a compensation chamber and a condenser connected to the evaporator by means of separate smooth-walled tubing of a relatively small diameter.

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Scheme of a traditional heat pipe Scheme of a LHP

liquid

vaporwick

heat supply

heat removal

liquid

wick

vapor

heat supply

heat removal


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Scheme of classification of heat-transfer devices by the main design features

• the condenser is located above the evaporator• separate smooth-walled tubing for vapor and liquid

Loop Thermosyphon Loop Heat Pipe

• the wick is located in the evaporator• separate smooth-walled tubing for vapor and liquid• the compensation chamber (reservoir) is combined with the evaporator

Conventional Heat Pipe

• single body• the wick is located along the whole length

Capillary Pumped Loop

• the wick is located in the evaporator• separate smooth-walled tubing for vapor and liquid• separate reservoir with an additional heater

Separate Tubing Heat Pipe

• the wick is located along the whole length• separate tubing for vapor and liquid

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saturator line liquid line

vapor removalchannels

TEMPERATURE

PR

ES

SU

RE wick

evaporator

compensationchamber

vapor line

condenser

1, vapor state over the evaporating menisci in a wick1-2, vapor motion in vapor-removal channels with superheating2-3, adiabatic vapor motion in vapor line3-4, vapor cooling and condensation in a condenser4-5, liquid supercooling in a condenser5-6, adiabatic liquid motion in a liquid line with allowance for the hydrostatic resistance6-7, liquid motion in a compensation chamber7-8, liquid motion in a wick

P1

P6

P8

PC

T6 T7 T4 T1

PEX

T3

T7T6

T1

T2

T4

T5

Scheme and diagram of working cycle of an LHP

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Conditions of an LHP serviceability

1. Condition of balance of the capillary head and the sum of pressure loses

in all sections of the working fluid circulation (hydrodynamic condition):

PC = P1-8 = PL+ PV + PG

2. Condition of correlation between the temperature and the pressure ofsaturated vapor above the surface of menisci in the evaporation zone and abovethe surface of the interface in the compensation chamber (start-up condition):

dP/dT (T1 - T7) P1 - P7 3. Condition of liquid supercooling (thermodynamic condition):

dP/dT (T5 - T4) P5 - P6 4. Condition of relationship between the internal volumes and the volumeof a liquid:

VCC VVL + VC

VL = VW + VLL + VCC + VCCH

{

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Correlation of volumes in an LHP

1. The volume of the compensationchamber VCC must be equal to or exceed the sum of the volumes of the vapor line VVL and the condenser VC

VCC VVL + VC

2. The volume of the liquid VL in an LHP must be equal to the sum of the volumes of the liquid in the wick VW, the liquid line VLL, the compensation chamber VCC and the centralchannel VCCH

VL = VW + VLL+ VCC + VCCH

VCC

VW

VCCH

VVL

VC

liquid level

liquid level

VLL

before start up after start up

liquid level

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Classification of LHPs

LHP design LHP dimensions Evaporator shape Evaporator design

• conventional (diode)• reversible• flexible• ramified

Condenser design

• pipe-in-pipe• flat coil• collector

• miniature• all the rest

Number of evaporators and condensers• one• two and more

• cylindrical• flat disk-shaped• flat rectangular

Temperature range

• cryogenic• low-temperature• high-temperature

• one butt-end compensation chamber• two butt-end compensation chambers• coaxial

Operating- temperature control

• without active control• with active control

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Metal-theramic wicks for LHPs

nickel

10 m

titanium

10 m


Material Nickel TitaniumEffective poreradius, m 0.5 - 2 3 - 10Porosity, % 65 - 75 55 - 70

Permeability, m2 10-14 10-13

Year of development 1972 1978

Specification

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Tested LHPs material- working fluid combinations

stainless steel

Body Wick Working fluid

nickel water, ammonia, acetone,pentane, freon-152A, freon 11,propylene

stainless steel titanium water, ammonia, acetone,pentane, freon-152A, toluene

stainless steel stainless steel ammonia

nickel titanium ammonia

nickel ammonianickel

copper watercopper


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LHPs with a high heat-transfer capacity

evaporator

condenser

vapor line

liquid line

condenser

evaporator


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Ammonia two-meter LHP with two butt-end compensation chambers


HEAT LOAD, W

EV

AP

OR

AT

OR

TE

MP

ER

AT

UR

E, 0 C

0 200 400 600 800 1000 1200 1400 160010

20

30

40

50

60

vertical position, evaporator above condenserhorizontal position

evaporator horizontal, above vertical condenser

vertical position, condenser above evaporator

ambient temperature 19±10Ccondenser cooling temperature 17±10C

CC1 CC2

Evaporator scheme

CC1

CC2

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Effective length, mm 450

Evaporator diameter, mm 20

Vapor line diameter, mm 6/4

Liquid line diameter, mm 4/3

Heating zone area, cm2 4.25

Max heat flux, W/cm2 130

Max heat transfer coef., W/m2 K 30 000


Ammonia High-Heat Flux LHP Specification


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Flexible LHPs


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Active zone length, mm 100

Vapor line diameter, mm 6

Liquid line diameter, mm 4

Max capacity, W 900

Min thermal resistance, 0C/W 0.02


SpecificationReversible LHP scheme


General view of ammonia RLHPevaporator

condenser

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Specification



Evaporator thickness, mm 13

Vapor/Liquid line diameter, mm 2/1.2

Condenser length, mm 720

Body material ss

Wick material nickel/titanium

Working fluid ammonia

Total mass, g 167

Max capacity, W 110/90

Min thermal resistance,0C/W 0.30/0.41

Year of development 1999 - 2001

LHPs with flat evaporators


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LHP with temperature active control

T, 0C

42,1

42,0

41,9

41,8-10 0 10 20 TCOOL, 0C

set point 420C

Q = 6…10 W

+0,1 0C

-0,1 0C

regulatingheater

controlunit

thermocouple

vapor line 2 mm

liquid line 2 mm

evaporator 8 x 120 mm

condenser

radiator

controlledtemperatureT


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Base design variants of ramified LHPsUral Branch / Institute of Thermal Physics (ITP)

CC1

COND

CC2

EV1 EV2CC1 CC2

EV2EV1

COND COND

CC

CC

COND1

COND2

CC

COND2

COND1

EVEV

EV2EV1

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Two evaporator-condenser LHP

condenser 1

evaporator 1


condenserliquid line

vapor line evaporator

compensation chamber

cooling jacket

•

TV

TCh1TCh2

• •

TCOOL1

•

•TL•

TCOOL2

Specification

Total length, mm 1000Evaporator diameter, mm 24Vapor line diameter, mm 6/4Liquid line diameter, mm 4/3Max capacity, W 1400Year of development 2002

evaporator 2

condenser 2

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TIME, s

TE

MP

ER

AT

UR

E, 0 C


Test results of ramified LHP

Q1 = 400W, Q2 = 200WG1 = 0.1kg/s, G2 = 0.05kg/s

= 90o

0 200 400 600 800 1000 1200 1400 1600

30

25

20

15

10

5

0

TvTlc1Tlc2TlTcool

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Miniature LHPs


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Effective length, mm 300 230

Evaporator diameter, mm 6 5

Lines diameter, mm 2.5 2

Active - zone length, mm 20 20

Condenser length, mm 60 60

Thermal interface, mm 20 x 20 20 x 20

Heat load, W 130 70

Evaporator temperature 98 70

Own thermal resistance, 0C/W 0.10 0.12

Total thermal resistance -

(evaporator-ambient), 0C/W 0.59 0.68

Year of development 2003 2002

Specification

Body-working fluid Copper- water SS-ammonia

General view of MLHP


evaporator

condenser

vapor line

liquid line

saddle

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Tests results of miniature LHPs

HEAT LOAD, W

TE

MP

ER

AT

UR

E, 0 C

0 20 40 60 80 100 120 140

40

20

120

60

80

100

SS-ammonia

Condenser cooling by water, 200C

HEAT LOAD, W

0 20 40 60 80 100 120 140 0.0

0.4

0.8

1.2

1.6

2.0

TH

ER

MA

L R

ES

IST

AN

CE

, 0 C/W

0 20 40 60 80 100 120 140

25

20

10

HEAT LOAD, W

HE

AT

TR

AN

SF

ER

CO

EF

. x 1

0 - 3

, W/m

2 0 C

0 20 40 60 80 100 120 140 20

40

60

80

HEAT LOAD, W


air, 200C

Copper-water

SS-ammoniaCopper-water Condenser cooling by

water, 200C air, 200C

HE

AT

TR

AN

SF

ER

CO

EF

. x 1

0 - 3

, W/m

2 0 C

Condenser cooling byair, 200C

Condenser cooling by water, 200C

air, 200C

SS-ammonia

Copper-water

15

30

35

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Comparison of operating characteristics HP (Fujikura) and LHP (ITP)


HP Fujikuraworking fluid - waterLeff - 150 mmLe - 50 mmLc - 250 mmT? - 500C

LHP ITPworking fluid - ammoniaLeff - 230 mmLe - 20 mmLc - 62 mmTe - 500C

Condenser water cooling 200C

Condenser air cooling 200C

LHP

LHP

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The first flight experiment with an LHP aboard the spacecraft «GORISONT» in 1989

The first application of an LHP aboard the spacecraft «OBZOR» in 1994


optical instrumentsarterial HP

LHP OI

LHP RSSLHP Rad

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Thermoregulation system with LHPs for the international program «MARS-96»

penetrator

TRS assembling

TRS LHP


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Cooling of the copper bus of an electrolysis-bath electrode

liquid line

cooling water

vapor line

current-carruing wire

bath

electrolyte

electrode

condenser


evaporator

saddle

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Cooling of quantum-electronic converters Cooling of powerful transistors


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25 W CPU coolers for a mobile computer


evaporator

CPU

liquid line

5.6

12

fan

60

96vapor line condenser

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45 W CPU Cooler for a Mobile PCUral Branch / Institute of Thermal Physics (ITP)

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Conclusion

Loop Heat Pipes are very promising and universalheat-transfer devices, whose potential of developmentand application has not been used in full measure.

Loop Heat Pipes - Development and Application

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