Loop Heat Pipes - Development and Application

1. Ural Branch / Institute of Thermal Physics (ITP) Loop Heat Pipes - Development and Application Yu. F. Maydanik

2. Ural Branch / Institute of Thermal Physics (ITP) 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

3. Ural Branch / Institute of Thermal Physics (ITP) 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.

4. Ural Branch / Institute of Thermal Physics (ITP) The first LHP scheme Specification evaporator 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 main wick vapor removal channels secondary wick compensation chamber liquid line vapor line condenser

5. Ural Branch / Institute of Thermal Physics (ITP) Identifications of the Loop Heat Pipe (LHP) 1. By the principle of operation A loop heat pipe is a hermetic heat-transfer device operating on a closed evaporation-condensation cycle, in which the circulation of vapor and liquid flows in the transportation section is realized along separate smooth-walled tubing, and the capillary structure (wick), localized in the heat-supply zone, acts simultaneously as a capillary pump, a thermal and a hydraulic gate. 2. By design A loop heat pipe is a hermetic heat-transfer device made in the form of a closed loop filled with a working fluid in the vapor and in the liquid 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.

6. Ural Branch / Institute of Thermal Physics (ITP) Scheme of a traditional heat pipe Scheme of a LHP heat supply heat supply wick vapor wick liquid liquid vapor heat removal heat removal

7. Ural Branch / Institute of Thermal Physics (ITP) Scheme of classification of heat-transfer devices by the main design features Loop Thermosyphon Loop Heat Pipe Capillary Pumped Loop • the wick is located in the evaporator • separate smooth-walled tubing for • vapor and liquid • separate reservoir with an additional • heater • the condenser is located above • the evaporator • separate smooth-walled tubing • for vapor and liquid Separate Tubing 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 • the wick is located along the whole • length • separate tubing for vapor and liquid

8. Ural Branch / Institute of Thermal Physics (ITP) Scheme and diagram of working cycle of an LHP compensation chamber T7 P1 T6 evaporator liquid line saturator line wick T1 PEX PRESSURE PC vapor removal channels P6 P8 T2 T6 T7 T4 T1 TEMPERATURE T3 1, vapor state over the evaporating menisci in a wick 1-2, vapor motion in vapor-removal channels with superheating 2-3, adiabatic vapor motion in vapor line 3-4, vapor cooling and condensation in a condenser 4-5, liquid supercooling in a condenser 5-6, adiabatic liquid motion in a liquid line with allowance for the hydrostatic resistance 6-7, liquid motion in a compensation chamber 7-8, liquid motion in a wick vapor line condenser T4 T5

9. Ural Branch / Institute of Thermal Physics (ITP) 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 = D PL+ D PV + D PG 2. Condition of correlation between the temperature and the pressure of saturated vapor above the surface of menisci in the evaporation zone and above the 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 volume of a liquid: VCC VVL + VC VL = VW + VLL + VCC + VCCH {

10. Ural Branch / Institute of Thermal Physics (ITP) Correlation of volumes in an LHP before start up after start up 1. The volume of the compensation chamber VCCmust 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 VCCand the central channel VCCH VL = VW + VLL+ VCC + VCCH liquid level VCC VW VCCH liquid level VVL VLL VC liquid level

11. Ural Branch / Institute of Thermal Physics (ITP) Classification of LHPs LHP dimensions Evaporator shape Evaporator design LHP design • cylindrical • flat disk-shaped • flat rectangular • conventional (diode) • reversible • flexible • ramified • one butt-end • compensation • chamber • two butt-end • compensation • chambers • coaxial • miniature • all the rest Operating- temperature control Temperature range Number of evaporators and condensers Condenser design • without active • control • with active • control • pipe-in-pipe • flat coil • collector • cryogenic • low-temperature • high-temperature • one • two and more

12. Ural Branch / Institute of Thermal Physics (ITP) titanium 10 m nickel 10 m Metal-theramic wicks for LHPs Specification Material Nickel Titanium Effective pore radius, m 0.5 - 2 3 - 10 Porosity, % 65 - 75 55 - 70 Permeability, m2 10-14 10-13 Year of development 1972 1978

13. Ural Branch / Institute of Thermal Physics (ITP) Tested LHPs material- working fluid combinations Body Wick Working fluid stainless steel 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 nickel ammonia copper copper water

14. Ural Branch / Institute of Thermal Physics (ITP) evaporator LHPs with a high heat-transfer capacity condenser vapor line evaporator condenser liquid line

15. Ural Branch / Institute of Thermal Physics (ITP) Ammonia two-meter LHP with two butt-end compensation chambers evaporator horizontal, above vertical condenser vertical position, evaporator above condenser CC1 horizontal position vertical position, condenser above evaporator 10 20 30 40 50 60 EVAPORATOR TEMPERATURE, 0C CC2 ambient temperature 19±10C condenser cooling temperature 17±10C 0 200 400 600 800 1000 1200 1400 1600 HEAT LOAD, W Evaporator scheme CC2 CC1

16. Ural Branch / Institute of Thermal Physics (ITP) Ammonia High-Heat Flux LHP Specification 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 Year of development 1997

17. Ural Branch / Institute of Thermal Physics (ITP) Flexible LHPs

18. Ural Branch / Institute of Thermal Physics (ITP) Reversible LHP scheme Specification Total length, mm 2000 Evaporator diameter, mm 24 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 Year of development 2000 General view of ammonia RLHP evaporator condenser

19. Ural Branch / Institute of Thermal Physics (ITP) LHPs with flat evaporators Specification Total length, mm 865 Evaporator diameter, mm 30 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

20. Ural Branch / Institute of Thermal Physics (ITP) LHP with temperature active control regulating heater control unit controlled temperature T liquid line  2 mm evaporator  8 x 120 mm thermocouple condenser T, 0C Q = 6…10 W vapor line  2 mm 42,1 +0,1 0C set point 420C 42,0 -0,1 0C 41,9 radiator 41,8 0 10 20 -10 TCOOL, 0C

21. Ural Branch / Institute of Thermal Physics (ITP) Base design variants of ramified LHPs CC1 CC2 CC CC2 CC1 EV1 EV2 EV1 EV2 EV1 EV2 COND COND COND CC CC EV EV COND1 COND1 COND2 COND2

22. Ural Branch / Institute of Thermal Physics (ITP) Two evaporator-condenser LHP cooling jacket TCOOL1 TCOOL2 • • condenser liquid line evaporator 2 evaporator • vapor line evaporator 1 TV TL • compensation chamber TCh2 TCh1 • • Specification Total length, mm 1000 Evaporator diameter, mm 24 Vapor line diameter, mm 6/4 Liquid line diameter, mm 4/3 Max capacity, W 1400 Year of development 2002 condenser 2 condenser 1

23. Ural Branch / Institute of Thermal Physics (ITP) Test results of ramified LHP 30 25 20 15 10 5 0 Q1 = 400W, Q2 = 200W G1 = 0.1kg/s, G2 = 0.05kg/s  = 90o Tv Tlc1 Tlc2 Tl Tcool TEMPERATURE, 0C 0 200 400 600 800 1000 1200 1400 1600 TIME, s

24. Ural Branch / Institute of Thermal Physics (ITP) Miniature LHPs

25. Ural Branch / Institute of Thermal Physics (ITP) General view of MLHP Specification SS-ammonia Copper- water Body-working fluid 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, W130 70 Evaporator temperature 98 70 Own thermal resistance, 0C/W0.10 0.12 Total thermal resistance - (evaporator-ambient), 0C/W 0.59 0.68 Year of development 2003 2002 evaporator saddle vapor line liquid line condenser

26. Ural Branch / Institute of Thermal Physics (ITP) Tests results of miniature LHPs 120 35 Copper-water Condenser cooling by water, 200C 30 100 air, 200C 25 80 TEMPERATURE, 0C HEAT TRANSFER COEF. x 10 -3, W/m20C 20 60 15 Condenser cooling by 40 Copper-water air, 200C SS-ammonia 20 10 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 HEAT LOAD, W HEAT LOAD, W SS-ammonia Copper-water Condenser cooling by water, 200C 2.0 80 SS-ammonia air, 200C 1.6 60 1.2 HEAT TRANSFER COEF. x 10 -3, W/m20C THERMAL RESISTANCE, 0C/W 0.8 40 Condenser cooling by water, 200C 0.4 air, 200C 0.0 20 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 HEAT LOAD, W HEAT LOAD, W

27. Ural Branch / Institute of Thermal Physics (ITP) Comparison of operating characteristics HP (Fujikura) and LHP (ITP) LHP LHP LHP ITP working fluid - ammonia Leff - 230 mm Le - 20 mm Lc - 62 mm Te - 500C HP Fujikura working fluid - water Leff - 150 mm Le - 50 mm Lc - 250 mm T? - 500C Condenser water cooling 200C Condenser air cooling 200C

28. Ural Branch / Institute of Thermal Physics (ITP) 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 instruments arterial HP LHP OI LHP RSS LHP Rad

29. Ural Branch / Institute of Thermal Physics (ITP) Thermoregulation system with LHPs for the international program «MARS-96» TRS LHP penetrator TRS assembling

30. Ural Branch / Institute of Thermal Physics (ITP) Cooling of the copper bus of an electrolysis-bath electrode liquid line cooling water condenser evaporator vapor line saddle current-carruing wire bath electrolyte electrode

31. Ural Branch / Institute of Thermal Physics (ITP) Cooling of quantum-electronic converters Cooling of powerful transistors

32. Ural Branch / Institute of Thermal Physics (ITP) 25 W CPU coolers for a mobile computer fan evaporator liquid line 60 CPU 5.6 12 vapor line 96 condenser

33. Ural Branch / Institute of Thermal Physics (ITP) 45 W CPU Cooler for a Mobile PC

34. Ural Branch / Institute of Thermal Physics (ITP) Conclusion Loop Heat Pipes are very promising and universal heat-transfer devices, whose potential of development and application has not been used in full measure.