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

2. 2 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. 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. 4 The first LHP scheme evaporator main wick vapor removal channels secondary wick 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 Ural Branch / Institute of Thermal Physics (ITP)

5. 5 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. 6 Scheme of a traditional heat pipe Scheme of a LHP liquid vapor wick heat supply heat removal liquid wick vapor heat supply heat removal Ural Branch / Institute of Thermal Physics (ITP)

7. 7 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

8. 8 Ural Branch / Institute of Thermal Physics (ITP) saturator line liquid line vapor removal channels TEMPERATURE PRESSURE wick evaporator compensation chamber vapor line condenser 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 P1P1 P6P6 P8P8 PCPC T6T6 T7T7 T4T4 T1T1  P EX T3T3 T7T7 T6T6 T1T1 T2T2 T4T4 T5T5 Scheme and diagram of working cycle of an LHP

9. 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): P C =  P 1-8 =  P L +  P V +  P G 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 (T 1 - T 7 )  P 1 - P 7 3. Condition of liquid supercooling (thermodynamic condition): dP/dT (T 5 - T 4 )  P 5 - P 6 4. Condition of relationship between the internal volumes and the volume of a liquid: V CC  V VL + V C V L = V W + V LL + V CC + V CCH {

10. 10 Ural Branch / Institute of Thermal Physics (ITP) Correlation of volumes in an LHP 1. The volume of the compensation chamber V CC must be equal to or exceed the sum of the volumes of the vapor line V VL and the condenser V C V CC  V VL + V C 2. The volume of the liquid V L in an LHP must be equal to the sum of the volumes of the liquid in the wick V W, the liquid line V LL, the compensation chamber V CC and the central channel V CCH V L = V W + V LL + V CC + V CCH V CC VWVW V CCH V VL VCVC liquid level V LL before start upafter start up liquid level

11. 11 Ural Branch / Institute of Thermal Physics (ITP) Classification of LHPs LHP design LHP dimensionsEvaporator shapeEvaporator 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

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

13. 13 Tested LHPs material- working fluid combinations stainless steel BodyWickWorking fluid nickelwater, ammonia, acetone, pentane, freon-152A, freon 11, propylene stainless steeltitaniumwater, ammonia, acetone, pentane, freon-152A, toluene stainless steel ammonia nickeltitaniumammonia nickelammonianickel copperwatercopper Ural Branch / Institute of Thermal Physics (ITP)

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

15. 15 Ammonia two-meter LHP with two butt-end compensation chambers Ural Branch / Institute of Thermal Physics (ITP) HEAT LOAD, W EVAPORATOR TEMPERATURE, 0 C 0 200 400 600 800 1000 1200 1400 1600 10 20 30 40 50 60 vertical position, evaporator above condenser horizontal position evaporator horizontal, above vertical condenser vertical position, condenser above evaporator ambient temperature 19±1 0 C condenser cooling temperature 17±1 0 C CC1 CC2 Evaporator scheme CC1 CC2

16. 16 Effective length, mm 450 Evaporator diameter, mm 20 Vapor line diameter, mm 6/4 Liquid line diameter, mm 4/3 Heating zone area, cm 2 4.25 Max heat flux, W/cm 2 130 Max heat transfer coef., W/m 2 K 30 000 Year of development 1997 Ammonia High-Heat Flux LHP Specification Ural Branch / Institute of Thermal Physics (ITP)

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

18. 18 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, 0 C/W 0.02 Year of development 2000 Specification Reversible LHP scheme Ural Branch / Institute of Thermal Physics (ITP) General view of ammonia RLHP evaporator condenser

19. 19 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, 0 C/W 0.30/0.41 Year of development 1999 - 2001 LHPs with flat evaporators Ural Branch / Institute of Thermal Physics (ITP)

20. 20 LHP with temperature active control T, 0 C 42,1 42,0 41,9 41,8 -10 01020 T COOL, 0 C set point 42 0 C Q = 6…10 W +0,1 0 C - 0,1 0 C regulating heater control unit thermocouple vapor line  2 mm liquid line  2 mm evaporator  8 x 120 mm condenser radiator controlled temperature T Ural Branch / Institute of Thermal Physics (ITP)

21. 21 Base design variants of ramified LHPs Ural Branch / Institute of Thermal Physics (ITP) CC1 COND CC2 EV1EV2 CC1 CC2 EV2 EV1 COND CC COND1 COND2 CC COND2 COND1 EV EV2EV1

22. 22 Two evaporator-condenser LHP condenser 1 evaporator 1 Ural Branch / Institute of Thermal Physics (ITP) condenser liquid line vapor line evaporator compensation chamber cooling jacket TVTV T Ch1 T Ch2 T COOL1 TLTL T COOL2 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 evaporator 2 condenser 2

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

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

25. 25 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, 0 C/W 0.10 0.12 Total thermal resistance - (evaporator-ambient), 0 C/W 0.59 0.68 Year of development 2003 2002 Specification Body-working fluid Copper- water SS-ammonia General view of MLHP Ural Branch / Institute of Thermal Physics (ITP) evaporator condenser vapor line liquid line saddle

26. 26 Tests results of miniature LHPs HEAT LOAD, W TEMPERATURE, 0 C 0 20 40 60 80 100 120 140 40 20 120 60 80 100 SS-ammonia Condenser cooling by water, 20 0 C HEAT LOAD, W 0 20 40 60 80 100 120 140 0.0 0.4 0.8 1.2 1.6 2.0 THERMAL RESISTANCE, 0 C/W 0 20 40 60 80 100 120 140 25 20 10 HEAT LOAD, W HEAT TRANSFER COEF. x 10 - 3, W/m 2 0 C 0 20 40 60 80 100 120 140 20 40 60 80 HEAT LOAD, W Ural Branch / Institute of Thermal Physics (ITP) air, 20 0 C Copper-water SS-ammonia Copper-water Condenser cooling by water, 20 0 C air, 20 0 C HEAT TRANSFER COEF. x 10 - 3, W/m 2 0 C Condenser cooling by air, 20 0 C Condenser cooling by water, 20 0 C air, 20 0 C SS-ammonia Copper-water 15 30 35

27. 27 Comparison of operating characteristics HP (Fujikura) and LHP (ITP) Ural Branch / Institute of Thermal Physics (ITP) HP Fujikura working fluid - water Leff - 150 mm Le - 50 mm Lc - 250 mm T? - 50 0 C LHP ITP working fluid - ammonia Leff - 230 mm Le - 20 mm Lc - 62 mm Te - 50 0 C Condenser water cooling 20 0 C Condenser air cooling 20 0 C LHP

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

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

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

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

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

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

34. 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.