Loop heat pipe

A loop heat pipe (LHP) is a two-phase heat transfer device that uses capillary action to remove heat from a source and passively move it to a condenser or radiator. LHPs are similar to heat pipes but have the advantage of being able provide reliable operation over long distance and the ability to operate against gravity. Different designs of LHPs ranging from powerful, large size LHPs to miniature LHPs (micro loop heat pipe) have been developed and successfully employed in a wide sphere of applications both ground based as well as space applications.

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Construction

The most common coolants used in LHPs are anhydrous ammonia and propylene^[1].

Mechanism

Limitations of heat pipes

Heat pipes are excellent heat transfer devices but their sphere of application is mainly confined to transferring relatively small heat loads over relatively short distances when the evaporator and condenser are at same horizontal level. This limitation on the part of the heat pipes is mainly related to the major pressure losses associated with the liquid flow through the porous structure, present along the entire length of the heat pipe and viscous interaction between the vapor and liquid phases, also called entrainment losses. For the applications involving transfer of large heat loads over long distances, the thermal performance of the heat pipes is badly affected by increase in these losses. For the same reason conventional heat pipes are very sensitive to the change in orientation in gravitational field. For the unfavorable slopes in evaporator-above-condenser configuration, the pressure losses due to the mass forces in gravity field adds to the total pressure losses and further affect the efficiency of the heat transfer process.

As a result of these limitations, different solutions involving structural modifications to the conventional heat pipe have been proposed. Some of these modified versions of heat pipe incorporated arterial tube with considerably low hydraulic resistance for the return of the liquid to the heat supply zone e.g. arterial heat pipes while others provided spatial separation of the vapor and liquid phases of a working fluid at the transportation section e.g. separated lines heat pipes.

Though these heat pipes were able to increase heat transport length and transferred significant heat flows but still they remain very sensitive to orientation in the gravity field. To extend functional possibilities of two-phase systems towards applications involving otherwise inoperable slopes in gravity field, the advantages provided by the spatial separation of the transportation line and usage of non-capillary artery were combined in the loop scheme. As a result, the loop scheme makes it possible to develop heat pipes with higher heat transfer characteristics while maintaining normal operation at any orientation in the mass force field. The loop principle forms the basis of the physical concept of the Two-Phase Loops (TPLs).

Origins

Loop heat pipes were patented in USSR in 1979 by Valery M. Kiseev, Jury F. Maidanik, Jury F. Gerasimov, all of the former Soviet Union. The patent for LHPs was filed in the USA in 1982 (Patent 4,467,861).

Applications

The first space application occurred aboard a Russian spacecraft in 1989. LHPs are now commonly used in space aboard satellites including; Russian Granat, Obzor spacecraft, Boeing’s (Hughes) HS 702 communication satellites, Chinese FY-1C meteorological satellite, NASA’s ICESat ^[2].

LHPs were first flight demonstrated on the NASA space shuttle in 1997 with STS-83 and STS-94.

Loop heat pipes are important parts of systems for cooling electronic components.

See also

• Heat Pipe
• Thermosiphon

External references

• http://www.google.com/search?q=cache:8rdQ3AN7n4YJ:nmp.nasa.gov/st8/tech_papers/ices-1999-01-2007.pdf+Loop+Heat+Pipes&hl=en&ct=clnk&cd=17&gl=us&client=firefox-a

References