Loop heat pipe
Encyclopedia
A loop heat pipe 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 pipe
s but have the advantage of being able to provide reliable operation over long distance and the ability to operate against gravity. They can transport a large heat load over a long distance with a small temperature difference. 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.
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 modifications incorporate arterial tubes with considerably low hydraulic resistance for liquid return to the heat source (arterial heat pipes), while others provide spatial separation of the vapor and liquid phases of the working fluid at the transportation section (separated line heat pipes).
Though these new forms of heat pipes are able to transfer significant heat flows and can increase heat transport length, they remain very sensitive to spatial orientation relative to gravity. To extend functional possibilities of two-phase systems towards applications involving otherwise inoperable slopes in gravity, the advantages provided by the spatial separation of the transportation line and the usage of non-capillary arteries are combined in the loop scheme. This scheme allows heat pipes to be created with higher heat transfer characteristics while maintaining normal operation in any directional orientation. The loop scheme forms the basis of the physical concept of Two-Phase Loops (TPLs).
. The patent for LHPs was filed in the USA in 1982 (Patent № 4515209).
.
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.
Capillary action
Capillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paint-brush, in a thin tube, or in porous material such as paper or in some non-porous material such as liquified carbon fiber, or in a...
to remove heat from a source and passively move it to a condenser
Condenser (heat transfer)
In systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, typically by cooling it. In so doing, the latent heat is given up by the substance, and will transfer to the condenser coolant...
or radiator
Radiator
Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in automobiles, buildings, and electronics...
. LHPs are similar to heat pipe
Heat pipe
A heat pipe or heat pin is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces....
s but have the advantage of being able to provide reliable operation over long distance and the ability to operate against gravity. They can transport a large heat load over a long distance with a small temperature difference. Different designs of LHPs ranging from powerful, large size LHPs to miniature LHPs (micro loop heat pipe
Micro loop heat pipe
A micro-loop heat pipe or MLHP is a miniature loop heat pipe in which the radius of curvature of the liquid meniscus in the evaporator is in the same order of magnitude of the micro grooves' dimensions; or a miniature loop heat pipe which has been fabricated using microfabrication techniques....
) have been developed and successfully employed in a wide sphere of applications both ground based as well as space applications.
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 evaporatorEvaporator
An evaporator is a device used to turn the liquid form of some chemical into its gaseous form. For example, an evaporator is used in an air conditioning system to allow the compressed cooling chemical to evaporate from liquid to gas, absorbing heat in the process.-Uses:As stated above, an...
and condenser
Condenser (heat transfer)
In systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, typically by cooling it. In so doing, the latent heat is given up by the substance, and will transfer to the condenser coolant...
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 modifications incorporate arterial tubes with considerably low hydraulic resistance for liquid return to the heat source (arterial heat pipes), while others provide spatial separation of the vapor and liquid phases of the working fluid at the transportation section (separated line heat pipes).
Though these new forms of heat pipes are able to transfer significant heat flows and can increase heat transport length, they remain very sensitive to spatial orientation relative to gravity. To extend functional possibilities of two-phase systems towards applications involving otherwise inoperable slopes in gravity, the advantages provided by the spatial separation of the transportation line and the usage of non-capillary arteries are combined in the loop scheme. This scheme allows heat pipes to be created with higher heat transfer characteristics while maintaining normal operation in any directional orientation. The loop scheme forms the basis of the physical concept of Two-Phase Loops (TPLs).
Origins
Loop heat pipes were patented in USSR in 1974 by Yury F. Gerasimov and Yury F. Maydanik (Inventor's certificate № 449213), all of the former Soviet UnionSoviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
. The patent for LHPs was filed in the USA in 1982 (Patent № 4515209).
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 ICESatICESat
ICESat , part of NASA's Earth Observing System, was a satellite mission for measuring ice sheet mass balance, cloud and aerosol heights, as well as land topography and vegetation characteristics...
.
LHPs were first flight demonstrated on the NASA space shuttle in 1997 with STS-83
STS-83
STS-83 was a mission of the United States Space Shuttle Columbia.-Crew:-Mission parameters:*Mass:**Orbiter Landing with payload: **MSL-1 Spacelab Module: *Perigee: *Apogee: *Inclination: 28.5°...
and STS-94
STS-94
STS-94 was a mission of the United States Space Shuttle Columbia, launched on 1 July 1997.-Crew:-Mission parameters:*Mass:**Orbiter Landing with payload: **MSL-1 Spacelab Module: *Perigee: *Apogee:...
.
Loop heat pipes are important parts of systems for cooling electronic components.
External references
- Thermacore, Inc. Loop Heat Pipe Technology, retrieved 2010-05-25
- http://nmp.nasa.gov/st8/tech_papers/ices-1999-01-2007.pdf
- http://www.ohiolink.edu/etd/view.cgi?ucin1152120112