Editing Heat pipe (section)

== Types ==

In addition to standard, constant conductance heat pipes (CCHPs), other types include:<ref>{{cite web|url=http://www.1-act.com/resources/heat-pipe-fundamentals/different-types-of-heat-pipes/|title=Heat Pipes - Different Kinds of Heat Pipes|website=www.1-act.com}}</ref>

* Vapor chambers (planar heat pipes), which are used for heat flux transformation, and surface isothermalization
* Variable conductance heat pipes (VCHPs), which use a non-condensable gas (NCG) to change the heat pipe effective thermal conductivity as power or the heat sink conditions change
* Pressure controlled heat pipes (PCHPs), a type of VCHP where the volume of the reservoir, or the NCG mass can be changed, to increase precision
* Diode heat pipes, which have a high thermal conductivity in the forward direction, and a low thermal conductivity in the reverse direction
* Thermosyphons, which return the liquid to the evaporator by gravitational/accelerational forces,
* Rotating heat pipes, which return the liquid to the evaporator by centrifugal forces

=== Vapor chamber ===

Thin planar pipes ([[heat spreader]]s or flat pipes) have the same primary components as tubular pipes.<ref>{{cite web|url=https://www.youtube.com/watch?v=PGsB1AtlpD4|title=Vapor Chamber Animation|last=Advanced Cooling Technologies Inc.|date=29 November 2013|via=YouTube}}</ref> They add an internal support structure or a series of posts to the vapor chamber to accommodate clamping pressures up to {{cvt|90|psi}}. This helps prevent collapse of the flat top and bottom when the pressure is applied.

The two main applications for vapor chambers are when high powers and heat fluxes are applied to a relatively small evaporator.<ref>{{cite web|url=http://www.1-act.com/vapor-chambers/|title=Vapor Chambers|publisher=Advanced Cooling Technologies}}</ref> Heat input to the evaporator vaporizes liquid, which flows in two dimensions to the condenser surfaces. After the vapor condenses, capillary forces in the wick return the condensate to the evaporator. Most vapor chambers are insensitive to gravity, and operate when inverted, with the evaporator above the condenser. In this application, the vapor chamber acts as a heat flux transformer, cooling a high heat flux from an electronic chip or laser diode, and transforming it to a lower heat flux that can be removed by natural or forced convection. With special evaporator wicks, vapor chambers can remove 2000 W over 4&nbsp;cm<sup>2</sup>, or 700 W over 1&nbsp;cm<sup>2</sup>.<ref>{{cite web|url=http://www.1-act.com/high-heat-flux-high-power-low-resistance-low-cte-two-phase-thermal-ground-planes-for-direct-die-attach-applications/|title=High Heat Flux, High Power, Low Resistance, Low CTE Two-Phase Thermal Ground Planes for Direct Die Attach Applications|website=Advanced Cooling Technologies}}</ref>

Another major use of vapor chambers is for cooling laptops. As vapor chambers are flatter and more two-dimensional, gaming laptops benefit more compared to traditional pipes. For example, the vapor chamber cooling in [[Lenovo|Lenovo's Legion 7i]] was a selling point (although only a few units were so equipped),<ref>{{Cite web|date=2020-08-28|title=Legion 7i falsely advertised: not all models have vapor chambers|url=https://spearblade.com/news/legion-7i-falsely-advertised/|access-date=2020-10-20|website=Spearblade|language=en-US}}</ref>

Compared to a one-dimensional tubular pipe, the width of a two-dimensional pipe allows thin devices to offer an adequate cross section for heat flow. Such pipes appear in "height sensitive" applications, such as notebook computers and surface mount circuit board cores. It is possible to produce flat pipes as thin as 1.0&nbsp;mm (only slightly thicker than a [[ISO/IEC 7813|credit card]]).<ref>{{cite web|url=https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1186&context=coolingpubs/|title=Modeling and Design Optimization of Ultra-Thin Vapor Chambers for High Heat Flux Applications |first=R. |last=Ranjan |display-authors=etal |publisher=Purdue University Cooling Technologies Research Center |id=Paper 186 |year=2012}}</ref>

=== Variable conductance ===

Standard heat pipes are constant conductance devices, where the heat operating temperature is set by the source and sink temperatures, and the thermal resistance from the source to the sink. The temperature drops linearly as the power or condenser temperature is reduced. For some applications, such as satellite or research balloon thermal control, the electronics is overcooled at low powers, or at the low sink temperatures. Variable conductance heat pipes (VCHPs) are used to passively maintain the temperature of the electronics being cooled as power and sink conditions change.<ref>{{cite web|url=http://www.1-act.com/vchps-for-passively-controlling-temperature/|title=VCHPs for Passively Controlling Temperature|publisher=Advanced Cooling Technologies}}</ref>

Variable conductance heat pipes add two elements:

* a reservoir
* a non-condensable gas (NCG)

The non-condensable gas is typically [[argon]], except that helium is used for thermosyphons. When the heat pipe is not operating, the non-condensable gas and working fluid vapor are mixed. When the pipe is operating, the non-condensable gas is swept toward the condenser by the flow of the working fluid vapor. Most of the non-condensable gas is located in the reservoir, while the remainder blocks a portion of the condenser. The VCHP works by varying the active length of the condenser. When the power or heat sink temperature is increased, the heat pipe vapor temperature and pressure increase. The increased vapor pressure forces more of the non-condensable gas into the reservoir, increasing the active condenser length and the conductance. Conversely, when the power or heat sink temperature is decreased, the heat pipe vapor temperature and pressure decrease, and the non-condensable gas expands, reducing the active condenser length and conductance.

The addition of a small heater on the reservoir, with the power controlled by the evaporator temperature, allows thermal control of roughly ±1-2&nbsp;°C. In one example, the evaporator temperature was maintained in a ±1.65&nbsp;°C control band, as power varied from 72 to 150 W, and heat sink temperature varied from +15&nbsp;°C to &minus;65&nbsp;°C.

VCHPs can be used when tighter temperature control is required.<ref>{{cite web|url=http://www.1-act.com/pchps-for-precise-temperature-control/|title=PCHPs for Precise Temperature Control|publisher=Advanced Cooling Technologies}}</ref> The evaporator temperature is used to either vary the reservoir volume, or the amount of non-condensable gas. VCHPs have demonstrated milli-Kelvin temperature control.<ref>{{cite web|url=http://www.1-act.com/pressure-controlled-heat-pipe-applications/|title=Pressure Controlled Heat Pipe Applications|publisher=Advanced Cooling Technologies}}</ref>

=== Diode ===

Conventional heat pipes transfer heat from the hotter to the colder end. Several designs act as a [[thermal diode]], transferring heat in one direction, while acting as an insulator in the other:<ref>{{cite web|url=http://www.1-act.com/diode-heat-pipes/|title=Diode Heat Pipes|publisher=Advanced Cooling Technologies|access-date=2013-12-03|archive-date=2016-04-20|archive-url=https://web.archive.org/web/20160420112745/http://www.1-act.com/diode-heat-pipes/|url-status=dead}}</ref>

* [[thermosyphon#Heat pipes|Thermosyphon]]s transfer heat only from the bottom to the top, where the condensate returns by gravity. When the thermosyphon is heated at the top, no liquid is available to evaporate.
* Rotating heat pipes, allow liquid to travel only by centrifugal forces from the evaporator to the condenser. No liquid is available when the condenser is heated.
* Vapor trap diode heat pipes.
* Liquid trap diode heat pipes.

{{anchor |Vapor Trap Diode|Vapor trap diode heat pipe}} A vapor trap diode is fabricated in a similar fashion to a variable conductance heat pipe, with a gas reservoir at the end of the condenser. During fabrication, the heat pipe is charged with the working fluid and a controlled amount of a non-condensable gas (NCG). During normal operation, the flow of the working fluid vapor from the evaporator to the condenser sweeps the non-condensable gas into the reservoir, where it does not interfere with the normal heat pipe operation. When the nominal condenser is heated, the vapor flow is from the nominal condenser to the nominal evaporator. The non-condensable gas is dragged along with the flowing vapor, completely blocking the nominal evaporator, and greatly increasing the thermal resistivity of the heat pipe. In general, there is some heat transfer to the nominal adiabatic section. Heat is then conducted through the heat pipe walls to the evaporator. In one example, a vapor trap diode carried 95&nbsp;W in the forward direction, and only 4.3&nbsp;W in the reverse direction.<ref name="1-act.com">{{cite web|url=http://www.1-act.com/variable-conductance-heat-pipes-for-variable-thermal-links/|title=Variable Conductance Heat Pipes for Variable Thermal Links|publisher=Advanced Cooling Technologies}}</ref>

{{anchor |Liquid Trap Diode|Liquid Trap Diode Heat Pipe|Liquid trap diode|Liquid trap diode heat pipe}}
A liquid trap diode has a wicked reservoir at the evaporator end of the heat pipe, with a separate wick that is not in communication with the wick in the remainder of the heat pipe.<ref>{{cite web|url=https://www.youtube.com/watch?v=ZEEbbIR9VfU|title=Liquid Trap Diode Heat Pipes Animation|author=Advanced Cooling Technologies|date=7 November 2013|website=YouTube}}</ref> During normal operation, the evaporator and reservoir are heated. The vapor flows to the condenser, and liquid returns to the evaporator by capillary forces in the wick. The reservoir eventually dries out, since there is no method for returning liquid. When the nominal condenser is heated, liquid condenses in the evaporator and the reservoir. While the liquid can return to the nominal condenser from the nominal evaporator, the liquid in the reservoir is trapped, since the reservoir wick is not connected. Eventually, all of the liquid is trapped in the reservoir, and the heat pipe ceases operation.

=== Thermosyphons ===

Most heat pipes use a wick to return the liquid from the condenser to the evaporator, allowing the heat pipe to operate in any orientation. The liquid is sucked up back to the evaporator by [[capillary action]], similar to the way that a sponge sucks up water when an edge is placed in contact with a pool of water. However the maximum adverse elevation (evaporator over condenser) is relatively small, on the order of 25&nbsp;cm long for a typical water heat pipe.

If, however, the evaporator is located below the condenser, the liquid can drain back by gravity instead of requiring a wick, and the distance between the two can be much longer. Such a gravity-aided heat pipe is known as a [[thermosyphon#Heat pipes|thermosyphon]].<ref>{{cite web|url=http://www.1-act.com/thermosyphons/|title=Thermosyphon Heat Exchanger, Cooling Systems & Reboilers by ACT|publisher=Advanced Cooling Technologies}}</ref>

In a thermosyphon, liquid working fluid is vaporized by a heat supplied to the evaporator at the bottom of the heat pipe. The vapor travels to the condenser at the top of the heat pipe, where it condenses. The liquid then drains back to the bottom of the heat pipe by gravity, and the cycle repeats. Thermosyphons are diode heat pipes; when heat is applied to the condenser end, there is no condensate available, and hence no way to form vapor and transfer heat to the evaporator.

Thermosyphon designs include<ref>{{cite web |date=21 October 2013 |title=Thermosyphon technology for Artificial Ground Freezing (AGF) |url=http://simmakers.com/thermosyphon-technology-ground-freezing/ |website=simmakers.com}}</ref> thermoprobe, [[thermopile]], depth thermosyphon, sloped-thermosyphon foundation, flat loop thermosyphon foundation, and hybrid flat loop thermosyphon foundation.

While a typical terrestrial water heat pipe is less than 30&nbsp;cm long, thermosyphons are often several meters long. The thermosyphons used to cool the Alaska pipe line were roughly 11 to 12&nbsp;m long. Even longer thermosyphons have been proposed for the extraction of geothermal energy. For example, Storch et al. fabricated a 53&nbsp;mm I.D., 92&nbsp;m long propane thermosyphon that carried roughly 6&nbsp;kW of heat.<ref>{{cite conference |first=T. |last=Storch |display-authors=etal |title=Wetting and Film Behavior of Propane Inside Geothermal Heat Pipes |conference=16th International Heat Pipe Conference |location=Lyon, France |date=May 20–24, 2012}}</ref> Their scalability to large sizes also makes them relevant for solar thermal <ref name="g975">{{cite journal | last1=Khanna | first1=Mohan Lal | last2=Singh | first2=Narinder Mohan | title=Industrial solar drying | journal=Solar Energy | publisher=Elsevier BV | volume=11 | issue=2 | year=1967 | issn=0038-092X | doi=10.1016/0038-092x(67)90046-1 | pages=87–89| bibcode=1967SoEn...11...87K }}</ref> and HVAC applications.<ref name="s337">{{cite journal | last=Yellott | first=J. I. | title=Passive solar heating and cooling systems | journal=ASHRAE J.; (Canada) | volume=20 | date=1978-01-01 | issue=1 | url=https://www.osti.gov/etdeweb/biblio/5132103 | access-date=2024-06-22 | page=}}</ref>

=== Loop ===

A [[loop heat pipe]] (LHP) is a passive two-phase transfer device. It can carry higher power over longer distances by having co-current liquid and vapor flow, in contrast to the [[counter-current flow]] in a conventional heat pipe.<ref name=NTRS>{{cite conference |last1=Ku |first1=Jentung |last2=Ottenstein |first2=Laura |last3=Douglas |first3=Donya |last4=Hoang |first4=Triem |title=Multi-Evaporator Miniature Loop Heat Pipe for Small Spacecraft Thermal Control – Part 2: Validation Results |conference=Multi-Evaporator Miniature Loop Heat Pipe for Small Spacecraft Thermal Control |location=Orlando, Florida |date=January 4–7, 2010 |publisher=American Institute of Aeronautics and Astronomics |hdl=2060/20110015223 |via=NASA Technical Reports Server}}</ref><ref>{{cite conference |author1=Ku, Jentung |author2=Paiva, Kleber |author3=Mantelli, Marcia |title=Loop Heat Pipe Transient Behavior Using Heat Source Temperature for Set Point Control with Thermoelectric Converter on Reservoir|conference=9th Annual International Energy Conversion Engineering Conference |date=31 July 2011 |hdl=2060/20110015224 |via=NASA Technical Reports Server}}</ref> This allows the wick in a loop heat pipe to be required only in the evaporator and compensation chamber. [[Micro loop heat pipe]]s have been employed in ground and space applications.

=== Oscillating or pulsating ===

An oscillating heat pipe (OHP), also known as a pulsating heat pipe (PHP), is only partially filled with liquid working fluid. The pipe is arranged in a serpentine pattern in which freely moving liquid and vapor segments alternate.<ref>{{Cite web | url=https://www.electronics-cooling.com/2003/05/an-introduction-to-pulsating-heat-pipes/ | title=An Introduction to Pulsating Heat Pipes|website=Electronics Cooling| date=May 2003}}</ref> Oscillation takes place in the working fluid; the pipe remains motionless. These have been investigated for many applications, including cooling photovoltaic panels,<ref name="r018">{{cite journal | last1=Alhuyi Nazari | first1=Mohammad | last2=Ahmadi | first2=Mohammad H. | last3=Ghasempour | first3=Roghayeh | last4=Shafii | first4=Mohammad Behshad | last5=Mahian | first5=Omid | last6=Kalogirou | first6=Soteris | last7=Wongwises | first7=Somchai | title=A review on pulsating heat pipes: From solar to cryogenic applications | journal=Applied Energy | volume=222 | date=2018 | doi=10.1016/j.apenergy.2018.04.020 | pages=475–484| bibcode=2018ApEn..222..475A }}</ref> cooling electronic devices,<ref name="PCPpipeElec">{{cite journal | last1=Behi | first1=Hamidreza | last2=Ghanbarpour | first2=Morteza | last3=Behi | first3=Mohammadreza | title=Investigation of PCM-assisted heat pipe for electronic cooling | journal=Applied Thermal Engineering | publisher=Elsevier BV | volume=127 | year=2017 | issn=1359-4311 | doi=10.1016/j.applthermaleng.2017.08.109 | pages=1132–1142| bibcode=2017AppTE.127.1132B }}</ref> heat recovery systems, fuel cell systems,<ref name="g453">{{cite journal | last1=Oro | first1=Marcos Vinício | last2=Bazzo | first2=Edson | title=Flat heat pipes for potential application in fuel cell cooling | journal=Applied Thermal Engineering | volume=90 | date=2015 | doi=10.1016/j.applthermaleng.2015.07.055 | pages=848–857| bibcode=2015AppTE..90..848O }}</ref><ref name="a705">{{cite book | last=Vasiliev | first=L. | title=Mini-Micro Fuel Cells | chapter=Heat Pipes in Fuel Cell Technology | series=NATO Science for Peace and Security Series C: Environmental Security | publisher=Springer Netherlands | publication-place=Dordrecht | date=2008 | isbn=978-1-4020-8293-1 | doi=10.1007/978-1-4020-8295-5_8 | pages=117–124}}</ref> HVAC systems,<ref name="r822">{{cite journal | last1=Nethaji | first1=N. | last2=Mohideen | first2=S. Tharves | title=Energy conservation studies on a split airconditioner using loop heat pipes | journal=Energy and Buildings | volume=155 | date=2017 | doi=10.1016/j.enbuild.2017.09.024 | pages=215–224| bibcode=2017EneBu.155..215N }}</ref> and desalination.<ref name="PCPPHPdesal">{{cite journal | last1=Khalilmoghadam | first1=Pooria | last2=Kiyaee | first2=Soroush | last3=Rajabi-Ghahnavieh | first3=Abbas | last4=Warsinger | first4=David M. | last5=Behshad Shafii | first5=Mohammad | title=An improved passive solar still integrated with pulsating heat pipes and phase change materials | journal=Solar Energy | volume=275 | date=2024 | doi=10.1016/j.solener.2024.112612 | page=112612| bibcode=2024SoEn..27512612K }}</ref> PHPs can be combined with [[phase change material]]s.<ref name=PCPpipeElec /><ref name=PCPPHPdesal />