Heat Pipes: Theory, Design and Applications

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This helps prevent collapse of the flat top and bottom when the pressure is applied. There are two main applications for vapor chambers. First, they are used when high powers and heat fluxes are applied to a relatively small evaporator. After the vapor condenses on the condenser surfaces, capillary forces in the wick return the condensate to the evaporator.

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Note that most vapor chambers are insensitive to gravity, and will still 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.


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Second, compared to a one-dimensional tubular heat pipe, the width of a two-dimensional heat pipe allows an adequate cross section for heat flow even with a very thin device. It is possible to produce flat heat pipes as thin as 1. Standard heat pipes are constant conductance devices, where the heat pipe operating temperature is set by the source and sink temperatures, the thermal resistances from the source to the heat pipe, and the thermal resistances from the heat pipe to the sink.

In these heat pipes, 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 will be 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. VCHPs have two additions compared to a standard heat pipe: 1. A reservoir, and 2. When the heat pipe is not operating, the NCG and working fluid vapor are mixed throughout the heat pipe vapor space. Most of the NCG is located in the reservoir, while the remainder blocks a portion of the heat pipe 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 NCG into the reservoir, increasing the active condenser length and the heat pipe conductance. Conversely, when the power or heat sink temperature is decreased, the heat pipe vapor temperature and pressure decrease, and the NCG expands, reducing the active condenser length and heat pipe conductance.

PCHPs have shown milli-Kelvin temperature control. Conventional heat pipes transfer heat in either direction, from the hotter to the colder end of the heat pipe. Several different heat pipes act as a thermal diode , transferring heat in one direction, while acting as an insulator in the other: [16]. When the nominal condenser is heated, the vapor flow is from the nominal condenser to the nominal evaporator.

The NCG 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 W in the forward direction, and only 4. 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.

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.

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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. 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. 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. 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.

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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. As discussed below, the thermosyphons used to cool the Alaska pipe line were roughly 11 to 12 m long.

Even longer thermosyphons have been proposed for the extraction of geothermal energy. For example, Storch et al. A loop heat pipe LHP is a passive two-phase transfer device related to the heat pipe. 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 heat pipe. Micro loop heat pipes have been developed and successfully employed in a wide sphere of applications both on the ground and in space.

An oscillating heat pipe, also known as a pulsating heat pipe, 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. Heat pipes employ evaporative cooling to transfer thermal energy from one point to another by the evaporation and condensation of a working fluid or coolant.

Heat pipes rely on a temperature difference between the ends of the pipe, and cannot lower temperatures at either end below the ambient temperature hence they tend to equalise the temperature within the pipe. When one end of the heat pipe is heated, the working fluid inside the pipe at that end evaporates and increases the vapour pressure inside the cavity of the heat pipe. The latent heat of evaporation absorbed by the vaporisation of the working fluid reduces the temperature at the hot end of the pipe.

The vapour pressure over the hot liquid working fluid at the hot end of the pipe is higher than the equilibrium vapour pressure over the condensing working fluid at the cooler end of the pipe, and this pressure difference drives a rapid mass transfer to the condensing end where the excess vapour condenses, releases its latent heat, and warms the cool end of the pipe. Non-condensing gases caused by contamination for instance in the vapour impede the gas flow and reduce the effectiveness of the heat pipe, particularly at low temperatures, where vapour pressures are low.

The speed of molecules in a gas is approximately the speed of sound, and in the absence of noncondensing gases i. In practice, the speed of the vapour through the heat pipe is limited by the rate of condensation at the cold end and far lower than the molecular speed. However, if the surface is close to the temperature of the gas, the evaporation caused by the finite temperature of the surface largely cancels this heat flux.

If the temperature difference is more than some tens of degrees, the evaporation from the surface is typically negligible, as can be assessed from the vapour pressure curves. In most cases, with very efficient heat transport through the gas, it is very challenging to maintain such significant temperature differences between the gas and the condensing surface.

Moreover, this temperature differences of course corresponds to a large effective thermal resistance by itself. The bottleneck is often less severe at the heat source, as the gas densities are higher there, corresponding to higher maximum heat fluxes. The condensed working fluid then flows back to the hot end of the pipe.

In the case of vertically oriented heat pipes the fluid may be moved by the force of gravity. In the case of heat pipes containing wicks, the fluid is returned by capillary action. When making heat pipes, there is no need to create a vacuum in the pipe. One simply boils the working fluid in the heat pipe until the resulting vapour has purged the non-condensing gases from the pipe, and then seals the end.

An interesting property of heat pipes is the temperature range over which they are effective. However, the boiling point of water depends on the absolute pressure inside the pipe.

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In an evacuated pipe, water vaporizes from its triple point 0. The main reason for the effectiveness of heat pipes is the evaporation and condensation of the working fluid. The heat of vaporization greatly exceeds the specific heat capacity. Almost all of that energy is rapidly transferred to the "cold" end when the fluid condenses there, making a very effective heat transfer system with no moving parts. The general principle of heat pipes using gravity, commonly classified as two phase thermosiphons , dates back to the steam age and Angier March Perkins and his son Loftus Perkins and the "Perkins Tube", which saw widespread use in locomotive boilers and working ovens.

Gaugler of General Motors in , who patented the idea, [29] [30] but did not develop it further.

George Grover independently developed capillary-based heat pipes at Los Alamos National Laboratory in , with his patent of that year [31] being the first to use the term "heat pipe", and he is often referred to as "the inventor of the heat pipe". Such a closed system, requiring no external pumps, may be of particular interest in space reactors in moving heat from the reactor core to a radiating system. In the absence of gravity, the forces must only be such as to overcome the capillary and the drag of the returning vapor through its channels.

Grover's suggestion was taken up by NASA , which played a large role in heat pipe development in the s, particularly regarding applications and reliability in space flight. This was understandable given the low weight, high heat flux, and zero power draw of heat pipes — and that they would not be adversely affected by operating in a zero gravity environment. The first application of heat pipes in the space program was the thermal equilibration of satellite transponders. This causes severe discrepancies in the temperature and thus reliability and accuracy of the transponders.

The heat pipe cooling system designed for this purpose managed the high heat fluxes and demonstrated flawless operation with and without the influence of gravity. The cooling system developed was the first use of variable conductance heat pipes to actively regulate heat flow or evaporator temperature. NASA has tested heat pipes designed for extreme conditions, with some using liquid sodium metal as the working fluid. Other forms of heat pipes are currently used to cool communication satellites. These papers were also the first to mention flexible, arterial, and flat plate heat pipes.

Publications in introduced the concept of the rotational heat pipe with its applications to turbine blade cooling and contained the first discussions of heat pipe applications to cryogenic processes. Starting in the s Sony began incorporating heat pipes into the cooling schemes for some of its commercial electronic products in place of both forced convection and passive finned heat sinks. Initially they were used in receivers and amplifiers, soon spreading to other high heat flux electronics applications.

During the late s increasingly high heat flux microcomputer CPUs spurred a threefold increase in the number of U. As heat pipes evolved from a specialized industrial heat transfer component to a consumer commodity most development and production moved from the U. Modern CPU heat pipes are typically made of copper and use water as the working fluid. The spacecraft thermal control system has the function to keep all components on the spacecraft within their acceptable temperature range.

This is complicated by the following:. Some spacecraft are designed to last for 20 years, so heat transport without electrical power or moving parts is desirable. Rejecting the heat by thermal radiation means that large radiator panes multiple square meters are required. Grooved wicks are used in spacecraft heat pipes, as shown in the first photograph in this section. The heat pipes are formed by extruding aluminum, and typically have an integral flange to increase the heat transfer area, which lowers the temperature drop.

Ammonia is the most common working fluid for spacecraft heat pipes.

Heat Pipes

Ethane is used when the heat pipe must operate at temperatures below the ammonia freezing temperature. The heat pipe is an aluminum extrusion, similar to that shown in the first figure. This book is a useful reference as well as an accessible introduction for those approaching the topic for the first time. The standard work on heat pipes; contains all information required to design and manufacture a heat pipe Suitable for use as a professional reference and graduate textRevised with greater coverage of key electronic cooling.

Descriptions Heat pipes are used in a wide range of applications, including electronics cooling, die-casting and injection molding, heat recovery and energy conservation, de-icing and manufacturing process temperature control, and in domestic appliances.


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Heat pipes are used in a wide range of applications, including electronics cooling, die-casting and injection molding, heat recovery and ene See More.