Heat pipe
A heat pipe is a heat exchanger that uses the evaporation heat of a medium to allow a high heat flow density . In this way, large amounts of heat can be transported over a small cross-sectional area.
A distinction is made between two types of heat pipe: the heat pipe and the two-phase thermosiphon . The basic functional principle is the same for both designs; the difference lies in the return of the gaseous working medium to the evaporator, i.e. H. to the point where heat is applied. The return transport takes place passively with both designs and therefore without aids such as a circulation pump .
The thermal resistance of a heat pipe at working temperature is significantly lower than that of metals. The behavior of the heat pipes therefore comes very close to the isothermal change in state. The temperature is almost constant over the length of the heat pipe. With the same transmission capacity, significantly lighter designs than with conventional heat exchangers are therefore possible under the same operating conditions. By carefully selecting the working medium for the heat pipe, operating temperatures from a few Kelvin to approx. 3000 Kelvin can be achieved.
In a heat pipe, the ability to transport energy depends largely on the specific evaporation enthalpy (in kJ / mol or kJ / kg) of the working medium and not on the thermal conductivity of the vessel wall or working medium. For reasons of efficiency, a heat pipe is usually operated just above the hot end and just below the boiling point of the working medium at the cold end .
Function and distinction
Structure and principle of operation
Heat pipes are usually elongated metal vessels that contain a hermetically sealed volume. It is filled with a working medium (e.g. water or ammonia), which fills the volume to a small extent in a liquid state and to a large extent in a gaseous state.
The part of the vessel that serves to absorb energy is called the evaporator, those that serve to release energy are called the condenser . The vaporizer can be at one end or in the middle.
- Heat input increases the temperature of the vessel and working medium until the boiling point of the working medium is reached; from then on the working medium begins to evaporate ; the temperature no longer rises; Instead, all the energy supplied is converted into heat of vaporization .
- As a result, the pressure in the heat pipe is increased locally above the liquid level, which leads to a low pressure gradient within the heat pipe. The resulting steam begins to distribute itself in the entire available volume, i. H. it flows wherever the pressure is lower; It condenses at the points where its temperature falls below the boiling point of the working medium . To do this, the steam must give off energy to the vessel and the vessel to the environment. This happens most strongly at the point where the condenser is located, in which active cooling can take place.
- The temperature no longer drops until the entire latent heat contained, the heat of condensation , has been released into the environment.
- The liquid part of the working medium returns to the evaporator by gravity ( thermosiphon ) or by capillary forces ( heat pipe ). For the latter to work, the proportion of working medium in the liquid state must be lower there.
Steam flows to the cooling zone, a film of condensation flows / flows / creeps back. The driving force is the force of adhesion (but read under Capillarity ).
Since the vapor and liquid of the working medium are in the same room, the system is located in the wet steam area . As a result, there is exactly a certain temperature in the heat pipe at a certain pressure. Since the pressure differences in heat pipes are very small, usually a few Pascal , the temperature difference between the evaporator and the condenser is also small and is a maximum of a few Kelvin . A heat pipe therefore has a very low thermal resistance . The area between the evaporator and condenser is practically isothermal .
Since the heat transport takes place indirectly via the material-bound transport of latent heat (evaporation / condensation heat), the area of application of a heat pipe is limited to the range between the melting temperature and the temperature of the critical point of the working fluid. All forces that act on the working medium also influence the actual heat transport performance. Gravity can supplement or partially cancel the capillary forces in heat pipes. The centrifugal force also acts in rotating hollow shafts constructed as heat pipes .
Distinction
With gravity-driven heat pipes ( two-phase thermosiphon or gravitational heat pipes ), the medium circles due to gravity . As a result, the heat transfer medium automatically flows back into the evaporator. The heat is often only supplied via the sump , i.e. up to the height of the liquid level. This depends on the film formation by the returning (liquid) medium. If thermosyphons are aligned at a flat incline, they can dry out if the condensed medium does not flow back quickly enough.
Heat pipes use the wick principle to guide the condensed fluid back to the evaporator. The process is therefore independent of the position; Heat pipes also work under weightlessness . Compared to thermosiphons, they hardly tend to dry out because the flow of liquid through the capillary is significantly improved, which leads to a higher transferable heat flow. The capillary structure also ensures that, unlike the thermosiphon, the heat can be supplied anywhere and at any height. Heat pipes are used wherever high heat flux densities are required in any orientation.
Working media
The working capacity of the working medium in a heat pipe depends on both thermodynamic properties such as the enthalpy of vaporization and fluidic properties such as kinematic viscosity and surface tension . The surface tension and the enthalpy of vaporization should be as large as possible and the viscosity as low as possible. This means that the optimal heat transfer medium for the operating point can be determined.
The Merit number ( Merit number , Me ) as a specific performance quantity can be calculated as:
So its unit is watts per square meter; But Me does not correspond to a real heat flux density .
The merit number should be as large as possible in the specified working range (temperature, heat flux density) of the heat pipe so that the heat transfer of the heat pipe is as large as possible. It should be noted that these properties depend on the temperature. Normally, Me is determined for several possible heat carriers and it then serves as a decision-making aid for choosing the right medium.
For very low temperatures, media are used that are gaseous under room conditions. Gases such as helium and nitrogen can be used to cover the temperature range close to absolute zero (0 K) down to around −20 ° C. Typical refrigerants such as ammonia or mixtures are also used. From 0 ° C water can be used as a heat transfer medium. Depending on the possible compressive strength (vapor pressure) of the heat pipe, water is sufficient up to a temperature range of 340 ° C. (Cf. critical point of water at 374 ° C.) From 400 ° C temperature, one speaks of high-temperature heat pipes. Alkali metals such as sodium and lithium are the best heat carriers here according to the merit number. The upper limit of the range is primarily limited by the strength of the material used for the heat pipe.
materials
Different materials are used depending on the external conditions. The behavior of the heat transfer medium towards the material also plays a role here. For example, sodium dissolves components out of steels, which would lead to failure of a heat pipe over a long period of time.
Heat pipe
In the lower temperature ranges, copper is mostly used because it is easy to shape and has a high thermal conductivity . In the case of high-temperature heat pipes, heat-resistant steels such as 1.4841 or nickel-based alloys are mainly used. The wick shape is largely dependent on the operating point. A wick with low flow resistance is used wherever the heat pipe is operated at the capillary force limit. Groove-shaped capillary structures are typical for this. In the case of high-temperature heat pipes, close-meshed wire mesh is usually used because of the high density of the heat transfer medium. Even simpler types are used in copper-water heat pipes, similar to copper conductors in electrical cables, mainly because of their inexpensive production.
Thermosiphon
For applications in construction, thermosiphons are mostly made of conventional structural steels.
History and Development
A first heat pipe was patented in 1944. However, at this point in time there was still no meaningful application. It was not until the 1960s that space travel was decisively developed that this idea was taken up again. Even today, heat pipes are used to cool the sun-facing side of satellites. The first high-temperature heat pipe was presented in 1964. Since then, the physical descriptions such as the properties of certain heat carriers, capillary structures and the analytical description of heat pipes have been significantly expanded. Research is still being carried out on heat pipes today because they represent an inexpensive and highly effective means of heat transfer.
application
Due to the flexible design and variability of properties, heat pipes are used in many areas today. In the past few years, they have been increasingly noticed by the public through their use in PCs and notebooks . The overall height of notebooks could be significantly reduced through the use of heat pipes, since the actual waste heat convectors on the heat pipes could be attached directly to the outer surfaces. The increased heat transfer made it possible to integrate more powerful graphics processors.
Much earlier, in the 1960s, heat pipes were used in space technology. Especially with satellites , the use of heat pipes minimizes the temperature gradient between the side facing and facing away from the sun.
Computer technology
Conventional heat sinks for cooling microprocessors are based purely on forced convection on cooling fins . In order to achieve the best possible heat transfer here, the ventilator or ventilator must sit as close as possible to the ribs because of the limiting thermal conductivity of the ribs. The heated air flows in the direction of the mainboard and increases the surface temperatures of the adjacent components. In addition, there is usually more space further away from the mainboard, which means that a cheaper shape can be used, which means that a larger surface can be achieved without increasing the mass of the heat sink. Furthermore, the heat dissipation in the housing is very non-directional. On the other hand, coolers that use heat pipes are not dependent on local proximity, because they allow a decoupling of heat absorption and output due to their function. You can therefore give off waste heat in a targeted manner into the air flow of the housing fans. Compared to the water cooling, which is often used as an alternative, the heat pipes do not require a circulation pump, which leads to additional noise generation.
Construction
Heat pipes have been used to stabilize the permafrost under the Trans-Alaska Pipeline since the 1970s . In conventional structures, two steel piles are lowered into the ground, which take up the load of the pipeline . In the area of permafrost, however, this is not easily possible because the 40–80 ° C warm oil thaws the soil locally through heat conduction via the piles. This would cause the piles to sink in and deform the pipeline. If the air temperature is sufficiently low, which usually occurs in the area of permafrost, it is possible to avoid this problem by using heat pipes. The heat is not conducted into the ground, but is released into the ambient air through cooling fins attached to the heat pipes . In addition, heat is extracted from the permafrost ground by thermosiphons, which means that it remains frozen and thus stable. This technology is also used on the Lhasa Railway to stabilize the embankment on permafrost.
The independent circulation of the working medium in heat pipes and thus the elimination of auxiliary energy leads to increased use in the field of geothermal energy use. In conventional geothermal probes, through a sunk in the ground line loop z. B. water is pumped and the geothermal energy obtained is transferred to a heat pump . With the carbon dioxide probes, there is no need for a double line or the pump energy for circulation.
Nowadays, they are also used successfully in areas where heat pipe technology is not directly suspected, such as vacuum tube collectors . They can also be found in heat recovery systems or simple heat exchangers.
High temperature heat pipes are used in allothermal biomass gasification . Here they transfer heat in the range of 850 ° C with almost no loss. Thanks to a sophisticated concept, the heat pipes make it possible to convert solid biomass such as wood chips directly into energy-rich product gas.
Motor vehicles
Despite constant further development, mechanical efficiencies of 37% are rarely exceeded for a modern Otto engine . Other energies are dissipated as heat losses via the cooling water and the exhaust gas heat. To cool the thermally highly stressed outlet valves, some of them are made hollow and some are filled with sodium. The sodium melts during operation, with the movement of the valve dissipating the heat from the particularly critical valve disc to the valve stem. Only then could powerful, air-cooled large aircraft engines be made operationally safe.
Controllable heat pipes
Since heat pipes, in addition to their low weight and small volume, have a thermal conductivity that is up to 1000 times as high as e.g. B. a copper rod , heat can easily be transported in a targeted manner in vehicles. The only drawback would be the problem of controllability, i.e. the ability to vary the thermal conductivity as desired, to switch it on or off. Two principles are suitable for the controllability of heat pipes:
External heat control of heat pipes
Two heat pipes, one each originating from the heat source and one from the heat sink , run parallel to one another at their ends at a small distance without touching one another. They are surrounded in this area by a body (coupler) made of a material that conducts heat well (e.g. copper or aluminum ) with two holes that guide the heat pipes as precisely as possible. The thermal conductivity of the entire system can be easily adjusted by pushing in or pulling out the coupler, since the contact area between the heat pipes and the coupler depends linearly on the depth of insertion. The coupling with external control by a small motor can be relocated to an easily accessible place, provided the (very low) additional thermal resistance of the longer heat pipes allows this detour.
Internal heat control of heat pipes
The internal heat transport in the heat pipe itself can also be controlled, using a valve or a throttle within the heat pipe as a control element. A throttle mounted rotatably and controlled from the outside by a small motor can vary both the outward and return flow of the heat-transporting medium from the heat source to the heat sink. Alternatively, a small solenoid valve located inside the heat pipe , implemented by a magnetic ball with a return spring, allows the heat flow through the heat pipe to be largely stopped or released again.
Compared to the valve, the throttle has the great advantage that the thermal conductivity can be controlled continuously, depending on the setting angle. The valve, on the other hand, only allows the heat pipe to be switched on or off, because it can only be opened or closed due to the control via an electromagnet. On the other hand, the disadvantage of the throttle valve is its control shaft, which has to be guided to the outside and which makes it difficult to achieve hermetic sealing . In motor vehicles in particular, this can lead to undesirably short maintenance intervals.
Use of controllable heat pipes in vehicles
In the car, excess heat can be transported to almost any point using heat pipe technology. The main source of heat is the exhaust system of the internal combustion engine. Enormous heat output is available immediately after the engine has started. (The exhaust gas temperature is several hundred ° C.) The heat can also be tapped from heated surfaces in the interior or exterior , from the power electronics or in the cooling or air conditioning circuit. It can then be used for interior air conditioning, seat heating, cooling water and engine oil heating or for batteries to reach operating temperature more quickly. The controllability of the transported heat is of great importance everywhere here, which is already clear in the comfort area.
Space travel
Heat pipes are often exposed to strong temperature fluctuations, which immediately results in fluctuations in the volume of the material. If the heat pipe is now on a material with a significantly different coefficient of thermal expansion (CTE for short), mechanical stresses occur which can damage the heat pipe or its external heat transfer surfaces. This fact is particularly problematic because of the enormous temperature fluctuations in space technology . The temperature difference between the side of a satellite facing the sun and the side facing away from the sun can be 130 Kelvin in places . Carbon fiber reinforced plastic (CFRP) has asserted itself as the basic material here for many years .
However, heat pipes are primarily not made of CFRP, but z. B. made of aluminum . The advantages of this element are, among other things, its low weight, its good suitability for the production of capillary structures, its optimal thermal conductivity and its chemical resistance to the most commonly used thermally conductive media. The coefficients of thermal expansion of the two materials differ, however, very strongly: that of CFRP with 1 · 10 −6 K −1 up to 3 · 10 −6 K −1 is only about 1/24 to 1/8 that of aluminum (24 · 10 - 6 K −1 ).
Composite materials made of aluminum and fiber composite material provide a possible remedy . The aluminum heat pipe is combined in various ways with fiber composite material, which has a very low or even negative CTE. In practice, it is either embedded in cavities or groove-like depressions, wrapped around the aluminum block as a kind of cage, or the aluminum is penetrated by this, i.e. directly integrated.
This technology achieves thermal expansion coefficients of the overall system of around 5 · 10 −6 K −1 (the CTE of the composite material counteracts that of aluminum), which makes the heat pipe technology also suitable for space travel.
Physical design
The equations for calculating the transferable power of a heat pipe usually contain coefficients that are to be selected on the basis of experimentally obtained data. The specific heat pipe properties such as the type of capillary structure, the type of heat transfer medium, the available vapor space, the operating temperature, etc. are decisive. With sufficiently well chosen equations and coefficients, the error between model and experiment can be kept in a narrow space. The initial steps in the design of a heat pipe are therefore the choice of the design and the establishment of a corresponding numerical heat pipe model to simulate the transferable power.
The model created is calibrated by means of an experimental check or the real limits are determined. If the tested heat pipe does not achieve the required performance, changes (e.g. changing the capillary structure) are carried out with the aim of increasing the performance. With a purely experimental procedure, a number of experiments that cannot be predicted is necessary.
For heat pipes of small and medium power (<1 kW), the essential equations are linear or can be linearized around a development point . Therefore one uses numerical optimization methods (e.g.) to limit the design effort. Such procedures reduce the number of experiments on calibration tests.
Particular attention is paid to the design on the operating limits. These physical boundary conditions are obtained from the parameters of the heat transfer medium. Precise knowledge of the heat transfer medium used is therefore essential. Operation is possible if the operating point (temperature, heat flow) is within these limits.
The following limits are usually taken into account:
- Viscosity limit
- It limits the heat flux density at working temperatures just above the melting point. The flow is severely impaired by the viscosity forces in the steam.
- Sound velocity limit
- The heat flow density can only be increased until the steam flow created by the pressure difference reaches the speed of sound .
- Interaction limit
- At high heat flux densities, liquid is entrained by the vapor, and partial drying of the capillary leads to a break in the liquid flow.
- Capillary force limit
- The capillary force limit is reached when the flow losses of the liquid heat transfer medium are greater than the existing capillary pressure.
- Boiling limit
- The liquid flow is restricted or comes to a standstill as a result of nucleate boiling in the capillary.
Optimization of heat pipes
Reduction of the temperature resistance
In addition to the optimization of material structures, etc. the efficiency of a heat pipe can also be significantly increased by modifying the liquids that act as heat-transferring media. Researchers at Tamkang University in Danshui (Taiwan) developed an aqueous solution that contains a certain amount of tiny nanoparticles and compared its properties in terms of temperature transfer behavior with those of conventional heat pipe liquids.
It became clear that using this solution as a heat-transporting medium in a heat pipe results in an improvement, i.e. a minimization, of the temperature resistance of 10% to 80%. The efficiency of this liquid is not only dependent on the type and internal structure of the heat pipe, but also on the concentration of the solution and the size of the nanoparticles. Various tests have shown that the smaller the diameter of the nanoparticles and the lower their concentration in the aqueous solution, the greater the temperature resistance of the heat pipe.
35 nm small silver particles serve as nanoparticles . The amount of particles in the solution varies between 1 mg and 100 mg per liter.
Non-wettable porous structure
A major advance in heat pipe technology was achieved in the 1990s through the safe decoupling of the condensate and steam flow using a so-called non-wettable porous structure , which led to a significant increase in the internal transmission capacity. The problem until then was that the returning condensate slowed down the opposing steam flow through collisions and thus negatively influenced the temperature transfer.
This non-wettable porous structure, used as a steam channel for the heat pipe, has the property of a lower surface tension than the heat-transporting medium itself (as condensate ). Thus, the porous structure can only be penetrated by the heat-transporting medium in the gaseous state, and any condensate remains outside.
The temperature transport is similar as mentioned above, through the heat circuit by steam and condensate channel instead. The non-wettable porous structure between the steam channel and the condensate channel forms the boundary between the evaporation area and the condensation area of the heat pipe.
The condensate evaporates through external heat supply and now moves through the above as a gas. Structure inside the heat pipe, the steam channel, through which it then reaches the condensation area. There, on a heat sink , the partition, again designed as a non-wettable porous structure, forms the transition point to the condensation area. Due to the pressure or concentration gradient, the gas diffuses to the outside and comes into contact with the outer walls of the heat pipe. At this point, heat energy is given off and the steam condenses. Here, due to its high surface tension, the condensate can only flow back through the condensate channel (capillary effect), at the end of which, at the location of the heat source , the temperature cycle begins again through external heat input.
Use of nanostructures
The development of an American research team from 2008 represents a further increase:
By using nanotechnology in the production of the capillary structure in heat pipes, the capillary effect on the corresponding working medium is again significantly increased. In the adjacent diagram you can clearly see that as the pore diameter of the capillary structure decreases, the achievable rise of the working fluid increases significantly. The medium water delivers the greatest success here .
In addition to the resulting greater acceleration effect on the working medium, this technology has a burden that the liquid transport within the heat pipe through very small structures, as one would like to realize in practice, in order to achieve the greatest possible success, again slowed down or even completely prevented because the pores have become too small to penetrate. Another disruptive factor are the undesirable inhomogeneities in the material (production-related), as well as the very high production costs.
Overheating protection for heat pipes
A certain strength of the outer jacket of a heat pipe is usually useful, not only to protect it from mechanical damage, but also to be able to withstand pressure differences between atmospheric pressure and internal pressure caused by the heat transfer medium.
However, problems can also arise if the heat pipe is exposed to excessive temperatures, i.e. if the heat energy supplied is greater than that which can be discharged to the outside in the condensation area (heat sink). This creates an inadmissibly high internal pressure, which can damage the outer jacket or even destroy the heat pipe. One possible remedy is a technology patented in 2005 that is supposed to prevent overheating through flexible outer material.
It is built on the inside from a known structure made up of two areas with different pore diameters (steam and condensate channel). In the middle area, the gaseous medium should be guided through a porous material structure with a large pore diameter and in the outer area the condensate (via the capillary effect) should be guided through a porous structure with a small pore diameter. The real difference to the usual heat pipe lies in the outer jacket itself. This does not consist of a rigid material, as usual, but of two elastic and also very thin foils, which are connected at their ends and lie on the outer capillary structure. Internal and external pressure compensate each other in normal operation in such a way that the foils lie parallel to one another at a predetermined distance and the outer porous structure is in direct contact with the heat source and heat sink via the foils .
If an unexpectedly high pressure builds up, be it due to the fact that more heat energy is supplied than removed, forces act on the outer skin of the heat pipe, which, due to its elastic properties, pushes it outwards. The resulting chamber fills with gaseous heat transfer medium. Mechanical damage to the heat pipe is avoided in this way. In addition, this phenomenon creates a thermal resistance between the capillary structure carrying the condensate and the outer skin, because the condensate and heat source are no longer in direct contact with one another, but are separated from one another by the gas. The intensity of the heating energy acting on the condensate, i.e. the absorbed thermal energy, is therefore reduced, which does not apply to the energy given off, since this is stored in the gas that is still in contact with the outer skin.
Another plus point for this technology with a foil cover are the smaller external dimensions of the heat pipe - as a result of the fact that the massive cover is dispensed with. In practice, it must be ensured that the heat pipe has to be better protected against mechanical influences than with other designs.
Work area expansion
The working point of a heat pipe is generally the temperature at which the heat-transporting medium condenses or evaporates . The potential areas of application of a heat pipe result from this specific property, which is the reason why a wide variety of heat-transporting media are used as working media in practice. Mixtures of different elements are often not dispensed with, through which the boiling point can be varied to any temperature.
Often, however, it makes more sense, either because some elements enter into undesired chemical reactions with the heat pipe material used, or not least for cost reasons, to adapt a desired element in such a way that it functions as a heat-transporting medium in many different temperature ranges. For this purpose, it is advisable to be able to adjust the boiling point as desired, which is achieved in practice with the aid of a buffer gas .
This so-called extension of the working range is based on the physical property of the pressure dependence of the boiling points of the elements. Now a further step is involved in the manufacturing process before the heat pipe is hermetically sealed :
After filling with the heat-transporting medium and evacuation of excess gases, a defined internal pressure is set by additionally filling the heat pipe with a gas, the so-called buffer gas. This forms a buffer zone within the heat pipe into which the working medium cannot penetrate. An important criterion for the selection of this buffer gas must be that it must not under any circumstances enter into chemical reactions with the heat pipe or with the heat-transporting medium in the later work area. For example, when using the working medium mercury, an inert buffer gas such as argon or helium could be used.
With this method, a desired internal pressure can be set, which varies the boiling point of the working medium and thus the working range of the heat pipe as desired.
In addition to being able to set various operating points using this process, it is also advantageous that any impurities that may arise in the heat pipe are flushed into the buffer zone and do not affect further operation, as this is outside the condensate and steam duct. The disadvantage, however, is the additional space required by the buffer zone. As a result, the heat pipe cannot be used for heat transport over its full length.
Manufacturing
Once the boundary conditions of the heat pipe have been worked out, these must also be taken into account during manufacture. The essential feature is the boiling temperature or the vapor pressure of the medium, since the heat pipe only begins to work when this temperature is reached. The boiling temperature can be adjusted thermodynamically via the vapor pressure . In most cases, the lowest possible boiling temperature is aimed for. In the case of water, for example, this would be the temperature of the triple point . If you take a look at the associated steam table , it becomes clear that in the case of water, an extremely low pressure is necessary in order to reduce the boiling temperature to room temperature, for example .
One of the most frequently used methods is mechanical evacuation of the heat pipe. A corresponding pump is connected and when a certain pressure (vacuum) is reached, the heat pipe is usually closed purely mechanically.
This process is complex and expensive. That is why another option is used by filling the heat pipe with the heat transfer medium itself, instead of evacuation using a vacuum pump . For this purpose, a filling pipe and a cooling pipe are attached to the heat pipe. The desired heat-transporting medium is introduced into the heat pipe through the filling pipe. After this process, the heat pipe is heated at the other end, so that the usual heat cycle is started. Now the medium filled in, which is initially present as condensate , begins to evaporate. The pressure that builds up as a result causes the media in the heat pipe to expand and, because of the cooling pipe, all undesirable gases, i.e. those that are not condensable, escape through the filling pipe.
The purpose of the cooling pipe becomes clear at this point: The heat-transporting medium, which in the meantime strives as a gas in the direction of the filling pipe , condenses through the cooling and moves back to the heat source as condensate due to the capillary effect of the outer pore structure of the heat pipe . Remaining gases, i.e. all those that are not condensable, do not enter the capillary structure, but are flushed outwards by the internal pressure.
The filling pipe is hermetically sealed when all non-condensable gases have been expelled and the stationary boundary between air and heat transfer medium is located directly on the filling pipe.
Above all, when using it, it should be noted that heat pipes are closed volumes. With this change of state ( isochoric ), heat input goes directly into the pressure. If the permissible temperature is exceeded, it can lead to a steam explosion . This is particularly important during further processing, since heat pipes are often soldered to the actual heat sink because of their better thermal conductivity. A lot of heat pipes are filled with harmful substances, so heat pipes should be disposed of properly and not opened. Opening also usually leads to loss of functionality.
See also
Web links
- Heat pipe basics (PDF file; 3 MB)
- Definition of the merit number and its use, English
swell
- Cooling arrangement for a rotating anode (centrifugal force)
- Allothermal fluidized bed gasification - options for realizing the heat input in fluidized beds (gravity, heat pipe; PDF file; 1001 kB)
- Adrian Bejan, Allan D. Kraus: Heat Transfer Handbook . Wiley & Sons, Hoboken NJ 2003, ISBN 0-471-39015-1 .
- Stephan Kabelac (Red.): VDI Heat Atlas . Published by the Association of German Engineers, VDI Society for Process Engineering and Chemical Engineering (GVC). 10th, revised and expanded edition. Springer-Verlag, Berlin et al. 2006, ISBN 3-540-25503-6 .
Individual evidence
- ↑ M. Groll: Heat pipes as components in energy technology. In: W. Fratzscher, K. Stephan (Ed.): Use of waste energy: technical, economic and social aspects. Akad.-Verlag, Berlin 1995, ISBN 3-05-501706-4 , p. 84. (edoc.bbaw.de ; PDF; 3.1 MB).
- ↑ a b W. M. Rohsenow: Handbook of Heat Transfer. Mcgraw-Hill Publ. Comp., 1998, ISBN 0-07-053555-8 .
- ↑ a b A. Faghri: Heat Pipe Science and Technology. Taylor and Francis, 1995, ISBN 1-56032-383-3 .
- ↑ a b c d e P. Dunn: Heat pipes. Pergamon Press, 1994.
- ↑ RS Gaugler: Heat Transfer Device. U.S. Patent 2,350,348
- ^ L. Trefethen: On the Surface Tension Pumping of Liquids or a Possible Role of the Candlewick in Space Exploration. GE Tech. Info., Ser. No. 615 D114, Feb. 1962.
- ^ GM Grover, TP Cotter, GF Erikson: Structures of Very High Thermal Conductivity. In: J. Appl. Phys. 35, 1964, p. 1990.
- ^ Christopher E. Heuer: The Application of Heat Pipes on the Trans-Alaska Pipeline. 6/1979.
- ^ The Trans-Alaska Pipeline Passive Cooling System .
- ↑ M. Schneider: Modeling and optimization of heat pipe plates for cooling of electronic circuits. IKE, 2007.
- ↑ a b c d e f Association of German Engineers VDI-Wärmeatlas Springer-Verlag, 2006.
- ↑ Experimental investigation of silver nano-fluid on heat pipe thermal performance .
- ↑ Experimental investigation of micro / nano heat pipe wick structures ( Memento of May 30, 2012 in the Internet Archive ) (PDF; 1.2 MB).
- ↑ a b c Patent: Flat heat pipe with overheating protection
- ↑ Construction and testing of a mercury heat pipe furnace for four-wave mixing inside the resonator ( page no longer available , search in web archives )