heat pipe temperature range

In solar thermal water heating applications, an individual absorber tube of an evacuated tube collector is up to 40% more efficient compared to more traditional "flat plate" solar water collectors.

The addition of a small heater on the reservoir, with the power controlled by the evaporator temperature, will allow thermal control of roughly 1-2C. It was originally conceived that the porous element boiler could be developed to provide a new concept of boiling water reactor design, see Figure9.

The choice of working fluid must be such that the heat pipe is operated at a temperature well beyond the viscous limit, even at start up.

Mercury has extremely attractive properties inherent in a liquid metal such as its high thermal conductivity.

It is further possible to control the temperature of operation of the pipe by introducing a controlled pressure of inert gas, such as helium or argon.

Rice, G., Dunn, P. D., (1992) 'Porous Element Boiling and Superheating'. [34] As satellites orbit, one side is exposed to the direct radiation of the sun while the opposite side is completely dark and exposed to the deep cold of outer space. This page has been accessed 34,932 times. Sodium, lithium, cesium, silver and a sodium-potassium compound (NaK) are often used in the high temperature range (750 K and above).

Grooved wicks are used in spacecraft heat pipes, as shown in the first photograph in this section. When the nominal condenser is heated, liquid condenses in the evaporator and the reservoir. In the case of a copper casing, that is around 1/80 of the original flux. Several different heat pipes act as a thermal diode, transferring heat in one direction, while acting as an insulator in the other:[20], 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. [5] Some heat pipes have demonstrated a heat flux of more than 23kW/cm2, about four times the heat flux through the surface of the sun. Grover, G. M., Cotter, T. P., and Erickson, G. R, (1964) Structures of very high thermal conductance.

Busse, C. A. This allows spacecraft heat pipes to be several meters long, in contrast to the roughly 25cm maximum length for a water heat pipe operating on Earth.

Academic Publishers, ISBN 7-80003-272 1/T 9. [18] In a pressure controlled heat pipe, the evaporator temperature is used to either vary the reservoir volume, or the amount of non-condensable gas in the heat pipe. Longevity of a heat pipe can be assured by selecting a container, a wick and welding materials that are compatible with one another and with the working fluid of interest.

| and s is the conductivity of the solid, l the conductivity of the liquid and the porosity of the wick structure. These limits were catogorized by Busse and are as follows.

The condensed working fluid then flows back to the hot end of the pipe.

The vast majority of heat pipes for room temperature applications use ammonia (213373K), alcohol (methanol (283403K) or ethanol (273403K)), or water (298573K) as the working fluid. With special evaporator wicks, vapor chambers can remove 2000 W over 4cm2, or 700 W over 1cm2.[14]. The low temperature range is from 200 to 550 K. Most heat pipe applications fall within this range. Faghri [2] provided a detailed procedure for the fabrication, processing, and testing of low, moderate, and high temperatures. 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. Typically, a vacuum pump is used to remove the air from the empty heat pipe. In these applications, distilled water is commonly used as the heat transfer fluid inside a sealed length of copper tubing that is located within an evacuated glass tube and oriented towards the sun. The choice of working fluid very much depends on the thermophysical properties of the fluid as well as the mode of operation of the device. Table 1 [1][2] lists some of the commonly used and proposed working fluids, their melting and boiling points at atmospheric pressure, and their useful ranges. These papers were also the first to mention flexible, arterial, and flat plate heat pipes. Figure1. Rejecting the heat by thermal radiation means that large radiator panes (multiple square meters) are required. [44] The first nuclear reactor to produce electricity using heat pipes was first operated on September 13, 2012, in a demonstration using flattop fission.[45]. "Heat Pipes", Fifth Edition, D. A. Reay, P.A.

f1 can be found from ESDU 81038, but is seen to have a value of 4 at B0 = 1.0 and a value of 8 at B0 = 10.0.

[url=. The working-fluid inventory of a heat pipe is the sum of the masses of the vapor and liquid phases, assuming the wick is full of liquid. At the hot interface of a heat pipe, a volatile liquid in contact with a thermally conductive solid surface turns into a vapor by absorbing heat from that surface. Finally, rotating heat pipes use centrifugal forces to return liquid from the condenser to the evaporator. [4][5] Water heat pipes are sometimes filled by partially filling with water, heating until the water boils and displaces the air, and then sealed while hot. Copper/water heat pipes have a copper envelope, use water as the working fluid and typically operate in the temperature range of 20 to 150C. where p is the maximum capillary pressure provided by the wick (see above), rn is the nucleate radius (= 2 106 m) and Z the thermal impedance of the wick. Porous element heating and vaporization. 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.

All Rights Reserved. [37] They are common in many consumer electronics like desktops, laptops, tablets, and high-end smartphones. where Av is the area of the vapor passageway. Non-condensing gases (caused by contamination for instance) in the vapor impede the gas flow and reduce the effectiveness of the heat pipe, particularly at low temperatures, where vapor pressures are low.

When the nominal condenser is heated, the vapor flow is from the nominal condenser to the nominal evaporator. The envelope was stainless steel, with an inner copper layer for compatibility. NASA has tested heat pipes designed for extreme conditions, with some using liquid sodium metal as the working fluid. To prevent this, each vertical support member has been mounted with four vertical heat pipe thermosyphons.[40]. Ammonia is the most common working fluid for spacecraft heat pipes. Nucleation sites, at which bubbles first form, are provided by scratches or rough surfaces and by the release of absorbed gas. George Grover independently developed capillary-based heat pipes at Los Alamos National Laboratory in 1963, 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".

Therefore, real world efficiencies of both designs are about the same. Pressure controlled heat pipes have shown milli-Kelvin temperature control. Such a gravity aided heat pipe is known as a thermosyphon.[23]. [citation needed], Since heat pipes were rediscovered by George Grover in 1963, extensive life tests have been conducted to determine compatible envelope/fluid pairs, some going on for decades. Heat pipe fabrication, processing, and testing involve several detailed procedures which are recommended to be strictly followed in order to achieve the highest quality possible. The cooling system developed was the first use of variable conductance heat pipes to actively regulate heat flow or evaporator temperature. The bottleneck is often less severe at the heat source, as the gas densities are higher there, corresponding to higher maximum heat fluxes. In a heat pipe life test, heat pipes are operated for long periods of time, and monitored for problems such as non-condensable gas generation, material transport, and corrosion. [19], Conventional heat pipes transfer heat in either direction, from the hotter to the colder end of the heat pipe. The saturated liquid vaporizes and travels to the condenser, where it is cooled and turned back to a saturated liquid. Working fluids are chosen according to the temperatures at which the heat pipe must operate, with examples ranging from liquid helium for extremely low temperature applications (24K) to mercury (523923K), sodium (8731473K) and even indium (20003000K) for extremely high temperatures. Filed 1963. The valve is removed after filling and sealing the heat pipe. Widely varying external conditions, such as, Limited electrical power available, favoring passive solutions, Long lifetimes, with no possibility of maintenance, Improving the efficiency of geothermal heating to prevent slippery roads during winter in cold climate zones, Increased efficiency of photovoltaic cells by coupling the solar panel to a heat pipe system. In heating, ventilation and air-conditioning (HVAC) systems, heat pipes are positioned within the supply and exhaust air streams of an air-handling system or in the exhaust gases of an industrial process, in order to recover the heat energy.

This eventually leads to "dry out", which in the case of constant heat flux heating can cause "Burn Out" of the evaporator containment.

Figure9. The vapor pressure over the hot liquid working fluid at the hot end of the pipe is higher than the equilibrium vapor 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 vapor condenses, releases its latent heat, and warms the cool end of the pipe. The heat pipe is partially filled with a working fluid and then sealed. [27] Oscillation takes place in the working fluid; the pipe remains motionless. Additionally, the tested set up seizes the recovered thermal heat to warm, for instance, water, Hybrid control rod heat pipes to shut down a nuclear reactor in case of an emergency and simultaneously transferring decay heat away to prevent the reactor from running hot, This page was last edited on 13 July 2022, at 21:10.

(Ref. The limit for each regime is presented below for a simple cylindrical geometry heat pipe, as illustrated in Figure3.

Wick structures used in heat pipes include sintered metal powder, screen, and grooved wicks, which have a series of grooves parallel to the pipe axis. Pressure controlled heat pipes (PCHPs) can be used when tighter temperature control is required. The increased vapor pressure forces more of the non-condensable gas into the reservoir, increasing the active condenser length and the heat pipe conductance. The heat pipe is a development of the thermosyphon, in which there is no wick structure and liquid is returned to the evaporator by gravity.

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.

Equations relating to the various limits of performance of a two-phase closed thermosyphon are given in ESDU data sheet 81038. The vapor travels to the condenser at the top of the heat pipe, where it condenses. The speed of molecules in a gas is approximately the speed of sound, and in the absence of noncondensing gases (i.e., if there is only a gas phase present) this is the upper limit to the velocity with which they could travel in the heat pipe. 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.

Grover, G. M., Cotter, T. P., and Erickson, G. R, (1964) Structures of very high thermal conductance, J Appl. Variable conductance heat pipe.

The other end extends into the oven where it draws heat to the middle of the roast. As vapor chambers are a flatter and more two-dimensional method of heat dissipation, sleeker gaming laptops benefit hugely from them as compared to traditional heat pipes. The above limitations are seen to relate to temperature, according to working fluid, in the manner illustrated by Figure3. For laminar flow conditions in the wick structure: where is the rate of heat transfer, l the liquid viscosity, Aw the cross sectional area within the wick, K the permeability of the wick, and l the liquid density. Grover and his colleagues were working on cooling systems for nuclear power cells for space craft, where extreme thermal conditions are encountered. Heat pipes began to be used in computer systems in the late 1990s,[38] when increased power requirements and subsequent increases in heat emission resulted in greater demands on cooling systems. The significant feature of a thermosyphon is that it is passive and does not require any external power to operate. The vapor pressure drop between the extreme end of the evaporator and the end of the condenser, represents a restriction in operation. Most manufacturers cannot make a traditional heat pipe smaller than 3mm in diameter due to material limitations. Figure8.

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. [1], The advantage of heat pipes over many other heat-dissipation mechanisms is their great efficiency in transferring heat. This can be counterintuitive, in the sense that if a heat pipe system is aided by a fan, then the heat pipe operation may break down, resulting in a reduced effectiveness of the thermal management systempotentially severely reduced. There are two main applications for vapor chambers. Depending on application there are several thermosyphon designs:[41] thermoprobe, thermopile, depth thermosyphon, sloped-thermosyphon foundation, flat loop thermosyphon foundation, and hybrid flat loop thermosyphon foundation. A loop heat pipe (LHP) is a passive two-phase transfer device related to the heat pipe.

The factor f3 is a function of the inclination of the heat pipe. However, problems with wetting the wick and wall present difficulties in using mercury in capillary heat pipes. If the temperature difference is more than some tens of degrees, the vaporization from the surface is typically negligible, as can be assessed from the vapor pressure curves. Rice, G., Dunn, P. D., Oswald, R. D., Harris, N. S., Power, B. D., Dennis, H. T. M., and Pollock, J. F. (1977) An industrial vapor vacuum pump employing a porous element boiler.

An experimental "in-pile" steam generator was designed, as illustrated in Figure8, in the hope that the concept may be demonstrated under nuclear heating conditions. For example, Storch et al. A heat pipe is a heat-transfer device that employs phase transition to transfer heat between two solid interfaces.[1]. About Us

The thermosyphon differs from the heat pipe, in having no wick structure. Ignition of the fuel mixture always takes place in the same part of Wankel engines, inducing thermal dilatation disparities that reduce power output, impair fuel economy, and accelerate wear. Heat pipes are used in some cases to avoid the risk of destabilization.

Thus a heat pipe can operate at hot-end temperatures as low as just slightly warmer than the melting point of the working fluid, although the maximum rate of heat transfer is low at temperatures below 25C (77F). Schematic of in-pile porous element steam generator. The onset of Boiling within the wick structure interferes with liquid circulation.

The vapor flows to the condenser, and liquid returns to the evaporator by capillary forces in the wick. [citation needed] Note/explanation: The condensation rate is very close to the sticking coefficient times the molecular speed times the gas density, if the condensing surface is very cold.

Faghri [2] presents thermophysical property data for most heat pipe working fluids and container materials along with polynomial temperature-property relations for the working fluids. Most of the non-condensable gas is located in the reservoir, while the remainder blocks a portion of the heat pipe condenser.

The heat pipe has three major operating zones, namely evaporator, adiabatic section and condenser, see Figure1. The heat pipes are formed by extruding aluminium, and typically have an integral flange to increase the heat transfer area, which lowers the temperature drop. where rv is the radius of the vapor passageway. Each heat pipe application has a particular temperature range in which the heat pipe needs to operate. Porous element pressurized reactor. Fulford, D., (1989) Variable Conductance Heat Pipes, PhD Thesis, University of Reading, U.K. Heat PipesGeneral Information in their Use, Operation and Design, Heat PipesPerformance of Capillary-driven Design, ESDU data sheet>, Heat-PipesProperties of Common Small-pore Wicks, ESDU data sheet, Heat PipesPerformance of Two-phase Closed Thermosyphons, ESDU data sheet >. As heat pipes evolved from a specialized industrial heat transfer component to a consumer commodity most development and production moved from the U.S. to Asia. Thermophysical properties of heat pipe working fluids: operating range between -60C and 300C, ESDU data sheet 80017, Aug. 1980.

Stanford Ollendorf. The condenser is shown above the adiabatic section. In an evacuated pipe, water vaporizes from its triple point (0.01C, 32F) to its critical point (374C; 705F), as long as the heat pipe contains both liquid and vapor. However, the boiling point of water depends on the absolute pressure inside the pipe. A compilation of the most up-to-date information concerning the compatibility of metals with working fluids for heat pipes is given in Table 2 [2]. The factor f1 is a function of a dimensionless parameter Kp, which is defined as. Figures of merit () for different working fluids in capillary driven heat pipes.

The maximum heat transfer under this condition is given by, where f1 is a function of the Bond Number, defined as. 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. Initially, it might be suspected that a water-charged heat pipe only works when the hot end reaches the boiling point (100C, 212F, at normal atmospheric pressure) and steam is transferred to the cold end. The porous element would consist of packed enriched UO2 coated particles contained in a porous ceramic dispenser tube. 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.

[16], 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.

Figure 1 presents various working fluid boiling points and classifies them into four categories: cryogenic, low, intermediate and high temperature ranges. The variable conductance heat pipe works by varying the active length of the condenser.

Cryogenic heat pipes operate between 4 to 200 K. Typical working fluids include helium, argon, oxygen, and krypton. [2], A typical heat pipe consists of a sealed pipe or tube made of a material that is compatible with the working fluid such as copper for water heat pipes, or aluminium for ammonia heat pipes. 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.

[29] Capillary-based heat pipes were first suggested by R. S. Gaugler of General Motors in 1942, who patented the idea,[30] but did not develop it further. Heat and Mass Transfer, 16, 169-186. This is because below the intended temperature range the working fluid will not undergo phase change, while above it, all of the working fluid in the heat pipe vaporizes and the condensation process ceases.

[32] He noted in his notebook:[33].

For example, water in an aluminium envelope will develop large amounts of non-condensable gas over a few hours or days, preventing normal operation of the heat pipe. The heat of vaporization greatly exceeds the specific heat capacity. Privacy Policy [43], The principle has also been applied to camping stoves. An interesting property of heat pipes is the temperature range over which they are effective.

When the power or heat sink temperature is increased, the heat pipe vapor temperature and pressure increase. | Cotter, T. P. (1965) Theory of Heat Pipes, LA 3246-MS, 26 March 1965. Modern CPU heat pipes are typically made of copper and use water as the working fluid. The vapor pressure of the liquid charge will be equal to that of the gas, provided operation is ensured to be of the nature illustrated by Figure2. The spacecraft thermal control system has the function to keep all components on the spacecraft within their acceptable temperature range. Because of the characteristics of the device, better efficiencies are obtained when the unit is positioned upright with the supply-air side mounted over the exhaust air side, which allows the liquid refrigerant to flow quickly back to the evaporator aided by the force of gravity. The use of a buffer gas to control vapor pressure and hence vapor temperature is seen to be a very effective method of temperature control.

The pipe is arranged in a serpentine pattern in which freely moving liquid and vapor segments alternate. Moreover, this temperature differences of course corresponds to a large effective thermal resistance by itself. Another major usage of vapor chambers is for cooling purposes in gaming laptops.

Chisholm, D. (1971) The Heat Pipe, Mills and Boon Ltd., London. Terms of Use They are now extensively used in many modern computer systems, typically to move heat away from components such as CPUs and GPUs to heat sinks where thermal energy may be dissipated into the environment.

In general, there is some heat transfer to the nominal adiabatic section.

NOTE: for laminar flow, i.e., Re < 2100 the Fanning friction factor quoted above is replaced by the Hagen-Poiseuille form, f = 16/Rev.

The heat pipe as we now know was originated by Grover in Los Alamos for use in thermionic direct conversion devices. In a thermosyphon, liquid working fluid is vaporized by a heat supplied to the evaporator at the bottom of the heat pipe. The concept of vaporization of a fluid in a heated porous element was developed firstly at Harwell by Dunn and Rice in the late 1960's for establishing a nuclear reactor design using this principle, and secondly at the University of Reading, leading to the successful submission of a PhD thesis by Rice (1971).

The heat pipe transfers a large volume of heat at low temperature to allow goods to be baked and other dishes to be cooked in camping-type situations. [1], The stated/recommended operating temperature of a given heat pipe system is critically important. [6], Heat pipes have an envelope, a wick, and a working fluid. After the vapor condenses on the condenser surfaces, capillary forces in the wick return the condensate to the evaporator. 5, pp. 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 doesn't interfere with the normal heat pipe operation.

These alkali metal heat pipes transferred heat from the heat source to a thermionic or thermoelectric converter to generate electricity. This page was last modified on 13 March 2014, at 00:27.

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