With extreme overfill, however, any excess fluid might collect as liquid in the condenser section and increase the thermal resistance, thereby decreasing the heat transport capability of the heat pipe. Heat is then conducted through the heat pipe walls to the evaporator. [citation needed]. 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. Table 1 Working fluids and temperature ranges of heat pipes.
In these heat pipes, the temperature drops linearly as the power or condenser temperature is reduced. This criterion is slightly over the optimum requirement because the meniscus recedes into the evaporator wick during normal operation. The envelope was stainless steel, with an inner copper layer for compatibility. "Heat Pipes", Fifth Edition, D. A. Reay, P.A. During fabrication, the heat pipe is charged with the working fluid and a controlled amount of a non-condensable gas (NCG). 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). In the case of heat pipes containing wicks, the fluid is returned by capillary action. 218, No. 
Sodium, lithium, cesium, silver and a sodium-potassium compound (NaK) are often used in the high temperature range (750 K and above). The vapor flows to the condenser, and liquid returns to the evaporator by capillary forces in the wick. Thermophysical properties of heat pipe working fluids: operating range between -60C and 300C, ESDU data sheet 80017, Aug. 1980. This may vary from cryogenic conditions (well below 0C) to high temperature operation (above 600C), in which case liquid metals are used (e.g., potassium, sodium or lithium). The heat pipe is partially filled with a working fluid and then sealed. The heat pipe has four major operating regimes, each of which sets a limit of performance in either heat transfer rate (axial or radial) or temperature drop. The wick is designed to provide a capillary pumping action, as described below. The non-condensable gas (NCG) reservoir is located above the main heat pipe. [25][26] This allows the wick in a loop heat pipe to be required only in the evaporator and compensation chamber. The short residence time for liquid heating and evaporation was exploited in further work associated with pyrolytic chemical reactions. where rv is the radius of the vapor passageway. Even longer thermosyphons have been proposed for the extraction of geothermal energy. The temperature of the vapor corresponds to the vapor pressure, and any temperature variation throughout the system is related directly to vapor pressure drop. The limit for each regime is presented below for a simple cylindrical geometry heat pipe, as illustrated in Figure3. In a thermosyphon, liquid working fluid is vaporized by a heat supplied to the evaporator at the bottom of the heat 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. Thermal conduction is still possible through the walls of the heat pipe, but at a greatly reduced rate of thermal transfer. This page was last modified on 13 March 2014, at 00:27. Some material/working fluids pairs that appear to be compatible are not. The valve is removed after filling and sealing the heat pipe. 
In general, there is some heat transfer to the nominal adiabatic section.
During the late 1990s increasingly high heat flux microcomputer CPUs spurred a threefold increase in the number of U.S. heat pipe patent applications. 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 working fluids in the medium temperature range, 450 to 750 K, are mercury and sulphur. This non-condensable gas is typically argon for standard Variable conductance heat pipes, and helium for thermosyphons. 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 equalize the temperature within the pipe). Initially they were used in receivers and amplifiers, soon spreading to other high heat flux electronics applications. J. This causes severe discrepancies in the temperature (and thus reliability and accuracy) of the transponders. [43], The principle has also been applied to camping stoves. [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.
by M. A. TangzeInt. 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.
The choice of liquid charge is related to the required operating temperature range of the heat pipe. Phys., 35, p. 1990. The driving pressure for liquid circulation within the heat pipe is given by the capillary force established within the wick structure, namely: where pl is the frictional pressure drop in liquid and pv is the factional pressure drop in the vapor.
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. [6], Heat pipes have an envelope, a wick, and a working fluid. In calculating the vapor pressure drop (pv ) it is important to ensure that the Mach Number M < 0.2 and incompressible flow conditions are assumed.
Additionally, with one broken heat pipe the heat pipe heat exchanger still remains operable. In one example, a vapor trap diode carried 95 W in the forward direction, and only 4.3 W in the reverse direction.[21]. [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]. [27] Oscillation takes place in the working fluid; the pipe remains motionless. 
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. There is also the potential of enhanced heat pipe performance, when operating in the capillary limit regime, with use of composite wick structure design. Contact Us
As a rule of thumb, the useful range extends from the point where the saturation pressure is greater than 0.1 atm and less than 20 atm. [22] During normal operation, the evaporator and reservoir are heated.
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. In case that one heat pipe breaks, only a small amount of liquid is released which is critical for certain industrial processes such as aluminium casting. (1) refer to conditions at the closed end of the evaporator. [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. DOI: 10.1016/0017-9310(73)90260-3. Principle of vaporization within a porous element. )are therefore inescapably and closely related.[3]. In the case of the elementary pipe design, liquid returns from the condenser via a wick structure. 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. Below 0.1 atm, the vapor pressure limit may be approached. Typically, a vacuum pump is used to remove the air from the empty heat pipe. 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. Water, which is perhaps the most widely used working fluid, has good thermophysical properties such as large heat of vaporization and surface tension, and has the added benefit of being safe to use during handling. (1994) Heat Pipes, 4th edn., Pergamon. However, under prolonged exposure to freezing temperatures the heat transfer fluid can still freeze and precautions must be taken to ensure that the freezing liquid does not damage the evacuated tube when designing systems for such environments. This increases efficiency, life span and safety. Due to the great adaptability of heat pipes, research explores the implementation of heat pipes into various systems: Heat pipes must be tuned to particular cooling conditions.
The vapor travels to the condenser at the top of the heat pipe, where it condenses. Therefore, the design of the heat pipe must account for the intended temperature range by specifying the proper working fluid. The heat pipe as we now know was originated by Grover in Los Alamos for use in thermionic direct conversion devices. The maximum heat flux as given in ESDU 81038: This condition relates to entrainment or flooding. Generally, gross heat transfer efficiencies of up to 75% are claimed by manufacturers. Heat pipes are designed for very long term operation with no maintenance, so the heat pipe wall and wick must be compatible with the working fluid. In practice, the speed of the vapor through the heat pipe is limited by the rate of condensation at the cold end and far lower than the molecular speed. [35] Publications in 1967 and 1968 by Feldman, Eastman,[36] and Katzoff first discussed applications of heat pipes for wider uses such as in air conditioning, engine cooling, and electronics cooling. Due to the very high heat transfer coefficients for boiling and condensation, heat pipes are highly effective thermal conductors. Working Fluids and Temperature Ranges of Heat Pipes, Capillary Wick Designs and Structures in Heat Pipes. Stanford Ollendorf. The second figure shows a typical grooved aluminium/ammonia variable conductance heat pipe (VCHP) for spacecraft thermal control. Figure8. Nucleation sites, at which bubbles first form, are provided by scratches or rough surfaces and by the release of absorbed gas. (See Friction Factors for Single Phase Flow. This pipe is referred to by Dunn and Reay as "gas-buffered" or "variable conductance" design. Advances in Heat Pipe Science and Technology, Ed.
The working fluid mass is chosen so that the heat pipe contains both vapor and liquid over the operating temperature range. Figures of merit () for different working fluids in capillary driven heat pipes. Rotating heat pipes, where the heat pipe is shaped so that liquid can only travel by centrifugal forces from the nominal evaporator to the nominal condenser. The heat pipe is a sealed system containing a liquid, which when vaporized transfers heat under isothermal conditions. Cotter, T. P. (1965) Theory of Heat Pipes, LA 3246-MS, 26 March 1965. The spacecraft thermal control system has the function to keep all components on the spacecraft within their acceptable temperature range. A point is reached when temperature difference exceeds the degree of superheat sustainable in relation to nucleate boiling conditions. 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. These alkali metal heat pipes transferred heat from the heat source to a thermionic or thermoelectric converter to generate electricity. However, the boiling point of water depends on the absolute pressure inside the pipe. The factor f3 is a function of the inclination of the heat pipe. 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. The device consists of a battery of multi-row finned heat pipe tubes located within both the supply and exhaust air streams.
Grooved wicks are used in spacecraft, instead of the screen or sintered wicks used for terrestrial heat pipes, since the heat pipes don't have to operate against gravity in space.
At low temperature range of operation of the working fluid, especially at start-up of the heat pipe, the minimum pressure at the condenser end of the pipe can be very small. Grooved wicks are used in spacecraft heat pipes, as shown in the first photograph in this section. NASA has tested heat pipes designed for extreme conditions, with some using liquid sodium metal as the working fluid. Figure 1 presents various working fluid boiling points and classifies them into four categories: cryogenic, low, intermediate and high temperature ranges. These thin planar heat pipes are finding their way into "height sensitive" applications, such as notebook computers and surface mount circuit board cores. Porous element pressurized reactor. It is seen that stable boiling can only be achieved in a porous media if a uniform flow regime is established. NOTE: for laminar flow, i.e., Re < 2100 the Fanning friction factor quoted above is replaced by the Hagen-Poiseuille form, f = 16/Rev. 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. see Figure4 for 1 versus Temperature for a range of fluids. Disclaimer. NOTE. The dispenser also provided a thermal barrier to prevent subcooled boiling at inlet and porous alumina, with small pore size (typically 1-5 m diameter pore) and low permeability, and produced high pressure drop compared to pressure drop across the heated porous media. 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). For example, Storch et al. With special evaporator wicks, vapor chambers can remove 2000 W over 4cm2, or 700 W over 1cm2.[14]. Heat pipes are also used to keep the permafrost frozen alongside parts of the QinghaiTibet Railway where the embankment and track absorb the sun's heat. When vertical f3 = 1. [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. The system recovers heat from the exhaust and transfers it to the intake. For laminar flow condition (i.e., Rev < 2000) the Hagen-Poiseuille equation may be applied, thus. SAE paper 2014-01-2160, by Wei Wu et al., describes: 'A Heat Pipe Assisted Air-Cooled Rotary Wankel Engine for Improved Durability, Power and Efficiency',[citation needed] they obtained a reduction in top engine temperature from 231C to 129C, and the temperature difference reduced from 159C to 18C for a typical small-chamber-displacement air-cooled unmanned aerial vehicle engine.
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