As discussed below, the thermosyphons used to cool the Alaska pipe line were roughly 11 to 12 m long. The second figure shows a typical grooved aluminium/ammonia variable conductance heat pipe (VCHP) for spacecraft thermal control. The low temperature range is from 200 to 550 K. Most heat pipe applications fall within this range. The vapor then travels along the heat pipe to the cold interface and condenses back into a liquid, releasing the latent heat. 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. For example, the vapor chamber cooling in Lenovo's Legion 7i was its most unique selling point (although it was misadvertised as all models having vapor chambers, while in fact only a few had[15]). where vv is the specific volume of the vapor; and vl is the specific volume of the liquid. [22] During normal operation, the evaporator and reservoir are heated. [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. 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. 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. Additionally, the container material may be soluble in the working fluid or may catalyze the decomposition of the working fluid at the expected operating temperature. Below 0.1 atm, the vapor pressure limit may be approached. Initially they were used in receivers and amplifiers, soon spreading to other high heat flux electronics applications. see Figure4 for 1 versus Temperature for a range of fluids. Cotter, T. P. (1965) Theory of Heat Pipes, LA 3246-MS, 26 March 1965. )are therefore inescapably and closely related.[3]. Starting in the 1980s 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. 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 one example, a vapor trap diode carried 95 W in the forward direction, and only 4.3 W in the reverse direction.[21]. Heat pipe limitsoperating envelope. Heat pipes are used in some cases to avoid the risk of destabilization. (1994) Heat Pipes, 4th edn., Pergamon. [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. 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. The varitation of f3 with both angle of inclination of the pipe and Bond number is given in Figure2 of ESDU 81038. 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]. In the event of nucleate boiling the relationship between bubble radius and pressure difference sustainable across the curved surface is given by: The degree of superheat Ts related to p is given by the Clausius-Clapeyron equation. 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. 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. A further advantage of the heat pipe is its application as a thermal transformer, see Figure5. where Av is the area of the vapor passageway. 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. The means for achieving this condition was brought about by the use of a dispenser region through which the liquid feed was fed into the element, see Figure8. Heat is then conducted through the heat pipe walls to the evaporator. When used outside of its design heat range, the heat pipe's thermal conductivity is effectively reduced to the heat conduction properties of its solid metal casing alone. The short residence time for liquid heating and evaporation was exploited in further work associated with pyrolytic chemical reactions. Note that most vapor chambers are insensitive to gravity, and will still operate when inverted, with the evaporator above the condenser. 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. 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. It was conceived that this reactor design would permit both boiling and superheating in a single pass through the porous element "fuel rods". 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. The cooling system developed was the first use of variable conductance heat pipes to actively regulate heat flow or evaporator temperature. 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. The recommended maximum rate of heat transfer, to avoid choked flow conditions (i.e., sonic limit) is given by. Using water as an example, the energy needed to evaporate one gram of water is 540 times the amount of energy needed to raise the temperature of that same one gram of water by 1C. 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. Additionally, with one broken heat pipe the heat pipe heat exchanger still remains operable. 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. 5, pp. Figure9. However, the boiling point of water depends on the absolute pressure inside the pipe. The device can therefore only operate with the condenser above the evaporator with gravity-assist liquid flow return. and s is the conductivity of the solid, l the conductivity of the liquid and the porosity of the wick structure. Working Fluids and Temperature Ranges of Heat Pipes, Capillary Wick Designs and Structures in Heat Pipes. The principle of vaporization within a porous element, compared to vaporization from a plain surface, is illustrated in Figure6 and 7. Above the evaporator, the flange is machined off to allow the adiabatic section to be bent. Porous element pressurized reactor. 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. This causes severe discrepancies in the temperature (and thus reliability and accuracy) of the transponders. The choice of pipe material, size, and coolant all have an effect on the optimal temperatures at which heat pipes work. This page has been accessed 34,932 times. When the condenser is located above the evaporator in a gravitational field, gravity can return the liquid. This non-condensable gas is typically argon for standard Variable conductance heat pipes, and helium for thermosyphons. 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 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. The working fluid mass is chosen so that the heat pipe contains both vapor and liquid over the operating temperature range. Modern CPU heat pipes are typically made of copper and use water as the working fluid. Work at Reading lead to the use of the porous element heater for such applications as a fast response vapor diffusion vacuum pump, jointly developed with AERE Harwell and Edwards High Vacuum Ltd. NOTE: for laminar flow, i.e., Re < 2100 the Fanning friction factor quoted above is replaced by the Hagen-Poiseuille form, f = 16/Rev. Table 1 Working fluids and temperature ranges of heat pipes. 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. Sodium, lithium, cesium, silver and a sodium-potassium compound (NaK) are often used in the high temperature range (750 K and above). The effective thermal conductivity varies with heat pipe length, and can approach 100kW/(mK) for long heat pipes, in comparison with approximately 0.4kW/(mK) for copper. The heat pipe has three major operating zones, namely evaporator, adiabatic section and condenser, see Figure1. The high effective conductivity of the heat pipe reduces the cooking time for large pieces of meat by one-half. 218, No. For instance, the parts can react chemically or set up a galvanic cell within the heat pipe. [19], Conventional heat pipes transfer heat in either direction, from the hotter to the colder end of the heat pipe. 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). When the power or heat sink temperature is increased, the heat pipe vapor temperature and pressure increase. Entrainment will cause a starvation of fluid flow from the condenser and eventual "dry out" of the evaporator. A pipe one inch in diameter and two feet long can transfer 3.7kW (12.500 BTU per hour) at 1,800F (980C) with only 18F (10C) drop from end to end. An alternative equation for the boiling limit is given in ETSU data sheet 79012 as. This means that while an individual evacuated tube has better insulation (lower conductive and convective losses) due to the vacuum created inside the tube, an array of tubes found in a completed solar assembly absorbs less energy per unit area due to there being less absorber surface area pointed toward the sun because of the rounded design of an evacuated tube collector. Figure7. where is the surface tension of the liquid, x is the characteristic dimension of the wick surface (2r, where r = effective radius of pore structure). To prevent this, each vertical support member has been mounted with four vertical heat pipe thermosyphons.[40]. The temperature of the vapor corresponds to the vapor pressure, and any temperature variation throughout the system is related directly to vapor pressure drop. One of its main features, namely isothermalization, is of major significance in this application. Variable conductance heat pipes have two additions compared to a standard heat pipe: 1. a reservoir, and 2. a non-condensable gas (NCG) added to the heat pipe, in addition to the working fluid; see the picture in the spacecraft section below. [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. Grover and his colleagues were working on cooling systems for nuclear power cells for space craft, where extreme thermal conditions are encountered. Figure3. |
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. Most manufacturers cannot make a traditional heat pipe smaller than 3mm in diameter due to material limitations. These thin planar heat pipes are finding their way into "height sensitive" applications, such as notebook computers and surface mount circuit board cores. The working fluids in the medium temperature range, 450 to 750 K, are mercury and sulphur. In addition to standard, constant conductance heat pipes (CCHPs), there are a number of other types of heat pipes,[11] including: Thin planar heat pipes (heat spreaders) have the same primary components as tubular heat pipes: a hermetically sealed hollow vessel, a working fluid, and a closed-loop capillary recirculation system. Figure1. In this case, the heat pipe is a thermosiphon. 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 non-condensable gas (NCG) reservoir is located above the main heat pipe. 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. When making heat pipes, there is no need to create a vacuum in the pipe. |
The porous element would consist of packed enriched UO2 coated particles contained in a porous ceramic dispenser tube. Ethane is used when the heat pipe must operate at temperatures below the ammonia freezing temperature. [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. While a typical terrestrial water heat pipe is less than 30cm long, thermosyphons are often several meters long. In connecting pipes, the heat transport occurs in the liquid steam phase because the thermal transfer medium is converted into steam in a large section of the collecting pipeline.[39]. [25][26] This allows the wick in a loop heat pipe to be required only in the evaporator and compensation chamber. Relative efficiencies of the evacuated tube system are reduced however, when compared to flat plate collectors because the latter have a larger aperture size and can absorb more solar energy per unit area. 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. Grover, G. M., U.S. Patent 3229759. With special evaporator wicks, vapor chambers can remove 2000 W over 4cm2, or 700 W over 1cm2.[14]. 7th Int. The factor f3 is a function of the inclination of the heat pipe. There is also the potential of enhanced heat pipe performance, when operating in the capillary limit regime, with use of composite wick structure design. Figures of merit () for different working fluids in capillary driven heat pipes. These alkali metal heat pipes transferred heat from the heat source to a thermionic or thermoelectric converter to generate electricity. 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 heat pipe conductance.
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