heat pipe temperature range

It was originally conceived that the porous element boiler could be developed to provide a new concept of boiling water reactor design, see Figure9. 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. A review of intermediate temperature fluid life tests experiments was reported by Anderson et al. In selecting the working fluid for a heat pipe or thermosyphon it is necessary to ensure that the device operates within the above defined limits. The thermosyphon differs from the heat pipe, in having no wick structure. Variable Conductance Heat Pipes (VCHPs) are used to passively maintain the temperature of the electronics being cooled as power and sink conditions change.[17]. Above the operating temperature, all the liquid has turned to gas, and the environmental temperature is too high for any of the gas to condense. Figure4. When vertical f3 = 1. [37] They are common in many consumer electronics like desktops, laptops, tablets, and high-end smartphones. fabricated a 53mm I.D., 92 m long propane thermosyphon that carried roughly 6kW of heat.[24]. When the condenser is located above the evaporator in a gravitational field, gravity can return the liquid. | 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. This increases efficiency, life span and safety. Properly designed solar thermal water heaters can be frost protected down to more than -3C with special additives and are being used in Antarctica to heat water. [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. 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. Dunn and Reay.)

Finally, rotating heat pipes use centrifugal forces to return liquid from the condenser to the evaporator. These thin planar heat pipes are finding their way into "height sensitive" applications, such as notebook computers and surface mount circuit board cores. Stanford Ollendorf. 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. The wick is designed to provide a capillary pumping action, as described below. 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. In a standard heat pipe, the condensed liquid is returned to the evaporator using a wick structure exerting a capillary action on the liquid phase of the working fluid. (1973) Theory of ultimate heat transfer limit of cylindrical heat pipes, Int. 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. The choice of pipe material, size, and coolant all have an effect on the optimal temperatures at which heat pipes work. 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. The recommended maximum rate of heat transfer, to avoid choked flow conditions (i.e., sonic limit) is given by. Thus in the case of the thermosyphon the condenser region must be above the evaporator region, angle in Figure1 being negative. [27] Oscillation takes place in the working fluid; the pipe remains motionless. The heat pipe is an aluminium extrusion, similar to that shown in the first figure. [1], Heat pipes contain no mechanical moving parts and typically require no maintenance, though non-condensable gases that diffuse through the pipe's walls, that result from breakdown of the working fluid, or that exist as original impurities in the material, may eventually reduce the pipe's effectiveness at transferring heat. Porous element pressurized reactor. Moreover, this temperature differences of course corresponds to a large effective thermal resistance by itself. 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. 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. 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 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 bottom flanged area is the evaporator. The envelope was stainless steel, with an inner copper layer for compatibility. All vapor properties in Eq. In the case of vertically oriented heat pipes the fluid may be moved by the force of gravity. It is seen that stable boiling can only be achieved in a porous media if a uniform flow regime is established. Academic Publishers, ISBN 7-80003-272 1/T 9. The associated evaporator heat flux with nucleate boiling is given by: where w is the effective thermal conductivity of the wick (metal plus liquid) and x is the thickness of wick structure. temperature pipe In general, there is some heat transfer to the nominal adiabatic section. Entrainment will cause a starvation of fluid flow from the condenser and eventual "dry out" of the evaporator. The second figure shows a typical grooved aluminium/ammonia variable conductance heat pipe (VCHP) for spacecraft thermal control. 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. [28], The main reason for the effectiveness of heat pipes is the vaporization and condensation of the working fluid. Kew, p. 10. 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]. 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. temperature pipes heat water cooling solution above The concept was developed using electrically heated porous elements, see Figure7. Due to the very high heat transfer coefficients for boiling and condensation, heat pipes are highly effective thermal conductors. Figures of merit () for different working fluids in capillary driven heat pipes. 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. Heat pipes and loop heat pipes are used extensively in spacecraft, since they don't require any power to operate, operate nearly isothermally, and can transport heat over long distances. Figure1. When the variable conductance heat pipe is operating, the non-condensable gas is swept toward the condenser end of the heat pipe by the flow of the working fluid vapor. Other forms of heat pipes are currently used to cool communication satellites. The latent heat of vaporization absorbed by the working fluid reduces the temperature at the hot end of the pipe. T. Storch et al., "Wetting and Film Behavior Of Propane Inside Geothermal Heat Pipes", 16th International Heat Pipe Conference, Lyon, France, May 2024, 2012.

The viscous and sonic limits are the same as for wicked heat pipes and the equation for the boiling limit and countercurrent flow limits are summarized below. 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. Rice, G., Dunn, P. D., (1992) 'Porous Element Boiling and Superheating'. where vv is the specific volume of the vapor; and vl is the specific volume of the liquid. 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. The heat pipe may be used to transfer heat under near isothermal conditions and may also be used to effect temperature control, as illustrated by Figure2.

When the power or heat sink temperature is increased, the heat pipe vapor temperature and pressure increase. For the heat pipe to transfer heat, it must contain saturated liquid and its vapor (gas phase). Sodium, lithium, cesium, silver and a sodium-potassium compound (NaK) are often used in the high temperature range (750 K and above). The reactor vessel would be fed with water through porous dispenser tubes. [22] During normal operation, the evaporator and reservoir are heated. | 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. 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. There is also the potential of enhanced heat pipe performance, when operating in the capillary limit regime, with use of composite wick structure design. Such a gravity aided heat pipe is known as a thermosyphon.[23]. 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. In one example, a vapor trap diode carried 95 W in the forward direction, and only 4.3 W in the reverse direction.[21]. In these heat pipes, the temperature drops linearly as the power or condenser temperature is reduced. Privacy Policy Therefore, real world efficiencies of both designs are about the same. 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. 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 liquid metals, having much higher surface tension give much higher degrees of superheat (e.g., 10C Ts < 54C compared to Ts 2C for NH3 and H2O). The heat transport rates for liquid-metal heat pipes are generally much higher than those in the other temperature ranges because the surface tension coefficients, latent heats of vaporization, and thermal conductivities of liquid metals are very high. 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. Pressure controlled heat pipes (PCHPs) can be used when tighter temperature control is required.

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. see Figure4 for 1 versus Temperature for a range of fluids. This is largely due to the vacuum that exists within the tube, which slows down convective and conductive heat loss. These limits were catogorized by Busse and are as follows. Figure2. 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. Initially they were used in receivers and amplifiers, soon spreading to other high heat flux electronics applications. The amount of heat that can be transferred for cryogenic heat pipes is quite low due to the small heats of vaporization, high viscosities, and small surface tensions of the working fluids. J. G. Y. Eastman, "The Heat Pipe" Scientific American, Vol. In the case of heat pipes containing wicks, the fluid is returned by capillary action. Most manufacturers cannot make a traditional heat pipe smaller than 3mm in diameter due to material limitations. The high effective conductivity of the heat pipe reduces the cooking time for large pieces of meat by one-half. [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. 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". 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. Both flat plate and cylindrical geometry porous element boilers were constructed, with stable boiling and superheat in a single pass, see Figure8. In addition, for a given heat input, it is necessary that a minimum temperature of the working fluid be attained; while at the other end, any additional increase (deviation) in the heat transfer coefficient from the initial design will tend to inhibit the heat pipe action. While a typical terrestrial water heat pipe is less than 30cm long, thermosyphons are often several meters long. A loop heat pipe (LHP) is a passive two-phase transfer device related to the heat pipe. )are therefore inescapably and closely related.[3]. [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. Nucleation sites, at which bubbles first form, are provided by scratches or rough surfaces and by the release of absorbed gas.

Figure9. Busse, C. A. [1], The advantage of heat pipes over many other heat-dissipation mechanisms is their great efficiency in transferring heat. The condenser is shown above the adiabatic section. 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. When one end of the heat pipe is heated, the working fluid inside the pipe at that end vaporizes and increases the vapor pressure inside the cavity of the heat pipe. Rice, G., Dunn, P. D., (1992) 'Porous Element Boiling and Superheating', 8th International Heat Pipe Conference,, Beijing Sept. 1992, Publ. The factor f3 is a function of the inclination of the heat pipe. 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. 218, No. [42] The cooking pins used water as the working fluid. Ammonia is the most common working fluid for spacecraft heat pipes. The maximum heat flux as given in ESDU 81038: This condition relates to entrainment or flooding. 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 device can therefore only operate with the condenser above the evaporator with gravity-assist liquid flow return. 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. This helps prevent collapse of the flat top and bottom when the pressure is applied. 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. Faghri [2] provided a detailed procedure for the fabrication, processing, and testing of low, moderate, and high temperatures.

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