For the heat pipe to transfer heat, it must contain saturated liquid and its vapor (gas phase).
[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. 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. [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. The working fluid mass is chosen so that the heat pipe contains both vapor and liquid over the operating temperature range. The other end extends into the oven where it draws heat to the middle of the roast. However the maximum adverse elevation (evaporator over condenser) is relatively small, on the order of 25cm long for a typical water heat pipe.
During operation, one end of the heat pipe is poked through the roast.
[42] The cooking pins used water as the working fluid. It was conceived that this reactor design would permit both boiling and superheating in a single pass through the porous element "fuel rods". However, this situation is more advantageous than underfilling the heat pipe, which may significantly reduce the maximum heat transfer.
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.
Figure4. These thin planar heat pipes are finding their way into "height sensitive" applications, such as notebook computers and surface mount circuit board cores. Schematic of in-pile porous element steam generator.
Below 0.1 atm, the vapor pressure limit may be approached. Thermophysical properties of heat pipe working fluids: operating range between -60C and 300C, ESDU data sheet 80017, Aug. 1980.
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.
Heat pipes are used in some cases to avoid the risk of destabilization.
[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. These alkali metal heat pipes transferred heat from the heat source to a thermionic or thermoelectric converter to generate electricity. The heat pipe is partially filled with a working fluid and then sealed. [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.
Finally, rotating heat pipes use centrifugal forces to return liquid from the condenser to the evaporator.
The factor f1 is a function of a dimensionless parameter Kp, which is defined as.
Porous element heating and vaporization. Specific power ratings in excess of 1 kW/cm3 of element were achieved both when vaporizing water and freon.
In the case of freon, evaporation and superheat was achieved uniformly with a porous element in excess of l m long.
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. The envelope was stainless steel, with an inner copper layer for compatibility. The vapor flows to the condenser, and liquid returns to the evaporator by capillary forces in the wick. 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. [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. However, problems with wetting the wick and wall present difficulties in using mercury in capillary heat pipes.
Due to the very high heat transfer coefficients for boiling and condensation, heat pipes are highly effective thermal conductors.
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 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.
Thermosyphons, which are heat pipes where the liquid is returned to the evaporator by gravitational/accelerational forces, Rotating heat pipes, where the liquid is returned to the evaporator by centrifugal forces.
The cooling system developed was the first use of variable conductance heat pipes to actively regulate heat flow or evaporator temperature.
[32] He noted in his notebook:[33].
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.
by M. A. TangzeInt.
Thus in the case of the thermosyphon the condenser region must be above the evaporator region, angle in Figure1 being negative.
A heat pipe heat exchanger contains several heat pipes of which each acts as an individual heat exchanger itself. 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].
The working fluids in the medium temperature range, 450 to 750 K, are mercury and sulphur. 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. For example, Storch et al. fabricated a 53mm I.D., 92 m long propane thermosyphon that carried roughly 6kW of heat.[24]. A point is reached when temperature difference exceeds the degree of superheat sustainable in relation to nucleate boiling conditions.
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.
The latent heat of vaporization absorbed by the working fluid reduces the temperature at the hot end of the pipe.
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.
NASA has tested heat pipes designed for extreme conditions, with some using liquid sodium metal as the working fluid. Pressure controlled heat pipes have shown milli-Kelvin temperature control.
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.
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.
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. The condenser is shown above the adiabatic section.
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.
Advances in Heat Pipe Science and Technology, Ed. The high effective conductivity of the heat pipe reduces the cooking time for large pieces of meat by one-half.
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. [27] Oscillation takes place in the working fluid; the pipe remains motionless.
An oscillating heat pipe, also known as a pulsating heat pipe, is only partially filled with liquid working fluid. The reservoir eventually dries out, since there is no method for returning liquid.
Below the operating temperature, the liquid is too cold and cannot vaporize into a gas. The heat pipe cooling system designed for this purpose managed the high heat fluxes and demonstrated flawless operation with and without the influence of gravity. Cryogenic heat pipes operate between 4 to 200 K. Typical working fluids include helium, argon, oxygen, and krypton.
The maximum rate of heat transfer under this restricted vapor pressure drop limit is given by: where Dv is the diameter of the vapor passageway, hlg is the enthalpy of vaporization, pv is the pressure, v the vapor density, and v the vapor dynamic viscosity.
Moreover, this temperature differences of course corresponds to a large effective thermal resistance by itself.
Pressure controlled heat pipes (PCHPs) can be used when tighter temperature control is required.
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. 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. At a temperature above the vapor pressure limit, the vapor velocity can be comparable with sonic velocity and the vapor flow becomes "choked".
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Similarly, a heat pipe with water as a working fluid can work well above the atmospheric boiling point (100C, 212F).
(See Friction Factors for Single Phase Flow. 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 factor f3 is a function of the inclination of the heat pipe.
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.
Eventually, all of the liquid is trapped in the reservoir, and the heat pipe ceases operation.
Rice, G., (1971) Porous Element Boiler, PhD Thesis, University of Reading, U.K.. 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, Proc. 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.
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. Therefore, real world efficiencies of both designs are about the same.
When the condenser is located above the evaporator in a gravitational field, gravity can return the liquid. Disclaimer. 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. Dunn, P. D. and Reay, D. A. 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.
A figure of merit () may be used to establish the relative performance of a range of prospective working fluids. The reactor vessel would be fed with water through porous dispenser tubes.
The first application of heat pipes in the space program was the thermal equilibration of satellite transponders.
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). When making heat pipes, there is no need to create a vacuum in the pipe.
The concept was developed using electrically heated porous elements, see Figure7.
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.
[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 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).
Grover, G. M., Cotter, T. P., and Erickson, G. R, (1964) Structures of very high thermal conductance, J Appl. Ethane is used when the heat pipe must operate at temperatures below the ammonia freezing temperature.
The liquid ammonia at the bottom of the thermosyphon is vaporized by heat absorbed from the ground, cooling the surrounding permafrost and lowering its temperature. DOI: 10.1016/0017-9310(73)90260-3.
Phys., 35, p. 1990. 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.
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.
Vacuum Conf, Vienna.
Therefore, the design of the heat pipe must account for the intended temperature range by specifying the proper working fluid. where rv is the radius of the vapor passageway. 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.
Most of the non-condensable gas is located in the reservoir, while the remainder blocks a portion of the heat pipe condenser. The vapor then travels along the heat pipe to the cold interface and condenses back into a liquid, releasing the latent heat.
In the absence of gravity, the forces must only be such as to overcome the capillary and the drag of the returning vapor through its channels.
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. During the winter, the air is colder than the ground around the supports. For example, in the Trans-Alaska Pipeline System residual ground heat remaining in the oil as well as heat produced by friction and turbulence in the moving oil could conduct down the pipe's support legs and melt the permafrost on which the supports are anchored. [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. When the heat pipe is not operating, the non-condensable gas and working fluid vapor are mixed throughout the heat pipe vapor space. Vertical heat pipes on either side of relevant formations prevent that heat from spreading any further into the surrounding permafrost. (1973) Theory of ultimate heat transfer limit of cylindrical heat pipes. 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.
Above 20 atm, the container thickness must increase to the point where the heat pipe becomes limited by the thermal resistance through the container.
The temperature of the vapor corresponds to the vapor pressure, and any temperature variation throughout the system is related directly to vapor pressure drop.
First, they are used when high powers and heat fluxes are applied to a relatively small evaporator. Ammonia is the most common working fluid for spacecraft heat pipes. 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).
Academic Publishers, ISBN 7-80003-272 1/T 9.
G. Y. Eastman, "The Heat Pipe" Scientific American, Vol. Using this characteristic dimension they have produced a table showing the degree of superheat for a range of candidate heat pipe working fluids, including ammonia, water and liquid metals (for high temperature operation). 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.
This causes severe discrepancies in the temperature (and thus reliability and accuracy) of the transponders. Grover and his colleagues were working on cooling systems for nuclear power cells for space craft, where extreme thermal conditions are encountered.
The heat pipe is a sealed system containing a liquid, which when vaporized transfers heat under isothermal conditions.
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.
Since the early 1990s, numerous nuclear reactor power systems have been proposed using heat pipes for transporting heat between the reactor core and the power conversion system.
), The vapor pressure drop over the length of the evaporator plus the adiabatic region, for turbulent flow (Re 2000) is given in ESDU 79012 as.
Heat exchangers transfer heat from a hot stream to a cold stream of air, water or oil. [citation needed]. 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).
[43], The principle has also been applied to camping stoves. 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. 7th Int.
One of its main features, namely isothermalization, is of major significance in this application.
The valve is removed after filling and sealing the heat pipe.
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