And fluid returns to evaporator by capillary action in wick. Capillary action is the physical phenomenon arising due to surface tension on the interface of immiscible media. At the condenser region, the vapor of the working fluid condenses and the latent heat is rejected via condensation. A capillary structure wick can be placed inside the heat pipe body to allow the condensed liquid phase of the working fluid wicking against the vapor flow due the capillary action. The condensed working fluid is returned to the evaporator by gravity, or by the heat pipes wick structure, creating capillary action. Here, the vapor condenses back to liquid and releases heat at the same time. Here is a link to the wikipedia page for capillary action. Capillary wick designs and structures in heat pipes.
Heat pipes employ phase change to transfer thermal energy from one point to another by the vaporization and condensation of a working fluid or coolant. Design considerations when using heat pipes electronics. The liquid evaporates into a gas that travels to the cooler end of the pipe. Due to capillary action, heat pipes can operate in microgravitational fields without any external force field or. The working fluid is selected to meet the desired temperature conditions and is usually a class i. A new approach to compute the porosity and surface.
Heat pipe limits advanced cooling technologies, inc. Capillary tube technology learning from nature beka. It is common, especially for small pipes 12 or 34 inch, to heat the entire joint and then apply solder to an edge on the side of the joint. It was also being drawn sideways, up, and around to fill the very small gap between the pipe and the fitting. Learn about acts standard heat pipe transport limits, which are dependent on variables including operating temperature, wick selection and fluid properties.
Capillary action is effected by the grooves on the inside of the heatpipe. For a heat pipe to function, the capillary pressure difference occurring in the heat pipe must always be greater than the summation of all the pressure. Heat pipes are offered in a variety of diameters ranging from 2mm to 12mm. How to prevent the loss of capillary water through evaporation. Both cylindrical and planar heat pipe variants have an inner surface lined with a capillary wicking material. Heat pipes operate at a considerably higher temperature and pressure in microgravity than they do on earth, explains constrained vapor. Capillary action sometimes capillarity, capillary motion, or wicking is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity. There is an wick lining inside the tube which acts as a medium for capillary action. The utvg has developed a product that creates capillary action and vaporization simultaneously which allows for the elimination of any type of sponge or filter materials. Heat pipes usually have a wick to return the condensate to the evaporator via capillary action. Although the heat transfer intensity is greatest in the bubble boiling, for most types of wick heat pipes, the bubble boiling is not desired because it interferes with the liquid wicking into the wick structure. The basic heat pipe is a closed container whfch contains a capillary wick structure. The heat pipe capillary limit typically sets the maximum heat pipe operating power. Depending on the shape of the grooves, there is a difference in performance.
The capillary pumping capability depends on surface tension and two radii of. The operating principles of heat pipes hp are expounded, along with a discussion of different types of hp, and the underlying theory, design, and fabrication of hp. Capillary action, sonic limitation, entrainment limitation, boiling. This effect is similar to placing a paper towel in coffee, and watching the fluid rise up the paper towel. Commonly, capillary phenomena occur in liquid media and are brought about by the curvature of their surface that is adjacent to another liquid, gas, or its own vapor. Heat pipes fall under the category of capillary driven devices. While small pores are needed at the liquidvapor interface to develop high capillary pressures, large pores are preferred within the wick so that the movement of. Ats engineering ebook advanced thermal solutions, inc. Lhps are twophase heat transfer devices using capillary action to remove heat from a source and to passively move it to a condenser or radiator 75. Pdf basic principles of heat pipes and history researchgate. C when the effects of gravity overcome the effects of the capillary action. Loop heat pipes and capillary pumped loops an applications perspective dan butler, jentung ku, theodore swanson nasagsfc, code 545, greenbelt, md 20771 301 2863478, email. Metal 3dprinted wick structures for heat pipe application. The best technique to keep capillary hang water in the soil, is to add a layer of loose surface soil.
Heat and mass transport in heat pipe wick structures. In heat pipe design, wick performance is often a limiting factor. Heat and mass transport in heat pipe wick structures brian d. 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. A variety of liquids and wicks are used to make a heat pipe, but the principle is the same. The capillary flow experiment cfes is a suite of fluid physics experiments whose purpose is to investigate capillary flows and phenomena in low gravity. For a custom solutions contact our team of engineers. The vapor in the condenser section loses heat to the integral heat sink and is converted back to liquid by the transfer of the latent heat of vaporization to the condenser. The middle section of the heat pipe l a, the adiabatic portion, has a very small temperature difference. In a zero gravity environment, capillary action is one mechanism of condensate removal. The vapor pressure changes along the heat pipe are due faghri, 1995 to friction, inertia and blowing evaporation and suction. A capillary structure wick can be placed inside the heat pipe body to allow the condensed liquid phase of the working fluid wicking against the vapour flow due the capillary action.
The simplest type of heat pipe consists of two basic parts, the body container and the working medium. Esa satellite cooling system makes paris metro more. Qmax is reduced when the heat pipe is bent, the capillary action is against gravity, required operating altitude above sea level increases, and often when the pipe is flattened a small amount of flattening typically wont affect it. The new approach to determine the thin film porosity and surface roughness is illustrated in fig. A heat pipe is composed of a vacuumtightened vessel, wick structure, and some amount of working fluid. Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a. Figure 2 shows the pressure drop distribution inside a heat pipe. This article is intended to offer design guidance when using heat pipes for the most prevalent types of electronics applications. Isobar heat pipe transfer system electronic heating. During the day, capillary water evaporates because of the suns heat hitting the soil. As heat is inputted at the evaporator, the fluid vaporizes, creating a pressure gradient. When any of these limitations is encountered, the capillary. Nasa keeping cool with heat pipes on the space station.
Heat pipes of various capillary wick structures are attractive in the area of spacecraft cooling and temperature stabilization because of their lightweight, zero maintenance, and reliability. Lhps are similar to heat pipes but have the advantages of providing reliable operation over long distances and having the ability to operate against gravity. Its probably better to refer to is as capillary action rather than capillary pressure because it is not an effect that acts over an area. The liquid is drawn back to the evaporator section by capillary action, where it is. Pdf the heat pipe is one of the remarkable achievements of thermal physics and heat transfer engineering in this. Effect of working fluid on the condenser temperature profile. Generally speaking, the larger the diameter, the more heat measured in watts. The condensate returns to the evaporator by means of capillary action in the wick. The effect can be seen in the drawing up of liquids between the hairs of a paintbrush, in a thin tube, in porous materials such as paper, in some nonporous materials such as liquefied. An important heat pipe design consideration is the amount of power a heat pipe is capable of transferring. The working principles of vapor chambers are identical to heat pipes.
When does the capacity of a heat pipe decrease to zero. Capillary action atoz guide to thermodynamics, heat. Heat pipe engineering ebook advanced thermal solutions. The liquid is then pumped back to the evaporator through the wick capillary action. Water, ammonia and sodium are used as working fluid, its selection depends on the operating temperature ranges. Gasloaded heat pipes have been applied in many diverse fields, and are useful when the temperature of a device must be held constant while a variable heat load is dissipated. How is sweating a pipe an example of capillary action. These twophase systems are typically used to cool areas or materials, even in outer space. Heat pipes are the most common passive, capillary driven of the twophase systems. Applications of heat pipes to cool pwbs and hybrid microcircuits hughes aircraft company r fullerton, california. Micro heat pipes for cooling electronic devices were proposed in 83.
A heat pipe consists of an evaporation zone, an adiabatic zone, and a condensation zone. Heat pipe capillary limit advanced cooling technologies. The capillary action of a heat pipe wick is analogous to how a paper towel will become soaked if only one corner is placed in water. The explanation of the capillary in the soil technology. The wick structure within the heat pipe is present to return condensate to the evaporator section. Capillary pumped loops cpls and loop heat pipes lhps are versatile twophase heat transfer devices which. On the other hand, in a heat pipe with a grooved capillary structure, a gravity heat pipe with bubble boiling is favorable. International heat pipe conferences have been conducted, two books on heat pipes have. A heatpipe is a sealed copper tube that is under vacuum, and can transfer heat rapidly away from the source. These cycles continues as long as there heat pipe is kept between two temperature. Capillary action sometimes capillarity, capillary motion, capillary effect, or wicking is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. The formula flows through a pointed felt tip thanks to capillary action. This book introduces operational and design principles for heat pipes. This pressure gradient pushes the vapor to flow along the pipe to the cooler section where it condenses, giving up its latent heat of vaporization.
Of course, a heat pipes capillary material made of copper, stainless steel or plastic is far more efficient than sugar, and produces an almost inexhaustible capillary force. The procedure to quantify the porosity is demonstrated on thinfilm of copper coated on to a. The wick provides the structure to develop the capillary action for the liquid returning from the condenser heat outputsink to the evaporator heat inputsource. Since the heat pipe contains a vacuum, the working fluid will boil and take up latent heat at well below its boiling point at atmospheric pressure.
As previously mentioned there is liquid vapor equilibrium inside the heat pipe. The vapor travels through the hollow core of the heat pipe to the other end of the heat pipe, where heat energy is being removed by a heat sink or other means. The inner surface of the heat pipe is lined with the wick that provides the capillary action for liquid moving from the condenser to evaporator, inside the vacuum isobar heat pipe structure. Heat transfer limitations of water wick heat pipe with sintered capillary structure heat pipe. The process continuously repeats so long as heat is applied. Liquid in contact with the evaporation zone turns into vapor by. Heat transfer is achieved by the wick structure and working fluid inside the isobar heat pipe. The present work aims at obtaining mass flow rate measurements in heat pipe. Heat pipes are tubes that have a capillary wick inside running the length of the tube, are evacuated and then filled with a refrigerant as the working fluid, and are permanently sealed. The whole liquid and vapor cycle requires no moving parts and the heat transfer process can repeat indefinitely. Heat pipes are one of the most efficient ways to move thermal energy from one point to another. Manufacturing costs are low with this type of heatpipe because the grooves are easier to make, however the technique is much more susceptible to gravity and can be orientation specific in use. Heat pipe contains a small amount of working fluid.
Condensation removal by capillary action thermal fluids. Today, heat pipes are used in a variety of applications from space to handheld devices that fit in your pocket. In effect, this operational range defines the region or combination of. A capillary structure wick can be placed inside the heat pipe body to allow the. Working fluids, wick configurations, reservoirs, and pipe materials are dealt with, and cleaning, assembly, evacuation, charging, and closure of hp are covered. In order for the capillary force to drive the vapor, the capillary pressure of the wick should exceed. The capillary tube technology is equally suitable for new buildschemes or for modernising existing buildings, irrespective of. Capillary action occurs because water is sticky, thanks to the forces of cohesion water molecules like to stay close together and adhesion water molecules are attracted and stick to other substances. They are utilized as capillary wicking medium to drive the circulation of working fluid in its liquid phase, as well as evaporationenhancement structures to releasecapture the working fluid in its vapor phase.
The high thermal conductivity enables a heat pipe to transfer, and dissipate heat to a more convenient location through a capillary action. Heat pipes sasecurity wiki fandom powered by wikia. The liquid then travels back to the original end of the heat pipe via the wick by capillary action. Mathematical model for heat transfer limitations of heat pipe. Heat pipe is constructed using a small hollow metal tube which is sealed under partial vacuum.