IndexDesign ConsiderationsContainerCapillary or Wick StructureEffect of Fluid ChargeEffect of Wick StructureEffect of Working FluidEffect of Inclination AngleHeat Pipes for Dehumidification and Air ConditioningApplicationsSpace TechnologyLaptop Solution Heat PipeCPU WorkstationsNotebooks and Mobile Devices Thermal Control of PCsHeat pipes are heat transfer devices that increase large amounts of heat and work on the principle of evaporation and condensation of a working fluid. Despite the wide application of heat pipe in microelectronics cooling systems, the trend of chip performance and power usage has been increasing every year, and a complete understanding of the mechanism has not yet been completed, although it has the ability to work against gravity and a maximum greater heat transport capacity. This workshop report provides a detailed literature review on various parameters influencing the operational characteristics of circular heat pipe. Furthermore, the thermal resistance and heat transfer capacity are influenced by the choice of working fluid, inclination angle, filling ratio, thermal properties and heat input. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essayThe term heat pipe", as the name suggests, a device for transferring heat from a source to a sink by evaporation and condensation of a fluid in a sealed system. The idea of ​​heat pipes was originally invented by R. S. Gaugler of General Motors Corporation in 1942, but it was not until its independent invention by GM Grover in the early 1960s that the remarkable properties of heat pipes were appreciated and serious development work took place is a heat transfer mechanism capable of transporting large amounts of heat with a very small temperature difference between the hot and cold interfaces. Thermal tube is an evaporation-condensation device for heat transfer in which the latent heat of Vaporization is exploited to transport heat over long distances with a correspondingly small temperature difference. Heat transport is accomplished by evaporating a liquid in the heat inlet region (called the evaporator) and subsequently condensing the vapor in a region of rejection of heat (called condenser). Design Considerations The three basic components of the heat tube are:•The container •The working fluid•The wick or capillary structureContainerThe function of the container is to isolate the working fluid from the external environment. It must therefore be leak-proof, maintain the pressure difference across its walls and allow heat transfer to and from the working fluid. The selection of container material depends on many factors. These are as follows:•Compatibility (both with the working fluid and the external environment)•Strength to weight ratio.•Thermal conductivity•Ease of fabrication, including welding, workability and ductility•Porosity•Wettability Most of the above it's self-explanatory. A high strength-to-weight ratio is more important in spacecraft applications. The material must be non-porous to prevent vapor diffusion. High thermal conductivity ensures minimal temperature drop between the heat source and the wick. Working Fluid A first consideration in identifying a suitable working fluid is the temperature range ofsteam operation. Within the appropriate temperature range, several possible working fluids may exist, and a variety of characteristics must be examined to determine the most acceptable of these fluids for the application under consideration. The main requirements are: • Compatibility with wick and wall materials. • Good thermal stability.• Wettability of wick and wall materials.• Vapor pressure neither too high nor too low in the operating temperature range.• High latent heat.• High thermal conductivity.• Low level of liquid and vapor viscosity. • High surface tension. • Acceptable freezing or pour point. The selection of the working fluid must also be based on thermodynamic considerations involving the various heat flow limitations that occur within the heat pipe such as viscous, sonic, capillary, entrainment, and nuclear boiling levels. In heat pipe design, a high value of surface tension is desirable to prevent the heat pipe from working against gravity and generating a high capillary driving force. In addition to the high surface tension, it is necessary for the working fluid to wet the wick and the container material, i.e. the contact angle must be zero or very small. The vapor pressure in the operating temperature range must be high enough to avoid high vapor velocity, which tends to create high temperatures. gradient and cause slab instability. Capillary structure or wick It is a porous structure made up of materials such as steel, aluminum, nickel or copper in various ranges of pore sizes. For their production, metal foams and more particularly felts are used, the latter being the most used. By varying the pressure on the felt during assembly it is possible to produce pores of various sizes. By incorporating removable metal mandrels, it is also possible to model an arterial structure in the field. Fibrous materials such as ceramics have also been widely used. They generally have smaller pores. The main disadvantages of ceramic fiber are that they have little rigidity and usually require continuous support by a wire mesh. Therefore, while the fiber itself may be chemically compatible with the working fluids, the support materials can cause problems. More recently, interest has turned to carbon fiber as a wick material. Carbon fiber filaments have many fine longitudinal grooves on their surface, have high capillary pressure and are chemically stable. A number of heat pipes that have been successfully constructed using carbon fiber wicks appear to exhibit increased heat carrying capacity. [2] To transport the working fluid from the condenser to the evaporator, a wick structure is incorporated into the upper and lower layers of the heat pipe. Since the transport relies on capillary effects, the effective radius of the structure should be small. This causes a larger (capillary) pressure difference across the heat pipe. However the radius should not be too small, as this causes low permeability of the wick due to friction effects. In addition to this geometric feature, the orientation of the heat pipe is also important. Bodily forces acting on the liquid can increase or decrease its flow. To maximize heat pipe potential, in general, an open-wick structure with gravity-assisted orientation is preferred. When the liquid must rise against gravity, a more compact wick is needed. The wick structure allows liquid to travel from one end of the heat tube to the othervia capillary action. Each wick structure has advantages and disadvantages. There are four common wick structuresa) Rectangular microgrooves.b) Supported foil channel.c) Screen covered artery.d) Wire and sintered metal. The effect of the filled fluid charge ratio is the fraction (by volume) of the heat pipe that is initially filled with liquid. There are two operating fill ratio limits. With a filling ratio of 0%, a heat pipe structure with only bare tubes and no working fluid, it is a pure conduction mode heat transfer device with very high unwanted thermal resistance. A 100% fully filled heat pipe is identical in operation to a single-phase radiator. The heat transfer performance of an OHP was apparently improved after the addition of alumina nanoparticles into the working fluid. Compared to pure water, the maximum decrease in thermal resistance was 0.14 °C/W (or 32.5%) which occurred at a fill ratio of 70% and a mass fraction of 0.9 % when the power consumption was 58.8 W [7]. In PHP, a considerably high filling ratio will hinder the pulsation of the bubble, and the heat transfer efficiency will not be favorable enough. The low filling ratio will easily allow the bubble to pulsate, but it is extremely easy for it to dry out. Therefore the most appropriate filling ratio is between 40% and 60%. Effect of Wick Structure A heat tube is a vessel whose internal walls are aligned with the wick structure. There are four common wick structures: • Groove • Wire mesh • Metal powder • Fiber/spring. The wick structure allows liquid to travel from one end of the heat tube to the other via capillary action. Each wick structure has its advantages and disadvantages. Each wick structure has its own capillary limit. Fig. 1 illustrates the actual test performance of four commercially produced wicks. It can be seen that the slotted heat tube has the lowest capillary limit among the four but works best under gravity-assisted conditions. Fig.2. The actual test results of heat pipe with different wick structure in horizontal and vertical. Effect of working fluid A first consideration in selecting a suitable working fluid is the temperature range of the operating steam within the approximate temperature band (50 to 1500 C) different working fluids can escape. A variety of characteristics must be examined in order to determine the most acceptable of these fluids for the application being considered. The primary requirements are: compatibility with heat pipe materials, thermal stability, wettability, reasonable vapor pressure, high latent heat and thermal conductivity, low liquid and vapor viscosity, and acceptable freezing point. The heat transfer performance of an OHP was apparently improved after the addition of alumina nanoparticles into the working fluid. Compared to pure water, the maximum decrease in thermal resistance was 0.14°C/W (or 32.5%). For the heat pipe with a volume concentration of nanoparticles of 0.10%, the thermal efficiency is 10.60% higher than that of the basic working fluid. The best heat transfer efficiency is a concentration of 100 ppm of silver nanofluid aqueous solution; the worst is a concentration of 450 ppm of aqueous silver nanofluid solution. Although nanofluid has the highest heat conduction coefficient which theoretically dissipates more heat. But a higher concentration will produce a higher viscosity. Effect of the angle ofinclination Orientation is important for the functioning of a heat pipe. Depending on the conditions, a heat pipe can operate in a horizontal or vertical position. For the horizontal position of a heat pipe, gravity has no effect. But in a vertical position, gravity can favor or oppose the operation of the heat pipe. The slope of a heat pipe is classified into two types; favorable tilt and adverse tilt. Favorable tilt is the tilt position where gravity helps the heat pipe function. With a favorable inclination, the condenser is placed above the evaporator. In this way the return of the liquid from the condenser to the evaporator is assisted by gravity. Therefore, the capillary pumping pressure can overcome larger pressure drops and this increases the heat transfer capacity of the heat pipe, in terms of capillary limit. Another type is adverse bias. In this tilted condition, the evaporator is positioned above the condenser. Therefore, the liquid in the condenser will have to overcome the force of gravity to return to the evaporator. This creates extra resistance to overcome the capillary pumping pressure. As a result, the heat transfer capacity of the heat pipe decreases. Therefore, it is preferable for a heat pipe to operate in a favorable inclined position if possible. An increase in heat transfer rate of 39% is achieved for 2% iron oxide nanoparticles when the tilt angle of the heat pipe is 90°. The efficiency of the heat pipe increases with increasing inclination angle because the gravitational force has a significant effect on the flow of the working fluid between the evaporator section and the condenser section. However, when the inclination angle of the heat pipe exceeds the value of 60° for deionized water and 45° for alcohol, the thermal efficiency of the heat pipe tends to decrease. The efficiency of the heat pipe increases as the value of the inclination angle increases. However, when the inclination angle of the heat pipe exceeds 30° for deionized water and 45° for copper nanofluid and copper nanofluid with n-butanol aqueous solution, the thermal efficiency of the heat pipe tends to decrease.[3] Heat Pipes for Dehumidification and Air ConditioningIn an air conditioning system, the colder the air passes over the cooling coil (evaporator), the more moisture condenses. The heat pipe is designed to have one section in the incoming hot flow and the other in the outgoing cold flow. By transferring heat from the warm return air to the cold supply air, heat pipes create the dual effect of pre-cooling the air before it goes to the evaporator and then heating it immediately. Fig.3 Operation of the heat pipe in the air conditioner Activated by the temperature difference and therefore without energy consumption, the heat pipe, thanks to its pre-cooling effect, allows the evaporator coil to operate at a lower temperature , increasing the humidity removal capacity of the air conditioning system by 50-100%. With lower relative humidity, indoor comfort can be achieved by setting a higher thermostat, resulting in net energy savings. Typically, for every 1°F increase in your thermostat setting, you get a 7% savings on your electricity costs. Furthermore, the pre-cooling effect of the heat pipe allows the use of smaller compressors. ApplicationsThe heat pipe has been, and is currently being studied, for a variety of applications, covering almost the entire spectrum of temperatures encountered in heat transfer processes. Heat pipes are used in a wide range of products such as air conditioners.