The aluminum has good compatibility with acetone and ammonia, but not with water. This results in very low thermal resistance (~0.3 K/W), indicating low temperature drop with respect to the given thermal load. The heat pipeembedded block can be directly connected to the fin, as shown in this figure. In some applications such as sever computer and telecommunication unit handle a large amount of data, block-to-block module is employed, as shown in Figure18. However, the capillary pressure is strongly limited, due to the fact that the scale of the grooves, which are machined through extrusion process, cannot be reduced beyond several tens of micrometers. On the other hand, the sintered particle wick has high capillary pressure as well as moderate-to-high effective thermal conductivity, due to the tailorable particle size and fused contact between particles. It should be noted that the effective length of a heat pipe is not its actual length; it is the heat pipe distance from the midpoint of the evaporator to the midpoint of the condenser. The flow in the wick is attributable to the same mechanism with the suction of water by a sponge. Can you provide a computer-generated model of how your heat exchanger will operate in my application? In this chapter, the general aspects of heat pipes are introduced. Thermal conductivity is the number of watts (W) conducted per meter (m) of thickness per temperature (kelvin (k)) difference between one side and the other of the material at that thickness. Licensee IntechOpen. However, when the pore size is too large, the capillary pressure becomes too small so that the gravity effect cannot be overcome, which in turn makes the heat pipe useless. Recently, it has been shown that the boiling limit does not occur with the nucleate boiling if the vapor bubble can escape the wick efficiently [4]. According to the purpose of this chapter mentioned above, the author considers fundamental aspects regarding heat pipe and phase change phenomena. As PhD students, we found it difficult to access the research we needed, so we decided to create a new Open Access publisher that levels the playing field for scientists across the world. What materials can be used to construct a heat pipe? In case of water-based heat pipe that operates at the room temperature, the inner pressure of heat pipe is typically set to be approximately 0.03 bar for maximizing the thermal performance. The pressure difference drives the vapor from evaporator to the condenser, where it condenses releasing the latent heat of vaporization to the heat sink. The longer the heat pipe, the higher the thermal conductivity. The design procedure of the heat pipe is as follows: The followed subsections will be devoted to each procedure. For these reasons, the particle size of the sintered particle wick typically ranges from 40 m to 300 m. The entrainment limit is related to the liquidvapor interface where counterflows of two phases are met. Aavid has the capability to design and manufacture thermal management solutions at the component, board, and system levels. Typically, the maximum allowable stress at any given temperature can only be one-fourth of the materials maximum tensile strength. The inverse of thermal conductivity is thermal resistance. Use of tubular heat pipe and flat-plate heat pipe. The grooved wick utilizes axial grooves directly sculptured on the envelope inner surface as the flow channel. The inner pressure of heat pipe should be properly adjusted according to its operating temperature. Elnaggar and Ezzaldeen Edwan. For a reliable operation of future electronics that have ultra-high heat flux amounts to 1000 W/m2, heat pipe and phase change heat transfer are essential. This suggests that the Kis also an important parameter for the boiling limit. It has an additional advantage in which the wide and straight (not tortuous) flow path can bring about high permeability. The mesh screen wick can have high capillary pressure and moderate permeability because numerous pores per unit length and the tightness of the structure can be controlled, where the permeability is a measure of the ability of a porous medium to transmit fluids through itself under a given pressure drop as follows: where Kis the permeability, Uis the mean velocity of flow inside porous media, is the viscosity, and dP/dxis the applied pressure gradient. Besides design of heat pipe itself, the interfaces of heat pipe with heat sinksource are also of significant interest because the interfacial contact thermal resistance is much larger than that of heat pipe itself. The heat pipe is also widely employed in solar heat collection, snow melting, heat exchanger and related energy applications, and pure science applications demanding ultra-precise temperature control. The block-to-fin applications are shown in Figure17. Indeed, nucleate boiling may not stop or retard the capillary-driven flow in porous media according to the literatures. Our team is growing all the time, so were always on the lookout for smart people who want to help us reshape the world of scientific publishing. Some of the more common heat pipe fluids used for electronics cooling operations are ammonia, water, acetone and methanol. K) the effective thermal conductivity of copper, depending on the length of the heat pipe.What materials can be used to construct a heat pipe?The heat pipe wall or shell material selection is driven by compatibility of the working fluid. Furthermore, the shape of the heat pipe does not necessarily have to be cylindrical, but it can be formed into various shapes such as disks, flat plates, and airfoils. Combining Equations (3) and (4) yields the following equation for the capillary limit: It should be underlined that the Kand Reff are related to the microstructure of the wick; hfg, , l, and lare the fluid properties; and Leff and Awrepresents the macroscopic geometry of the heat pipe. Finally, applications of heat pipe to electronics cooling are presented. Various kinds of working fluids and their operating temperature ranges and corresponding inner pressures are shown in Figure10. Open Access is an initiative that aims to make scientific research freely available to all. On the contrary, there is a definite limitation of thermal performance for heat pipe, beyond which the heat transfer rate cannot be increased for a reliable operation. The sintered particle wick is made of slightly fusing microsized metal particles together in the sintering process. By Mohammad Reza Safaei, Marjan Gooarzi, Omid Ali Akb HeadquartersIntechOpen Limited5 Princes Gate Court,London, SW7 2QJ,UNITED KINGDOM, Heat Pipes for Computer Cooling Applications. In addition, design method for heat pipe is presented. The fluid in a heat pipe will boil at any temperature above its freezing point. When the vapor flow rate exceeds the sonic velocity, chocked flow is achieved and the heat pipe will not operate isothermal.Entrainment limit: This occurs when the sheer force of the vapor flowing from the evaporator to the condenser section of the heat pipes at the vapor-wick interface causes liquid droplets to be entrained and carried to the condenser section. The heat passes through the metal envelope and vaporizes the liquid. It happens when the circulation rate of working fluid increases so that the pressure drop along the entire flow path reaches the developed capillary pressure. The boiling limit also becomes important when the operating temperature is high because bubble nucleation is more likely to happen in high superheat. As an example, a typical value of effective thermal conductivity of a copperwater heat pipe with 0.5-m length and 1/2 inch diameter is around 10,000 W/mK, which is much larger than those of thermally conductive metals such as copper (~377 W/mK) or aluminum (~169 W/mK). The maximum hoop stress in the heat pipe wall is given as follows [1]: where fmax is the maximum stress in the heat pipe wall; Pis the pressure differential across the wall, which causes the stress; dois the heat pipe outer wall; and tis the wall thickness. The heat pipe is a thermal superconductor of which thermal conductivity amounts to several thousands of Watts per meter-Kelvin. Provided that the ambient air is at 20C, the chip temperature is only 26C, which enables designers to easily come up with plausible and fascinating thermal solution. Its based on principles of collaboration, unobstructed discovery, and, most importantly, scientific progression. Thermal conductivity can be calculated as follows: Keff= Effective thermal conductivity (W/mK), Leff= Effective length = (Levaporator+ Lcondenser)/2 + Ladiabatic(m), T = Temperature difference between evaporator and condenser sections (k). The last term is related to the wick microstructure, thus, in regard to the wick design, we have to maximize this term. As shown in Figure1, the heat pipes have been conventionally used for PCs, laptops, telecommunication units, solar collectors, small energy systems such as geothermal pipes, and satellites. The performance of applicability in electronics of heat pipe is strongly dependent on the geometry, working fluid, and microstructure of wick. The contact thermal resistance between the evaporator and the heat source and that between the condenser and the heat sink is relatively large. How?Temperature control, speed control and fan-failure alarms can be integrated into each heat exchanger. The capillary limit is also called the wicking limit. The goodness of the heat transfer module is characterized by the effective thermal conductivity (keff) or thermal resistance (Rth) of the module. The capillary limit occurs when the liquid flow along the axial direction cannot afford the evaporation rate due to the limited capillary pressure. In addition, the wick also acts as a thermal flow path because the applied heat is transferred to the working fluid through the envelope and wick. When the inertia force becomes significant, the thermal performance is significantly deviated from the prediction by Equation (12), in other words, is degraded much. In some circumstances, the drag imposed by the vapor on the returning liquid can be large enough to entrain the flow of condensate in the wick structures, resulting in dry out. Combining Equations (9) and (10) yields: The maximum thermal performance of heat pipe is given in Equation (6). Exceeding the entrainment limit may prevent the working fluid from returning from the condenser section to the evaporator section, as a result the heat pipe will not operate.Are heat pipes reliable?Yes, mainly because they have no moving parts. The heat pipeembedded heat spreader is shown in Figure16. Allow Aavid to review the application in order to recommend the best solution.Do you build custom designs?Aavid utilizes a broad scope of technologies to deliver fully optimized, custom solutions as well as our standard offerings.Can you provide a computer-generated model of how your heat exchanger will operate in my application?Yes, Aavid can utilize CFD (computational fluid dynamics) programs such as Aavid SmartCFD to model the performance of a heat exchanger within the enclosure.Why should we buy a heat exchanger from Aavid?Understanding of the total thermal circuit is crucial to a product's success. The Kand Reff values for representative wick structure are shown in Figure9. The pressure drop of working fluid consists of that of liquid flow path (Pl), that of vapor flow path (Pv), additional pressure drop imposed by counterflow at the phase interface (Plv), and the gravitational pressure drop (Pg). Each fluid has its vapor pressure profile with respect to the temperature. The thermal performance of heat pipe is generally determined by capillary limit, which can be readily predicted based on simple analytic method represented by Equation (6). After a working fluid is selected, the heat pipe wall or shell material is selected based on its chemical compatibility with the working fluid to prevent corrosion or chemical reaction between the fluid and the heat pipe wall or shell material. In case of grooved wick, the effective thermal conductivity is high due to the sturdy thermal path. We are a community of more than 103,000 authors and editors from 3,291 institutions spanning 160 countries, including Nobel Prize winners and some of the worlds most-cited researchers. In this regard, the boiling limit becomes dominant when the effective heat pipe length is relatively small, and vice versa. For example, the thermal conductivity of copper is 390 W/m-k, but for heat pipes, it can range from 1,500 W/mk to 50,000W /mk. Important parameters in heat pipe are considered, and theoretical model for predicting the thermal performance of the heat pipe is introduced. Typically, five selections can be considered: no wick (for thermosiphon), mesh screen wick, grooved wick, sintered particle wick, and heterogeneous type wick. The high thermal performance of the heat pipe is originated from the latent heat of vaporization, which typically amounts to millions of Joules per 1 kg of fluid. (Nanoscale wicks are available. Therefore, the ratio between Kand Reff captures a trade off between those two competing effects. This term is often called the capillary performance of wick. Small diameter heat pipes have a higher effective thermal conductivity than larger diameter heat pipes. Boiling limit is also important in high operation temperature. There is a good method for distinguishing those two limitations (see Figure8). The thermal conductivity of a solid metal stays constant because the heat transfers through molecules that are the same and therefore offer the same thermal resistance. Another fundamental problem also exists in which the nucleate boiling within the wick does not necessarily represent a heat transfer limit unless bubbles cannot escape from the wick, as indicated by several researchers [2]. These features can be provided by installing a solid-state control board and/or integrating the feature into the fan itself.How do you seal the core element in the Aavid HXi Heat Exchanger series?Aavid uses an RTV sealant to provide a cohesive gasket around both the inner core cassette and the core flange assembly. The internal pressure of the heat pipe is the saturation pressure of the fluid at the corresponding fluid temperature. The thermal performance of the heat pipe is limited by one of various mechanisms depending on the working temperature range and geometry of the heat pipe. Performance Evaluation of Nanofluids in an Incline School of Mechanical Engineering, Yeungnam University, Gyeongsan, South Korea. The finer the pore radius of a wick structure, the higher against gravity the heat pipe can operate. Figure7 shows the thermal capacity of a heat pipe (copperwater, 1 cm diameter, 30 cm long) determined by various limiting mechanisms with respect to temperature. where dvis the vapor core diameter, Qmax is the maximum axial heat flux, vis the vapor density, vis the vapor-specific heat ratio, hfg is the latent heat of vaporization, Rvis the gas constant for vapor, and Tvis the vapor temperature. When the capillary limit happens, dry out occurs in the evaporator, while more fluid is vaporized than that can be supplied by the capillary action of the wick. Sonic limit: The maximum flow rate of the working fluid vapor flow rate traveling from the heat pipe evaporator to condenser. The thermal resistance of the heat pipe can be estimated based on the thermal resistance network, as shown in Figure13. Aavid is the only heat pipe manufacturer in the world that can claim over 40 years of heat pipe reliability and life test data.Are heat pipes expensive?Compared with traditional (and less effective) heat transfer methods such as aluminum extrusions and cast heat sinks, heat pipes can have a higher initial cost. The capillary pumping pressure must overcome three basic pressure drops within the heat pipe, namely, vapor pressure drop, liquid pressure drop and gravitational/body force pressure drops.Boiling limit: The boiling limit occurs when the maximum radial heat flux (W/cm2) is exceeded resulting the rate of working fluid vaporization to exceed the rate at which the liquid condensate is returning from the condenser section of the heat pipe. Due to the extremely high effective thermal conductivity, the heat pipe can handle a large amount of heat transfer with a negligible temperature drop. The heat pipe working fluid is selected based on the operating temperature range of the application. In this equation, is the surface tension coefficient, Reff is the effective pore radius of the wick structure, lis the liquid viscosity of working fluid, Leff is the effective length of heat pipe, Kis the permeability, Awis the cross-sectional area of the wick, lis the liquid density of working fluid, m.is the mass flow rate, gis the gravitational constant, and is the orientation angle with respect to the horizontal plane. Coatings such as Herresite or E-Coat can be added to heat exchangers to provide environmental protection to the unit (minimum volumes apply).What is the difference between the HX, HXi and HXc technologies?Each technology offers its own merits in regard to size, efficiency, adaptability for customization and power capability. In more demanding applications, however, the overall cost of heat pipes is competitive with other alternatives. Figure3 clearly illustrates the superiority of the heat pipe. In case of other cooling modules, there is no heat transfer limitation, implying that increasing heat transfer rate just keeps increasing the temperature drop and worsening the situation. Recently, the application of heat pipe even includes smart phones, vehicle headlight, gas burner, LED products, and agricultural systems, as shown in Figure2. Publishing on IntechOpen allows authors to earn citations and find new collaborators, meaning more people see your work not only from your own field of study, but from other related fields too. The working principle of the heat pipe is based on two phase flows pumped by capillary pressure formed at the wick. The heat pipe utilizes phase change heat transfer inside enveloped structures, where the working fluid evaporates in heated zone, and vapor moves to the condenser, and the condensed liquid is pumped back through microporous structure call wick. Regarding the boiling limit, it has been postulated that the boiling limit occurs as soon as the bubble nucleation is initiated. Txis the heat source temperature, and Tcf is the heat sink temperature. The material compatibility with working fluid is shown in Figure12. The capillary limit is related to the ability of the wick to move the liquid through the required pressure drop. The subscripts s, l, and irepresent the shell, liquid, and interface, respectively. As illustrated in Equation (6), key parameters for capillary limit are Kand Reff. The use of heat pipe to electronics cooling is diversified into portable devices, VGA, mobile PC, LED projector and related devices, telecommunication repeater, and so on. However, the effective thermal conductivity is low because the screens are not thermally connected to each other. Why should we buy a heat exchanger from Aavid. Copper is shown to be compatible with water, acetone, and methanol. However, sometimes another performance index, the thermal resistance, is more important when the heat transfer rate is not of an important consideration while the temperature uniformization is more important. The initial cost is also partially offset by improvements in system reliability and increased life of cooler running electronics. First, the working principle of heat pipe is introduced. Especially, the use of heat pipes for electronics cooling has recently been increasing abruptly because the heat pipe is an attractive passive cooling scheme, which can offer high effective thermal conductivity and large heat transport capability. The safety criterion is given as follows: where Yis the yielding stress of the container material. As shown, the type of wick has its pros and cons. Therefore, the thermal performance of the heat pipe is strongly dependent on the wick structure. It should be noted that the wick provides the capillary pumping of working fluid, which must be steadily supplied for the operation of heat pipe as well as the flow passage of the working fluid. The heat pipe design starts with working fluid selection, followed by wick type and container material selections, determining diameter and thickness, wick design, and heat sinksource interface design. *Address all correspondence to: cbyon@ynu.ac.kr. The diameter becomes a major geometric parameter upon consideration of the vapor velocity. The heat pipe has to be anyhow connected to the heat sink for the final heat dissipation to the air. This situation happens under the condition where the pressure drop along the entire flow path is equal to the developed capillary pressure. Attributable to these characteristics, the heat pipe is regarded as an ultimate candidate for addressing the thermal problem of concurrent high-power-density semiconductor industry, which encompasses solar cell, LEDs, power amplifiers, lasers, as well as electronic devices. In this chapter, general aspects of heat pipes for electronics cooling are introduced. The second step is to select the wick type. The heat pipe consists of metal envelope, wick, and working fluid. Therefore, the working fluid should be selected under the consideration of the operating temperature of heat pipe. Have questions about heat pipes or need assistance with thermal management, contact us. Literally, the heat pipe is apparently just a pipe without any accessories for operating it. The main cause of heat pipe failures is gas generation in the heat pipe, but this can be completely avoided by proper cleaning and assembly procedures. In large quantities, the cost of heat pipes drops significantly and often makes them the most economical solution to a cooling application.Can heat pipes work against gravity?Yes, this occurs whenever the evaporator is located above the condenser. )Not all types of passive heat transfer can operate against gravity. Effective cooling technology is a crucial requirement for a reliable operation of electronic components. The boiling limit is known to occur when the bubble nucleation is initiated in the evaporator section. The next two subsections will be devoted to the models for capillary limit and boiling limit, respectively. To fulfill this criterion, the following equation should be satisfied. When the heat is applied at the evaporator by an external heat source, the applied heat vaporizes the working fluid in the heat pipe. The second paragraphed term is about the macroscopic geometry of the heat pipe. The next step is to determine the diameter of the heat pipe. The thermal conductivity will vary with the heat pipe length, unlike solid metals. The first paragraphed term is a combination of the fluid properties, suggesting that the capillary limit of heat pipe is proportional to this term. In addition, large pore size represents significant effect of inertia force. Even though Equation (7) is simple and in a closed form, it is difficult to implement this equation in which these parameters are quite arbitrary, and thus, it is difficult to exactly predict those values. As mentioned, the capillary limit occurs when the liquid flow along the axial direction cannot afford the evaporation rate. Some researchers even insisted that the nucleate boiling in the moderate temperature heat pipe wicks is not only tolerable but could also produce performance enhancement by significantly increasing the heat transfer coefficient over the conduction model and consequently reducing the wick temperature drop [3]. By Mohamed H.A. In this regard, various types of wick structures have been developed, such as mesh screen wick, grooved wick, and sintered particle wick. It should be noted that the maximum developable capillary pressure is inversely proportional to the characteristic length of the pore structure. In the meantime, depletion of liquid by evaporation at the evaporator causes the liquidvapor interface to enter into the wick surface, and thus a capillary pressure is developed there. That is why heat pipes are not recommended for applications where cooling can be performed by simple conductive heat sinks. The application of heat pipe to electronics cooling can be classified by the configuration: heat pipeembedded spreader, block-to-block, block-to-fin, and fin-to-fin applications. How? Figure of merit numbers of working fluids. In Equation (7), important design parameters related to the wick microstructure are rband Pc, which are bubble radius and capillary pressure, respectively. As the heat pipe is like a pressure vessel, it must satisfy the ASME vessel codes. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Contact our London head office or media team here. This is often called the figure of merit of working fluid. The effective thermal conductivity goes down by the ratio of the cross-sectional area. The reason we select the wick type prior to choosing the material is that the manufacturable microstructure is dependent on the material. The sonic limit also typically takes place during unsteady start-up at low temperature, when the chocked flow regime is reached at the sonic speed of the vapor. The types of heat pipe applications to electronics cooling are as follows: use of flat-plate heat pipe, heat pipeembedded heat spreader, block to fin, block to block, and fin to fin. This occurs through wick structures that pump working fluid through capillary pressure developed in the porous wick. This chapter provides the most valuable opportunity for all readers from industry and academia to share the professional knowledge and to promote their ability in practical applications. Generally, the vapor pressure drop (Pv) and interfacial pressure drop (Plv) are negligible when compared with others; thus, the equation reduces to the following: where the left-hand side represents Pc, the first term on the right-hand side is Pl, and the second term corresponds to Pg. Therefore, the designers choose the wick type in accordance with the corresponding suitable applications. The permeability Kis proportional to the pore characteristic length, whereas Reff is inversely proportional to the pore size. The vapor pressure increases as the temperature increases, and when the vapor pressure reaches the pressure of environment, boiling occurs. In some applications, the fin-to-fin module is also used. It should be noted that, in the heat pipe, the limitation on the fluid transport represents the heat transfer limit because the heat transfer rate is given as the multiplication of latent heat coefficient and mass flow rate of working fluid. In addition, the heat pipe is a passive cooling module, which accompanies no power consumption or moving parts. As shown in this figure, the viscous limit, the sonic limit, and the entrainment limit do not play an important role in determining the thermal capacity of the heat pipe, unless the temperature is very low (< 20C). Did you know that KTK is the thermal management supplier to the top medical imaging companies? Figure5 shows three representative types of wick structures: mesh screen wick (it is also often termed as fiber mesh or wrapped screen), grooved wick, and sintered particle (or sintered powder) wick. Therefore, new light should be shed on the model for the boiling limit. The boiling limit occurs when the bubble barricades the liquid flow onto the heated surface. It is reported as W/mk. Heat pipes will not operate until the temperature rises above the freezing temperature of the fluid. Performance limitations with respect to the temperature. Likewise, the working fluid circulates in a closed loop inside the envelope, while evaporation and condensation simultaneously take place for heat absorption and dissipation, respectively.
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