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Effective cooling of electronic components is an important issue for successful functionality and high reliability of the electronic devices. The rapid developments in microprocessors necessitate an enhanced processing power to ensure faster operations. The electronic devi- ces have highly integrated circuits that produce a high heat flux, which leads to increase in the operating temperature of devices, and this results in the shortening of life time of the electronic devices [1]. Consequently, the need for cooling techniques to dissipate the associ- ated heat is quiet obvious. Thus, heat pipes have been identified and proved as one of the There is an increasing demand for efficient cooling techniques in computer industry to dissipatetheassociated heat from thenewly designed and developed computer processors to accommodate for their enhanced processing power and faster operations. Such a demand necessitates researchers to explore efficient approaches for central processing unit (CPU) cooling. Consequently, heat pipes can be a viable and promising solution for this challenge. In this chapter, a CPU thermal design power (TDP), cooling methods of electronic equipments, heat pipe theory and operation, heat pipes components, such as the wall material, the wick structure, and the working fluid, are presented. Moreover, we review experimentally, analytically and numerically the types of heat pipes with their applications for electronic cooling in general and the computer cooling in particular. Summary tables that compare the content, methodology, and types of heat pipes are presented. Due to the numerous advantages of the heat pipe in electronic cooling, this chapter definitely leads to further research in computer cooling applications.

Figure 1 Effective cooling of electronic components is an important issue for successful functionality and high reliability of the electronic devices. The rapid developments in microprocessors necessitate an enhanced processing power to ensure faster operations. The electronic devi- ces have highly integrated circuits that produce a high heat flux, which leads to increase in the operating temperature of devices, and this results in the shortening of life time of the electronic devices [1]. Consequently, the need for cooling techniques to dissipate the associ- ated heat is quiet obvious. Thus, heat pipes have been identified and proved as one of the There is an increasing demand for efficient cooling techniques in computer industry to dissipatetheassociated heat from thenewly designed and developed computer processors to accommodate for their enhanced processing power and faster operations. Such a demand necessitates researchers to explore efficient approaches for central processing unit (CPU) cooling. Consequently, heat pipes can be a viable and promising solution for this challenge. In this chapter, a CPU thermal design power (TDP), cooling methods of electronic equipments, heat pipe theory and operation, heat pipes components, such as the wall material, the wick structure, and the working fluid, are presented. Moreover, we review experimentally, analytically and numerically the types of heat pipes with their applications for electronic cooling in general and the computer cooling in particular. Summary tables that compare the content, methodology, and types of heat pipes are presented. Due to the numerous advantages of the heat pipe in electronic cooling, this chapter definitely leads to further research in computer cooling applications.

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Figure 1. Heat pipe operation [10]. where, AP, is the maximum capillary force inside the wick structure; AP, is pressure drop required to return the liquid from the condenser to the evaporation section; AP, is the pressure drop to move the vapor flow from the evaporation to the condenser section; and AP, is the pressure drop caused due to the difference in gravitational potential energy (may be positive, negative, or zero, depends on the heat pipe orientation and a direction).
Figure 1. Heat pipe operation [10]. where, AP, is the maximum capillary force inside the wick structure; AP, is pressure drop required to return the liquid from the condenser to the evaporation section; AP, is the pressure drop to move the vapor flow from the evaporation to the condenser section; and AP, is the pressure drop caused due to the difference in gravitational potential energy (may be positive, negative, or zero, depends on the heat pipe orientation and a direction).
The heat pipe has many advantages compared with other cooling devices such as the follow- At horizontal orientation @=0, equation (5) will become
The heat pipe has many advantages compared with other cooling devices such as the follow- At horizontal orientation @=0, equation (5) will become
Figure 2. Metal sintered powder wick [15]. As shown in the Figure 2, this type of the wick has a small pore size, resulting in low wick permeability, leading to the generation of high capillary forces for antigravity applications. The heat pipe that carries this type of wick gives small differences in temperature between evaporator and condenser section. This reduces the thermal resistance and increases the effective thermal conductivity of the heat pipe.
Figure 2. Metal sintered powder wick [15]. As shown in the Figure 2, this type of the wick has a small pore size, resulting in low wick permeability, leading to the generation of high capillary forces for antigravity applications. The heat pipe that carries this type of wick gives small differences in temperature between evaporator and condenser section. This reduces the thermal resistance and increases the effective thermal conductivity of the heat pipe.
El-Genk and Lianmin [25] reported on the experimental investigation of the transient respons¢ of cylindrical copper heat pipe with water as working fluid. The copper heat pipe with coppe: screen wick consisted of two layers of 150 meshes. Results showed that the temperature of the vapor was uniform along the heat pipe whereas the wall temperature drop was very smal (maximum variation less than 5 K) between the evaporator section and the condenser section The steady-state value of the vapor temperature was increased when the heat input wa: increased or the cooling water flow rate was decreased. Said and Akash [26] experimentally
El-Genk and Lianmin [25] reported on the experimental investigation of the transient respons¢ of cylindrical copper heat pipe with water as working fluid. The copper heat pipe with coppe: screen wick consisted of two layers of 150 meshes. Results showed that the temperature of the vapor was uniform along the heat pipe whereas the wall temperature drop was very smal (maximum variation less than 5 K) between the evaporator section and the condenser section The steady-state value of the vapor temperature was increased when the heat input wa: increased or the cooling water flow rate was decreased. Said and Akash [26] experimentally
Figure 6. Schematic of the flat plate heat pipe: (a) geometry of the heat pipe and (b) cross-sectional view of the heat pipe [27]. Wang and Vafai [27] presented an experimental investigation on the thermal performance of asymmetric flat plate heat pipe. As shown in Figure 6, the flat heat pipe consists of four sections with one evaporation section in the middle and three condenser sections. The heat transfer coefficient and the temperature distribution were obtained. The results indicated that the temperature was uniform along the wall surfaces of the heat pipe, and the porous wick of the evaporator section had significant effect on the thermal resistance. The heat transfer coefficient was also found to be 12.4 W/m? °C at the range of input heat flux 425-1780 W/m2.
Figure 6. Schematic of the flat plate heat pipe: (a) geometry of the heat pipe and (b) cross-sectional view of the heat pipe [27]. Wang and Vafai [27] presented an experimental investigation on the thermal performance of asymmetric flat plate heat pipe. As shown in Figure 6, the flat heat pipe consists of four sections with one evaporation section in the middle and three condenser sections. The heat transfer coefficient and the temperature distribution were obtained. The results indicated that the temperature was uniform along the wall surfaces of the heat pipe, and the porous wick of the evaporator section had significant effect on the thermal resistance. The heat transfer coefficient was also found to be 12.4 W/m? °C at the range of input heat flux 425-1780 W/m2.
Figure 7. (a) Geometry of different cross-sectional shapes of micro-heat pipe: (i) square star groove, (ii) hexagonal star groove, (iii) octagonal star groove, and (iv) equilateral triangle. (b) Schematic diagram of optimally charged equilateral triangular and star-groove micro-heat pipes [29]. Micro-heat pipes differ from conventional heat pipes in the way that they replace wi structure with the sharp-angled corners, which play an important role in providing capilla1 pressure for driving the liquid phase. Hung and Seng [29] studied the effects of geometr design on thermal performance of star-groove micro-heat pipes. As shown in Figure 7, thre different types of cross-sectional shapes of micro-heat pipes such as square star (4 corners hexagonal star (6 corners), and octagonal star (8 corners) grooves with corner width w, we: considered. Accordingly, the corner apex angle 20 was varied from 20° to 60°. At steady-sta mode, one-dimensional mathematical model was developed to yield the heat and fluid flo characteristics of the micro-heat pipe. Results indicated that the geometrical design of the sta zroove micro-heat pipes provides a better insight on the effects of various geometric parameters, such as cross-sectional area, total length, cross-sectional shape, number of corner and acuteness of the corner apex angle.
Figure 7. (a) Geometry of different cross-sectional shapes of micro-heat pipe: (i) square star groove, (ii) hexagonal star groove, (iii) octagonal star groove, and (iv) equilateral triangle. (b) Schematic diagram of optimally charged equilateral triangular and star-groove micro-heat pipes [29]. Micro-heat pipes differ from conventional heat pipes in the way that they replace wi structure with the sharp-angled corners, which play an important role in providing capilla1 pressure for driving the liquid phase. Hung and Seng [29] studied the effects of geometr design on thermal performance of star-groove micro-heat pipes. As shown in Figure 7, thre different types of cross-sectional shapes of micro-heat pipes such as square star (4 corners hexagonal star (6 corners), and octagonal star (8 corners) grooves with corner width w, we: considered. Accordingly, the corner apex angle 20 was varied from 20° to 60°. At steady-sta mode, one-dimensional mathematical model was developed to yield the heat and fluid flo characteristics of the micro-heat pipe. Results indicated that the geometrical design of the sta zroove micro-heat pipes provides a better insight on the effects of various geometric parameters, such as cross-sectional area, total length, cross-sectional shape, number of corner and acuteness of the corner apex angle.
Figure 8. Schematic of an oscillating heat pipe [33].
Figure 8. Schematic of an oscillating heat pipe [33].
where, g is the acceleration of gravity, p, vapor density, LL, is the effective viscosity of vapor for laminar case is merely the dynamic viscosity, c, specific heat, and k, is thermal conduc- tivity of vapor.
where, g is the acceleration of gravity, p, vapor density, LL, is the effective viscosity of vapor for laminar case is merely the dynamic viscosity, c, specific heat, and k, is thermal conduc- tivity of vapor.
where, u and v are components of velocity in x and y directions, respectively. R, and R, are distributed resistance components in x and y directions, respectively. A dis tributed resistance is a proper method to estimate the effect of porous media.
where, u and v are components of velocity in x and y directions, respectively. R, and R, are distributed resistance components in x and y directions, respectively. A dis tributed resistance is a proper method to estimate the effect of porous media.
*H, horizontal orientation; I, inclined orientation; **SS, steady state; and T, transient. Table 2. An overview of some mathematical studies on heat pipes.
*H, horizontal orientation; I, inclined orientation; **SS, steady state; and T, transient. Table 2. An overview of some mathematical studies on heat pipes.
Figure 9. Coordinate system of the heat pipe [38].
Figure 9. Coordinate system of the heat pipe [38].
Figure 10. Heat pipe heat sink solution for cooling desktop PCs [47]. Due to the high effective thermal conductivity of heat pipes compared to that of traditiona heat sinks, heat pipes have been proposed and selected for electronic cooling. Therefore, th heat pipe transfers and dissipates the heat very fast. Many researchers focused their studie: on using the heat pipe for cooling of electronic devices, and all of them proved that the hea pipe is the best tool for cooling the electronic devices such as desktop and notebook computers Cooling fins equipped with heat pipes for high power and high temperature electronic circuit: and devices were simulated by Legierski and Wiecek [43], and the superiority of the proposec¢ system over the traditional devices was demonstrated. Kim et al. [44] developed a coolins module in the form of remote heat exchanger using heat pipe for Pentium-IV CPU as a mean: to ensure enhanced cooling and reduced noise level compared to the fan-assisted ordinar\ heat sinks. Saengchandr and Afzulpurkar [45] proposed a system that combines the advan tages of heat pipes and thermoelectric modules for desktop PCs. As shown in Figure 10, the usage of the heat pipes with heat sink could enhance the thermal performance [46].
Figure 10. Heat pipe heat sink solution for cooling desktop PCs [47]. Due to the high effective thermal conductivity of heat pipes compared to that of traditiona heat sinks, heat pipes have been proposed and selected for electronic cooling. Therefore, th heat pipe transfers and dissipates the heat very fast. Many researchers focused their studie: on using the heat pipe for cooling of electronic devices, and all of them proved that the hea pipe is the best tool for cooling the electronic devices such as desktop and notebook computers Cooling fins equipped with heat pipes for high power and high temperature electronic circuit: and devices were simulated by Legierski and Wiecek [43], and the superiority of the proposec¢ system over the traditional devices was demonstrated. Kim et al. [44] developed a coolins module in the form of remote heat exchanger using heat pipe for Pentium-IV CPU as a mean: to ensure enhanced cooling and reduced noise level compared to the fan-assisted ordinar\ heat sinks. Saengchandr and Afzulpurkar [45] proposed a system that combines the advan tages of heat pipes and thermoelectric modules for desktop PCs. As shown in Figure 10, the usage of the heat pipes with heat sink could enhance the thermal performance [46].
Figure 11. Prototype: (a) aluminum base plate, (b) copper fin, (c) CEOHP. [49]. thermal and mechanical management of the system was improved using the heat pipe. Kim et al. [44] presented the heat pipe cooling technology for CPU of desktop PC. They had developed a cooler using heat pipe with heat sink to decrease the noise of the fan. Results showed that the usage of heat pipe for desktop PC CPU cooling would increase the dissipated heat without the need for high speed fan. Thus, the problem of the noise generated by the traditional heat sink cooling was solved. Additionally, Closed-end Oscillating Heat pipe CEOHP) used for CPU cooling of desktop PC was presented by Rittidech and Boonyaem [48]. As shown in Figure 11, the CEOHP kit is divided into two parts, i.e., the evaporator is 0.05 m ong and a condenser section is 0.16 m long with and a vertical orientation. They selected R134a as the working fluid with filling ratio of 50%. The CEOHP kit should transfer at least 70 W of heat power to work properly. The CPU chip with a power of 58 W was 70°C. The results indicate that the cooling performance increases when the fan speed increases, where the fan speed of 2000 and 4000 rpm were employed. The thermal performance using CEOHP cooling module was better than using conventional heat sink. Recently, heat sinks with finned U-shape heat pipes have been introduced for cooling the high: frequency microprocessors such as Intel Core 2 Duo, Intel Core 2 Quad, AMD Phenom series and AMD Athlon 64 series, as reported by Wang et al. [49], Wang [50], and Liang and Hung [51]. Wang et al. [49] experimented on the horizontal twin heat pipe with heat sink. The hea input was transferred from CPU to the base plate and from the base plate to the heat pipes anc heat sinks simultaneously. The heat was dissipated from fins to the surrounding by forcec convection. As shown in Figure 12, experiments were conducted in two stages, in which the first stage measured the temperature for heat pipes only to calculate its thermal resistance. The second stage aimed to measure the temperature for heat sink without and with heat pipes ir order to calculate their thermal resistances. It was observed that 64% of the total dissipatec heat was transported from CPU to the base plate and then to fins, whereas 36% was transferrec from heat pipes to fins. The lowest value of the total thermal resistance for the heat pipes witl heat sink was 0.27°C/W.
Figure 11. Prototype: (a) aluminum base plate, (b) copper fin, (c) CEOHP. [49]. thermal and mechanical management of the system was improved using the heat pipe. Kim et al. [44] presented the heat pipe cooling technology for CPU of desktop PC. They had developed a cooler using heat pipe with heat sink to decrease the noise of the fan. Results showed that the usage of heat pipe for desktop PC CPU cooling would increase the dissipated heat without the need for high speed fan. Thus, the problem of the noise generated by the traditional heat sink cooling was solved. Additionally, Closed-end Oscillating Heat pipe CEOHP) used for CPU cooling of desktop PC was presented by Rittidech and Boonyaem [48]. As shown in Figure 11, the CEOHP kit is divided into two parts, i.e., the evaporator is 0.05 m ong and a condenser section is 0.16 m long with and a vertical orientation. They selected R134a as the working fluid with filling ratio of 50%. The CEOHP kit should transfer at least 70 W of heat power to work properly. The CPU chip with a power of 58 W was 70°C. The results indicate that the cooling performance increases when the fan speed increases, where the fan speed of 2000 and 4000 rpm were employed. The thermal performance using CEOHP cooling module was better than using conventional heat sink. Recently, heat sinks with finned U-shape heat pipes have been introduced for cooling the high: frequency microprocessors such as Intel Core 2 Duo, Intel Core 2 Quad, AMD Phenom series and AMD Athlon 64 series, as reported by Wang et al. [49], Wang [50], and Liang and Hung [51]. Wang et al. [49] experimented on the horizontal twin heat pipe with heat sink. The hea input was transferred from CPU to the base plate and from the base plate to the heat pipes anc heat sinks simultaneously. The heat was dissipated from fins to the surrounding by forcec convection. As shown in Figure 12, experiments were conducted in two stages, in which the first stage measured the temperature for heat pipes only to calculate its thermal resistance. The second stage aimed to measure the temperature for heat sink without and with heat pipes ir order to calculate their thermal resistances. It was observed that 64% of the total dissipatec heat was transported from CPU to the base plate and then to fins, whereas 36% was transferrec from heat pipes to fins. The lowest value of the total thermal resistance for the heat pipes witl heat sink was 0.27°C/W.
Figure 12. Heat sink without and with embedded heat pipes [50]. The investigations by Elnaggar et al. [52] on the experimental and finite element (FE) simula- tions of vertically oriented finned U-shape multi-heat pipes for desktop computer cooling are shown in Figure 13a. The total thermal resistance was found to decrease with the increase in heat input and coolant velocity. Moreover, the vertical mounting demonstrated enhanced thermal performance compared with the horizontal arrangement. The lowest total thermal resistance achieved was 0.181°C/W with heat load of 24 W and coolant velocity of 3 m/s. This study was further pursued by Elnaggar et al. [53] to determine the optimum heat input and the cooling air velocity for vertical twin U-shape heat pipe with the objective of maximizing the effective thermal conductivity as shown in Figure 13b.
Figure 12. Heat sink without and with embedded heat pipes [50]. The investigations by Elnaggar et al. [52] on the experimental and finite element (FE) simula- tions of vertically oriented finned U-shape multi-heat pipes for desktop computer cooling are shown in Figure 13a. The total thermal resistance was found to decrease with the increase in heat input and coolant velocity. Moreover, the vertical mounting demonstrated enhanced thermal performance compared with the horizontal arrangement. The lowest total thermal resistance achieved was 0.181°C/W with heat load of 24 W and coolant velocity of 3 m/s. This study was further pursued by Elnaggar et al. [53] to determine the optimum heat input and the cooling air velocity for vertical twin U-shape heat pipe with the objective of maximizing the effective thermal conductivity as shown in Figure 13b.
Figure 13. Finned U-shape heat pipe for desktop computer cooling [53, 54]. (a) Finned U-shape multi-heat pipe [53]. (b) Finned U-shape twin heat pipe [54].
Figure 13. Finned U-shape heat pipe for desktop computer cooling [53, 54]. (a) Finned U-shape multi-heat pipe [53]. (b) Finned U-shape twin heat pipe [54].
Table 3. A summary of studies on heat pipe with heat sink for CPU PC cooling. A summary of studies on heat pipe with heat sink for CPU PC cooling are listed in Table 3.
Table 3. A summary of studies on heat pipe with heat sink for CPU PC cooling. A summary of studies on heat pipe with heat sink for CPU PC cooling are listed in Table 3.
Figure 14. Laptop’s cooling using a heat pipe with heat sink [56].
Figure 14. Laptop’s cooling using a heat pipe with heat sink [56].
Related topics:
Mechanical EngineeringMaterials ScienceElectronics CoolingComputer Coolingheat pipe
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