CHAPTER 2 LITERATURE REVIEW

Transcription

1 31 CHAPTER 2 LITERATURE REVIEW The cooling of electronics has emerged as a challenging and constraining problem during the last several decades. The economic market demands ever faster clock speeds and simultaneously smaller physical enclosures. Since computer chip heat fluxes (rate of heat transfer per unit area) increase with increasing clock speeds and decreasing chip sizes, these demands have led to skyrocketing heat flux removal demands. Such challenges posed to make the thermal management of electronics as a vital technology in the continued development of 21st century microelectronic systems. Indeed, thermal management of many systems that are likely to be developed in the next several years cannot be done with the current state of technology. In the meanwhile increasing the performance of the present cooling technology is essential to meet the nearby development of the present status in the electronics industries. Present electronic cooling technology scenario, air cooling is the most common technique. Innovative air cooling techniques allowed heat dissipation rates of W by the late 1990s. Nowa-days heat dissipation rate is increased to W for CPU. Cooling systems for computers may be categorized as air-cooled, hybrid-cooled, liquid-cooled, or refrigeration-cooled. In electronic systems thermal management, excess heat from the small chips has to be dissipated. Hence extended surface called heat sink has been used to remove that excess heat. Over the years heat sinks have been used as the method to enhance the effective heat transfer in various industrial applications where space and weight are an important constraint such as in the electronic industry for cooling of electronic devices and other heat exchanging devices. In air-cooled system, forced convective heat transfer is a

2 32 dominated phenomenon in which heat is removed directly from arrays of electronic modules and packages. Among the novel methods for thermal management of the high heat fluxes found in microelectronic devices, forced air cooled heat sinks are the most effective at heat removal especially for desktop computer. Thus the thermal and flow characteristics of heat sinks have been the interest of many investigators. A large number of recent investigations have studied the forced air cooled heat sinks for electronics cooling as well as to compare the flow and heat transfer characteristics of heat sinks. A comprehensive review of these investigations conducted over the past decade is presented in this chapter.the critical review of literature on thermal management of CPU cooling has been carried out. The CPU cooling has been studied broadly on experimental and numerical. The thermal performance of the heat pipe and heat sink for CPU cooling under different conditions has been studied by many researchers. Also the literature on variety of heat sinks used in different electronic applications has been identified. 2.1 Studies on Experimental and Numerical analysis of Heat pipes Many of the researchers have focused on heat pipe cooling and few of them discussed about liquid cooling for CPU. Zhao and Avedisian (1997) have presented an experimental study of heat transfer from an array of copper plate fins supported by a copper heat pipe and cooled by forced air flow. The results are compared to an identical array of copper fins, but supported by a solid copper rod. The primary variable is the height of the fin stack, while the fin pitch, air flow rate, surface area and fin shape are fixed. The results show that for some conditions, fins of fixed pitch supported by a heat pipe dissipate higher heat transfer rates for the same surface temperature than fin arrays supported by a solid rod. The difference in heat transfer rates decreases as the height of the fin stack decreases. The maximum steady state heat fluxes and

3 33 total powers have been measured to be 80 W cm 2 and 800 W, respectively, for the tallest fin stack studied (10.16 cm) for an approach air velocity of 5.9 m s 1 and a surface temperature rise above the ambient of 160. The fin stacks supported by a solid copper rod has dissipated 30 W cm 2 and 300 W for the same conditions. For the smallest height examined (2.54 cm) no significant advantage has been realized by using a heat pipe to support the fin stack. A simplified analysis is also presented to predict surface temperature for a known heat input for both heat pipe supported and solid rod supported plate fin arrays. Marongiu et al. (1998) have discussed the investigation of microheat pipes and other high thermal conductivity materials that has been incorporated into Multi Chip Modules (MCM). The parameters that affect the heat dissipation capabilities such as fin material, fin height, heat pipe configuration and pumping power have been changed and analyzed using Icepak. An experimental investigation of the thermal performance of a flat plate heat pipe has discussed by Wang and Vafai (2000). The results indicate that the temperature along the heat pipe wall surfaces is quite uniform. The results also indicate that the porous wick of the evaporator section creates the main thermal resistance resulting in the largest temperature drop, which consequently affects the performance of the heat pipe. The idea of the heat pipe time constant is introduced in this work to describe the transient characteristics of the flat plate heat pipe and an empirical correlation for the time constant in terms of input heat flux is presented. Correlations for the maximum temperature rise and maximum temperature difference within the heat pipe are also presented. The experimental results at steady state are compared with the analytical results and found to be in good agreement. This

4 34 work constitutes the first detailed experimental investigation of a flat plate heat pipe Studies of heat pipe cooling for Pentium IV CPU Kim et al. (2003) have proposed cooling using heat pipe, and recommended Pentium IVCPU cooling module on change of fan revolution speed and have looked for possibility of reducing acoustic noise. They have developed cooler using heat pipe and the thermal stability has been tested and compared to heat sink with fan. To overcome the poor cooling performance, an acoustic module in the form of remote heat exchanger using heat pipe has been developed. Cooling performance of different cooling modules has been compared and analyzed and have been concluded that the heat sink cooling module has provided the most excellent thermal performance at high fan speed, but that the heat pipe cooling module provided better thermal performance than heat sink below 2950 rpm. It has been concluded that the heat pipe cooling module has produced low acoustic noise when compared to heat sink cooling module. The CEOHP cooling module is described by Rittidech et al. (2005). It consists of two main parts, i.e. the aluminum housing and the CEOHP. The house casing has been designed to be suitable for CEOHP. The CEOHP design employed copper tubes: two sets of capillary tubes with an inner diameter of m, an evaporator length of 0.05 and a condenser length of 0.16 m and each of which has six meandering turns. The evaporator section has been embraced in the aluminum housing and attached to the thermal pad of Pentium 4 CPU, model SL 6 PB, 2.26 GHZ, while the condenser section has been embraced in the cooling fin housing and cooled by forced convection. R134a has been used as the working fluid with filling ratio of 50%. In the experiment, the CPU chip with a power of 58 W has 70 C and fan speed of 2000 and 4000 rpm. It is found that, if fan speed increases the

5 35 cooling performance increases. The CEOHP cooling module has better thermal performance than conventional heat sink Studies of heat pipe cooling for desktop computer CPU Miller et al.(2007) have described the design of a high performance single phase liquid cooling system which has been used to cool single or multiple heat sources within the computer system. The liquid cooling system consists of copper cold plates, heat exchanger, centrifugal pump, flexible tubing and a coolant has been integrated for long operating life. Thecooling system described copper cold plates with meso scalechannels to pick up heat from the CPU and GPU typeheat sources and highly efficient liquid-to-air heat exchangers with flatcopper tubes and plain fins to transfer the heat toair by forced convection. A water based coolant is usedfor high thermal performance and additives are used to provideburst protection to the cooling system at temperatures down to 40 C and corrosion protection to critical components. A highlyreliable compact pump is used to circulate the fluid ina closed loop. The overall system is integrated using assemblymethods and materials that enable very low fluid permeation forlong life. An experimental investigation on the thermal resistance of a heat sink with horizontal embedded heat pipes has conducted by Wang et al.(2007). Heat sink with embedded heat pipes separates heat from CPU to both the base plate and the heat pipes, and then removes heat from fins to the surrounding. This experimental approach measures the thermal resistances of interface between CPU and base plate, base plate, CPU to heat pipes, heat pipes and fins through the thermal resistance analysis. In this paper, the first step is to measure the thermal performance of a heat pipe and the next step is the measurement of the thermal performance of the heat sink with and without the function of heat pipes. These results are limited to the case of sink-processor assemblies installed horizontally and two U-shaped embedded heat pipes

6 36 inserted into the heat sink. The results show that two heat pipes embedded in the base plate carry 36% of the total dissipated heat from the CPU, while 64% of the heat is delivered from the base plate to the fins. Feroz and Uddin (2009) have described the heat transfer performance of parallel miniature heat pipes (MHPs) of 2.8 mm ID used for cooling desktop computer with different working fluids which is presented in this paper. In cooling desktop processors, MHPs consist of six single tube heat pipes connected by a copper block at the evaporator section and fifteen parallel copper sheets used as external fins at the condenser section. Acetone and ethanol are used as working fluid. The copper block is placed above the heat source (on the top of the processor) and the condenser section is provided with external fins perpendicular to the MHPs. Heat transfer characteristics of MHPs using different working fluids are determined experimentally, based on the principle of phase. The experimental results show that the maximum and steady state temperature of the processor has been reduced significantly by using MHPs with acetone as the working fluid instead of a conventional cooling fan. An additional use of a fan at the condenser section results in much lower processor surface temperature for both working fluids. 2.2 Studies on Experimental and Numerical analysis of impinging jet cooling on heat sinks Nowadays, micro-jets impinging cooling devices have found a large number of applications in cooling of electronics. Micro-jet impingement is a very efficient method for removing a large amount of heat from a uniformly heated plate. Heat transfer and fluid flow characteristics of a single jet impinging on a heated surface has been the subject of numerous investigations for many years. Due to the wide application of jet impingement cooling and its high heat transfer coefficient, many literature reviews have been done on this subject. Most of those literature reviews were on

7 37 experimental work. So far, little analytical and numerical work has been reported due to the nature of this very complex problem. Most of the numerical investigations were conducted on two- dimensional flows in a single jet with large dimensions Experimental Studies of Air Impinging Jets Lin et al. (1997) experimentally performed a confined slot jet impingement for electronic cooling applications. They explored the parametric effects of jet Reynolds numbers and jet separation distance on heat transfer characteristics of the heated target surface. With the measurement of jet mean velocity and turbulence intensity distributions at nozzle exit, two jet flow characteristics at nozzle exit; initially laminar and transitional/turbulent regimes were classified. As for the investigation of heat transfer behavior on stagnation, local and average Nusselt number, the effect of jet separation distance was not significant; while the heat transfer performance increased with increasing jet Reynolds number. A concept of effective cooling length was introduced to evaluate the average Nusselt number on a finite-length target surface. The existing numerical results were reasonably consistent with their experimental data. Single-phase force convection in deep rectangular micro channels is studied experimentally by Harms et al. (1999). They reported that decreasing the channel width and increasing the channel depth to allow better flow and heat transfer performance. For single channel, the experimental Nusselt number was higher than predicted numerically for all flow rates. Nishino et al. (1996) have provided the detailed turbulence statistics in the stagnation region of an axisymmetric jet impinging vertically on a flat plate. The measurements have been made in a submerged water jet facility at a Reynolds number of approximately 13,000 based on the nozzle exit velocity

8 38 and the nozzle diameter. Particle-tracking velocimetry was used for the measurement of highly turbulent flows near the stagnation point. The axial mean momentum balance was examined to clarify the effect of the turbulent normal stress on the axial mean momentum transport. It has been found that the turbulent normal stress of the axial component has made a substantial contribution to the increase in the static pressure near the impingement wall. Detailed distributions of the turbulent stresses and the triple correlations of velocity fluctuations are presented. The state of turbulence has been studied by means of an invariant map of the turbulent stress anisotropy. It has been revealed that the turbulence is close to an axisymmetric state in the stagnation region. The budget equation for the turbulent kinetic energy evaluated from the present data shows that the net negative production takes place in the vicinity of the wall. This negative production is compensated by the pressure diffusion. Jang et al. (2003) have experimentally investigated the heat transfer enhancement of a microchannel heat sink subject to an impinging jet. In order to evaluate the cooling Performance of the microchannel heat sink subject to an impinging jet, temperature distributions are measured by using a microthermal sensor array manufactured through simple and convenient microfabrication processes. In the paper, the experimental results for the thermal resistance of the microchannel heat sink subject to an impinging jet are compared with numerical results. In order to show the heat transfer enhancement of the microchannel heat sink subject to an impinging jet, its thermal resistance is compared with that of a microchannel heat sink with a parallel flow. Under the condition that the pumping power is 0.072W, the thermal resistance of the microchannel heat sink subject to an impinging air jet is experimentally obtained to be 6.1 C/W, which is enhanced by about 48.5% compared with that of the microchannel heat sink with a parallel flow. In addition, the microchannel heat sink subject to an impinging jet is shown to

9 39 be superior to a manifold microchannel heat sink as a cooling device for advanced electronic equipment with high heat generation and compact size Numerical Studies of Air Impinging Jets In this study, the effects of jet-to-plate distance on heat and flow characteristics are described. Yongmann et al. (2002) have studied the unsteady heat transfer caused by a confined impinging jet using direct numerical simulation (DNS). The time-dependent compressible Navier- Stokes equations are solved using high-order numerical schemes together with high-fidelity numerical boundary conditions. A sixth-order compact finite difference scheme is employed for spatial discretization while a thirdorder explicit Runge-Kutta method is adopted for temporal integration. Extensive spatial and temporal resolution tests have been performed to ensure accurate numerical solutions. The simulations cover several Reynolds numbers and two nozzle-to-plate distances. The instantaneous flow fields and heat transfer distributions are found to be highly unsteady and oscillatory in nature, even at relatively low Reynolds numbers. The fluctuation of the stagnation or impingement Nusselt number, for example, can be as high as 20 percent of the time-mean value. The correlation between the vortex structures and the unsteady heat transfer is carefully examined. It is shown that the fluctuations in the stagnation heat transfer are mainly caused by impingement of the primary vortices originating from the jet nozzle exit. The quasi-periodic nature of the generation of the primary vortices due to the Kelvin-Helmholtz instability is behind the nearly periodic fluctuation in the impingement heat transfer, although more chaotic and nonlinear fluctuations are observed with increasing Reynolds numbers. The Nusselt number distribution away from the impingement point, on the other hand, is influenced by the secondary vortices which arise due to the interaction between the primary vortices and the wall jets. The unsteady vortex separation from the wall in the higher Reynolds

10 40 number cases leads to a local minimum and a secondary maximum in the Nusselt number distribution. These are due to the changes in the thermal layer thickness accompanying the unsteady flow structures. Tien and Huang (2005) have investigated the feasibility and effectiveness of air impingement cooling on pin-fin heat sinks applied in personal computers numerically. The effects of fin height, fin width, base plate thickness of the heat sink, and the ratio of the vertical spacing between the nozzle and the heat sink to jet diameter (z/d) have been discussed. For a PC with 68.4 W CPU, it has been found that the cooling effect by using air impingement on a heat sink with z/d = 4 and fin width 4.13 mm for the jet Reynolds number (Re) between to is comparable with the cooling effect achieved by using traditional coolers. When the fin width is increased to 5.3 mm, the performance by jet impingement is improved significantly. Specifically, the cooling performance by jet impingement for Re = and fin width 5.3 mm surpasses that by using a fan-heat sink coolers. Moreover, by changing the design of the PC chassis, the fin height and z/d is increased from mm to mm and from 4 to 8, respectively. It is found that the cooling effect is further enhanced. When handling CPU s with higher power (> 80 W), jet impingement cooling along with proper heat sink design and PC chassis design offer another possible choice. Mikhail et al. (1982) studied laminar 2D flow from a row of impinging slots where they investigated two types of input velocity profiles, parabolic and uniform. They reported that the parabolic profile results in higher values of both the average Nusselt and the local Nusselt number in the stagnation region. In addition, their results also showed that the average Nusselt number increases as the nozzle-to-plate spacing decreases. Law and Masliyah (1984) used a 2-D numerical model to study a laminar impinging jet. These types of jets are less commonly used than

11 41 turbulent impinging jets. However, they can be encountered in practice especially when the jet-to-plate spacing Zn is small and very high stagnation pressure is not desirable. They studied experimentally and numerically the flow characteristics of a confined laminar jet impinging on a flat plate. Baughn and Shimizu (1989) reported that the jet-to-plate distance not only affects the heat transfer rate, but also has a significant effect on the local heat transfer coefficient distribution. They used a uniformly heated plate in conjunction with liquid crystals for measurement of temperature distribution. They found that for a jet-to-plate distance of 6 jet diameters and a Reynolds number of around 20,000, the confinement and the jet-outlet conditions had a marginal influence on the rate of heat transfer. 2.3 Studies on Experimental and Numerical analysis of various types of heat sinks for electronics cooling In this section, many researchers are investigated the performance of heat sinks for electronics cooling by experimentally. In these literatures, not only the performance of various geometries of heat sinks like plate fin, pin fin, elliptical fin, strip fin, but also microchannel heat sinks, porous medium heat sinks and heat sinks with metal foam are discussed Experimental and numerical studies of Plate fin heat sinks Chiang (2005) presents an effective method for predicting and optimizing the cooling performance of Parallel-Plain Fin (PPF) heat sink module based on the Taguchi method. The numerical simulative analyses of the PPF heat sink module have been constructed to understand the affecting situation of its related modeling parameters. The design parameters evaluated are the outline design of the heat sink module and the wind capacity of fan, and the highest temperature (or thermal resistance) of this module is

12 42 considered as the performance characteristics. Taguchi method for the design of experiment (DOE) and the analysis of variance (ANOVA) are applied to find the optimized design parameters efficiently. From the numerical simulative analyses, the optimum design parameters to obtain the lowest value of the highest temperature (or thermal resistance) are found, and the highest temperature value has decreased to C and about 15.01% improvement. The result of the analyses of the noise factors has shown that the two noticeable variable factors are the wind capacity of the electric fan and the gap of fin flake. By using Taguchi method for design of experiment (DOE) and the analysis of variance (ANOVA), four noticeable variable factors will be obtained: number of opening slot, copper base surface area, wind capacity of the fan, and the height of the fin flake respectively. Arularasan and Velraj (2008) have selected an optimal heat sink design in their research work, preliminary studies on the fluid flow and heat transfer characteristics of a parallel plate heat sink have been carried through CFD modeling and simulations. The geometric parameters fin height, fin thickness and fin pitch have been considered in this work. In this research work, optimal design of the heat sink is carried out on a parallel plate heat sink using CFD study. Experimental studies have been performed with a parallel plate heat sink to validate the heat sink model. These results and conclusions drawn in this paper benefit the design engineers involved in electronics cooling. Kim et al. (2008) have compared the thermal performances of the two types of heat sinks most commonly used in the electronic equipment cooling: plate-fin and pin-fin heat sinks. In order to obtain the fluid flow and thermal characteristics of heat sinks, an experimental investigation is conducted. Based on the experimental results of the present study and the available data from the existing literature, the correlations of the friction factor and the Nusselt number are suggested for each type of heat sink.

13 43 Correlations for the pin-fin heat sinks are newly developed, while correlations for the plate-fin heat sinks are selected from previous models. By using the appropriate correlations, thermal resistances of the optimized plate-fin and pin-fin heat sinks are compared under fixed pumping power conditions. Finally, a contour map, which depicts the ratio of the thermal resistances of the optimized plate-fin and pin-fin heat sinks as a function of dimensionless pumping power and dimensionless length, is presented. The contour map indicates that the optimized plate-fin heat sinks possess lower thermal resistances than optimized pin-fin heat sinks when dimensionless pumping power is small and the dimensionless length of heat sinks is large. On the contrary, the optimized pin-fin heat sinks have smaller thermal resistances when dimensionless pumping power is large and the dimensionless length of heat sinks is small. Bin et al. (2008) have employed the slotted fin concept to improve the air cooling performance of plate-fin in heat sinks. Numerical simulations of laminar heat transfer and flow pressure drop have been conducted for the integral plate fin, discrete plate fin and discrete slotted fin heat sinks. It is found that the performance of the discrete plate fin is better than that of the integral continuum plate fin and the performance of slotted fin is better than that of the discrete plate fin at the same pumping power of the fan. A new type of heat sink characterized by discrete and slotted fin surfaces with thinner fins and smaller spaces between the fins is then proposed. A Preliminary computation shows that this type of heat sink may be useful for the next generation of higher thermal load CPUs. The limit of cooling capacity for air-cooling techniques is also addressed. Kim and Kim (2009) have experimentally studied the effects of cross-cuts on the thermal performance of heat sinks under the parallel flow condition. To find the effects of the length, position, and number of cross-

14 44 cuts, heat sinks with one or several cross-cuts ranging from 0.5 mm to 10 mm have been fabricated. The pressure drop and the thermal resistance of the heat sinks are obtained in the range of 0.01 W<Pp < 1 W. Experimental results show that among the many cross-cut design parameters, the cross-cut length has the most significant influence on the thermal performance of heat sinks. The results also show that the heat sinks with a cross-cut are superior to heat sinks containing several cross-cuts in the thermal performance. The friction factor and Nusselt number correlations for heat sinks with a crosscut are suggested by using experimental results. Through the optimization process, the thermal performances of three types of heat sinks, a plate-fin heat sink, a square pin-fin heat sink, and a cross-cut heat sink, are compared under the constraint of a fixed pumping power and a fixed heat sink volume. Finally, a contour map that shows the optimum type of heat sink as a function of the dimensionless pumping power and heat sink length is obtained. Sivasankaran et al.(2010) have studied the experimental investigation of parallel plate fin and cross cut pin fin heat sinks where the heating element is placed asymmetrically. The thermal performance, heat transfer coefficient and efficiencies have been compared for various heat sinks. It is concluded that the average heat transfer coefficient of parallel plate fins is higher than that of cross cut pin fins. The fan distance is varied in the experimental work to find the optimum distance for maximum efficiency for both parallel plate and cross cut pin fin heat sinks. Kim et al.(2010) have conducted the thermal optimization of a plate-fin heat sink with the fin thickness varying in the direction normal to the fluid flow. The model used for this optimization is based on the volume averaging theory (VAT). It is found that the thermal resistance of the platefin heat sink can be reduced by allowing the fin thickness to increase in the direction normal to the fluid flow. In the case of a water-cooled heat sink, the

15 45 thermal resistance decreases by as much as 15%. The amount of the reduction increases as the pumping power increases or as the length of the heat sink decreases. In the paper the authors have suggested that due to its high thermal performance, the variable-thickness-fin heat sink is expected to be suitable as a next generation cooling solution Studies on pin fin, elliptical fin and micro channel heat sinks Peterson and Chang (1997) have presented the results of a heat transfer analysis of two-phase heat dissipation using a high-conductivity porous-channel heat sink. In the analysis, a consistent set of conservation equations based on phase-averaged properties of the fluid are derived from the conventional ones and solved numerically by the finite volume method. The results indicate that the high conductivity and large solid-fluid contact area of the porous channel result in a high heat transfer performance of twophase heat dissipation, which may be an alternative to cooling techniques for microelectronics with high heat flux. Jonsson and Moshfegh (2001) have conducted tests in a wind tunnel with seven types of heat sinks including plate fin, strip fin, and pin fin heat sinks. In the case of strip fin, and pin fin heat sinks, both in-line and staggered arrays have been studied. The pin fin heat sinks have circular and square cross-sections. For each type, tests have been run with fin heights (H) of 10, 15, and 20 mm while the heat sink width (B) has been kept constant and equal to 52.8 mm. In total, 42 different heat sinks have been tested. The width of the wind tunnel duct (CB) is varied in such a way that results are obtained for B/CB=0.84, 0.53, and The wind tunnel height (CH) is varied similarly, and data are recorded for H/CH=1, 0.67, and 0.33 while the duct Reynolds number varies between 2000 through An empirical bypass correlation has been developed for the different fin designs. The correlation predicts the Nusselt number and the dimensionless pressure drop

16 46 and takes into account the influence of duct height, duct width, fin height, fin thickness, and fin-to-fin distance. The correlation parameters are individual for each fin design. Further, a physical bypass model for plate fin heat sinks has been developed to describe the bypass effect. Kim et al.(2001) have been numerically carried out to investigate the thermal characteristics of an aluminum foam heat sink. An aluminum foam heat sink placed horizontally in a channel is modeled as a hydraulically and thermally anisotropic porous medium. A uniform heat flux is given from the bottom of the heat sink. Cold air is supplied from the top opening of the channel and exhausts to the channel outlet. Comprehensive numerical solutions are acquired to the governing Navier-Stokes and energy equations, using the Brinkman-Forchheimer extended Darcy model for the region of heat sink. It is assumed that the solid is in the local thermal equilibrium with the fluid. Details of flow and thermal fields are examined over wide ranges of the principal parameters: the Darcy number Da, the anisotropic permeability ratio K*(=K 2 /K 1 ), and the anisotropic effective thermal conductivity ratio k*(=k 2 /k 1 ). The results indicate that the anisotropy in permeability and effective thermal conductivity yields a significant change in the heat transfer rate. The same numerical approach that involves the anisotropic porous medium model is also applied to the thermal analysis of a conventional pinfin heat sink. Yang and Fu (2001) have presented the fluid and thermal analysis of the heated electrical component through a numerical solution of the Navier- strokes equations for the forced convection by a fan. In this paper, the computed pressure and heat flux distributions along the surface of the electronic component have been discussed. This study provides detail about flow structure and heat transfer mechanism which may give fundamental knowledge in designing electronic cooling.

17 47 Ryu et al.(2002) have presented the analysis of heat transfer and fluid flow in square pin fin heat sinks through an experimental method. Twenty aluminum square pin fin heat sinks have a mm mm base size is tested with inlet velocity ranging from 1 m/s to 5 m/s. In each test, the heat sink is heated uniformly at the bottom. From the experimental results, the effects of the height of the heat sink and the ratio of the spacing between the adjacent fins of the fin pitch on the pressure drop across the heat sink and the thermal resistance of the heat sink are investigated. In addition, the design guidelines for shrouded square pin fin heat sinks are suggested considering both pumping power and thermal resistance. Hetsroni et al.(2002) have presented the experimental investigation of a heat sink for cooling of electronic devices. In the study the operating temperature is kept at a relatively low level of about K, using a dielectric liquid that boils at a lower temperature, while reducing the undesired temperature variation in the both streamwise and transverse directions. The experimental study is based on systematic measurements of temperature, flow and pressure, infrared radiometry and high-speed digital video imaging. The heat sink has parallel triangular microchannels with a base of 250 lm. Experiments on flow boiling of Vertrel XF in the microchannel heat sink have been performed to study the effect of mass velocity and vapor quality on the heat transfer, as well as to compare the twophase results with a heat sink cooled by single-phase water flow. Qu and Mudawar (2002) have investigated the pressure drop and heat transfer characteristics of a single-phase micro-channel heat sink by experimentally and numerically. The heat sink has been fabricated from oxygen-free copper and fitted with a polycarbonate plastic cover plate. The three-dimensional heat transfer characteristics of the heat sink have been analyzed numerically by solving the conjugate heat transfer problem

18 48 involving simultaneous determination of the temperature field in both the solid and liquid regions. They have also presented and discussed a detailed description of the local and average heat transfer characteristics of the heat sink. The measured pressure drop and temperature distributions show a good agreement with the corresponding numerical predictions. These findings demonstrate that the conventional Navier Stokes and energy equations can adequately predict the fluid flow and heat transfer characteristics of microchannel heat sinks. Xuet al.(2005) have demonstrated a new silicon microchannel heat sink, composing of parallel longitudinal microchannels and several transverse microchannels, which separate the whole flow length into several independent zones, in which the thermal boundary layer is in developing. The redeveloping flow is repeated for all of the independent zones and thus the overall heat transfer is greatly enhanced. Meanwhile, the pressure drops are decreased compared with the conventional microchannel heat sink. Both the benefits of enhanced heat transfer and decreased pressure drop ensure the possibility to use larger hydraulic diameter of the microchannels so that less pumping power is needed, which is attractive for high heat flux chip cooling. The above idea fulfilled in microscale is verified by a set of experiments. The local chip temperature and Nusselt numbers are obtained using a high resolution Infrared Radiator Imaging system. Preliminary explanation is given on the decreased pressure drop while enhancing heat transfer. The dimensionless control parameter that guides the new heat sink design and the prospective of the new heat sink are discussed. Gingras and Gosselin (2008) have presented a conceptual design of a heat sink combining a porous medium whose matrix is highly conductive and a fin. A simplified model is presented to estimate the performance of the system, relying on Darcy law and local thermal

19 49 equilibrium. The objective is to minimize the hot-spot temperature under global mass constraint by using an optimization procedure based on genetic algorithms. The design variables are the porosity and material of each layer of the porous medium, the fin material, height, and width, the aspect ratio of the heat sink, and the shape of a weightless upper corner deflector which reduces the width of the inlet and outlet air slots which remove the less useful mass. Results show that the optimal porous layers have been generally of copper, independent of the mass constraint. However, the fin is mostly beneficial for heavier designs, while the deflector becomes more important when lightness is required. These two special features show their efficiency by allowing a mass reduction of 95% with a decrease of only 24% in the cooling performance. Yang and Peng (2009) have demonstrated the numerical simulation of the compound heat sink and have provided physical insight into the flow and heat transfer characteristics. The governing equations are discretized by using a control-volume-based finite-difference method with a power-law scheme on an orthogonal nonuniform staggered grid. The coupling of the velocity and the pressure terms of momentum equations are solved by the SIMPLEC algorithm. The well-known RNG k - two-equations turbulence model is employed to describe the turbulent structure and behaviour. The compound heat sink is composed of a plate fin heat sink and some pins between plate fins. The objective of this investigation is to examine the effects of the types and the arrangements of the pins. It is found that the compound heat sink has better synthetical performance than the plate fin heat sink. Moreover, the compound heat sink which is composed of a plate fin heat sink and circular pins performs better than the square ones. Seyf and Layeghi (2010) have carried out a numerical analysis to determine flow and heat transfer characteristics of an elliptical pin fin heat

20 50 sinks with and without metal foam inserts. The effects of metallic foam properties at various Reynolds numbers are studied numerically. Different metallic foams with various porosities and permeabilities are used in the numerical analysis. The Darcy Brinkman Forchheimer and classical Navier Stokes equations, together with corresponding energy equations are used in the numerical analysis of the flow field and heat transfer in the heat sink with and without metal foam inserts, respectively. A finite volume code with point implicit Gauss Seidel solver in conjunction with algebraic multigrid method is used to solve the governing equations. The code is validated by comparing the numerical results with available experimental results for a pin fin heat sink without porous metal foam insert. The effects of air flow Reynolds number and metal foam porosity and permeability on the overall Nusselt number, pressure drop, and the efficiency of the heat sink are investigated. The results indicate that the structural properties of metal foam insert can significantly influence both flow and heat transfer in a pin fin heat sink Experimental and CFD analysis of Heat sinks for desktop computers In this chapter, several researchers have worked on conjugate heat transfer, i.e. simultaneous conduction and convection, problem at electronic systems via CFD. This study makes use of CFD for the conjugate heat transfer simulations in a whole computer chassis. Icepak and FLUENT are used simultaneously for the CFD calculations. In the last decade, there has been a great decrease in the cost of CFD applications with the development of new workstations and personal computers. CFD is a very powerful tool in the sense that qualitative issues and quantitative data can be obtained if CFD is complemented by experiments. The number of data that one can obtain is limited in experiments, but CFD is very useful to offer a large set of data. Therefore, CFD can be used to minimize the number of experiments and

21 51 design alternatives. Although known for a long time, Computational Fluid Dynamics (CFD) has not been used for electronics cooling for a long period. Linton and Agonafer (1994) have presented an alternative approach to modeling box cooling inelectronic packages and have compared the results of detailed CFD modelling of a heat sink with experimental data. A finitecontrol-volume simulation code is used to simulatean IBM desktop Personal Computer. Only the geometry, the overallair flow rate, the turbulent viscosity and the power dissipationsfrom each card must be specified. The simulation code predictsthe flow distribution inside the PC, the convection coefficients, theturbulence effects, and the temperatures. Then they have presented a technique for representing the heat sink in a coarse manner for less time consuming simulations. Their coarse model agrees well with the detailed model without losing the characteristics of the heat sink. Lee and Mahalingam (1994) have used a computational fluid dynamics (CFD) tool to evaluate the velocity and the temperature fields of air flow in a computer system enclosure. Simulations focused on the six printed circuit board regions, where approximately 37 W of power has been generated on the component side of the board. Three-dimensional, steady-state, turbulent air flow is assumed. The actual air flow rate at fans has been determined by matching the predicted pressure rise across the fan to the given fan characteristics. Both straight and swirl flow cases at fans were considered. Experiments have been performed under actual life conditions to verify the computational data. The results demonstrate a reasonable agreement between the temperatures obtained by simulations and measurements. In all comparisons, average errors have been less than 10%. The introduction of the swirl at fans has no significant effect on temperature. Recommendations on improvements of the simulation have also been presented.

22 52 Sathyamurthy and Runstadler (1996) have studied planar and staggered heat sink performance with a commercially available software, FLUENT. Their computational results agree well with the experimental ones. They have found that the thermal performance of staggered fin configuration is superior over planar fin configuration. However, the pressure drop requirements for the staggered fin heat sink have been greater than those for the planar case. Christopher et al.(1996) have studied the system level electronic packaging thermal design not only computationally but also verifying it experimentally. After the verification, they have worked on redesign of an inlet plenum. Their implemented modification resulted in a 56 % reduction of the surface temperature. Tucker (1997) has compared several commercial and noncommercial CFD codes in the systems related to electronics cooling. THEBES, FLOTHERM, FLUENT, FLOTRAN, FIDAP/Icepak, CFX4, PHOENIX/HOTBOX and STAR-CD have been discussed mainly with noncommercial programs originating from Massachusetts Institute of Technology (MIT). General thermo-fluid capabilities, user friendliness and peripheral aspects were checked and for all comparisons with measurement, agreement has been found to be within 30%. Finally, he has concluded that none of the programs have superior performance over the others. Biswas et al.(1999) have also used Icepak to study the airflow in a compact electronic enclosure. Their aim has been to investigate the pressure loss due to the presence of the inlet and outlet grilles. They have considered for the use of fan curves obtained from the manufacturer since the fan curve may be modified if the fan is not closely ducted.

23 53 David Lober (1999) has discussed some thermal management considerations involved in choosing an enclosure and demonstrates the use of CFD thermal modeling software to optimally integrate a computer system into an existing enclosure and reduce the design cycle time. In the study it has been described that the thermal management is one of the primary concerns in selecting the enclosure for the cobra system. The enclosure, rear exhaust fan and front bezel cooling vents have been optimized by using CFD software. System integration requirements have been determined and an appropriate enclosure has been selected for the system. System fan performance and noise level tests have been performed to select the most appropriate cooling fans for the enclosure. David Lober has been concluded that the thermal modeling which has been completed early in the design, allows for a customized cooling scheme to be implemented with no added cost. The result of the modeling and optimization studies allows the design time to be minimized. Chang et al.(2000) have reported the results of CFD analysis to cool the 30W socketed CPU of a desktop computer with minimum air flow rate and minimum heat sink size. In the paper, the methodology of CFD analysis for the heat sink, duct design has been described and experimental results have compared to CFD results. A skive fin heat sink, vertical fin heat sink and various types of duct have been analyzed to reduce CPU case temperature around 10 C relative to an unducted design. Based on the research, it is concluded that it is feasible to use only the power supply fan to cool CPUs up to 30W by using ducted heat sink. Subramanyam and Crowe (2000) have described to evaluate designs of electronic cooling heat sinks by using thermal finite element analysis (FEA) and computational fluid dynamics (CFD). In the paper, quantitative data from wind tunnel testing compare well with model results. Rabid design of electronic cooling heat sinks that meet customer requirements

24 54 have been optimally designed with the help of CFD, FEA and experimental (wind tunnel testing and infrared imaging) tools. Yu and Webb (2001) have analyzed the flow and heat transfer inside a computer cabinet for the high power conditions expected in desktop computers. The design has been in a total chassis power of 313 W. In the research, CFD (Icepak) has been used to identify a cooling solution for a desktop computer, which uses an 80W CPU. In their model, motherboard, PCI/AGP cards and memory have been modeled as zero thickness rectangular plates with heat generated uniformly on the component side. The HDD and DVD have been modeled as solid blocks generating a specified amount of heat inside the volume. Finally the power supply and the CPU heat sink have been modeled as a volume resistance. The design of the CPU heat sink is able to meet the CPU temperature specification and to cool the chassis with one case fan and power supply fan of minimizing chassis air flow requirements. 40W PCI card, different case fan size and different ducting positions have been studied in the paper. Kobayashi et al.(2001) have described a thermal design and simulation method for a closed cabinet with a heat exchanger. Since the system has a total power of 630 W, the air inside the cabinet has an increase of 30 K in temperature. So a heat pipe and heat exchanger system has been installed. In addition, the air flow path has also been changed which manages to reduce the inner air temperature rise to 15 K. A numerical model has been developed for the improvement of the inner temperature rise and the optimum structure has been examined. The numerical results have been compared with the measured values and they have been in good agreement. Moffat (2002) claims that the flow and heat transfer situations encountered in electronics cooling applications are much more challenging than those in heat exchangers and as complex as those encountered in gas

25 55 turbine blade cooling. Since it is almost impossible to get a detailed solution of the thermal and flow fields in a complicated electronic box, like a computer chassis, new areas are emerging for thermal design of such systems. Although known for years, Computational Fluid Dynamics simulations have not entered the electronics cooling area for a long time. Before the last decade, it has been very expensive to perform CFD calculations, but with the introduction of high power workstations and personal computers, the cost of such computations has been drastically reduced. Lin and Huang (2002) have been related with generating a small forward curved fan for the cooling management of laptop computers. This is an integrated study including fan design, mockup manufacture, experimental verification and numerical simulation. Then, the fan performances have been verified by both experimental and numerical approaches. As a commercial code for the numerical simulations, STAR-CD has been used and numerical results have been in good agreement with the experimental ones. The flow rate has been found to be within 2% to 6% error range. Yeh (2002) has employed CFD to check the feasibility of a proposed cooling scheme of using a fan card to cool the printed circuit boards within the rack. A commercial CFD package, Ideas-ESC has been used for the analysis. The main idea of the cooling scheme is to utilize cooling fans to keep the boards at a specific temperature. Since the air flow through the printed circuit boards is questionable, CFD analysis is necessary. Eveloy et al.(2003) have used Flotherm software to provide a perspective on the current capabilities of CFD as a design tool to predict the component temperature on printed circuit boards. Their computations predict the component operating temperature in an accurate range of 3 ºC to 22 ºC, with up to 35 % error. They suggest that the component junction temperature will need to be experimentally measured when used for strategic product

26 56 design decisions. They think that the source of error is due to the turbulence models employed. They suggest using flow visualization in the early design phase to identify aerodynamically sensitive regions on the board, where temperature distributions should be handled with care. Kim (2003) has used Pentium-IV PC and its CPU thermal design power has sharp increase.being a conventional cooling method, aluminum extruded heat sink has disadvantages like poor cooling performance, acoustic noise increase and weight rise and so cooling module in the form of remote heat exchanger using heat pipe is developed. Especially, using system fan exhausting heat inside to cool the CPU, reduces acoustic noise with lowered quantities of fan and makes it possible to reduce manufacturing unit price. The paper proposes cooling using heat pipe and recommends Pentium-IV CPU cooling module on change of fan revolution speed, and looks for possibility of reducing acoustic noise. Kim and Webb (2003) have analyticallyevaluated thethermal performance of plate fin, round pin-fin and offset strip-finheat sinks with a duct-flow type fan arrangement. Heat sinks of 65mm 60mm plan area 50 mm height with a 4300-RPM DC fan (60mm 15mm) have been chosen for the performance comparison. At constant temperature,6 mm thick heat sink base plate is assumed sothat the thermal spreading resistance is not involved. The operating pointon the fan curve is based on the flow pressuredrop impedance curve through a heat sink using the frictionfactor correlation for the chosen heat sink. The loss coefficientsat both the entrance and the exit of the heat sinkare included in the flow impedance curve. The operating pointis defined by the balance point of the flow impedancecurve and the fan performance curve. After determining the operatingair velocity, the convective thermal resistance of heat sinks isevaluated from the Nusselt number correlation for the chosen heatsink. Results obtained show that the