thermal analysis of cpu with composite pin fin heat sinks

R.Mohan et. al. / International Journal of Engineering Science and Technology Vol. 2(9), 2010, 4051-4062

THERMAL ANALYSIS OF CPU WITH COMPOSITE PIN FIN HEAT SINKS

R.Mohan Department of Mechanical Engineering,

Sona College of Technology, Salem-636005 Tamil Nadu, India.

Dr.P.Govindarajan

Principal and Head of Mechanical Department Department of Mechanical Engineering,

Sona College of Technology, Salem-636005 Tamil Nadu, India.

Abstract: This paper describes about pin fin and slot parallel plate heat sinks with copper and carbon carbon composite(CCC) base plate material mounted on CPUs. The parameters such as fin geometry, base plate material, base plate thickness, number of fins, fin thickness are considered and primarily in this paper fin geometry, base plate thicknesses, base plate materials are optimized for improving the thermal performance of a heat sink in the next generation. In this research work, the thermal model of the computer system with various fin geometry heat sink design has been selected and the fluid flow, thermal flow characteristics of heat sinks have been studied. The plate, pin and Elliptical fin geometry heat sinks have been used with base plate to enhance the heat dissipation. In this study a complete computer chassis with different heat sinks are investigated and the performances of the heat sinks are compared.

Keywords: Forced Cooling of Electronic Devices, Computational Fluid Dynamics, Pin fin Heat Sink and copper & CCC base plate.

1. INTRODUCTION Todays rapid IT development like internet PC is capable of processing more data at a tremendous speed. This leads to higher heat density and increased heat dissipation, making CPU temperature rise and causing the shortened life, malfunction and failure of CPU. Electronic portable devices, especially desktop PC, CPU have become challenging and popular nowadays. The new wave of computer technology making a crucial impact on modern world and desktop computer is widely employed in state-of-the-art industry. The failure rate of electronic components grows as an exponential function with their rising temperature. Power dissipation would be a major bottleneck to development of the micro electronic industry in the next 5 to 10 years. The performance level of electronic systems such as computers are increasing rapidly, while keeping the temperatures of heat sources under control has been a challenge. Many cooling techniques such as cooling by the heat pipes, cold water, and semiconductor and even by liquid nitrogen were proposed and adopted. Liquid nitrogen cooling is very expensive and not suitable for conventional use. Due to cost constraints, conventional air cooling technology with a fan, heat sink combination widely used to cool desktop computers. The air cooling technique is always significant and worthy of further study. The challenging aspect for improving heat sink performance is the effective utilization of relatively large air-cooled fin surface areas when heat is being transferred from a relatively small heat source (CPU) with high heat flux. When the heat loads are small, thermal conduction through aluminum plates is sufficient to spread the heat into finned heat sinks for convection from the fins into the air. In recent years, as the heat loads have increased, better heat conductors such as copper plates are used to improve the spreading of heat from heat sources into the heat sinks. To meet the next generation, CPU needs the thermal requirements with a low profile heat sink. Therefore new heat sinks with larger extended surfaces, highly conductive materials and more coolant flow are keys to reduce the hot spots. There has been much success in the thermal design of complex electronics system using CFD. Linton and Agonafer [13] simulated an entire desktop PC with one fan using Phoenics code. Lee and Mahalingam [12] used flotherm code to simulate detailed flow and temperature fields within a computer chassis with two fans. Similar work was also done by wong and Lee [15]. Yu.C.W and Webb.R.L [16] analyzed the flow and heat transfer inside a computer cabinet for the high power conditions expected in desktop computers. In this research CFD (Icepak) has

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been used to identify a cooling solution for a desktop computer. 40W PCI card, different case fan size and different ducting positions have been studied. Chang.J.Y ,etal [4] reports 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 this paper the methodology of CFD analysis for the heat sink, duct design has been described and experimental procedures to validate the predictions. David Lober [6] discussed some thermal management considerations involved in choosing an enclosure and demonstrated the use of CFD thermal modeling software which optimally integrated a computer system into an existing enclosure and reduced the design cycle time. Savithri subramanyam and Keith E.Crowe [13] described to evaluate designs of electronic cooling heat sinks by using thermal finite element analysis (FEA) and computational fluid dynamics (CFD). In a system as complex as a desktop computer, CFD is a good approach to explain various design quickly with reasonable accuracy. This research work stands to the challenges posed by increasing chip heat flux, smaller enclosures, and stricter performance and reliability standards. All of the changes expected in future desktop factors will make air cooling of the CPU more difficult. In this paper the heat sink with base plate and fan, proper vents for air flow are designed and implemented for better performance. In this study, various geometry heat sink with base plate is used to cool central processing units (CPUs) of desktop computers are investigated.

2. CFD SIMULATION APPROACH

The CPU heat sink with base plate is attached to the CPU together with a fan. The mainboard and all the other components are enclosed in a chassis. There are many other heat sources in addition to CPU. Some of them are on the mainboard (e.g., northbridge chip), some of them are attached to the mainboard (e.g., memory modules) and some of them are in the chassis volume (e.g., DVD).

2.1. CFD chassis model

The CFD 3D chassis model is shown in figure 1. The chassis is modeled using standard dimensions of a common ATX chassis by hollow blocks and internal components are represented as lumped objects. During modeling, all the components inside the chassis are standard sized components and exact dimensions are obtained by measurement. The CPU is modeled as a 2D area which dissipates 80W. The 25mm x 25mm cross sectional area of CPU is taken which is commercially available AMD CPU. For simplicity, the mother board, chipset card are modeled as zero thickness with heat generated uniformly. The CPU fan is modeled as a lumped parameter model and does not have blades. Ram cards are fixed on the motherboard. They are also heat sources and accurate dimensioning of space between ram cards is difficult. Therefore these things are not considered for study. Power supply is a very complex geometry which includes lot of electric components, wiring and heat sinks. It is assumed as a lumped media which exerts a resistance on the cooling air flow streams. SMPS (Switch Mode Power Supply) and few miscellaneous cards are modeled and lots of small electronic components on these cards are not modeled.

Fig. 1 Computer Chassis model

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The computer cases have small holes which are used to allow inlet air for cooling and discharge hot air through outlet. The modeling of these holes in accurate dimensioning is difficult and computationally expensive. Therefore it is modeled as a zero thickness flow resistance. The HDD (Hard Disk Drive), DVD, CD are modeled as solid blocks generating a specified amount of heat uniformly inside the volume. The closer view of geometric details of the CPU heat sink is shown in the Figure 1. The scope of this study is investigation of temperature distributions on CPU heat sinks. The thermal boundary conditions for the objects inside the chassis are listed in table1 and 2.

TABLE 1

Interior Conditions

Object name Material Heat dissipation Rate(w) CPU Silicon 80

CPU Heat sink Al Cu -

CD Al 15 DVD Al 15 HDD Al 20

POWER SUPPY Porous 75 Miscellaneous Card FR4 20

TABLE 2

Fan Conditions Name of the Fan Pressure Rise Heat Flow Rate

CPU Heat sink fan

30 Pa

30CFM

Case Fan 40 Pa

40CFM

A total of 225W of heat is dissipated. The fans inside the domain are modeled as circular surfaces which add momentum source to the flow. The added momentum source is given as the pressure rise across the fan versus the flow rate curve. The relationship between the pressure and the flow rate is taken linearly. The boundary condition for the power supply is different. The power supply is geometrically very complicated. Therefore it is modeled by simplifications. The power supply is a rectangular box which is a resistance to flow. The resistance is different in y-direction. A closer view of surface grid on one of the CPU heat sink is shown in figure 2.

Fig.2. Surface grid on one of the CPU heat sink

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2.2 Governing Equations Time-independent flow equations with turbulence are solved. The viscous dissipation term is omitted. Therefore, the governing equations for the fluid flow and heat transfer are the following form of the incompressible continuity equations, Navier stokes equations x-y and z direction momentum, and energy equations together with the equation of state. The continuity Equation

The X, Y, Z Momentum Equations

The Energy Equation

Equation of state

P= Where is the density, u, v and w are velocity components, v is the velocity vector, P is the pressure, B terms are the body forces, h is the total enthalpy and terms are the viscous stress components. The Reynolds averaging is employed to handle the turbulence effects. In the Reynolds averaging, the solution variables are decomposed into mean and fluctuating components. For the velocity components u = u + u , where u and u are the mean and fluctuating velocity components for x direction. Likewise, for the pressure and the other scalar quantities = + where is a scalar such as pressure or energy. The Reynolds Averaged NavierStokes (RANS) equations are solved together with the Boussinesq approximation. 2.3. Boundary Conditions While the NavierStokes equations are solved inside the domain, no-slip boundary condition is applied to all the walls in the domain. Therefore, at all of the surfaces u = v = w =0. It is assumed that the system fan does not drive a flow cell around the computer chassis and the heat transfer mechanism at the chassis outer walls is natural convection. Heat transfer coefficients at the outer walls are estimated from the empirical correlations. In order to use the correlations, the average wall temperature must be prescribed. To do that, a first cut analysis must be run. As the typical values of the natural convection heat transfer coefficient lie between 2 and 25 W/ m2 K , a value of 5 W/ m2 K is selected to be the heat transfer coefficient at the computer chassis walls. The analysis results by taking the ambient temperature as 27 C gives an average temperature of 33 C at the walls, and then heat transfer coefficients are calculated using this value and the available correlations with the uniform surface temperature assumption, and with the definitions of the Rayleigh number and the average Nusselt number as:

Ra = ; Nu =

Where is L the characteristic length, k, h, g, , and are the fluid thermal conductivity, the convection heat transfer coefficient, the gravitational acceleration, the volumetric thermal expansion coefficient, the thermal diffusivity, and the kinematic viscosity respectively. Ts and T are the surface and the ambient temperatures. Here, Ra is less than 109, therefore, the flow is laminar. Using the correlations for laminar natural convection on the vertical plate by taking the thermal conductivity of air as k = 0.027 W/ m K , the heat transfer coefficient, 4 W/ m2 K. Similarly, for the horizontal top plate the Rayleigh number is calculated as 1.5 x 105 , where the characteristic length is calculated from AL

p , where A is the plate surface area, and P is the plate perimeter. The average Nusselt

number is correlated to the Rayleigh number with Nu = 0.54 Ra0.25 which gives 0.04 W / m2 K . The calculated heat

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transfer coefficients are applied to all of the exterior walls of the chassis except the bottom horizontal wall which sits on the ground that is considered to be adiabatic. 2.4 Convergence issues The persistent results are obtained from only a well converged, well posed and grid independent simulation. The order of magnitude residuals drop is used for convergence. Two different convergence tolerances are compared, one is 10-3 for flow and 10-7for energy, and the other is 10-4 for flow and 10-8 for energy. Running the solver such that residuals fall one more order of magnitude means that more iteration is done to improve the solution quality. It should be noted that, convergence criteria must assure that the results do not change as the iterations proceed. There is a common way of implementing this. Scalar change of some values like temperature is displayed as well as the residual monitors. When the scalar values stay at a certain number and do not change as the iterations continue, then it can be stated that the solution is converged. It was seen that this trend is achieved when the momentum residuals fell below 10-4and energy residual fell below 10-8 .Therefore in this paper all the models use the convergence criteria of 10-4for the flow variables and 10-8for the energy. Emre Ozturk and Ilker Tari was used the convergence criteria of 10-4for the flow variables and 10-7for the energy. 2.5. Discretization Fluent uses the finite volume method, and error arises due to discretization of the governing equations. Interpolations are made to find values at the cell faces, whereas all the information is stored at the cell centers. Interpolations have to be done for discretization. There are numerous schemes, and the easiest one is the first-order upwinding. The advantage of this scheme is that it converges easily. The disadvantage is that it is only first-order accurate. It is suggested to use second-order schemes for unstructured grids. In our cases, the first-order upwinding are used for simulation. But Emre Ozturk (2007) used the first order and the second order upwinding schemes. It is suggested that the ranges of the local temperature values on the heat sink are similar in both cases; therefore, the first-order method is computationally less costly is used in all simulations. 2.6. Grid Selection The mesh is the key component of a high quality solution. The total number of cells generated is kept around 1.167 million for the entire model. The Tetrahedral / Hybrid element scheme T grid type meshing is used in this simulation. The only way to establish grid independent solutions is to setup a model with a finer mesh and analyze it to see if there are major differences in scalar quantities and vectors. Emre Ozturk (2007) is used 1.5 million cells. The results are compared with the default 900 000 cell model. The mesh density increase mostly concentrated within the nonconformal mesh around the heat sink. From the results the temperature distributions are similar. Emre Ozturk is suggested that 900 000 cells are enough for the models to be grid independent. The density distribution of the mesh is concentrated around the CPU heat sink, for example in the Elliptical heat sink with base plate case has 0.511 million cells out of 1.167 cells are in the nonconformal mesh of the heat sink. 2.7. Turbulence Modeling and Radiation Effects The default turbulence model of all calculations is the algebraic turbulence model. It is the computationally least expensive one since no extra equations are solved in addition to continuity, momentum, and energy equations. The RNG k- model is used as a test case. Radiation heat transfer helped the heat sink cool by 0.2 to 0.5 K. therefore it is concluded that radiation could be ignored for forced cooling of CPUs. 3. HEAT SINK SELECTION The slot parallel plate fin heat sinks in two arrays, pin fin heat sinks of variable fin thickness and base plates are used to cool the CPU. Unfortunately, significant modeling and run time is needed to represent small pins with complex meshing. In the course of preliminary numerical simulation work, three variable thickness fin geometries of same base area, same fin height, and same fin pitch are simulated. The 54mm x 65mm heat sink base plate size is selected for this work. The different shape of extruded fins and a 3.5mm thick base plate is finished of aluminum materials. In addition to enhance the heat transfer 2.5mm to 5 mm thick copper and CCC base plate has been provided as a spreader to conduct heat from CPU processor. For all fin geometries 2.5 mm fin pitch, 40mm fin

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height, 54mm x 65mm heat sink base plate, 5mm clearance between the power supply and the fin tips of the heat sink at the flow rate of 30CFM. The fin thickness is varied from 3 to 5mm for plate heat sink model. Similarly 4 to 5mm for pin fin heat sink model. The thermal performance of the heat sink is modeled using gambit. The heat sink solid model in the chassis numerical model is done and the CFD software solves the heat transfer problem for the heat sink. The flow of air is parallel to the heat sinks and vertical flow of air is pulled upward by a fan mounted at the top of the heat sink with clearance.

4. RESULTS AND DISCUSSIONS

The chassis model with slot parallel plate heat sink with base plates are analyzed by CFD simulations. The results have been obtained by varying the heat sink model and keeping the entire computational domain same. The three different thicknesses of heat sink are considered with variable base plate, the 5mm CCC slot parallel plate heat sink temperature distributions are as shown in figure 3.

Fig 3.1Temperature distributions on 5mm thick pin fin Heat sinks with 5mm CCC base

Fig 3.2Temperature distributions on 5mm thick Plate fin Heat sinks with 5mm CCC base

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For all of the heat sinks, it is viewed that their centers are the hot spots since the heat source corresponds to the closeness of the base center. The fans installed on the heat sinks are identical with dimensions. The fans have hubs where air cannot pass through and it makes the center parts hotter. In the current simulations, the swirl of the fan is not modeled since the fans are lumped parameter models. For real cases, the center would not be as hot as the present simulations predict, due to the swirl. It is observed that the upper right and left part of the heat sink has lower temperature when compared to centre part of the heat sink. This is due to more air flow circulation in sides of heat sink and also exhaust fan sucks the hot air which is nearer to side of the heat sink. The cooling becomes less efficient at other sides of the heat sink. It is observed that the conduction rate is high in CCC base plate rather than copper base plate and also it is enhanced by increasing the number of fins. Since for same heat source CPU the bottom heat sink temperature is decreased for increasing number of fins. In this work parallel plate, pin fin and elliptical fin heat sinks with variable base plates are used and the performances are compared. Although the heat sink dimensions are similar, CCC base plate heat sinks enables higher conduction rates, and heat is conducted to the whole heat sink in a more efficient way. When the computer chassis is investigated, it is observed that only the upper right part of the heat sink has a free path for the air flow. Therefore, air driven by the CPU fan can travel to that side and the effect of which can also be seen in the temperature distributions of figure3.On the other sides of the CPU, air returns to the proximity of the heat sink by hitting the wall, the fan sucks the returning relatively hot air and the cooling becomes less efficient at these sides of the heat sink as can be observed in figure 4.

Fig 4 path lines and temperature distribution for 3mm thick Plate fin Heat sinks

It is also observed that these results that modeling not only the CPU- heat sink assembly but the whole chassis is important for predicting heat sink performance. To investigate this issue further, everything inside the chassis is fixed and the heat sink model is changed. The mesh is kept same, to able to compare the results with the detailed chassis model. The model with CPU heat dissipation values also resembles the experimental setup. The air can bounce of the chassis walls and recirculate in the chassis, but the temperature distribution is much more symmetric compared to the detailed whole chassis model. It is viewed that the 5 mm thick base plate heat sinks are performed well when compared to 2.5 mm thick base plate heat sinks. It is viewed that the CCC base plate heat sinks are performed well when compared to copper thick base plate heat sinks. It is noticed that the performance of heat sink is increased by increasing the thickness of fins instead of increasing number of fins. In the case of large number fins, it is noticed the small pitch between fins does not permit air to cool the hottest centre part of the heat sinks. The performance of parallel plate heat sinks is not affected by making 1 mm slot of 20mm of plate fin. However, the air flow path is better and heat transfer rate does not change and if heat transfer area is decreased, then the thermal resistance is not significantly changed. The reduction in heat sink material and weight creates worth for the manufacture. The heat transfer rate is enhanced by increasing thickness of fin. The heat transfer rate is enhanced by increasing thickness of fin up to 25% in 5mm base plate heat sink models and up to 10% in 2.5 mm base plate thickness heat sink models.

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TABLE 3 Maximum and minimum temperatures for the three heat sinks

with Copper base plate of thickness 5 mm and 2.5mm

Slot Plate heat sink with.5 mm Cu Base plate thickness

Slot Plate heat sink with 2.5 mm Cu Base plate thickness

Fin Height 40 40 40

Fin Height 40 40 40

fin Thickness for first 20 mm

3 4 5


3 4 5

Second 20 mm with 1 mm slot

2 3 4

Second 20 mm with 1 mm slot 2 3 4

Fin Pitch 2.5 2.5 2.5

Fin Pitch 2.5 2.5 2.5

Air flow rate 30CFM 30CFM 30CFM


Pressure drop 40Pa 40Pa 40Pa


Temperature of base plate

327 328 332 Temperature of base plate

328 330 333

Temperature of tip of heat sink

314 316 322 Temperature of tip of heat sink

314 317 321

TABLE 4

Maximum and minimum temperatures for the three heat sinks with CCC base plate of thickness 5 mm and 2.5mm

Slot Plate heat sink with.5 mm CCC Base plate thickness

Slot Plate heat sink with 2.5 mm CCC Base plate thickness

Fin Height 40 40 40

Fin Height 40 40 40


3 4 5


3 4 5

Second 20 mm with 1 mm slot

2 3 4

Second 20 mm with 1 mm slot 2 3 4

Fin Pitch 2.5 2.5 2.5

Fin Pitch 2.5 2.5 2.5





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326 328 331 Temperature of base plate

327 329 331


314 318 322 Temperature of tip of heat sink

315 318 322

TABLE 5 Maximum and minimum temperatures for the Pin fin heat sinks

with Copper base plate of thickness 5 mm and 2.5mm

Pin fin heat sink with 5 mm Cu Base plate thickness

Pin fin heat sink with 2.5 mm Cu Base plate thickness

Fin Height 40 40

Fin Height 40 40

fin Thickness 4 5 fin Thickness 4 5 Fin Pitch 2.5 2.5

Fin Pitch 2.5 2.5

Air flow rate 30CFM 30CFM


Pressure drop 40Pa 40Pa



337 339.5 Temperature

of base plate

339 342


322 325 Temperature

of tip of heat sink

322 326

TABLE 6

Maximum and minimum temperatures for the Pin fin heat sinks with CCC base plate of thickness 5 mm and 2.5mm

Pin fin heat sink with 5 mm CCC Base plate thickness

Pin fin heat sink with 2.5 mm CCC Base plate thickness

Fin Height 40 40

Fin Height 40 40

fin Thickness 4 5 fin Thickness 4 5 Fin Pitch 2.5 2.5

Fin Pitch 2.5 2.5






335 338 Temperature

of base plate

338 341


322 326 Temperature

of tip of heat sink

322 327

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TABLE 7 Comparison of Experimental and simulation results of slot plate heat sinks

With2.5 & 5mm Copper base plate

STUDY

Parameter

2.5 MM BASE PLATE HEAT SINK MODELS

5 MM BASE PLATE HEAT SINK MODELS

Fin thickness at Bottom 3 mm 3 mm 5 mm 3 mm 3 mm 5 mm

Fin Thickness at Top 2 mm 2 mm 4 mm 2 mm 2 mm 4 mm

Fin Material Al/cu Al/cu Al/cu Al/cu Al/cu Al/cu CFD Simulation (K)

T 14 13 12 13 12 10

Experimental (K)

T 14.65 12.3 11.3 11.5 11.6 9.6

% T ERROR -4.64 5.38 5.83 11.5 3.33 4

TABLE 8 Comparison of Experimental and simulation results of slot plate heat sinks

With2.5 & 5mm CCC base plate

STUDY

Parameter

2.5 MM BASE PLATE HEAT SINK MODELS

5 MM BASE PLATE HEAT SINK MODELS

Fin thickness at Bottom 3 mm 3 mm 5 mm 3 mm 3 mm 5 mm

Fin Thickness at Top 2 mm 2 mm 4 mm 2 mm 2 mm 4 mm

Fin Material Al/ccc Al/ccc Al/ccc Al/ccc Al/ccc Al/ccc CFD Simulation (K)

T 12 11 11 12 10 9

Experimental (K)

T 12.45 10.6 9.7 12.2 9.1 8.72

% T ERROR -3.75 3.636 11.8 -1.67 9 3.11

TABLE 9 Comparison of numerical and simulation results of Pin fin heat sinks

with 2.5 & 5 mm Copper base plate

STUDY

Parameter

2.5 mm Base plate Pin fin Heat sink Models

5 mm Base plate Pin fin Heat sink Models

Fin thickness 5 mm 4 mm 5 mm 4 mm

Fin Material Al/cu Al/cu

Al/cu

Al/cu

CFD Simulation (K)

T 17 16 15 14.5

Experimental (K)

T 16.3 14.8 14.4 13.6

%T ERROR 4.12 7.5 4 6.21

TABLE 10

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Comparison of numerical and simulation results of Pin fin heat sinks with 2.5 & 5 mm CCC base plate

STUDY

Parameter

2.5 mm Base plate Pin fin Heat sink Models

5 mm Base plate Pin fin Heat sink Models

Fin thickness 5 mm 4 mm 5 mm 4 mm

Fin Material Al/ccc Al/ccc Al/ccc Al/ccc

CFD Simulation (K)

T 16 14 13 12

Experimental (K)

T 15.2 12.8 12 11.3

%T ERROR 5 8.57 7.69 5.83

In this work base plate thickness is increased from 2.5 to 5mm and base plate material is changed while keeping the fin lengths constant. The difference between the maximum and the minimum temperatures on the CCC base plate heat sink is around 9 C, where as the copper base plate heat sink is around 10 C. It is observed that for small thickness of CCC base plate heat sinks are performed well rather than increasing the thickness of copper base plate. The performance of heat sinks are enhanced, the weight reduction, space limitation is advantages of using CCC base plate heat sinks. CCC base plate heat sink performs well when compared to copper base plate heat sink. By increasing the base plate thickness and changing the material of base plate the performance of heat sink is enhanced. It is also observed that by adding the base plate the heat conduction rate is enhanced in place of increasing the fin height. The performances of 2.5 mm ccc base plate heat sinks are obviously comparable to 5 mm copper base plate heat sinks. 4.1 Comparison with experimental data This test setup is not the whole computer chassis system, but a smaller domain, in order to simplify the experiments. The base plate is attached to different heat sinks models and centre of the base plate is heated with heat loads 80W. Since the test setup is an open domain, the atmospheric temperature is the temperature of the air blown to the heat sink. The atmospheric air is passed around the heat sink which is heated that is exhausted by blower and also the pressure drop has been noted. In this setup other heat sources are not considered for simplify the experiments. In this experimental setup the CPU fan and Exhaust fan are not used in stead of those fans the blower is used to cool the heat sink models. After steady state the bottom and top surface temperatures are measured using thermocouples which are attached at bottom base plate and top fin surfaces at different points. Although the experimental comparison quantitatively, it would be considered as qualitative one is shown in table

5. CONCULSION Number of fins and their distribution, fin material and base plate thickness are investigated for enhancing the heat dissipation rate from CPU. Improvements on heat sink designs are possible by the use of CFD. It is possible to design a new heat sink with suitable base plate which has better thermal performance and uses less material using CFD simulations. The heat sink base thickness is also a parameter for increasing the performance of heat sink. If base plate material is selected to be CCC rather than copper or aluminum, then the thermal resistance of the heat sink is decreased. When the base plate thickness was varied from 2.5mm to 5mm, the heat sink performed better. Due to space limitations of heat sink in a computer it is not possible to increase the height of the heat sink. Therefore, the base plate is attached with heat sink to enhance the performance rather than increasing the height of heat sink. In the current study, it is observed that stacking too many fins is not a solution for decreasing the hot spots on the heat sink since they may prevent the passage of air coming from the fan to the hottest centre parts of the heat sink. In this paper, three thicknesses of heat sinks with base plate are selected and analyzed. From which the optimal design of heat sink is selected which gives more heat transfer rate. It is observed that the velocity field around the heat sink is affected from the presence of the other components inside the chassis as well as the chassis walls which redirect the hot air back into CPU heat sink. If the heat sink is

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plate fin type, plate fins can reduce the recirculation. It is also possible to increase intake of cooler air by using heat sink fan. The CPU is considered as the largest heat source, which is cooled directly drawing cooler ambient air from outside the chassis to the CPU fan with a duct is a viable option that has been recently implemented by many chassis manufacturers. The present study together with Emre ozturk (2007) outlines the details of CFD simulation steps for a computer chassis thermal management solution by concentrating on CPU cooling. These results and conclusion obtained in this present work are found to be in good agreement with conclusion obtained by Emre ozturk (2007). In this study, it is suggested that slot parallel plate heat sink model with CCC base plate, especially 5mm pin fin heat sink with 5mm CCC base plate will benefit the design engineers involved in electronic cooling. In this study a complete computer chassis with different heat sinks has been investigated and the performances of the heat sinks are compared.

REFERENCES

[1] Wong.H and Lee.T.Y, 1996. Thermal Evaluation of a PowerPC 620 Microprocessor computer, IEEE transactions on components,

Packaging, and manufacturing Technology-Part A, volume 19, pp.469-477. [2] Arularasan R and Velraj R, 2008. CFD analysis in a heat sink for cooling electronic devices, International Journal of the computer, the

internet and management, Volume 6, pp.1-11. [3] Bejan.A and Ledezma.G.A, 1995. Thermodynamic optimization of cooling techniques for electronic packages, International journal of

heat and mass transfer, Volume 39,pp. 1213-1221. [4] Chang.J.Y, et al, 2000. Identification of minimum air flow design for a Desktop computer using CFD modeling, 7th intersociety

conference on thermal and thermomechanical Phenomena in Electronic systems, volume 1, pp.330-338. [5] David Miller, et al, 2007 Closed loop liquid cooling for high performance computer systems, ASME Inter pack, pp 1-7. [6] David lober, 1999. Optimizing the intergration of an electronics system into an existing enclosure using CFD modeling techniques,

International journal of microcircuits and electronic packaging, Volume 22, pp.146-151. [7] EMRE ZTRK, 2007. CFD modeling of forced cooling of computer Chassis Engineering applications of computational fluid

mechanics , Volume 1, pp 304-313. [8] Hetstoni.G, et al, 2002. A uniform temperature heat sink for cooling of electronic devices, International Journal of Heat and Mass

transfer, Volume 45, pp. 3275-3286. [9] Kwang-soo kim, et al 2003. Heat pipe cooling technology for desktop PC CPU,Applied thermal Engineering, Volume 23, pp 1137-1144. [10] Man-in baek and Jung milee Thermal design for notebook PC by using thermal analysis, LG Electronics, LG production Research centre. [11] Lee.T.Y and Mahalingam.M, 1994. application of a CFD tool for system level thermal simulation IEEE transactions on components,

Packaging, and manufacturing Technology-Part A, volume 17, pp.564-571. [12] Linton.R.L and Agonafer.D , 1994. Thermal model of a PC,ASME Journal of Electronic Packaging , Volume 116, pp.134-137. [13] Savithri subramanyam and Keith E.Crowe,2000. Rapid design of heat sinks for electronic cooling computational and experimental tools,

IEEE Symposium, pp. 243-251. [14] Wong.H and Lee.T.Y, 1996. thermal Evaluation of a PowerPC 620 Microprocessor computer, IEEE transactions on components,

Packaging, and manufacturing Technology-Part A, volume 19, pp.469-477. [15] Yang.R.J, Fu.L-M, 2001. Thermal and flow analysis of a heated electronic component, International journal of heat and mass transfer,

volume 44, and pp. 2261-2275, [16] Yu.C-W and Webb.R.L, 2001. Thermal design of a desktop computer system using CFD analysis, Seventeenth IEEE SEMI- THERM

SYMPOSIUM, pp. 18-26. [17] Frank P. Incropera, David P. Dewitt, Fundamentals of Heat and Mass Transfer, (Fourth Edition, John Wiley & Sons) 1996. [18] Wikipedia, the free encyclopedia Computer cooling.

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