FLOW SIMULATION TO STUDY THE EFFECT OF FLOW TYPE ON THE PERFORMANCE OF MULTI MATERIAL PLATE FIN HEAT SINKS

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1 FLOW SIMULATION TO STUDY THE EFFECT OF FLOW TYPE ON THE PERFORMANCE OF MULTI MATERIAL PLATE FIN HEAT SINKS N.V.S. Shankar 1, Rahul Desala 2, VeerlaSrinivas Babu 3, P. Vamsi Krishna 4, M. M. Rao 5 1 Asst. Professor, Dept. of Mechanical Engg., SCET, AP, India, Shankar.publications@gmail.com 2 Graduate Student, Dept. of Mechanical Engg., SCET, AP, India, rahuldesala@gmail.com 3 Graduate Student, Dept. of Mechanical Engg., SCET, AP, India, srinivasbabu.v@gmail.com 4 Associate Professor, Dept. of IPE, GITAM University, AP, India, vamsikrishna@gitam.in 5 Principal, SCET, AP, India, dmmrao54@gmail.com Abstract Heat Sinks, in Electronic systems, are devices that cool the hotter body by dissipating the heat to a fluid medium, generally air. These are used to cool devices such as high-power semiconductor devices, and optoelectronic devices such as high power lasers and light emitting diodes (LEDs). These are primarily heat exchangers that are used to exchange the heat from the component to the surroundings so as to avoid the problem of overheating. There are different types of heat sinks 1.Extruded heat sinks 2.Flooded-fin heat sinks 3. Integrated vapour chamber heat sinks. The most effective heat sink is the one which can dissipate a large amount of heat. In this paper, numerical simulation using CFD techniques is carried out for different types of heat sinks. Multi-material heat sink in a computer cabinet is considered so as to study their performance under different flow conditions. Assembly model of cabinet is initially generated. Motherboard, Rams, Chipset, Chipset heat sink, Processor heat sink and the rest of the components are modelled and then assembled to the cabinet. A total of 180W maximum heat dissipation was given as input for analysis. The total heat consisted of heat profile of the processor, 20W heat dissipation each for Ram and chipset. 80mm axial flow fans with 80cfm were used for inlet and outlet air exits. 40 mm fan was modeled on the chipset and 80mm fan on the processor. Full case flow simulation was carried out and the results were presented. Index Terms: Heat Sink, Flow Simulation, Full Case Flow Simulation, CFD *** INTRODUCTION The advancements in computing technology led to higher data processing rates at tremendous speeds and smaller form factor. This is leading to higher processor temperatures and thus higher heat dissipation requirement. Higher processor temperatures lead to malfunctioning of CPU. Thus the major problem in electronic systems may be defined as increasing the performance of the processor while keeping the temperature to a minimum extent. Thus better ways for heat dissipation are required. Many cooling solutions [1] like using heat pipes, water cooling and even cooling by using liquid nitrogen have been developed. The main criteria to be considered when designing an air cooled heat sink is the effective utilization of the fin surface for transfer of heat from a relatively small heat source like CPU (with large heat generation rate) with high heat flux. The technological advancements have led to increase in heat loads. Thus better heat conductors such as copper plates, carbon carbon composites [1], doped Aluminum [10, 11] are used to improve heat spreading from the heat source into heat sinks. The type of heat sink and air flow in the heat sink also affect the amount of heat dissipation. Material used for the heat sink is another important factor that influences the efficiency of the heat sink. A desktop computer CPU is a complex system involving a lot of heat transfer. Processor, Chipset and Rams are considered as major heat sources. Experimental testing for study of heat dissipation in a computer cabinet is a costly affair. Thus prediction of flow pattern is of great interest as it helps in studying the heat dissipation process. While designing a desktop, full case flow simulation is very much necessary to understand the flow pattern and heat transfer happening in the Available 233

2 system. This data can be used for optimizing the design for better heat transfer. R. Mohan,et al [1] discussed the process of optimizing the pin fin and slot parallel plate fin heat sink for thermal performance. Thermal performance when using copper and carbon-carbon composite material for heat base were presented in their work. Dong-Kwon, et al [2] experimentally compared the performance of plate-fin and pin-fin heat sinks subjected to impinging flow. Jei Wei [3] gave an overview of the thermal design and cooling technology development of Fujitsu s high performance servers. Thermal management is outlined in terms of server cabinet, system board and CPU package. The challenges before cooling solutions were also discussed. Chyi-Tsong Chen, et al [4] demonstrated how FEM and GA can be together applied to optimize the shape of heat sink for better heat dissipation. Takeshi hirasawa, et al [5] addressed the problems in manufacturing heat sinks that rise due to size, shape and materials being used. Flat micro heat pipes are now being used in lot of applications like personal computers. Yuichi Kimura, et al [6] presented the results of steady state analysis on flat micro heat pipe. The steady-state heat transfer characteristics of this heat pipe have been experimentally confirmed in detail, and a prediction method for its maximum heat transfer rate is proposed. Sukhvinder Kang, et al [7] presented a physics based analytical model to predict the thermal behavior of pin fin heat sinks in transverse forced flow. Giovanni Cortella[8] showed how CFD can be used in refrigerator systems design. Ram Viswanath, et al [9] addressed the multidimensional problem in which materials and process improvements in packaging and heat-sink technology are required to minimize thermal resistance while maintaining an optimal cost for the thermal solution. Keller and Kurtis [10, 11] discussed the advantages of using cast heat sinks of aluminum doped with zinc. Tom Kowalski, et al [12] discussed the use of FNM and CFD to design the complex electronic cabinet used for high speed internet connection. M. Davis, et al [13] discussed the use of thermoelectric materials in cooling solutions for heat sources of small form factors. In the present work, performance comparison of multimaterial plate fin heat sink with copper base and aluminum plate fins, located in a desktop computer as shown in fig 1, during flooded and impinging flow is presented. A total of 180W heat dissipation is planned. The heat profile of processor [15, 16], 20W heat dissipation from Ram and Chipset [1] were given as input. A full case flow simulation is carried out and the results are compared. Processor temperature is the important factor considered in present work. Figure 1: Modeled cabinet with processor, heat sink and Ram (one side cover plate was made transparent for showing inner parts) 2. HEAT SINK TYPES Different types of heat sinks have been designed as cooling solution for heat dissipation problem. These heat sinks are classified based on various criteria. The classification of heat sinks is presented in fig 2. Active Passive Heat Sink Figure 2: Heat sink classification Extruded Flooded Integrated Vapour Chamber When air is forced to flow over the fins by use of a special fan on the heat sink, then it is called as active heat sink. Figure 3 shows an active heat sink. When no extra fan is used for circulating air over the fins and the air circulation is purely due to the case fan, the heat sink is referred as passive. Figure Available 234

3 4 shows a passive heat sink. Passive heat sinks are generally seen on north bridge chipset in common home desktops. Figure 5 shows an integrated vapor chamber heat sink. In addition to the above classification, heat sinks are further classified as extruded, flooded and Integrated Vapor Chamber heat sinks. Extruded heat sinks are generally made by extruding Aluminum or other material. This Heat sink is a single piece. The fins are rarely rectangular. Figure 6 shows an Extruded Heat sink. When higher power dissipation is the requirement, then flooded heat sinks are used. In Flooded heat sinks, the ratio of the fin thickness to fin pitch can be as low as 1:3[9]. Figure 3 shows a flooded heat sink. It can be observed that the fins are very close to each other. Integrated vapor chamber heat sink, on the other hand, uses heat pipe. The problem of resistance to heat spreading is well tackled by the usage of heat pipes. Figure 3: Active and flooded heat sink Figure 6: An active extruded heat sink Figure 4: Passive heat sink on a mother board In present work, the heat sink considered, has a Copper base and Aluminum plate fins. Studies are conducted when this heat sink was subjected to flooded and impinging air flows. The images of the heat sink models considered are show in figures 7 & 8. Figure 5: Integrated Vapour Chamber heat sink Figure 7: Designed flooded plate fin heat sink with Al fins and Cu base Available 235

4 Figure 8: Designed plate fin heat sink with impinging flow, Al fins and Cu base. Figure 10: Air density distribution in cabinet for parallel flow flooded cooler With the designed heat sinks, a full case flow simulation was carried out and the results are presented in the next section. 3. RESULTS AND DISCUSSIONS The chassis models with the flooded and impinging flow heat sinks are analyzed using CFD simulation for air flow patterns and heat dissipation. Four fans are used in simulation, of which 2 case fans and 1 processor fan were 80 mm axial flow with 80 cfm and other was a 40 mm chipset fan. Two orientations of flooded heat sink i.e. parallel flow and perpendicular flow are initially considered. When the fan of the flooded heat sink is placed parallel to the inlet case fan, it is observed that the temperature of the Ram is around 74 o C. The cut plot showing the orientation of the heat sink and the flow pattern for this case is given in fig 9. Figure 10 and 11 show the flow velocity and density of air in parallel flow pattern. Figure 11: Air velocity distribution in cabinet for parallel flow flooded cooler Figure 9: Cut plot showing temperature and velocity distribution for parallel flow flooded cooler Figure 12: Cut plot showing temperature and velocity distribution for perpendicular flow flooded cooler Available 236

5 of the processor heat sink and chipset heat sink for this orientation are presented in figures 16 to 18. Figure 13: Air density distribution in cabinet for perpendicular flow flooded cooler Figure 15: Comparison of temperatures in parallel and perpendicular flow orientations Figure 16: Temperature distribution on processor heat sink with flooded flow Figure 14: Air velocity distribution in cabinet for perpendicular flow flooded cooler In Figures 12, 13 and 14, the cut plots containing the perpendicular flow pattern are presented. From the above plots it can be observed that there is higher air velocity for perpendicular flow between rams when compared with parallel flow. This resulted in lower temperatures for RAM. A comparative graph of predicted temperatures of various components in both the cases is given in fig 15. Based on the results it can be observed that the latter is an optimum orientation. The results of these are taken into consideration when comparing the performance with those of impinging flow. Surface plots showing temperature distribution and heat flux Figure 17: Heat flux on processor heat sink with flooded flow Available 237

6 Figure 18: Temperature distribution on Chipset heat sinks with flooded flow on processor heat sink Figure 21: Temperature distribution on Chipset heat sink with impinging flow on processor heat sink Figure 19: Temperature distribution on processor heat sink with impinging flow Figure 22: Cut plot showing the air flow pattern in cabinet with impinging flow on CPU heat sink Figure 20: Heat flux on processor heat sink with impinging flow Figure 23: Comparison of temperature in impinging flow and flooded flow Available 238

7 As mentioned earlier, CFD analysis with impinging flow on multi-material heat sink was also performed. Figures 19 to 22 show the results of the analysis when impinging flow on heat sink was considered. Based on the surface plots of the processor heat sinks, it can be observed that there is a slight lower temperature during flooded flow. A comparative plot of temperatures of various components is presented in figure 23. Processor temperature is the main factor of comparison as processor is the component dissipating maximum amount of heat in the system. Based on the plot it can be observed that with flooded flow, the processor and Ram temperatures are slightly lower, but the chipset temperatures are high. Based on these observations, it can be concluded that there is performance gain achieved using a flooded heat sink but at the cost of increased temperature of other components. In order to overcome this disadvantage, a better chipset fan or chipset heat sinks are to be used. 4. CONCLUSION A study of effect of air flow on the performance of a multi material heat sink, with copper base and aluminium plate fins, using CFD analysis is performed. Two types of air flows are considered: Flooded flow and impinging flow. Initially flow simulation is carried for two orientations of the flooded flow heat sink. It is found that when the heat sink is perpendicular to the inlet case fan axis, the temperatures of all the other heat generating components are low. Thus this orientation is considered for comparison with imping flow configuration results. Comparing the results of impinging flow and flooded flow, it is observed that, there is a performance gain with flooded flow heat sink at the cost of rise in temperature of other components. REFERENCES [1] R. Mohan and Dr. P. Govindarajan, 2010, Thermal analysis of CPU with composite pin fin heat sinks, international journal of engineering science and technology Vol.2, issue 9, pp [2] Dong-Kwon Kim, Sung Jin Kim, Jin-Kwon Bae, 2009, "Comparison of thermal performances of plate-fin and pin-fin heat sinks subject to an impinging flow", International Journal of Heat and Mass Transfer 52, pp [3] Jie Wei, 2007, Thermal management of Fujitsu s Highperformance servers, FUJITSU Sci. tech J., Vol 43-1, pp [4] Chyi-Tsong Chen, Ching-Kuo Wu, Chyi Hwang, 2008, Optimal design and control of CPU heat sink processes, IEEE Transactions on Components, Packaging and Manufacturing Technology - TCPMT, vol. 31, no. 1, pp [5] Takeshi hirasawa, Kenya Kawabata and Masaru Oomi, 2005, Evolution of heat sink technology, Furukawa Review No.27, pp [6] Yuichi Kimura, Yoshio Nakamura, Junji Sotaniand Masafumi Katsuta, 2005, Steady and Transient Heat Transfer Characteristics of Flat Micro Heat pipe, No. 27, pp.3-8 [7] Sukhvinder Kang, Maurice Holahan, 2003, The Thermal Resistance of Pin Fin Heat Sinks in Transverse Flow, Proceedings of IPACK0, July 6 11, Maui, Hawaii, USA [8] Giovanni Cortella, 2002, CFD-aided retail cabinet designs, Computers and Electronics in Agriculture, pp [9] Ram Viswanath, Vijay Wakharkar, AbhayWatwe and VassouLebonheur, 2000, Thermal performance challenges from silicon to systems, Intel technology journal Q3, pp1-16 [10] Keller, Kurtis, 1998, "Cast 3D Heatsink Design Advantages", IEEE ITherm '98, Seattle, WA, May 27-30, pp [11] Keller, Kurtis, 1998 "Low Cost, High Performance, High Volume Heatsinks", IEMT-Europe. [12] Tom Kowalski and Amir Radmehr, 2000, Thermal analysis of an electronics enclosure: coupling flow network modeling (FNM) and computational fluids dynamics (CFD), Sixteenth Annual IEEE Semiconductor Thermal Measurement and Management Symposium, San Jose, CA [13] M. Davis, R. Weymouth, P. Clarke, Thermoelectric CPU cooling using High efficiency liquid flow heat exchangers, Hydrocool Pty Ltd, Proceedings of the COMSOL Users Conference, Taipei [14] Asad ALEBRAHIM and Adrian BEJAN, Entropy Generation Minimization in a Ram-Air Cross-Flow Heat Exchanger,Int.J. Applied Thermodynamics, ISSN , Vol.2, No.4, pp [15] Intel Core i7-900 Desktop Processor Extreme Edition Series and Intel Core i7-900 Desktop Processor Series Available 239

8 Datasheet, Volume 1, February 2010, Document # [16] Intel Core i7-900 Desktop Processor Extreme Edition Series and Intel Core i7-900 Desktop Processor Series Datasheet, Volume 2, October 2009, Document number: Dr. M. Muralidhar Rao is currently working as Principal, Swarnandhra College of Engineering and Technology, Seetharampuram. He has over 32 years of experience and has many national and international publications. He also guided research projects. BIOGRAPHIES N. V. S. Shankar is currently working as Asst. Professor in Department of Mechanical Engineering, Swarnandhra College of Engineering and Technology, Seetharampuram. He has a total of 8 years work experience consisting both academic and industrial. Rahul Desala is currently pursuing his IV year B. Tech. (Mechanical) at Swarnandhra College of Engineering and Technology, Seetharampuram affiliated to JNTU, Kakinada. V. SrinivasBabu is currently pursuing his IV year B. Tech. (Mechanical) at Swarnandhra College of Engineering and Technology, Seetharampuram affiliated to JNTU, Kakinada. Dr. P. Vamsi Krishna is working as Associate Professor in Industrial Production Engineering Department, GITAM Univesrsity, Visakhapatnam. He has 10 years of experience in teaching and research. Available 240