13 CHAPTER 2 LITERATURE REVIEW 2.1 INTRODUCTION The performance of heat sinks has been the focus of many investigations in recent years and the subject has been treated analytically, numerically and experimentally. Most of the works has dealt with heat sinks in order to minimize the equivalent thermal resistance to improve the heat transfer from the electronic packages. The optimal design of different types of heat sinks is also addressed in some research studies using parametric optimization using various optimization techniques. There are few investigations being done on a flat plate heat sink to find its electromagnetic emitted radiations. Depending upon the objectives of this study, the literature review is divided into three main sections such as: (i) Optimization of heat sink design with focus on the radiations emitted (ii) Optimization of heat sink design with focus on thermal resistance (iii) Multi objective optimization using Taguchi method and Grey Relational Analysis.
14 In this chapter, sections 2.2 and 2.3 reviews the experimental and simulation studies of flat plate heat sinks for the study for reduction of emitted radiations and thermal resistance respectively. Section 2.4 reviews the study on multi objective optimization of flat plate heat sink using Taguchi based grey relational analysis. 2.2 REDUCTION OF EMITTED RADIATION There are many studies on emitted radiations from a heat sink are found in the open literature. The common objective of these studies was to design a heat sink for minimizing emitted radiations. The influence of heat sink geometry parameters are studied on emitted radiations. Few experimental investigations were found in the literature and some studies tried to use HFSS simulation software to predict the amount of emitted radiations from the heat sink. Some of these studies are described below as representations from each category of research work. Brench (1994) examined the variations in the radiation characteristics of heat sink with respect to their geometries by applying 3D Finite Difference Time Domain (FDTD) technique and found that there is an additional increase in radiated emissions at the resonant frequency of the heat sink. Das and Roy (1998) observed that the radiating efficiency not only depends on the clock frequency of the Application specific integrated circuit (ASIC) but also on the shape of the heat sink. Also, they found that the measured radiated emissions are severe when the monopole length becomes quarter of the wavelength. The vertical polarized emissions were found to be higher than the horizontal polarization and the highest emission was observed when vertical length of the heat sink became one sixth of the wavelength. It was concluded that circular heat sink can be modeled as a monopole.
15 John Parry (2000) presented an approach for modeling of heat sink and studied the effect of grounded heat sink and examined the common mode RF coupling between die and heat sink and concluded that heat sink radiates clock harmonics throughout the frequency spectrum wherein, the grounding reduces the effect of emitted radiation. Bruce Archambeault et al (2001) discussed the effects of EMI by heat sink and presented a method to suppress its effects by grounding techniques. Georgerian and Mantrose (2003) studied the concept of wavelength based heat sink fin length selection for the minimization of radiated emissions and simultaneously they presented the relationship for the base temperature rise with the variation in fin length, for maximizing the cooling of electronic packages. Junwei Lu and Dawson (2006) studied the EMC computer modeling techniques for CPU heat sink simulation and presented HFSS model to simulate a heat sink. They found that a resonant frequency of 2.6 GHz with a reflection coefficient of -8.3db for the Intel CPU heat sink, which is closer to IEEE and Bluetooth wireless communication systems, and concluded that heat sink behaves like an efficient radiator for these range of frequencies. Junwei Lu and Xian Duan (2007) presented finite element frequency domain analysis for electromagnetic radiation emitted from high power microelectronic circuits connected to heat sink and also investigated various grounding options of the heat sink and compared various grounding options. Mohammad Golkhab and Mohammad Tavakoli Bina (2008) studied the effect of reduction in space for the want of compactness in
16 geometry and specified the effect of more EMI problems when the components become much closer and identified that EMI problems are more with higher compactness. Philippe Sochoux et al (2008) modeled heat sink using Micro stripes (Flomerics) to find emitted radiations from the heat sink. They investigated the variations in radiation for the design factors like height and number of fins and concluded that there is a considerable variation in radiation for the variations in the total number of fins. The influence of other design parameters like length and width of heat sink, fin thickness and base height are not studied. From the detailed review of open literature, the heat sink in the CPU behaves like an antenna and radiates emissions when it get coupled to high frequencies. A shape optimization of heat sink will reduce the emitted radiation from the heat sink. It is identified that there is further scope to apply standard optimization techniques using HFSS simulation results to identify the optimal geometry parameters. 2.3 REDUCTION OF THERMAL RESISTANCE There are several studies on optimizing thermal characteristics of a heat sink found in the literature. The common objective of these studies was to design a heat sink for minimizing thermal resistance. The influence of heat sink geometry parameters has been studied on thermal resistance. Some studies tried to incorporate fan power in the form of pressure drop in their optimization work. Few experimental investigations were found in the literature and some studies tried to use CFD simulation software to predict the thermal resistance and other performance parameters of a heat sink.
17 Goldberg (1984) constructed three air cooled forced convection heat sinks and tested each one. Each heat sink had a different fin thickness, with the channel to fin width ratio restricted to unity and flow limited to laminar regime. The air flow for each heat sink was adjusted to provide a rate of 30 lit/min. As expected, the design with the largest pressure drop and smallest channel width yielded the smallest thermal resistance. Only, experimental observation was provided in the literature. Azar et al (1992) performed experimental studies on narrow channel heat sink with air flow arrangement of side-in-side-exit (SISE) and top-in-side-exit (TISE) and found no significant difference in heat sink performance. They performed some experiments with tip clearance and found that the use of heat sinks with tip clearance did not lead to a significant improvement in thermal performance. Sathyamurthy el al (1996) investigated planar and staggered parallel plate arrays and obtained good agreement between their numerical simulation results and experiments. Their results illustrated that the thermal performance of the staggered fin configuration was better than the planar fin configurations. Teertstra et al (1999) presented an analytical model for predicting the average heat transfer rate for forced convection, air-cooled plate fin heat sink for use in the design and selection of heat sink for electronic applications. Model has been developed based on the limiting cases of fully developed and developing flow between isothermal parallel plates. Fin effects are included in the model to account for temperature variations between the fins and the base plate and these fin effects have been shown to be significant in certain heat sink configurations. The average Nusselt number is calculated as a function of the heat sink geometry and fluid velocity. Measurements were performed for
18 an air cooled, high aspect ratio heat sink prototype and the model was found to be in good agreement with the experimental results. Copeland (2000) presented an analysis of simultaneously developing flow for the performance calculations of a plate fin heat sink. They combined laminar fully developed theory to the developing flow (hydraulic and thermal) theory of Shah and London (1978). They also addressed the influence of spreading resistance in their model. Visser et al (2000) described the use of mathematical optimization techniques to minimize the heat sink mass and thermal resistance using five design variables namely, heat sink height, thickness, extrusion length, base thickness and number of fins for the heat sink. The numerical simulations have been performed using Q-Fin software and optimization was carried out using dynamic-q method. This method is specifically designed to handle constrained problems where the objective and constraint functions are expensive to evaluate. The paper illustrates how the parameters considered influence the heat sink mass and how mathematical optimization techniques can be used by the heat sink designer to design compact heat sinks for different types of electronic enclosures. Culham and Muzychka (2001) presented a procedure for optimization of heat sink design parameters based on minimization of the entropy generation associated with heat transfer and fluid friction. All relevant design parameters for plate fin heat sinks, including geometric parameters, heat dissipation, material properties and flow conditions are simultaneously optimized to characterize a heat sink to minimize entropy generation and in turn results in a minimum operating temperature. In addition, a novel approach for incorporating forced convection through the
19 specification of a fan curve is integrated into the optimization procedure, providing a link between optimized design parameters and the system operating point. Shah et al (2002) examined the effect of the shape of the heat sink fins. The pressure gradient at the center of the heat sink and near the base tends to be substantial and this significantly reduces air flow and hence transport in that regions. Different fin shapes and air flow rates were studied with the objective of searching for an optimal heat sink design to improve the thermal performance. An optimum heat sink shape is reported that results in a lower operating temperature and pressure gradient. Narasimhan et al (2003) developed, demonstrated and validated a boundary layer methodology for the application of compact, porous block models for the hydrodynamic behavior of parallel plate heat sinks in laminar flow. They compared the porous block data with the results obtained from several hundred laminar flow CFD simulations. Shih et al (2005) presented a formal systematic optimization process to plate-fins heat sink design for dissipating the maximum heat generation from electronic component by applying the entropy generation rate to obtain the highest heat transfer efficiency. The design investigations demonstrate that the thermal performance with horizontal inlet cooling stream is slightly superior to that with vertical inlet cooling stream. However, the design of vertical inlet stream model yields to a less structural mass required than that of horizontal inlet stream model under the same amount of heat dissipation. In the analysis, the constrained optimization of plate-fins heat sink design with vertical inlet stream model is developed to achieve enhanced thermal performance. The heat sink used on AMD Thunderbird 1GHz
20 processor has been examined and redesigned by presenting optimization methodology. The thermal performance is considerably improved. Also, the size and mass of the heat sink are apparently reduced. Khan et al (2006) studied the entropy generation minimization (EGM) procedure employed to optimize the overall performance of micro channel heat sinks using Newton-Raphson method. Chyi-Tsong Chen et al (2008) has done the parameter optimization of plate fin heat sink (with side cooling fan) using genetic algorithm and developed a direct adaptive control scheme for the CPU heat sink process using a bounded single neuron controller to regulate the temperature of a selected control point. But the authors did not consider the other influencing parameters like length of the heat sink and also mass optimization was not reported. The study done by Arularasan et al (2008) presents the modeling and simulation of parallel plate heat sink without cooling fan using Computational Fluid Dynamics (CFD) and Design of Experiments (DOE) considering the factors like number of fins, fin length, fin height and base height of the heat sink. From the study, the percentage contribution of each heat sink design factor towards minimization of thermal resistance has been investigated. From the detailed review of open literature, the heat sink design with minimum thermal resistance is of paramount importance in the design of electronics cooling. Also, there is further scope for improvement in cooling methods by adopting standard optimization techniques.
21 2.4 TAGUCHI METHOD AND GREY RELATIONAL ANALYSIS There are several studies on Taguchi based grey relational analysis found in the literature. The common objective of these studies is to optimize multiple performance characteristics of a system towards multiobjective goals. The objective of parameter design is to optimize the settings of process parameter values for improving performance characteristics to identify the product parameter values under the optimal process parameter values. In addition, it is expected that the optimal process parameter values obtained from the parameter design are insensitive to the variation of environmental conditions and other noise factors. Therefore, the parameter design is the key step in Taguchi method to achieve high quality without increasing cost. Basically, classical parameter design developed by Fisher (1925) is complex and not easy to use. Especially, a large number of experiments had to be carried out when the number of process parameters increases. Whereas the Taguchi method uses a special design of orthogonal arrays to study the entire parameter space with a small number of experiments only. Deng (1989) explained grey relational analysis based on grey system theory. It is used for solving the complicated inter-relationships among the multiple responses. A grey relational grade is then obtained for analyzing the relational degree of the multiple responses. The use of the grey analysis based on an orthogonal array in Taguchi method for optimizing the multi response process is attempted by Lin et al (2002). It has been showed that grey based Taguchi method can optimize the multi-response processes through the settings of the process parameters (Lin and Tarng 1998).
22 Wang and Tong (2003) proposed a method to optimize the parameter design problem. This study utilizes grey relational analysis from grey system theory, to develop a procedure for improving the ordered categorical response in a dynamic system, based on Taguchi s parameter design. This procedure can determine effectively the optimal factor level combination for an ordered categorical response in a dynamic system. Chen and Syu (2003) utilized Taguchi method with grey relational analysis for the optimization of process parameters on the bio-film thickness and substrate utilization rate. Wang et al (2001) presented a new method that uses grey relational analysis and fuzzy clustering to form part facilities. The main objective is to identify part families based on a new similarity coefficient which considers processing time, lot size, machine usability, etc., by using grey relational analysis. Lin (2004) has studied the use of the Taguchi method and grey relational analysis to optimize turning operations with multiple performance characteristics. Lin and Lin (2005) reported the use of grey fuzzy logic based on orthogonal array for optimizing the electrical discharge machining process with multi responses. An orthogonal array, grey relational coefficient, grey fuzzy reasoning grade and analysis of variance are applied to study the performance characteristics of the machining process. Ko-Ta Ching et al (2006) have studied the optimum design parameters of pin-fin heat sink using the grey-fuzzy logic based on the orthogonal arrays. The effects of design parameters and the optimum design parameters for a Pin-Fin heat sink (PFHS) with the multiple thermal performance characteristics have been investigated by using the grey-fuzzy
23 logic based on the orthogonal arrays. Various design parameters, such as height and diameter of pin-fin and width of pitch between fins were considered. Chih-Chung-Chou et al (2009) studied a grey-based fuzzy algorithm with the orthogonal arrays and employed it to find the optimal designing parameters setting for a heat sink with the Parallel-Plain Fin (PPF) on the multiple thermal performance characteristics. The proposed algorithm, coupled the grey relational analysis with the fuzzy logic to obtain a greyfuzzy reasoning grade for evaluating the multiple performance characteristics according to the grey relational coefficient of each performance characteristic. In their study, the designing parameters of the heat sink include the height of fin, the width of fins, the width of slot, the number of slot and the air speed were considered. The design of experiment (DOE) adopts the L 16 orthogonal arrays table which is four levels and five factors type of factorial design. The average convective heat transfer coefficient, the thermal resistance and the pressure drop are considered as the multiple thermal performance characteristics and explored in the experiment. In addition, the response table, response graph and the analysis of variance (ANOVA) are used to find the optimal settings and the influence of designing parameters on the multiple performance characteristics. The results of confirmation tests with the optimal settings of designing parameters have obviously shown that the multiple thermal performance characteristics are effectively improved through these procedures. From the detailed review of literature, it is observed that the study of the combined optimization of the electromagnetic emitted radiations and the thermal resistance of the heat sink has not been reported in literature.
24 Figure 2.1 shows the flowchart with respect to design and performance improvement of flat plate heat sinks by using multi objective optimization. Figure 2.1 Flowchart of multi objective optimization of heat sink design 2.5 SUMMARY An extensive literature review on modeling and optimization of flat plate heat sink is performed. It is observed that there is no comprehensive study carried on multi objective optimization of flat plate heat sinks towards the minimization of electromagnetic emitted radiations with reduction in thermal resistance. In most studies the experimental investigations were found to be in good agreement with the simulation results for both the electromagnetic emitted radiations and thermal resistance of flat plate heat sink. It is also observed that there is no metamodel presently available to predict the emitted
25 radiations of the flat plate heat sink using either experimental investigations or numerical simulations. In this research, the Taguchi based grey relational analysis is used for parameter optimization of a flat plate heat sink considering both the reduction of thermal resistance and emitted radiations.