A thermally driven two phase cooling device for CPU cooling

A THERMALLY DRIVEN TWO-PHASE COOLING DEVICE FOR CPU COOLING MARK AARON C. CHAN (B.Sc. (Hons.), De La Salle University-Manila) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2009 Acknowledgements To God To My Family To My Country and To my mentors: Prof. Christopher R. Yap Prof. Ng Kim Choon -Mark Aaron 8/19/09 I List of Publications Scientific journals: • Ng, K.C, Yap, C.R., Chan, M.A., A Universal Performance Chart for CPU Cooling Devices, Heat Transfer Engineering, 2008. • Chan, M.A., Yap, C.R., Ng, K.C., Modeling and Testing of an Advanced Compact Two-Phase Cooler for Electronics Cooling, International Journal of Heat and Mass Transfer, 2009. • Chan, M.A., Yap, C.R., Ng, K.C., Pool Boiling Heat Transfer of Water on Finned Surfaces at Near Vacuum Pressures, ASME Journal of Heat Transfer, 2010. Conference paper: • Chan, M.A., Yap, C.R., Ng, K.C., Modeling and Testing of an Integrated Evaporator-Condenser Device for CPU Cooling, Proceedings of the ASME Summer Heat Transfer Conference, 2008. • Ng, K. C., Yap, C.R., Chan, M.A, Experimental Analysis of Pool Boiling Heat Transfer on Extended Surfaces at Near Vacuum Pressures, The Third International Symposium on Physics of Fluids, 2009. Patent: • Ng, K.C., Yap, C.R., Elsharkawy, I., Chan, M.A., Cooling Device for Electronic Components, PCT Patent Application No. PCT/SG2008/000137, 2008. II Table of Contents Acknowledgements……………………………………………………………………… .I List of Publications……………………………………………………………………… .II Table of Contents…………………………………………………………………………III Summary……………………………………………………………………………….….V List of Tables…………………………………………………………………………….VII List of Figures………………………………………………………………………… .VIII Nomenclature……………………………………………………………………….….XVII Chapter : Introduction 1.1 History 1.2 Trend 1.3 Cooling Requirements 1.4 Goals of Thermal Packaging 1.5 Scope of the Study Chapter : Literature Review 2.1 Fundamentals of CPU Package Thermal Characteristics . 2.2 CPU cooling methods . 2.2.1 Forced Convection 10 2.2.2 Thermoelectric Refrigeration 11 2.2.3 Heat pipe . 11 2.2.4 Nucleate pool boiling 14 2.2.5 Vapor Compression Refrigeration 16 2.2.6 Micro-channel . 17 2.2.7 Impinging Jet/Spray Cooling 19 2.3 Fundamentals of Nucleate Pool Boiling Heat Transfer 21 2.4 Physical Mechanism of Boiling . 23 2.5 Enhanced Boiling Surfaces 25 2.5.1 Porous coated boiling surfaces 26 2.5.2 Finned surfaces 28 2.6 Working Fluid for CPU Cooling 31 2.7 The Heat Transfer Coefficient 31 Chapter : Experiments on Pool Boiling at Sub-Atmospheric Pressures 33 3.1 Pool boiling chamber . 34 1.1 Variable autotransformer 34 3.2 Heater chamber 37 3.2.1 Heat Leak Test 38 3.3 Measurement and Data acquisition 40 3.4 Vacuum pump and components . 40 3.5 Boiling surface test pieces 42 3.5.1 Finned boiling surfaces . 43 III 3.5.2 Porous coated boiling surfaces 44 3.6 Experimental Procedure . 44 3.7 Measurement Error . 47 3.8 High Speed Visualization . 48 3.9 Comparison of experimental results with empirical correlations . 49 3.10 Effect of Fin Thickness 57 3.11 Effect of Fin Spacing 62 3.12 Effect of Fin Height 67 3.13 Effect of Pin Fin Design . 70 3.14 Heat Transfer Augmentation ratio 72 3.15 Method for analyzing boiling performance of fins 73 3.15.1 Effect of Fin Thickness Based on Total Surface Area in Contact 75 3.15.2 Effect of Fin Space Based on Total Surface Area in Contact . 80 3.15.3 Effect of Fin Height Based on Total Surface Area in Contact 84 3.16 Confined boiling heat transfer correlation 88 Chapter : High Speed Visualization of Pool Boiling at Near Vacuum Pressures 95 4.1 Pool Boiling of Water on a Plain Surface at Low Pressures 96 4.2 Pool Boiling of Water on an Array of Plate Fin at Low Pressures 104 4.3 Pool Boiling of Water on Pin Fins at Low Pressures . 111 Chapter : Fundamental Experiment of Pool Boiling on Porous Surfaces 118 5.1 Effect of Pressure on Boiling in Porous Media 118 5.2 Effect of Thermo-Physical Properties 121 5.3 Effect of Layer Thickness 123 5.4 Augmentation ratio . 126 5.5 Effect of Grooves on Porous Surfaces . 128 5.6 Visualization of Pool Boiling on Porous Surfaces . 130 Chapter : Design and Modeling of a Compact Two-Phase Thermosyphon . 144 6.1 Working Principle of a Compact Two-Phase Thermosyphon 147 6.2 Governing Equations 148 6.3 Experimental Investigation of a Compact Two-Phase Thermosyphon 154 6.3.1 Experimental Apparatus and Procedure 154 6.3.2 Optimization of the Curvilinear Fin Array 158 6.3.3 Performance of the Two-Phase Thermosyphon 159 6.4 Tubular Two-Phase Thermosyphon Cooler . 170 6.4.1 Testing Methodology 172 6.4.2 Comparative Analysis of CPU coolers . 174 6.5 Performance Rating of CPU Coolers . 175 6.5.1 A Generic Model . 177 6.5.2 Number of Transfer Units (NTU) . 179 6.5.3 Temperature ceiling to thermal design limit ( Φ ) . 180 6.5.4 Ratio of heat transfer to overall device characteristic length ( Π ) 181 6.5.5 Figure of Merit (FOM) 182 6.5.6 The Universal Performance Chart . 182 Chapter : Conclusion 188 References…………………………………………………………………………… .190 Appendix……………………………………………………………………… .………… 198 IV Summary Since the dawn of the computer age, there has been tremendous advancement in terms of technology and global demand for electronic computing devices. The need for increased processing speed/functionality on a single chip and miniaturization of each unit has led to an unprecedented development of thermal management technologies. Given that the upward trend of heat generated by these devices is highly coupled with its performance, the lack of advanced thermal management technologies could ultimately impede the progress in the world’s computing technology. In order to accommodate heat fluxes as high as 1.0 MW/m2 from future microprocessors, the present study developed a novel cooling solution with the essential attributes of compactness, low-cost, high cooling capacity, low power consumption, and orientation-free design. To achieve both feats of compactness and high cooling capacity, the mechanism of phase change was utilized for the proposed cooler. Much focus was given on the design of surface enhancement for nucleate boiling heat transfer, which is the primary conveyor of heat from the electronic device to the ambient. Fundamental boiling experiments were conducted on finned and porous surfaces using water as the working fluid. Significant insight in enhanced boiling heat transfer at near vacuum pressures was gathered. High speed visualization further confirmed the results by qualitative analysis of bubble nucleation, growth, coalescence, and departure at various operating conditions and boiling surfaces. It was found that the heat transfer enhancement offered by narrowly spaced fins is mainly due to the effect of confinement. This ultimately led to the development of a V nucleate pool boiling correlation for confined boiling, which introduces a key dimensionless parameter that is Bond number. Numerical methodology was developed to effectively design a compact two-phase thermosyphon for CPU cooling. Simulation results have been shown to be in good agreement with experiments. It was also found that the two major thermal “bottlenecks” in the cooler are air-side convection and evaporator boiling resistances. Finally, a universal performance chart is proposed to provide a platform for comparison of various CPU coolers. This considers not only the overall thermal resistance but also key parameters such as compactness and CPU temperature levels. VI List of Tables TABLE 1.1. 2005 ITRS TECHNOLOGY REQUIREMENT FOR SINGLE CHIP PACKAGES (SIA, 2005). TABLE 3.1. SURFACE AREA AND BOILING HEAT TRANSFER PERFORMANCE OF VARIOUS FIN DESIGN. TABLE 3.2. SURFACE AREA AND SURFACE AVERAGED BOILING HEAT TRANSFER PERFORMANCE OF VARIOUS FIN DESIGN. TABLE 6.1. PHYSICAL DIMENSIONS OF THE TWO-PHASE COOLER. TABLE 6.2. COMPARISON OF VARIOUS CPU COOLING DEVICES. TABLE 6.3. CPU COOLING DEVICE’S PERFORMANCE FACTORS WITH COOLING LOAD RANGING FROM 70-100 WATTS AND A TDP OF 70 OC. VII List of Figures FIGURE 1.1. THE CHRONOLOGICAL EVOLUTION OF MODULE LEVEL HEAT FLUX IN MAINFRAME COMPUTERS FIGURE 1.2. INTEL SINGLE-CORE (BLUE) AND MULTI-CORE (PINK) CPU HEAT FLUXES PROPORTIONAL TO CLOCK FREQUENCY. FIGURE 2.1. SCHEMATIC OF CPU PACKAGE. FIGURE 2.2. SINGLE AND TWO PHASE HEAT REMOVAL TECHNIQUES EMPLOYED FOR ELECTRONIC COOLING. THE GRAY AREA INDICATES COOLING METHODS THAT CAN OPERATE WITH BOTH SINGLE AND TWO-PHASE WORKING FLUID. FIGURE 2.3. INTEL STOCK COOLER (FIN-FAN). FIGURE 2.4. COMMERCIALLY AVAILABLE MINIATURE HEAT PIPE COOLERS. ZALMAN CPS-9500 (LEFT) AND THERMALRIGHT INFERNO FX 14 (RIGHT). FIGURE 2.5. TWO-PHASE CLOSED THERMOSYPHON FROM WEBB ET AL (2002). FIGURE 2.6. KYROTECH SUPER G™ COMPUTER. FIGURE 2.7. MICRO-CHANNEL COOLING SYSTEM FROM CHANG ET AL (2006). FIGURE 2.8. BOILING CURVE FOR A HORIZONTAL PLAIN SURFACE. FIGURE 2.9. BUBBLE AGITATION AND LIQUID PUMPING EFFECT INDUCED BY ADJACENT BUBBLES. FIGURE 2.10. REMOVAL OF THERMAL BOUNDARY LAYER ON A HOT SURFACE. FIGURE 2.11. EVAPORATION FROM THIN-FILM LIQUID MICRO-LAYER AND FROM SURROUNDING SUPERHEATED LIQUID. FIGURE 3.1. PICTORIAL VIEW OF THE LOW PRESSURE BOILING TESTING FACILITY. FIGURE 3.2. DETAILED SCHEMATIC OF THE BOILING/HEATER CHAMBER. FIGURE 3.3. HEAT LEAK AS A FUNCTION OF TEMPERATURE DIFFERENCE BETWEEN HEATER AND AMBIENT. FIGURE 3.4. CHRONOLOGICAL PRESSURE LEVEL IN THE BOILING AND HEATER CHAMBER. FIGURE 3.5. SCHEMATIC AND PICTORIAL VIEW OF BOILING TEST PIECE. VIII FIGURE 3.6. FINNED BOILING TEST PIECE. FIN HEIGHT, L=15MM, FIN SPACE, G=1MM, FIN THICKNESS, T=1MM. FIGURE 3.7. POROUS COATED BOILING TEST PIECES. COATING THICKNESS OF 2.5 MM AND PORE DENSITY OF 60 PPI. FIGURE 3.8. HIGH SPEED CAMERA AND HIGH-LUMEN LIGHTING FOR BOILING VISUALIZATION. FIGURE 3.9. BOILING CURVES OF POOL BOILING ON A PLAIN COPPER SURFACE AT SUB-ATMOSPHERIC PRESSURES OF 2, 4, AND KPA WITH PREDICTIONS FROM COOPER’S CORRELATION (COOPER, 1984). FIGURE 3.10. HEAT TRANSFER COEFFICIENT OF A COPPER PLAIN SURFACE AT VARIOUS SUB-ATMOSPHERIC PRESSURES. FIGURE 3.11. EXPERIMENTAL AND PREDICTED HEAT TRANSFER COEFFICIENTS COMPARED. FIGURE 3.12. CRITICAL HEAT FLUX OF WATER AT VARIOUS SATURATION PRESSURES. FIGURE 3.13. BOILING CURVES FOR FINNED SURFACE HAVING DIFFERENT FIN THICKNESSES WITH CONSTANT FIN SPACING, (G) = 0.5 MM, FIN HEIGHT, (L) = 15 MM, AT PRESSURES OF KPA AND KPA. HEAT FLUXES ARE BASED ON THE PROJECTED HEATER SURFACE AREA. FIGURE 3.14. HEAT TRANSFER COEFFICIENT VERSUS HEAT FLUX (BASED ON THE PROJECTED HEATER SURFACE AREA) FOR FINNED SURFACE HAVING DIFFERENT FIN THICKNESSES WITH CONSTANT FIN SPACING, (G) = 0.5 MM, FIN HEIGHT, (L) = 15 MM, AT PRESSURES OF KPA AND KPA. FIGURE 3.15. BOILING CURVES FOR FINNED SURFACE HAVING DIFFERENT FIN SPACING WITH CONSTANT FIN THICKNESS (T) = 1.0 MM, FIN HEIGHT (H) = 15 MM, AT KPA PRESSURE. THE PLAIN SURFACE DATA IS MEASURED WITH THE SAME APPARATUS. HEAT FLUXES ARE BASED ON THE PROJECTED HEATER SURFACE AREA. FIGURE 3.16. BOILING CURVES FOR FINNED SURFACE HAVING DIFFERENT FIN SPACING WITH CONSTANT FIN THICKNESS (T) = 1.0 MM, FIN HEIGHT (L) = 15 MM, AT KPA PRESSURE. THE PLAIN SURFACE DATA IS MEASURED WITH THE SAME APPARATUS. HEAT FLUXES ARE BASED ON THE PROJECTED HEATER SURFACE AREA. FIGURE 3.17. HEAT TRANSFER COEFFICIENT VERSUS HEAT FLUX (BASED ON THE PROJECTED HEATER SURFACE AREA) FOR FINNED SURFACE HAVING DIFFERENT FIN SPACES WITH CONSTANT FIN THICKNESS, (T) = 1.0 MM, FIN HEIGHT, (L) = 15 MM, AT PRESSURES OF KPA AND KPA. IX Yu, C. K. and D. C. Lu. Pool boiling heat transfer on horizontal rectangular fin array in saturated FC-72, International Journal of Heat and Mass Transfer, 50, pp. 3624-3637. 2007. Yuan, L., Y. K. Joshi and W. Nakayama. Effect of Condenser Location and Imposed Circulation on the Performance of a Compact Two-phase Thermosyphon, Nanoscale and Microscale Thermophysical Engineering, 7, pp. 163-179. Zuber N., Hydrodynamic aspects of boiling heat transfer, AEC Report AECU-4439, Physics and Mathematics. 1959. - 199 - Appendix A Figure A.1. Omega thermistors specifications. - 200 - Figure A.2. Porvair metal foam specifications for various pore densities. Figure A.3. Pictures of metal foams having various pore densities. - 201 - Figure A.4. Graph of surface area against pore density. Figure A.5. Specifications of the high speed video camera. - 202 - Figure A.6. Voltage and current measurement accuracy specification of Fluke multimeter. - 203 - Figure A.7. Top and side view of the PTFE chamber separator. - 204 - Figure A.8. Top, side, and bottom view of the plain surface test piece. - 205 - Figure A.9. Top view of the compact two-phase cooler. Figure A.10. Side view of the compact two-phase cooler. - 206 - Figure A.11. Experimental set-up of the cooler test rig (air flow bench). - 207 - Figure A.12. Side view of the finned surface with height of 0.75 mm. - 208 - Figure A.13. Side and top view of the finned surface with g = 0.5 mm, t= 0.5 mm, and h=15mm. - 209 - Figure A.14. Side and top view of the finned surface with g = 1.0 mm, t= 1.0 mm, and h=15mm. - 210 - Figure A.15. Side and top view of the finned surface with g = 2.0 mm, t= 1.0 mm, and h=15mm. - 211 - Figure A.16. Side and top view of the pin finned surface with g = 0.5 mm, t= 1.0 mm, and h=15mm. - 212 - Figure A.17. Side and top view of the pin finned surface with g = 0.75 mm, t= 0.75 mm, and h=15mm. - 213 - Figure A.18. Side and top view of the compact two-phase thermosyphon. - 214 - [...]... that can have single phase forced convection and/or phase change heat transfer Single phase Forced convection (air) Two- phase Micro-channel Heat pipe Jet impinging Thermoelectric refrigeration Nucleate pool boiling Spray cooling Vapor-compression refrigeration Figure 2.2 Single and two phase heat removal techniques employed for electronic cooling The gray area indicates cooling methods that can operate... transfer mechanisms (e.g boiling, condensation, and impinging jets/sprays) 2.2 CPU cooling methods The imminent thermal limit of conventional fin-fan cooler is a reality, and the surge in CPU heat fluxes has sparked a wave of research for the “next generation CPU cooler” There are vast array of CPU cooling solutions that are being studied and developed for commercial application There are basically two types... the thermally conductive die bond material and on to the heat spreader casing, mainly conductive resistance Alternatively, the flow of heat from the casing, through the thermal interface material and heat sink/cooler, and finally to the ambient air, must overcome external resistance As heat flows from the CPU to the package surface/heat spreader, it encounters several resistances including the material... cooling capacity, geometry, and power consumption -6- Chapter 2 : Literature Review 2.1 Fundamentals of CPU Package Thermal Characteristics The thermal performance of a chip packaging is typically compared on the basis of the overall thermal resistance (chip-to-ambient) This is generally defined as, RT = (T chip − Tambient ) Qchip (2.1) where Tchip and Tambient are the CPU die and ambient air temperatures,... BOILING HEAT TRANSFER COEFFICIENTS AT VARIOUS SATURATION TEMPERATURES XIV FIGURE 6.15 AIR PRESSURE DROP THROUGH THE COOLER AT VARIOUS FLOW RATES FIGURE 6.16 CONVECTION THERMAL RESISTANCE OF THE CURVILINEAR FIN ARRAY AT VARIOUS AIR FLOW RATES FIGURE 6.17 STEADY STATE TEMPERATURE LEVELS AT VARIOUS HEAT LOADS (HORIZONTAL ORIENTATION) FIGURE 6.18 THERMAL RESISTANCES AT VARIOUS HEAT LOADS (HORIZONTAL ORIENTATION)... dissipation that exceeds 140 kW (US Department of Commerce, 2009) An array of large industrial cooling fans was used to maintain component temperatures low enough for reliable operation Despite the effort, at best it would still encounter tube failures at a rate of one tube every two days, mainly caused by thermal stress This -1- elucidates the importance of thermal management for reliable operation and... which augments the boiling performance of enhanced surfaces A working CPU cooler prototype was modeled and designed employing enhanced surfaces The prototype was tested at various operating conditions (e.g heat flux, air flow rate, and pressure) and its performance was compared with coolers in the market and literature A performance chart was developed to access various CPU coolers based on their cooling. .. commercially available copper/water heat pipe assembly for CPU cooling These cooler has 6-8 miniature heat pipes, a heat sink dimension of 90-146 mm length, 124 mm width, and 142-161 mm height, and weight over 750 grams The bulky size, tortuous design, and relatively heavy weight are proofs that heat pipe technology has already reached its maximum performance, and this is the most that manufacturers can achieve... et al (2003), Pastukov et al (2007), and Singh et al (2007) indicate that overall thermal resistance of an air-cooled loop and water-cooled loop heat pipe is 0.5 K/W, and with further integration of thermoelectric cooler and two- phase thermosyphons the thermal resistance is further reduced to 0.29 K/W The vapor chamber is a rectangular flattened heat pipe and mainly used for spreading heat from a small... small heater source to a larger area for heat rejection It replaces the thermally conductive base material of a conventional fin-fan heat sink to reduce heat spreading resistance Numerical and experimental investigations by Koito et al (2006), Lu et al (2006), and Rullie`re et al (2007) reveal that a temperature difference of only 45 K is incurred from a 96 W/cm2 heat source (heater size 1.5 cm2) to a point . PACKAGES (SIA, 2005). TABLE 3.1. SURFACE AREA AND BOILING HEAT TRANSFER PERFORMANCE OF VARIOUS FIN DESIGN. TABLE 3.2. SURFACE AREA AND SURFACE AVERAGED BOILING HEAT TRANSFER PERFORMANCE. Chan, M .A. , Yap, C.R., Ng, K.C., Modeling and Testing of an Advanced Compact Two-Phase Cooler for Electronics Cooling, International Journal of Heat and Mass Transfer, 2009. • Chan, M .A. ,. A THERMALLY DRIVEN TWO-PHASE COOLING DEVICE FOR CPU COOLING MARK AARON C. CHAN (B.Sc. (Hons.), De La Salle University-Manila) A THESIS SUBMITTED FOR THE DEGREE