Poh Seng (PS) Lee, PhD Associate Professor Micro Thermal Systems (MTS) Group Department of Mechanical Engineering National University of Singapore

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1 Poh Seng (PS) Lee, PhD Associate Professor Micro Thermal Systems (MTS) Group Department of Mechanical Engineering National University of Singapore Website: 1

2 Outline Introduction to Background and Motivations Current Research Interests High Performance Liquid Cooling Stabilized Two Phase Microchannel Cooling Future Research Focus Enhanced Air Cooling High Performance Heat Exchangers Applications 2

3 Micro Thermal Systems (MTS) Group Principle Investigator: A/Prof. PS Lee Research Fellows (4x) Research Engineers (2x) Graduate Students (6x) 3

4 Research Setups Liquid Flow Loops (4x) For liquid and twophase cooling studies Compact and Desktop Wind Tunnels For air cooling study Micro PIV System Flow field measurements in micro devices 4

5 Research Resources Hardware High speed camera High speed data acquisition systems 3 axis measurement microscope Research grade IR Camera Software NUS SVU Clusters In house Workstation CFD Packages (Fluent, CFX) 5

6 Motivations Miniaturization of electronic devices Thermal issues fast becoming bottleneck Heat transfer rate of 10 ~ to 50 W/cm 2 with acceptable noise levels. sustaining Moore s Law and hence to the continued growth of the electronics industry Source: Intel 6

7 Background Order of Magnitude for Heat Transfer Coefficients Oblique Fin Liquid Cooling & Stabilized Two- Phase Microchannel Cooling 7

8 Background H L W Pioneering work by Tuckerman and Pease (1981), R tot ~ 0.09 C/W Channel size: 50 ~ 1000 m Single or two phase operation Very high heat transfer coefficients h sp 1/D h ~ 10,000 W/m 2 K h tp ~ 40,000 W/m 2 K (latent heat) Highly compact, low acoustic noise etc. Tradeoffs High pressure drop High lateral temperature gradient t Wc Ww 8

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10 Introduction Microchannel Conventional microchannel: with oblique fins: Obliquely cuts Oblique fin Oblique channel Oblique fins promote: Periodic thermal boundary layer redevelopment Secondary flow generation Both which contribute to very effective convective heat transfer 10

11 Introduction Velocity contours Temperature contours Liquid flow direction Liquid flow direction Conventional Oblique Fins Conventional Oblique Fins 11

12 Comparison of heat transfer performance Heat transfer coefficient,h (x1000 W/m 2 K) streamwise position, X (m) Heater temperature, T ( C) streamwise position, X (m) conventional microchannel enhaned microchannel conventional microchannel enhanced microchannel 12

13 Comparison of pressure drop Local coordinate system Static pressure, P (Pa) Static pressure, P (Pa) Flow direction x 1 =0 x 1 =0.22 x 1 =0.5 x 1 =0.72 x 1 = streamwise position, X (m) Dimensionless distance, x 1 ' conventional microchannel enhanced microchannel Conventional microchannel Enhanced microchannel 13

14 Experimental Validation Conventional Conventional 0 Oblique fins Silicon test pieces 100µm Oblique fins 0 1mm Oblique fins Copper test pieces Aluminum test pieces 14

15 Experimental setup 15

16 Experimental Validation: Heat Transfer Performance Global heat transfer performance Local heat transfer performance 14 Average Nusselt number, Nu ave conventional microchannel - experiment 5 enhanced microchannel -experiment conventional microchannel -simulation enhanced microchannel - simulation Reynolds number, Re Local Nusselt number, Nu x Dimensionless channel length, X' conventional microchannel Flow direction enhanced microchannel 16

17 Pressure drop, dp (Pa) Experimental results: Pressure drop conventional microchannel - experimetal enhanced microchannel - experimental conventional microchannel - simulation enhanced microchannel - simulation ENu, Ef E Nu = Nu EM /Nu CM heat transfer enhancement factor pressure drop penalty Reynolds number, Re E f = f EM /f CM Reynolds number, Re At Re < 400, pressure drop penalty is small and negligible. At Re > 400, slightly higher pressure drop is incurred but should be manageable for the same micropump. 17

18 Hotspot mitigation 18

19 Temperature contour: single hotspot 12.7 Inlet Outlet Conventional microchannel Enhanced microchannel with variable fin pitch Inlet Outlet Uniform pitch

20 Temperature contour: multiple hotspots 12.7 Inlet Outlet 12.7 Conventional microchannel Inlet Outlet Enhanced microchannel with variable fin pitch Uniform pitch

21 Comparison of heat transfer and pressure drop Tempeature ( C) Conventional microchannel Enhanced microchannel - uniform fin pitch Enhanced microchannel - variable fin pitch Pressure drop (kpa) Maximum temperature Temperature gradient 15 C or more reduction in hot spot temperature when cooled by enhanced microchannel heat sinks. 0 conventional microchannel Enhanced microchannel with uniform fin pitch Enhanced microchannel with variable fin pitch ~15% pressure drop increment for the enhanced microchannel heat sinks compared to conventional microchannel heat sink. 21

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23 Stabilized Two Phase Cooling Technologies Stepped Fin Liquid Inlet Flow reversal (Unstable operation) Straight Fin Vapor has room to expand span wise (Stable operation) Vapor Confined 23

24 Stabilized Two Phase Cooling Technologies Expanding and stepped channels enlarged space along downstream Significantly reduced temperature and pressure fluctuations: stabilized boiling Significantly reduced pressure drop lower pumping power required, smaller pump can be used, more compact system 24

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26 Enhanced Air Cooling Pin Fins Cone Fins Convective thermal conductance 9 pin fin cone fin pin fin cone fin Pressure drop ha (W/K) Pressure drop (Pa) Velocity (m/s) Velocity (m/s)

27 High Performance Heat Exchangers 27

28 Applications Microprocessors Integrated circuit 3D ICs Electric vehicle Hybrid electronic vehicle High power battery pack Evaporation and condensation heat exchangers Fan coil unit in HVAC system Waste heat capture from industrial waste water or waste steam Concentrate photovoltaic Solar energy harvesting EV Battery Thermal Management High Performance IGBT/Power Electronics Cold Plate Awards: 2009 Tan Kah Kee Young Inventors Award (Defence Science category) 2011 Asia Pacific Clean Energy Summit s Top 10 Defense Energy Technology Solutions Award 2011 (MCCC) Golden Green Award 2011 Institution of Engineers Singapore s Prestigious Engineering Achievement Award 2013 NUS Faculty of Engineering Young Faculty Research Award 28