1. Numerical Investigation of Flow Boiling in Double-layer Microchannel Heat Sink

2. ContentsContents 1. INTRODUCTION 2. MATHEMATICAL MODELING 3. RESULTS AND DISCUSSIONS 2

3. INTRODUCTIONINTRODUCTION Moore’s Law High heat flux Tiny surface area Higher rate of heat dissipation Thermal management Moore, Gordon E., Cramming more components onto integrated circuits, Electronics Magazine (1965), p. 4. 3

4. Classical air cooling Micro-channel heat sink Low heat transfer coefficient Large surface-to-volume ratio Higher pressure drop Microchip cooling techniques INTRODUCTIONINTRODUCTION Thermal boundary layer development 4

5. INTRODUCTIONINTRODUCTION Definition of micro-channel 5

6. INTRODUCTIONINTRODUCTION Flow in micro- channel Single-phase flow Multi-phase flow Gas – liquidLiquid - liquidLiquid - solidGas - solid e.g. Oil - water Nanofluids e.g. CuO - water e.g. Air - dust particle e.g. Air - water 6

7. INTRODUCTIONINTRODUCTION S. S. Bertsch, E.A. Groll, S.V. Garimella, Effect of heat flux, mass flux, vapor quality, and saturation temperature on flow boiling heat transfer in microchannels, Int. J. of Multiphase Flow, vol. 35 (2009) pp. 142-154. Boiling curve for R-134a Flow boiling High heat flux after ONB Small wall temperature rise 7

8. INTRODUCTIONINTRODUCTION Average heat transfer coefficient vs heat flux for FC-72 Hear transfer coefficient Increases after ONB Decreases after CHF Y. Wang, K. Sefiane, Effects of heat flux, vapour quality, channel hydraulic diameter on flow boiling heat transfer in variable aspect ratio micro-channels using transparent heating, Int. J. of Heat and Mass Transfer, vol. 55 (2012), pp.2235-2243. 8

9. INTRODUCTIONINTRODUCTION Double-layer micro-channel Invented by Vafai and Zhu (1997) Provide more coolant in the upper layer More uniform surface temperature distribution K. Vafai, L. Zhu, Analysis of two-layered micro-channel heat sink concept in electronic cooling, International Journal of Heat and Mass Transfer, vol. 42, pp. 2287-2297, 1999. 9 Absence of flow boiling in DL- MCHS

10. MATHEMATICAL MODELING Multi-phase flow Different properties of primary and secondary phase Mass, momentum and energy transfer at the interface Properties Density Viscosity Thermal conductivity Specific heat capacity 10

11. MATHEMATICAL MODELING Vapor density Vapor quality Void fraction Relation between void fraction and vapor quality 11

12. MATHEMATICAL MODELING Void fraction equation Continuity equation (Mixture) 12

13. MATHEMATICAL MODELING Conservation of energy equation (Solid domain) 13 Momentum equation (Mixture)

14. MATHEMATICAL MODELING Temperature Interfacial heat flux Source/sink for enthalpy Conservation of Energy equation (vapor phase) 14 Mass transfer takes place due to evaporation and condensation

15. MATHEMATICAL MODELING Evaporation Condensation Source/sink for mass = evaporation coefficient = evaporation coefficient W. H. Lee. "A Pressure Iteration Scheme for Two-Phase Modeling". Technical Report LA-UR 79-975. Los Alamos Scientific Laboratory, Los Alamos, New Mexico. 1979. W. H. Lee. "A Pressure Iteration Scheme for Two-Phase Modeling". Technical Report LA-UR 79-975. Los Alamos Scientific Laboratory, Los Alamos, New Mexico. 1979. = condensation coefficient = condensation coefficient 15

16. MATHEMATICAL MODELING Initial conditions Velocity Temperature Inlet conditions Inlet temperature Mass flow rate inlet 16 Inlet void fraction

17. MATHEMATICAL MODELING Outlet conditions Pressure outlet Boundary conditions Bottom wall Other outer walls 17

18. MATHEMATICAL MODELING Boundary conditions Solid-coolant interface Energy balance 18 Heat loss is calculated from heat loss characterization curve

19. RESULTS AND DISCUSSIONS 19 Void fraction distribution along the length Heat loss characterization curve Void fraction

20. RESULTS AND DISCUSSIONS 20 Two phase heat transfer coefficient vs. Heat flux q (W/cm^2) h (W/cm^2.K)

21. RESULTS AND DISCUSSIONS 21 Two phase heat transfer vs. Wall temperature rise above saturation temperature Tw-Tsat (K) Q (W) CHF

22. THANK YOU 22