Development of a High Performance Heat Sink with Counter Flow Diverging Microchannels


Student thesis: Doctoral Thesis

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Award date5 Nov 2021


In the emerging electronic era, supercomputers are pushing the limits of computing power, the revolutionary development of electric vehicles is moving forward, and the demand for hyperscale data centers is surging. More efficient, energy-saving and reliable supporting cooling systems are urgently needed for the flourishing electronic industry. Two-phase flow boiling in microchannels is well recognized as a prominent approach for high density heat dissipation. However, the inherent defects of two-phase flow boiling in microchannels, such as relatively low critical heat flux due to the dry out of liquid film near the channel exit, serious instability during the vigorous liquid-vapor phase change process and very high two-phase flow pressure drop at high heat flux, still hinder its extensive commercial applications.

This study develops a novel high performance heat sink with counter flow diverging microchannel (CFDM) manifold to further enhance the flow boiling performance. A unique and significant heat exchange between the neighboring channels is demonstrated to dramatically alter the two-phase flow pattern transition in the counter flow microchannels. In particular, the channel-to-channel heat transfer may remove more energy from the neighboring channel than that imposed from the bottom wall, the void fraction evolvement along the flow direction may be retarded and a uniform void fraction distribution is possible. Therefore, this additional heat transfer mechanism in the present design enables significant enhancement in heat transfer and two-phase flow performance: up to 45.1% increment of heat transfer coefficient (HTC), 73.8% reduction of pressure drop and 123.1% increment of coefficient of performance (COP) compared with those of traditional co-current flow design.

More comprehensive tests are conducted to examine the effects of inlet temperature and mass flux on the CFDM heat sink. Attributed to the innovative combination of the diverging microchannel and counter-flow manifold, the CFDM heat sink enables extensive channel-to-channel heat transfer, especially near the end or inlet of the channels. The inlet subcooling plays a significant role in the neighboring chilling effect, larger inlet subcooling leads to high heat transfer performance in CFDM. Meanwhile, mass flux also enhances the heat transfer of CFDM for both single-phase and two-phase regimes. Flow visualization reveals that nearly uniform void fraction distribution along the channel with a relatively high bubble slug ratio is possible. Under a higher mass flux of 600 kg/m2s and lower inlet temperature of 50 ℃, the pressure drop increment becomes nearly negligible comparing to that of single-phase convection. Besides, an ultra-high coefficient of performance of 75675 can be attained under a low inlet temperature and low mass flux.

In order to further improve the CFDM heat sink, this study combines the macroscopic optimal flow organization through counter flow diverging manifold and the microscopic surface modification of microscale artificial cavities as well as nanoscale coating structures. The unique hybrid strategy realizes a great improvement of heat transfer performance. With highly stable two-phase flow, the present study achieves a 4.8 kW total effective heat dissipation rate on a cooling area of 3 cm×4 cm without any sign of reaching critical heat flux (CHF). Meanwhile, attributed to extremely low pressure penalty surge during two-phase flow regime, an unprecedented COP of over 150,000 is achieved.

Overall, with the excellent flow boiling performance, the present study offers a robust and highly promising counter flow diverging microchannel heat sink for a variety of applications requiring high heat flux dissipation.

    Research areas

  • flow boiling, counter flow, microchannel, high energy-efficiency, micro-nano structure