Two-phase flow regimes in a U-shaped diabatic manifolded-microgap channel

Embedded cooling — an emerging thermal management paradigm for electronic devices — has motivated further research in compact, high heat flux, cooling solutions. Reliance on phase-change cooling and the associated two-phase flow of dielectric refrigerants allows small fluid flow-rates to absorb large heat loads. Our previous research as well as work of others has shown that dividing chip-scale microchannels into parallel arrays of channels with novel manifold designs can produce very high chip-scale heat transfer coefficients with low pressure drops. In such manifolded microchannel coolers, the coolant typically flows at relatively high velocities through U-shaped microgap channels, producing centripetal acceleration forces on the fluid that can be several orders of magnitude larger than gravity. The impact of such large accelerations on the two-phase flow regimes in microgap channels, and their associated transport rates, are not well understood. Moreover, because the channels are very small and optically inaccessible, the flow regimes occurring in such manifolded microchannels have yet to be imaged and documented. The present effort analyzes the effects of such high centripetal acceleration on two-phase flow characteristics, including flow morphology. Since the available literature deals almost exclusively with macroscale and miniscale channels, the differences between macroscale and microscale two-phase flows are identified and discussed. The paper also shows, using dimensionless numbers to characterize the prevailing two-phase flow regimes, that results of previous U-channel visualizations in miniscale geometries, approximately an order of magnitude larger in every geometrical dimension than the microgap channels of interest, can provide insight into the flow regimes occurring in very high-performance microgap channels.