Two-phase Flow Morphology and Local Wall Temperatures in High-Aspect-Ratio Manifold Microchannels

Manifold microchannel heat sinks can dissipate high heat fluxes at moderate pressure drops, especially during two-phase operation. High-aspect-ratio microchannels afford a large enhancement in heat transfer area; however, the flow morphology in manifold microchannels during two-phase operation, as well as the resulting thermal performance, are not well understood. In this work, a single manifold microchan- nel representing a repeating unit in a heat sink is fabricated in silicon with a bonded glass viewing window. Samples of different channel lengths (750 μm and 1500 μm) and depths (125 μm, 250 μm, and 10 0 0 μm) are considered; channel and fin widths are both maintained at 60 μm. Subcooled fluid (HFE-7100) is delivered to the channel at a constant flow rate such that the fluid velocity at the inlet is ~1.05 m/s in all cases. A high-speed camera is used to visualize the two-phase flow in the channel through the glass sidewall; an infrared camera measures the temperature distribution on the opposite channel sidewall. The flow visualizations reveal that vapor nucleation occurs at stagnation regions below the manifold near the inlet plenum and at both corners adjacent to the channel base. For deep chan- nels (10 0 0 μm), at sufficiently high heat fluxes, vapor completely covers the base of the channels and liquid does not re-wet the surface in this region. This newly identified vapor blanketing phenomenon causes a significant decrease in performance and an increase in the measured channel wall temperatures. This study reveals the critical role of the two-phase flow morphology in manifold microchannel heat sink design