TWO-PHASE FLOW REGIMES AND HEAT TRANSFER IN A MANIFOLDED-MICROGAP

Abstract

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. Previous research 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. Furthermore, the manifolded microchannels consist of high aspect ratio rectangular channels, short length to hydraulic diameter ratios (L/Dh < 100), and step-like inlet restrictions. The existing literature provides only limited information on each of these effects, and nearly no information on the combined effects, on fluid flow and heat transfer performance.

This study provides fundamental insights into the impact of such channel features and coupled fluid forces on two-phase flow regimes and their associated transport rates. Moreover, because the flows in manifolded microchannel chip coolers are very small and optically inaccessible, a custom visualization test section was developed. The visualization test section was supported by a custom two-phase flow loop, which incorporated three power supplies, two chillers, and two dozen thermofluid sensors with live data monitoring. The fluid flow and wall heat transfer in the visualization test section was simultaneously imaged with a high-speed optical camera and a high-speed thermal camera. The results were reported with maps of the flow regime, wall temperature, wall temperature fluctuation, superheat, heat flux, and heat transfer coefficients under varying heat fluxes and mass fluxes for three manifold designs. Variations in flow phenomena and thermal performance among manifold designs and between R245fa and FC-72 were established. The post-annular flow regime of annular-rivulet was associated with a precipitous decline in wall heat transfer coefficients. The current experimental campaign is the first in the open literature to study the thermal and hydrodynamic characteristics of manifolded-microgap channels in such detail.