For a given heat sink thermal resistance and ambient temperature, the temperature of an electronic device rises fairly linearly with increasing device heat flux. This relationship is especially problematic for defense electronics, where heat dissipation is projected to exceed 1000 ${\hbox {W}}/{\hbox {cm}}^{2}$ in the near future. Direct and indirect low-temperature refrigeration cooling facilitate appreciable reduction in the temperature of both coolant and device. This paper explores the benefits of cooling the device using direct and indirect refrigeration cooling systems. In the direct cooling system, a microchannel heat sink serves as an evaporator in a conventional vapor compression cycle using R134a as working fluid. In the indirect cooling system, HFE 7100 is used to cool the heat sink in a primary pumped liquid loop that rejects heat to a secondary refrigeration loop. Two drastically different flow behaviors are observed in these systems. Because of compressor performance constraints, mostly high void fraction two-phase patterns are encountered in the R134a system, dominated by saturated boiling. On the other hand, the indirect refrigeration cooling system facilitates highly subcooled boiling inside the heat sink. Both systems are shown to provide important cooling benefits, but the indirect cooling system is far more effective at dissipating high heat fluxes. Tests with this system yielded cooling heat fluxes as high as 840 ${\hbox {W}}/{\hbox {cm}}^{2}$ without incurring critical heat flux (CHF). Results from both systems are combined to construct an overall map of performance trends relative to mass velocity, subcooling, pressure, and surface tension. Extreme conditions of near-saturated flow, low mass velocity, and low pressure produce “micro” behavior, where macrochannel flow pattern maps simply fail to apply, instabilities are prominent, and CHF is quite low. One the other hand, systems with high mass velocity, high subcooling, and high pressure are far more stable and yield very high CHF values; two-phase flow in these systems follows the fluid flow and heat transfer behavior as well as the flow pattern maps of macrochannels.

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