A hybrid (pump-assisted and capillary) two-phase loop (HTPL) is experimentally investigated to characterize its thermal performance under stepwise heat input conditions. An integration of mechanical pumping with capillary pumping is achieved by using planar evaporator(s) and a two-loop design separating liquid and vapor flows. The evaporator(s) use a sintered copper grooved wick bonded with a liquid screen artery. No active flow control of the mechanical pumping is required because of the autonomous capillary pumping due to the self-adjusting liquid menisci to variable heat inputs of the evaporators. Unlike other active two-phase cooling systems using liquid spray and microchannels, the HTPL facilitates a passive phase separation of liquid from vapor in the evaporator using capillary action, which results in a lower flow resistance of the single-phase flows than two-phase mixed flows in fluid transport lines. In this work, a newly developed planar form-factor evaporator with a boiling heat transfer area of 135.3cm2 is used aiming for the power electronics with large rectangular-shaped heat sources. This paper presents the experimental results of the HTPLs with a single evaporator handling a single heat source and dual evaporators handling two separate heat sources, while using distilled water as the working fluid for both cases. For the single evaporator system, the temperature results show that the HTPL does not create a big temperature upset under a stepwise heat load with sudden power increases and decreases. The evaporator thermal resistance is measured to be as low as 0.5Kcm2/W for the maximum heat load of 4.0 kW. A cold-start behavior characterized by a big temperature fluctuation was observed at the low heat inputs around 500 W. The HTPL with dual evaporators shows a strong interaction between the evaporators under an asymmetric heat load of the total maximum heat input of 6.5 kW, where each evaporator follows a different heat input schedule. The temperatures of the dual-evaporator system follow the profile of the total heat input, while the individual heat inputs determine the relative level of the temperatures of the evaporators.

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