The pool boiling phenomena, which may occur at high heat flux in the planar heat pipe such as vapor chamber and thermal ground plane (TGP), were visually investigated in the vertical orientation by capturing the bubble behavior within a narrow gap between two plates using distilled water at 1 atm. The size of two copper plates is 80×80 mm2 and the 10×10 mm2 ceramic heater, which simulates the high power chip, was bonded to the backside of the bottom plate. The gap between two plates was adjusted from 3 mm to 0.5 mm using spacers. It was observed that the generated bubble spreads laterally throughout the channel between the plates as decreasing the gap. Thin liquid film forms beneath the flattened bubble and the thin film evaporation improves heat transfer at the narrow gaps of 3 and 1 mm. However, further reduction of the gap down to 0.5 mm brings about an increase of the dried region, resulting in the deterioration of boiling heat transfer.

[This work was supported by the Civil-Military Technology Cooperation Program of the Institute of Civil-Military Technology Cooperation (ICMTC), with a grant funded by the Defense Acquisition Program Administration and the Ministry of Trade, Industry and Energy (Grant No. 18CM5017).]

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