We investigated the influence of surface structuring and wettability on heat transfer during
nucleate boiling of water in a confined space. Such conditions are important in compact
cooling systems, where the space available for bubble formation, vapor removal, and liquid
return is limited. The confined space was defined as a gap formed by an upper confinement
plate for adjusting the gap height and a lower boiling surface. Reference, superhydrophilic,
and superhydrophobic surfaces were prepared for both the boiling surface and the
confinement plate. The surfaces were fabricated by mechanical preparation, laser texturing,
gold coating, and hydrophobization. The experiments were performed during saturated
boiling of distilled water at atmospheric pressure. The gap height between the boiling surface
and the confinement plate was varied with respect to the capillary length of water, from
0.25 mm to 50 mm. For all combinations of boiling surfaces and confinement plates, the
critical heat flux (CHF) and heat transfer coefficient (HTC) were determined. As the gap
height decreased, the heat-transfer performance deteriorated due to restricted vapor removal
and hindered liquid supply. Compared with reference surfaces, structured surfaces improved
the boiling heat-transfer performance. The most favorable combination for gap boiling was
a superhydrophobic confinement plate with a superhydrophilic boiling surface, which
improved CHF by up to 118% and HTC by up to 803% compared with the reference
combination, as it enabled a more favorable separation of pathways for liquid supply and
vapor removal.
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