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Opposite effects of thermal oxidation on pool boiling heat transfer of copper bare and reentrant cavity tubes in the low heat flux regime
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DOI：10.1016/j.icheatmasstransfer.2026.111417.png)
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Abstract
En 中文
This study experimentally investigates the pool boiling heat transfer performance of commercial copper tubes—specifically smooth bare tubes and Turbo-E tubes featuring reentrant cavities—within a low heat flux regime (0–70 kW/m2). This operational range is critically relevant to industrial waste heat recovery systems and steam-generating heat pumps, where maximizing the heat transfer coefficient (HTC) at low wall superheats is paramount. While surface aging induced by thermal oxidation is conventionally understood to degrade boiling performance on planar metallic surfaces, this research elucidates a divergent, geometry-dependent evolutionary trend. Through a rigorous protocol of repeated boiling cycles interspersed with air exposure, the HTC of bare copper tubes was observed to decline by approximately 15-20% due to the oxidation-induced deactivation of surface micro-cavities (clogging). In stark contrast, the oxidized Turbo-E tubes exhibited a significant and monotonic performance enhancement, achieving up to a 51% increase in HTC relative to the non-oxidized baseline. Advanced surface characterization, including Raman spectroscopy and high-speed visualization, confirms that this enhancement is driven by a spontaneously formed “3D-biphilic” oxide structure. This structure is characterized by a wettability contrast where the outermost fin surfaces become Cu₂O-rich (comparatively hydrophobic), while the inner cavity regions become CuO-rich (superhydrophilic). This depth-wise wettability gradient optimizes the suction-evaporation mechanism by promoting liquid imbibition into the cavities and facilitating rapid vapor release from the fin tips. These findings challenge the prevailing view of oxidation as solely detrimental, suggesting instead that thermal aging can be strategically leveraged to enhance the efficiency of reentrant-cavity heat exchangers in industrial applications.
Keywords:
pool boiling
heat transfer enhancement
thermal oxidation
reentrant cavities
biphilic surface
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