Enhanced evaporation heat transfer via capillary-assisted thin-film formation on TiO₂-CNT-Cu-nanocomposite-coated fin-grooved surfaces

Thin-film evaporation is an efficient heat-transfer mechanism for low-pressure thermal systems; however, its performance strongly depends on the balance between capillary liquid supply and drainage. In this study, capillary-assisted evaporation on fin-grooved copper surfaces coated with a TiO₂–CNT–Cu nanocomposite was experimentally and numerically investigated. Three enhanced surface configurations including bare fin-grooved surfaces, nanocomposite-coated smooth surfaces, and nanocomposite-coated fin-grooved surfaces were tested in a semi-flooded evaporator–condenser loop using water under sub-atmospheric pressures of 0.79–1.07 kPa, which represent the typical evaporation pressure range for the practical operation of adsorption chillers and low-pressure thermal systems. The nanocomposite coating significantly enhanced surface hydrophilicity and capillary rise; however, its impact on evaporation heat transfer was strongly dependent on fin-groove geometry. For uncoated fin-grooved surfaces, decreasing fin pitch monotonically enhanced evaporation performance by extending the thin-film region. In contrast, when the nanocomposite coating was applied, an optimal fin pitch of 1.0 mm was identified, yielding evaporation heat-transfer coefficients up to 57 times higher than those of a bare smooth surface. Narrower coated grooves exhibited excessive liquid accumulation, leading to thicker liquid films and partial suppression of thin-film evaporation. A three-dimensional VOF–CSF CFD model was developed to study the liquid capillary meniscus under different grooves and hydrophilic conditions. The numerical simulations revealed that this performance degradation originates from coating-induced drainage resistance and altered meniscus morphology within narrow grooves. These findings highlight a critical design trade-off between surface wettability and geometric permeability in coated fin-groove evaporators and provide practical guidelines for designing high-performance capillary-assisted evaporation systems.