3D heterogeneous wetting microchannel surfaces for boiling heat transfer enhancement

Abstract The manipulation of surface wettability, such as the usage of hydrophobic/hydrophilic hybrid surfaces, is a promising method to enhance boiling heat transfer. However, most studies on wettability manipulation are performed on a 2D bare plain surface without considering the influence of microstructures. In order to disclose the synergistic effects of heterogeneous wettability and microstructures, three types of 3D heterogeneous wetting microchannel surfaces (i.e., TS1, TS2, and TS3) that exhibit different wettability combinations of the inner surface and fin top surface are designed, fabricated, and characterized. The inner surfaces of different microchannels exhibit the same porous structures with a contact angle of 113.2° while the fin top surfaces prepared by different technologies exhibit contact angles of 88.6° (TS1), 8.6° (TS2), and 156.1° (TS3). Saturated boiling of water on the surfaces is experimentally investigated. The maximum heat transfer coefficient (HTC) of 365.0 kW/m 2 K is obtained with TS3 and is 6.3 times that of the 2D bare plain surface (BS). The highest critical heat flux (CHF) of 162.7 W/cm 2 is achieved with TS2 and increases by 60% when compared with that of the BS. The synergistic effects of wettability and microstructures on the boiling performance are summarized in terms of two aspects: (1) combinations of microstructures with hydrophobicity contribute to a high HTC at low heat fluxes and small superheat for the onset of boiling since microstructures provide potential bubble nucleation sites and hydrophobicity reduces the energy barrier of the phase change; (2) combinations of microstructures with hydrophilicity lead to a high CHF since microstructures exhibit a strong capillary pumping ability for liquid returning to the heated surfaces and hydrophilicity enhances liquid affinity with respect to the heated surface. Two bubble growth patterns termed ‘oblate bubble growth’ and ‘conical bubble growth’ are identified based on the observations and are observed as closely related to the synergistic effects of wettability and microstructures.