Flow boiling heat transfer enhancement on plasma-engineered copper surfaces in a macrochannel under saturated conditions

Saturated flow boiling represents one of the most effective methods for dissipating high heat fluxes; nevertheless, the efficiency is limited by hydrodynamic instabilities and premature CHF. This study systematically examines the influence of cold atmospheric plasma surface modification of copper on flow boiling behavior in a macrochannel. Plasma treatment transformed the initially nearly hydrophobic copper surface (contact angle ≈ 90.1°) into a stable hydrophilic one (contact angle ≈ 55.8°). Surface characterization using AFM, SEM, EDX, and FT-IR showed a moderate rise in roughness (Ra ≈ 28 nm) along with oxygen-rich polar functionalities. These modifications enhanced liquid spreading, accelerated rewetting, and facilitated early activation of nucleation sites. Consequently, the ONB occurred at substantially lower wall superheats, with reductions of approximately 57–63% compared with untreated copper. Flow boiling measurements revealed substantial thermal gains: the HTC increased by about 127–155%, while the CHF was augmented by roughly 114–147% relative to the bare surface. Based on the experimental database, a new empirical correlation was developed for predicting the two-phase heat transfer coefficient, accounting for the combined effects of surface roughness, porosity, and wettability. Repeated heating–cooling cycles demonstrated that plasma-modified surfaces retain structural integrity and functional enhancements. Overall, cold atmospheric plasma treatment provides a robust, durable, and scalable route for improving CHF and HTC in high heat flux systems.