Mixed dropwise-filmwise condensation heat transfer on biphilic surface

Abstract

Condensation heat transfer on biphilic surface is investigated. The surface periodically populates hydrophobic stripes each having a coating layer thickness δp and a width WDWC, and hydrophilic stripes each having a width WFWC. The proposed model includes dropwise condensation on hydrophobic stripe, filmwise condensation on hydrophilic stripe, and droplet detachment radius rmax criterion for heat-mass coupling between the two wettabilities regions. The rmax is the minimum of detachment radii determined by droplet removal modes of double-sides-suction DSS, one-side-suction OSS and sliding, where DSS is a special case of OSS for droplet located at hydrophobic stripe centerline. Simulation results matched the measured heat transfer data well. Optimal width of hydrophobic stripe WDWCo is found to be dominated by δp and WFWC, but other parameters weakly influence WDWCo. Interfaced by a δp−WFWC transition curve, a heat transfer regime map is presented to contain Regime I for possible heat transfer enhancement and Regime II for heat transfer deterioration. Regime I enhances heat transfer if WDWC approaches WDWCo, but may deteriorate heat transfer if WDWC deviates WDWCo too much. The maximum heat transfer enhancement ratio is 1.67 compared with purely hydrophobic surface. Regime II always deteriorates heat transfer. Our work provides a general guideline to design biphilic surface for performance improvement.