Visualization of Steam Flow Condensation in Hydrophobic and Hydrophilic Mini-Gaps

An experimental, flow visualization study was conducted for flow condensation in 1-mm diameter mini-gaps with a visualization window. The hydrophilic channel was composed of bare copper, while the hydrophobic channel was coated with Teflon AF™. An open loop steam experimental apparatus was constructed, and single-phase validation tests are presented. For flow condensation in the hydrophilic gap, filmwise condensation was observed for all cases with a mass flux range of 30–100 kg/m²s. In contrast, the hydrophobic channel promoted dropwise condensation at mass fluxes of 30 and 50 kg/m²s. Cycles of droplet nucleation, growth and coalescence, and sweeping were identified at both mass fluxes in the hydrophobic channel at steady state. Droplets departed at a smaller sizes and more frequently at the higher mass flux, demonstrating the effects of shear forces on droplet sweeping. Based on flow visualizations, three droplet sweeping methods were identified: interfacial shear stress, upstream droplets, and absorbing into rivulets.