Study of flow condensation enhancement with hydrophobic and hydrophilic patterns

Abstract

There is a need for improved condensers in many applications, such as power generation, distillation, and industrial applications. The study examined the enhancement of filmwise flow condensation on hydrophobic and hydrophilic surfaces, as increased heat transfer would ultimately reduce condenser size and weight. Steam was condensed in 1.06-mm hydraulic diameter mini-gaps, installed in an open loop steam system. Single-phase experiments successfully validated the experimental measurement approach to measure condensation heat transfer coefficients. Six mini-gaps were tested for average qualities of 0.2 to 0.95, mass fluxes of 50 to 200 kg/m2s, and effects of hydrophobicity and hydrophilicity, which included hydrophilic copper, hydrophobic Teflon AFTM coated, and four surfaces with combined Teflon and hydrophilic patterns.

The geometry of the hydrophobic and hydrophilic patterned surfaces was determined with a simple analytical model. The surfaces were studied with one, two, three, and seven hydrophilic areas aligned in the flow direction, and were created by dip-coating surfaces with Teflon AFTM and then plasma etching. Condensation behavior on the patterned surfaces was comparable to that of the Teflon surface, within the uncertainties. The concept was that liquid film would develop on the hydrophilic areas of the patterned surfaces, but the data suggested that instead shear forces played a role sweeping the droplets. Overall, the data suggested that dropwise condensation was promoted on all Teflon coated surfaces, thus enhancing flow condensation heat transfer.

For the hydrophilic copper mini-gap, heat transfer coefficients were a strong function of quality, as well as a function of mass flux at higher qualities, which demonstrated the development and growth of a liquid film as quality decreased. The measured condensation heat transfer coefficients were well predicted by the Kim et al. (2013) correlation, with a mean average error of 8.9%. In contrast, the Teflon coated surface exhibited substantially different behavior; there was no condensation heat transfer dependence on mass flux or quality, which suggested that dropwise condensation was promoted and sustained throughout the flow condensation process. Heat transfer coefficients of 280,000 to 425,000 W/m2K were observed on the copper coated surface, with enhancements 3.2 to 13.4× that of the copper channel; higher enhancement factors were found at lower qualities.