Condensation of steam over nano-engineered surfaces, 9th International Conference on Boiling and Condensation Heat Transfer

Nano-engineered surfaces have recently been studied as a promising solution for several heat transfer applications. In particular, modification of surface wetting properties for achieving enhanced condensation heat transfer coefficients is an extremely interesting field of research. Superhydrophilic surfaces can be obtained by properly roughening the surface to a micro-/ nano- scale, while hydrophobic ones are obtained by lowering the surface free energy. In the present work, chemical procedures to obtain a modification of the wetting properties over aluminum substrates are presented. Moreover, the influence of such properties on condensation heat transfer applications is investigated. A new thermosyphon test loop for the analysis of pure steam flow condensation, at different vapor velocities, is described. Heat transfer data measured during condensation in the new apparatus over untreated, superhydrophilic and hydrophobic surfaces are hereinafter presented and discussed. It is shown that condensation is negatively affected by rough surfaces presenting superhydrophilic properties, because of condensate adhesion to the substrate which leads to an enhancement of the liquid film thickness. On the contrary, condensation over hydrophobic treated surfaces leads to an augmentation of the two-phase heat transfer coefficient when compared to the one on the untreated surface. This may be due to the presence of a non-zero slip velocity condition at the solid-liquid interface, due to the water repellency properties of the surface, which leads to a reduction of the condensate film thickness. Thinning the condensate thickness means a reduction of the driving thermal resistance, and thus an enhancement of the condensation heat transfer coefficient.