Experimental analysis of steam condensation over conventional and superhydrophilic vertical surfaces

Abstract

Nano-engineered surfaces have been recently studied as a promising solution for many heat transfer applications. In particular, it is known that superhydrophobic surfaces, obtained by combining low surface free energy with micro-/nano-scale surface roughness, can promote dropwise condensation mode, while superhydrophilic ones, obtained for example by roughening the substrate to a micro-/nano-scale morphology, showed promotion of film formation during condensation. In the open literature there is a lot of information regarding the fabrication and characterization of these surfaces, but very few results on the heat transfer performance are reported.In this paper, a new experimental apparatus for investigation of condensation of pure steam, flowing at different velocities, is presented. Filmwise condensation is investigated over untreated aluminum surfaces placed inside a rectangular narrow channel. The effect of wall subcooling and vapor velocity on the two-phase heat transfer coefficient is experimentally and theoretically analyzed. Condensation tests are also performed over a superhydrophilic surface, aiming at analyzing the effects of the wetting properties of the substrate on the process. A comparison between the heat transfer coefficients measured on the superhydrophilic surface and the ones obtained on the untreated sample shows a penalizing effect of the hydrophilic treatment.