Effects of Surface Structure and Wettability Modification on Condensate Mobility and Heat Transfer on Flat and Tubular Surfaces

Abstract

Dropwise condensation has long been known to offer heat transfer coefficients potentially an order of magnitude higher than filmwise condensation as mobile droplets can more readily be removed from condenser surfaces (traditionally via gravitational forces) taking with them the thermal resistance imposed by liquid layers on condensers. However most condensers in industrial applications operate under the filmwise condensation mode since traditionally there have not been dropwise condensation promoting materials or surface treatments with the combination of durability and performance necessary for adoption by industry. Recently, renewed and increased interest in the area of condensation enhancement has led to the development of several types of superhydrophobic surfaces intended to dramatically increase condensate droplet mobility. Many of the surface architectures proposed by literature to produce superhydrophobic surfaces require complex micro and nano fabrication techniques that are not feasible for scale-up to mass production and produce rather delicate surfaces. This study describes two methods intended to increase droplet mobility and thereby condensation heat transfer coefficients while focusing on durability and feasible fabrication. In the first method described, custom alumina nano-particle composite hydrophobic coatings were developed from a surface treatment originally designed at Auburn University for silicon surfaces called Repellix, which uses a vapor deposition process that is readily scalable. These custom coatings were adapted from the original Repellix process to produce surface treatments with increased durability suitable for use on metallic substrates. These custom developed surface treatments were applied to solid hemi-cylindrical test surfaces fabricated from four of the most common power generation plant condenser tube materials, namely, Admiralty brass, cupronickel, titanium and Sea-Cure stainless steel. Flat and hemi-cylindrical 304 stainless steel control surfaces were also fabricated and coated. Results show that the performance enhancement, measured in rate of heat transfer spikes corresponding to condensate roll-off events, was best for the titanium surface which produced 64% more events than the next most active material when coated using the most durable surface treatment tested in this work. The second method for increasing droplet mobility and thereby condensation heat transfer coefficients presented in this work employs asymmetric triangular saw-toothed profile ratchet surface features in several size, profile and wettability configurations. These surface features while formed in silicon via gray scale lithography for use in this study could easily be formed in metallic surfaces using common modern fabrication techniques. Effects of ratchet size, profile, orientation and surface wettability were examined. Hydrophobic asymmetric ratcheted surfaces studied were observed to produce directional water droplet growth due to surface tension forces during deposited droplet experiments as well as directional water condensate motion during coalescence events under certain conditions. Condensation heat transfer coefficients up to 57% higher on larger ratchets compared to flat surfaces also suggests that some fluid motion is promoted by larger hydrophilic ratchets condensing a highly wetting fluid (FC-72). Experimental data was compared to a model that predicts dropwise heat transfer coefficients by examining the heat flow through individual droplets to propose a modification to the model that increases its accuracy for predicting heat transfer coefficients on textured hydrophobic surfaces. Detail on development, fabrication, thermal performance and observations on effects on droplet and condensate motion for the two methods introduced above are presented in this work.