Effects of Hydrophobic Surface Treatments on Dropwise Condensation and Freezing of Water

Abstract

The effects of hydrophobic coatings on the dropwise condensation process for a flat horizontal copper plate held at a temperature much lower than the surrounding air was investigated. Extensive qualitative and quantitative data were taken to describe these

effects in terms of dropsize distributions, heat and mass transfer coefficients, and digital imaging of the microscopic condensing droplets in terms of various extensive properties. Conditions of the tests were laminar flow (Reynolds numbers of 600 to 1600), condensing surface temperature of -10.0°C, air temperatures of 1.5 to 7.5°C, and relative humidities of 50 to 90%. Digital imaging of the condensation process was done using a medium power microscope coupled to a solid-state camera and a computer-based image capture system. Three different treatments were used on the polished copper condensing surface; no coating, a silicone conformal coating, and a long-chain sulfur based coating. Results for the heat transfer data showed an increase of approximately 33% for condensation over a dry surface for the conditions of the study. The mechanism of condensation appeared consistently to be dropwise. A method to predict the Nusselt number, nucleation time, mean cluster size and the droplet surface density in terms of extensive properties that are known a priori was also developed based on the experimental data.

A condensation model was developed based on heat transfer relationships for the condensation process and the psychometric qualities of the ambient air. The inputs to the model were the surface temperature of the condensing system, the contact angle of the liquid phase in contact with the surface, the moisture content and temperature of the air stream, and the velocity of the air flowing over the condensing surface. Comparisons were made between the experimental results and those predicted by the model for dropsize distributions and mass of condensate on the surface. The results from the mode!

were within ±25% of the experimental values.

It was found that the application of a hydrophobic coating on a surface that is to be operated at a temperature below the freezing point of water will delay the onset of ice nucleation by 15 to 35% over an uncoated surface.