Enhancing Condensers for Geothermal Systems: The Effect of High Contact Angles on Drop Wise Condensation Heat Transfer

Abstract

Phase change heat transfer is notorious for increasing the irreversibility of, and therefore decreasing the efficiency of, geothermal power plants. Its significant contribution to the overall irreversibility of the plant makes it the most important source of inefficiency in the process. The focus of the poster presentation is to educate attendees on the research being done within the mechanical engineering department at the University of Nevada – Reno to increase the heat transfer rates in condensers by exploiting the phenomenon of drop wise condensation. Recent studies here have shown the promotion of drop wise condensation in the lab by means of increasing the surface energy density of a tube with nanotechnology. The use of nanotechnology has allowed the creation of surface treatments which discourage water from wetting a tube surface during a static test. To measure surfaces’ wettability, contact angle measurements with water are taken. These surface treatments are unique in that they create high- contact angles on the condensing tube surfaces to promote drop wise condensation. These hydrophobic surface treatments were employed to produce different contact angles with different treatment thicknesses to induce the phenomenon of drop wise condensation. The increased values of the resulting contact angles were well in excess of the typical 90° used in existing drop wise condensation models. For this reason a new single droplet model was introduced to predict performance. This model considers the resistances due to interfacial heat transfer, surface treatment, conduction through the drop and to drop curvature. The model incorporates the contact angle between the drop and the condensing surface in its calculation of overall heat transfer. The single droplet model was then applied to a population balance model for small drops to predict performance. The larger drop distribution commonly used in other models developed by Lefevre and Rose (Le Fevre, 1966) was adopted to complete the model. The model is used to illustrate and predict the higher heat transfer coefficients associated with contact angles much larger than 90° in drop wise condensation. An experimental apparatus was constructed to quantify the heat transfer ability of the treated tubes. Heat transfer data was collected on a number of different tubes with varying contact angles and varying surface treatment thicknesses. The contact angles produced by the coatings used in this study range from < 90° (i.e. filmwise condensation) to upwards of 140°. A camera was used in conjunction with contact angle measurement software to take accurate contact angle measurements of the droplet on the treated condensing surface. These measurements were taken both before and after the experimental runs to determine the degree of deterioration, if any, of the surface treatment’s ability to maintain contact angles during exposure to condensing vapors. These empirical results are compared to the results of the model.