Synergism of Binary Mixtures Wettability and Cover Porosity on Enhanced Pool Boiling Heat Transfer

The wettability of the system in capillary covers is an important element to increase the boiling heat transfer on the coverings. The sessile drop methodology allows the evaluation of either the surface energy of solids or the interfacial energy of liquids, and from both the system’s wettability. This methodology was tested with an experimental set up built for this study. The surface energies calculated for solids and metal foils used for construction of capillary coverings were in accordance with previous experimental results. The same methodology is used for measuring interfacial energies of the liquids used for increasing boiling heat transfer like ethanol-water mixtures. The mixture with 16% ethanol by weight had the lowest contact angle (associated to the lowest interfacial energy) and produced the highest convective heat transfer coefficient, h. Thus, the maximum for h correlates with an increase in the wettability of this system. This behavior is related to that observed as the critical micelial concentration (cmc) for surfactants, that produce the lowest interfacial energy of the liquid. Thus a set of experiments was developed to correlate the binary mixture behavior around the concentration with maximum heat transfer coefficient with the cmc boiling behavior. The surfactant sodium lauryl sulfate (SLS) produced an increase of the wettability of the solid surface with the addition of 100 ppm (or less) that is its cmc. The h values increase with the addition of SLS up to 100 ppm but do not change if the concentration of surfactant is higher than that value. The maximum heat transfer coefficient is obtained with the cmc of SLS in water, and with the 16% by weight ethanol-water mixture, both having the highest wettability. Porous coverings were tested with two covering’s thickness. A synergistic effect is found for the appropriate cover thickness combined with either a 16% by weight ethanol-water mixture or water with the cmc of SLS.