The boiling of fluids such as water is an energy-intensive step that is at the heart of many industrial processes, including most power plants, many chemical production systems, and even the cooling systems of electronic equipment. Improving the efficiency of systems that heat and evaporate water can significantly reduce their energy usage. Now researchers at MIT have found a way to tailor surfaces to materials used in such systems. The combination of three different sizes of surface modification increases the efficiency. Dr. Youngsup Song, a recent MIT graduate, Professor Evelyn Wang, and four other MIT professors present the new findings in the journal Advanced Materials. “If the boiling surface has a lot of bubbles, it means that the boiling is very efficient, but if there are too many bubbles on the surface, they can clump together and form a vapor film on the boiling surface,” Song said. Heat transfer from surface to water. “If you have steam between the surface and the water, it affects the heat transfer efficiency and reduces the critical heat flux value,” he said. Adding a series of microscopic cavities or indentations to the surface is one way to control the amount of air bubbles in the surface. The way they are formed on the surface effectively holds them in place of the dents, preventing the formation of a heat-resistant film from scattering. In this experiment, the researchers created cavities in the center of a series of pillars on the surface of the material. These pillars, combined with nanostructures, promote wicking of the liquid from the bottom to the top, a method that provides more surface area exposed to the water, thereby enhancing the boiling process. Three surface texture “layers”—a cavity separation layer, pillars, and nanoscale textures—combined to greatly improve the efficiency of the boiling process, Song said. The nanostructure facilitates evaporation under the bubble, while the column-induced capillary action provides liquid to the bottom of the bubble. This maintains a layer of liquid water between the boiling surface and the steam bubbles, increasing the maximum heat flux. This laboratory process is still far from an industrial-scale process, but may still have practical value, such as for thermal management of electronic devices.
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