Designing a surface that makes boiling water more efficient
The key to the new surface treatment is to add texture at several different size scales. The electron microscope image shows millimeter-scale pillars and dents (first two images), whose surface is covered with tiny nanometer-scale protrusions (bottom two images) to increase the efficiency of the boiling reaction. Credit: Massachusetts Institute of Technology
Boiling water or other liquids is an energy-intensive step at the heart of many industrial processes, including most power plants, many chemical production systems, and even cooling systems for electronics.
Increasing the efficiency of a system that heats and evaporates water can significantly reduce its energy use. Now, researchers at MIT have found a way to do just that, with surface treatments specifically designed for the materials used in the system.
The increase in efficiency comes from the combination of three different types of surface modification, at different size scales. New findings described in journal Advanced Material in a paper by recent MIT graduate Youngsup Song Ph.D. ’21, Ford Professor of Engineering Evelyn Wang, and four others at MIT. The researchers note that these preliminary findings are still at a laboratory scale, and more work is needed to develop practical industrial-scale processes.
There are two key parameters that describe the boiling process: heat transfer coefficient (HTC) and critical heat flux (CHF). In material design, there is generally a tradeoff between the two, so anything that increases one of these parameters tends to make the other worse. But both are critical to the efficiency of the system, and now, after years of work, the team has come up with a way to significantly improve both properties at the same time, through the combination of different textures added to the surface of the material.
“Both parameters are important,” said Song, “but raising the two parameters together is a bit difficult because they have an intrinsic trade off.” The reason, he explains, is “because if we have a lot of bubbles on the boiling surface, that means very efficient boiling, but if we have too many bubbles on the surface, they can join together, which can form a vapor film over the boiling surface.” The film introduces resistance to heat transfer from the hot surface to the water. “If we have steam between the surface and the water, it prevents heat transfer efficiency and lowers the CHF value,” he said.
Song, who is now a postdoc at Lawrence Berkeley National Laboratory, did much of the research as part of his doctoral thesis work at MIT. While the various components of the new surface treatment he developed have previously been studied, the researchers say this work is the first to show that these methods can be combined to overcome the tradeoff between the two competing parameters.
Adding a series of micro-scale cavities, or dents, to a surface is a way to control the way bubbles form on that surface, keeping the bubbles effectively pinned to the dent location and preventing them from spreading into the heat-resistant film. In this work, the researchers fabricated an array of 10 micrometer-wide dents separated by about 2 millimeters to prevent film formation. But the separation also reduces the concentration of bubbles on the surface, which can reduce the boiling efficiency. To compensate for that, the team introduced a surface treatment on a much smaller scale, creating tiny bumps and bumps at the nanometer scale, which increases the surface area and increases the evaporation rate below the bubble.
In this experiment, a cavity is created in the middle of a series of pillars on the surface of the material. These pillars, combined with the nanostructure, increase the absorption of the liquid from the bottom to the top, and this improves the boiling process by providing more surface area exposed to the water. In combination, the three “tiers” of surface texture—void separation, pile, and nanoscale texture—provide greatly improved efficiencies for the boiling process, Song said.
The photo shows how bubbles rising from a heated surface are “pinned” in specific locations due to the special surface texture, instead of spreading over the entire surface. Credit: Massachusetts Institute of Technology
“The micro cavities determine the position of the emergence of bubbles,” he said. “But by separating the cavities by 2 millimeters, we separated the bubbles and minimized the coalescence of the bubbles.” At the same time, the nanostructure increases evaporation under the bubble, and the capillary action caused by the pillar supplies the liquid to the bottom of the bubble. It maintains a layer of liquid water between the boiling surface and the steam bubbles, which increases the maximum heat flux.
Although their work has confirmed that a combination of these types of surface treatments can work and achieve the desired effect, this work was carried out under small-scale laboratory conditions that cannot easily be scaled up to practical devices, Wang said. “This kind of structure that we created was not meant to be scaled in its current form,” he said, but rather was used to prove that such a system could work. One next step is to find an alternative way to create this type of surface texture so that this method can be more easily scaled to practical dimensions.
“Showing that we can control the surface in this way to get an upgrade is the first step,” he said. “Then the next step is to think of a more scalable approach.” For example, although the pillars on the surface in this experiment were fabricated using the clean-room method commonly used to manufacture semiconductor chips, there are other, easier ways to fabricate such structures, such as electrodeposition. There are also a number of different ways to produce surface nanostructure textures, some of which may be easier to scale.
There may be some significant small-scale applications that could use this process in its current form, such as the thermal management of electronic devices, an area that is becoming more important as semiconductor devices get smaller and the management of their heat output becomes increasingly important. “There must be a space where this is very important,” Wang said.
Even such an application will take time to develop because typically thermal management systems for electronics use a fluid other than water, known as a dielectric fluid. These fluids have different surface tension and other properties than water, so the dimensions of the surface features must be adjusted. Working on these differences is one of the next steps for ongoing research, Wang said.
This same multiscale styling technique can also be applied to different fluids, Song said, by adjusting the dimensions to account for the different properties of the fluids. “Details like that can be changed, and that could be our next step,” he said.
The invention improves heat transfer in boiling
Youngsup Song et al, Three-tier Hierarchical Structure for Extreme Pond Boiling Heat Transfer Performance, Advanced Material (2022). DOI: 10.1002/adma.202200899
Provided by the Massachusetts Institute of Technology
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