MIT engineers are designing new surface treatments that make water boil more efficiently.
New surface treatments could save energy for systems used in many industries.
At the heart of a wide range of industrial processes, including most power plants, many chemical production systems, and even cooling systems for electronics, is an energy-intensive step of boiling water or other liquids.
They could significantly reduce energy consumption by improving the efficiency of systems that heat and evaporate water. MIT researchers have now found a way to do just that with a specially designed surface treatment for the materials used in these systems.
Three different types of surface modifications, in different sizes, together account for the increased efficiency. The new findings are described in a paper published in the journal Advanced Materials by recent MIT graduate Youngsup Song PhD ’21, the Ford Evelyn Wang Professor of Engineering, and four others at MIT. The scientists caution that this initial discovery is still at the laboratory scale, and more effort is needed to develop a practical process on an industrial scale.
High-speed video of the researchers’ test setup shows water boiling on a specially treated surface, causing bubbles to form at certain discrete points instead of spreading in a film across the surface, resulting in more efficient boiling. The video has been slowed down 100 times to see more details. Credit: Courtesy of the researchers
Heat transfer coefficient (HTC) and critical heat flux (CHF) are two key parameters that describe the boiling process. There is generally a trade-off between the two in material design, so anything that improves one of these parameters tends to degrade the other. But both are critical to the system’s performance, and now, after years of work, through their combination of different textures added to the material’s surface, the team of scientists has come up with a way to significantly improve both properties at the same time.
“Both parameters are important,” says Song, “but improving both parameters together is kind of difficult because they have inherent trade-offs.” The reason for this, he explains, is “because if we have a lot of bubbles on the boiling surface, it means that boiling is very efficient, but if we have too many bubbles on the surface, they can coalesce together, which can form a vapor film on the boiling surface. This film creates 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 says.
Song, now a postdoctoral fellow at Lawrence Berkeley National Laboratory, performed much of the research as part of his doctoral work at MIT. Although the various components of the new surface treatment he developed have been studied before, the researchers say this work is the first to show that these methods can be combined to overcome the trade-off between the two competing parameters.
The key to the new surface treatment is the addition of textures at several different size scales. Electron microscope images show millimeter-scale pillars and dimples (top two images) whose surfaces are coated with small nanometer-sized ridges (bottom two images) to improve the efficiency of the boiling reaction. Credit: Courtesy of the researchers
Adding a series of micro-scale cavities or dimples to a surface is a way to control how bubbles form on that surface, effectively keeping them pinned to the dimple locations and preventing them from spreading into a heat-resistant film. In this work, the researchers created an array of 10-micrometer-wide indentations separated by about 2 millimeters to prevent film formation. But this separation also reduces the concentration of bubbles at the surface, which can reduce boiling efficiency. To compensate for this, the team introduced surface treatment on a much smaller scale, creating small nanometer-scale bumps and ridges, which increases the surface area and promotes the rate of evaporation below the bubbles.
In these experiments, cavities were made in the centers of a series of pillars on the surface of the material. These pillars, combined with nanostructures, encourage the flow of liquid from the base to their tips, and this improves the boiling process by providing more surface area exposed to the water. Combined, the three “levels” of surface texturing—cavity separation, pillars, and nanoscale texturing—provide greatly increased efficiency for the boiling process, Song says.
“These microcavities define the position where bubbles appear,” he says. “But by separating these cavities by 2 millimeters, we separate the bubbles and minimize bubble coalescence.” At the same time, the nanostructures promote evaporation below the bubbles, and capillary action induced by the pillars delivers liquid to the base of the bubbles. This maintains a layer of liquid water between the boiling surface and the steam bubbles, which increases the maximum heat flow.
The photo shows how bubbles rising from a heated surface are “pinned” in certain places due to a special texture of the surface, instead of spreading over the entire surface. Credit: Courtesy of the researchers
Although their work confirmed that the combination of these types of surface treatments can work and achieve the desired effects, this work was done in small lab settings that cannot easily be scaled up to practical devices, Wang says. “These types of structures we’re making are not meant to be scaled in their current form,” she says, but rather used to demonstrate that such a system could work. One next step will be to find alternative ways to create these types of surface textures so that these methods can be more easily scaled to practical dimensions.
“Showing that we can control the surface in this way to get an improvement is the first step,” she says. “Then the next step is to think about more scalable approaches.” For example, although the surface pillars in these experiments were created using cleanroom methods typically used to manufacture semiconductor chips, there are other, less demanding ways to create such structures, such as electrodeposition. There are also a number of different ways to create surface nanostructured textures, some of which may be more easily scalable.
There may be some significant small-scale applications that could use this process in its current form, such as thermal management of electronic devices, an area that is becoming increasingly important as semiconductor devices become smaller and control of their heat output becomes increasingly important. “There’s definitely a place where that’s really important,” Wang says.
Even these types of applications will take some time to develop because typically thermal management systems for electronics use fluids other than water, known as dielectric fluids. These liquids have different surface tension and other properties than water, so the dimensions of the surface features must be adjusted accordingly. Working out these differences is one of the next steps for ongoing research, Wang says.
The same multiscale structuring technique can be applied to different liquids, Song says, by adjusting the dimensions to account for the different properties of the liquids. “Those kinds of details can be changed, and that could be our next step,” he says.
Reference: “Three-Level Hierarchical Structures for Extreme Pool Boiling Heat Transfer Performance” by Youngsup Song, Carlos D. Díaz-Marín, Lenan Zhang, Hyeongyun Cha, Yajing Zhao, and Evelyn N. Wang, 20 June 2022, Advanced Materials. DOI: 10.1002/adma.202200899
The team also included Carlos Diaz-Martin, Lennan Zhang, Hongyun Cha and Yajing Zhao, all from MIT. The work was supported by the Advanced Research Projects Agency-Energy (ARPA-E), the Air Force Office of Scientific Research, and the Singapore-MIT Research and Technology Alliance and used the MIT.nano facilities.
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