MIT engineers design 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 generation plants, many chemical production systems and even refrigeration systems for electronics, is a step that consumes energy with the boiling of water or other fluids.
They could significantly reduce their energy use by improving the efficiency of systems that heat and evaporate water. MIT researchers have now found a way to do this, with a surface treatment specially designed for the materials used in these systems.
Three different types of surface modifications, at different size scales, account for the increase in efficiency. The new findings are described in an article published in Advanced Materials magazine by recent MIT graduate Youngsup Song PhD ’21, Ford engineering professor Evelyn Wang and four others at MIT. Scientists warn that this initial finding is still on a laboratory scale and more effort is required to develop a practical process on an industrial scale.
The researchers ’high-speed video of the test setup shows boiling water on a specially treated surface, causing bubbles to form at separate specific points instead of spreading on a film across the surface, giving instead of a more efficient boil. Video slowed down 100 times to show more details. Credit: courtesy of researchers
Heat transfer coefficient (HTC) and critical heat flow (CHF) are two key parameters that describe the boiling process. In general, there is a trade-off between the two in the design of materials, so anything that improves one of these parameters tends to make the other worse. But both are crucial to the efficiency of the system, and now, after years of work, by combining different textures added to the surface of a material, the team of scientists has found a way to significantly improve both properties at once.
“Both parameters are important,” Song says, “but improving both parameters together is a bit tricky because they have intrinsic offsets.” The reason for this, he explains, is “because if we have a lot of bubbles on the boiling surface, that means the boiling is very efficient, but if we have too many bubbles on the surface, they can stick together, which can form a steam. film on the boiling surface “. This film introduces resistance to heat transfer from the hot surface to the water. “If we have steam between the surface and water, this impedes heat transfer efficiency and reduces the value of CHF,” he says.
Song, who is now a postdoctoral researcher at the Lawrence Berkeley National Laboratory, conducted much of the research as part of his doctoral dissertation work at MIT. Although the different components of the new surface treatment he developed had been previously studied, the researchers say this work is the first to show that these methods could be combined to overcome the compensation between the two competing parameters.
The key to the new surface treatment is to add textures at different size scales. Electron microscope images show millimeter-scale pillars and dentures (the first two images), the surfaces of which are covered with small nanometer-scale ridges (the two lower images) to improve the efficiency of the reaction. boiling. Credit: courtesy of researchers
Adding a series of microscale cavities, or dentures, to a surface is a way to control the way bubbles form on that surface, effectively keeping them fixed to the locations of the dentures and preventing them from spreading on a surface. heat resistant film. In this work, the researchers created a series of teeth 10 micrometers wide separated by about 2 millimeters to prevent the formation of films. But this separation also reduces the concentration of bubbles on the surface, which can reduce the boiling efficiency. To compensate for this, the team introduced a much smaller scale surface treatment, creating small bumps and ridges on a nanometer scale, which increases the surface area and promotes the rate of evaporation under the bubbles.
In these experiments, the cavities were made in the centers of a series of pillars on the surface of the material. These pillars, combined with nanostructures, promote the absorption of liquid from the base to the top, and this improves the boiling process by providing more surface exposed to water. In combination, the three “levels” of surface texture (cavity separation, sticks, and nanoscale texture) provide much improved efficiency for the boiling process, Song says.
“These micro cavities define the position where the bubbles come out,” he says. “But by separating these cavities by 2 millimeters, we separate the bubbles and minimize the coalescence of the bubbles.” At the same time, the nanostructures favor evaporation under the bubbles, and the capillary action induced by the pillars supplies liquid to the base of the bubble. 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 coming out of a heated surface are “fixed” in specific places due to the special texture of the surface, rather than spreading all over the surface. Credit: courtesy of researchers
While his work has confirmed that the combination of this type of surface treatment can work and achieve the desired effects, this work was done in small-scale laboratory conditions that could not be easily extended to practical devices, says Wang . “These kinds of structures we’re making aren’t meant to be scaled into their current form,” he says, but were used to demonstrate that this system can work. The next step will be to find alternative ways to create these types of surface textures so that these methods can be more easily extended to practical dimensions.
“Showing that we can control the surface this way for improvements is a first step,” he says. “So the next step is to think of more scalable approaches.” For example, although surface pillars in these experiments were created using clean room methods commonly used to produce semiconductor chips, there are other less demanding ways to create these structures, such as electrodeposition. There are also several different ways to produce the textures of the surface nanostructure, 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 shrink and the management of its heat production is increasingly important. “There’s definitely a space where that’s really important,” Wang says.
Even such applications will take some time to develop because thermal management systems for electronics typically use liquids other than water, known as dielectric liquids. These liquids have a different surface tension and other properties than water, so the dimensions of the surface characteristics should be adjusted accordingly. Working on these differences is one of the next steps for ongoing research, Wang says.
This same multiscale structuring technique could also be applied to different liquids, Song says, adjusting the dimensions to take into account the different properties of the liquids. “That kind of detail can be changed, and that can be our next step,” he says.
Reference: “Three-Tier Hierarchical Structures for Extreme Pool Boiling Heat Transfer Performance” by Youngsup Song, Carlos D. Diaz-Marin, Lenan Zhang, Hyeongyun Cha, Yajing Zhao, and Evelyn N. Wang, June 20, 2022, Advanced Materials. DOI: 10.1002 / adma.202200899
The team also included Carlos Diaz-Martin, Lenan Zhang, Hyeongyun Cha and Yajing Zhao, all at MIT. The work was supported by the Agency for Advanced Research-Energy Projects (ARPA-E), the Air Force Scientific Research Office and the Singapore-MIT Research and Technology Alliance, and make use of MIT.nano facilities.