Surfaces designed to boil water more efficiently

Boiling water or other fluids is an energy-intensive step at the heart of a wide range of industrial processes, including most power plants, many chemical production systems, and even cooling systems for the electronic.

Improving the efficiency of systems that heat and evaporate water could significantly reduce their energy consumption. Now, researchers at MIT have found a way to achieve this, with a surface treatment specially adapted to the materials used in these systems.

The improved efficiency comes from a combination of three different types of surface modifications, at different size scales. The new findings are described in the journal Advanced Materials in an article by recent MIT graduate Youngsup Song PhD ’21, Ford Professor of Engineering Evelyn Wang and four others at MIT. The researchers note that this initial discovery is still at the laboratory scale and that further work is needed to develop a practical industrial-scale process.

Two key parameters describe the boiling process: heat transfer coefficient (HTC) and critical heat flux (CHF). In material design, there is usually a trade-off between the two, so anything that improves one tends to make the other worse. But both are important for the efficiency of the system, and now, after years of work, the team has succeeded in significantly improving both properties at the same time, thanks to their combination of different textures added to the surface of a material.

“Both parameters are important,” Song says, “but improving both parameters together is a bit tricky because they have an inherent trade-off.” The reason is, he explains, “because if we have a lot of bubbles on the boiling surface, it means the boiling is very efficient, but if we have too many bubbles on the surface, they can fuse, which may form a vapour. 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 the water, it inhibits the efficiency of heat transfer and lowers the value of CHF,” he says.

Song, who is now a postdoc at Lawrence Berkeley National Laboratory, did much of the research as part of his doctoral dissertation at MIT. While the various components of the new surface treatment he developed had been studied before, the researchers say this work is the first to show that these methods could be combined to overcome the trade-off between the two competing parameters.

Adding a series of microscopic cavities, or bumps, to a surface is a way to control how bubbles form on that surface, effectively keeping them pinned to the bump locations and preventing them from spreading into a heat-resistant film. In this work, the researchers created a network of bumps 10 micrometers wide separated by about 2 millimeters to prevent film formation. But this separation also reduces the concentration of bubbles on the surface, which can reduce boiling efficiency. To compensate for this, the team introduced a much smaller-scale surface treatment, creating tiny nanoscale bumps and ridges, which increase surface area and promote the rate of evaporation beneath the bubbles.

In these experiments, the cavities were made in the center of a series of pillars on the surface of the material. These pillars, combined with nanostructures, promote the evacuation of liquid from the base to their tops, which improves the boiling process by providing a greater surface area exposed to the water. In combination, the three “levels” of surface texture — cavity separation, studs, and nanoscale texturing — provide greatly improved efficiency for the boiling process, Song explains.

“These micro-cavities define the position where the 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 under the bubbles, and the capillary action induced by the pillars feeds liquid to the bottom of the bubbles. This maintains a layer of liquid water between the boiling surface and the vapor bubbles, which improves maximum heat flow.

Although their work confirmed that combining these types of surface treatments can work and achieve the desired effects, this work was done in small-scale laboratory conditions that could not easily be scaled up to practical devices, explains Wang. “These kinds of structures that we make are not meant to be scaled up in their current form,” she says, but rather were used to prove that such a system can work. A next step will be to find other ways to create these types of surface textures so that these methods can more easily be extended to practical dimensions.

“Showing that we can control the surface in this way to achieve improvement is a first step,” she says. “Then the next step is to think about more scalable approaches.” For example, although the pillars on the surface of these experiments were created using cleanroom methods commonly used to produce semiconductor chips, there are other less demanding ways to create such structures, such as electroplating. There are also a number of different ways to produce the surface nanostructure textures, some of which may be more easily scalable.

Some important small-scale applications 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 thermal management their heat production becomes more and more important. “There’s definitely a space there where it’s really important,” Wang says.

Even these types of applications will take some time to develop because thermal management systems for electronics typically use liquids other than water, called dielectric liquids. These liquids have different surface tension and other properties than water, so the dimensions of the surface features should be adjusted accordingly. Working on these differences is one of the next steps in ongoing research, Wang says.

This same multi-scale structuring technique could also be applied to different liquids, Song says, by adjusting the dimensions to account for different liquid properties. “These kinds of details can be changed, and that can be our next step,” he says.

Reference: Song Y, Díaz-Marín CD, Zhang L, Cha H, Zhao Y, Wang EN. Hierarchical three-tier structures for extreme heat transfer performance in pool boiling. Advanced mat. 2200899.doi:10.1002/adma.202200899

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