The Blueprint: How researchers found a way to boil water faster

Youngsup’s words have been edited for brevity and clarity.

Interesting Engineering: Talk us through how this happened

Youngsup Song: The team were able to improve the two key parameters that are conducive to the boiling process, the heat transfer coefficient (HTC) and the critical heat flux (CHF). This is quite a development as there’s generally a tradeoff between the two, so anything that improves one of these parameters tends to make the other worse.

IE: And why was this so tricky?

Both parameters are important, but enhancing both parameters together is kind of tricky because they have an intrinsic trade-off.

If we have lots of bubbles on the boiling surface, that means boiling is very efficient, but if we have too many bubbles on the surface, they can coalesce together, which can form a vapor film over the boiling surface.

That film introduces resistance to the heat transfer from the hot surface to the water. If we have vapor in between the surface and water, that prevents the heat transfer efficiency and lowers the CHF value.

IE: So, how did you do it?

By adding a series of microscale cavities (dents) to a surface, we were able to control the way bubbles form on that surface. This effectively keeps the bubbles pinned to the dents and prevents them from spreading out into a heat-resisting film.

The microcavities were then positioned at the ideal length to optimize the process.

Those micro cavities define the position where bubbles come up. But by separating those cavities by two millimeters, we separate the bubbles and minimize the coalescence of bubbles.

IE: Why is this exciting?

The engineered surface can improve the efficiency and maximum capacity (a.k.a. critical heat flux or CHF) of boiling heat transfer.

In general, the efficiency and maximum capacity have a trade-off; whereas here, we achieved a way of significantly enhancing both of them by engineering surface structures in a multiple dimensions from nanometer to millimeter.

IE: Could this make a difference on a domestic or even industrial scale?

In this work, we demonstrated the strategy to enhance boiling by surface engineering. These kinds of surfaces, however, were fabricated by clean-room processes (silicon wafer-based processes) and experimental tests were done at laboratory scale.

So now, the first step when looking to apply our method to a domestic or industrial level application will be to find a scalable way of creating a similar surface textures, so our method could more easily be scaled up to practical dimensions.

IE: What environmental impact could this have?

Enhanced boiling could save a significant amount of energy for electricity generation in power plants. It also has the potential to be used in the thermal management of highly integrated electronics.

But to apply our method to such applications, more details need to be addressed, e.g., scalable texturing technique, optimization for different types of fluid (we tested with water while dielectric fluids are required for electronics), and optimizing a surface for high pressure condition (which is typical for power plant boilers). Once these details have been looked at in detail, the impact could be significant, especially considering the size of the energy and IT industries.

IE: Is this result exactly what you set out to achieve, or did you have other aims?

Surprisingly, this is the first work we’ve undertaken where we obtained the exact results that we expected! We aimed to overcome the intrinsic trade-off between the efficiency and maximum heat flux – and the experimental outcome was exactly what we expected.

From there, we could confirm that our design strategy is solid enough to provide surface design guidelines for enhanced boiling.

IE: Which direction do you want to take this experiment in next?

First, we need to investigate a method to create these kinds of surface structures in a scalable manner. Then, we also need to test boiling with different types of fluids and higher pressure conditions so that we can optimize a boiling surface for a specific application.

IE: Tell us a bit about you and your background

I am originally from Korea and am currently living in Berkeley, California while I investigate thermal energy storage at the Berkeley Laboratory.

I am broadly interested in thermal fluids and interfacial phenomena for water and energy applications. My research is currently focusing on the prediction of supercooling of phase change materials for thermal energy storage applications.

To find answers and satisfy my curiosity, I design, machine, clean-room process, code, experiment, and analyze through my research.

And when I’m not in the lab, you’ll probably find me seeking caffeine around campus, taking a walk with my family in Tilden Park, playing soccer, or working out in the gym.

IE: How did you get to where you are now?

My PhD research group, Device Research Lab (DRL), is directed by Prof. Evelyn Wang in the Mechanical Engineering Department at MIT. The DRL combines micro and nanoscale heat and mass transport processes with the development of novel nanostructured materials. By doing this, we create innovative solutions in thermal management, thermal energy storage, solar thermal energy conversion, and water desalination.

I received my M.S. degree in the Nano Transducers Laboratory (NTL), Mechanical Engineering Department at Yonsei University under the guidance of Prof. Jongbaeg (JB) Kim. The NTL specializes in the modeling and development of MEMS devices exploiting the interfacial properties of nanomaterials.

Based on micro/nano machining and nanomaterial-synthesis capabilities (e.g., carbon nanotubes, graphene, GaN nanowires, WO₂ nanowires etc.), the NTL has been developing functional MEMS/NEMS devices, piezoelectric/electrostataic energy harvesters, flexible/stretchable sensors, and gas sensors.

Also pior to joining the DRL, I worked at Korea Institute of Materials Science (KIMS) in Electrochemistry Department for four and a half years. KIMS is a research institute funded by South Korea government, where they conduct comprehensive research on material science. The major research areas are metallurgy, powder & ceramics, surface science, and composites.

Study Abstract:

Boiling is an effective energy-transfer process with substantial utility in energy applications. Boiling performance is described mainly by the heat-transfer coefficient (HTC) and critical heat flux (CHF). Recent efforts for the simultaneous enhancement of HTC and CHF have been limited by an intrinsic trade-off between them—HTC enhancement requires high nucleation-site density, which can increase bubble coalescence resulting in limited CHF enhancement. In this work, this trade-off is overcome by designing three-tier hierarchical structures. The bubble coalescence is minimized to enhance the CHF by defining nucleation sites with microcavities interspersed within hemi-wicking structures. Meanwhile, the reduced nucleation-site density is compensated for by incorporating nanostructures that promote evaporation for HTC enhancement. The hierarchical structures demonstrate the simultaneous enhancement of HTC and CHF up to 389% and 138%, respectively, compared to a smooth surface. This extreme boiling performance can lead to significant energy savings in a variety of boiling applications.

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