The boiling crisis and how to avoid it

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It’s rare for a teen to be in love with thermodynamics, but those consumed by such a passion may be lucky enough to end up in a place like MIT. Madhumitha Ravichandran definitely does. A Ph.D. As a nuclear science and engineering (NSE) student, Ravichandran first encountered the laws of thermodynamics as a middle school student in Chennai, India. “They made sense to me,” she says. “As I looked at the refrigerator at home, I wondered if maybe one day I could build energy systems that use the same principles.

She now uses her knowledge of thermodynamics in research in the laboratory of NSE assistant professor Matteo Bucci, her doctoral supervisor. Ravichandran and Bucci gain important insights into the “boiling crisis” – a problem that has preoccupied the energy industry for a long time.

When she arrived at MIT in 2017, Ravichandran was well prepared for this work. As a student at the Indian Sastra University, she did research on “two-phase flows” and examined the transitions of water between liquid and gaseous form. During an internship at the Bucci laboratory in early 2017, she further investigated droplet evaporation and related phenomena. It was an eye-opening experience, explains Ravichandran. “Back at my university in India, only 2-3 percent of engineering students were women, and there weren’t any women in the faculty. It was the first time I’ve faced social inequalities because of my gender, and I’ve been through a few.” struggles to say the least. “

MIT provided a welcome contrast. “The freedom that I was given made me very happy,” she says. “I was always encouraged to explore my ideas and I always felt included.” She was doubly happy because in the middle of her internship she learned that she had been accepted into the graduate program at MIT.

As a Ph.D. Student has followed a similar path in her research. She continues to study cooking and heat transfer, but Bucci added urgency to this work. You are now investigating the boiling crisis mentioned above, which affects nuclear reactors and other types of power plants that rely on steam generation to power turbines. In a light water nuclear reactor, water is heated by fuel rods in which nuclear fission has taken place. Heat dissipation is most efficient when the water flowing past the bars is boiling. However, if too many bubbles form on the surface, which envelop the fuel rods with a layer of vapor, the heat transfer is greatly reduced. Not only does this reduce power generation, but it can also be dangerous as the fuel rods must be constantly cooled to avoid a dreaded core meltdown accident.

Nuclear power plants are operated with low rated outputs in order to offer a sufficient safety margin and thereby prevent such a scenario from occurring. Ravichandran thinks these standards are too cautious as people are still unsure of the conditions under which the boiling crisis will be triggered. This is damaging the economic viability of nuclear power, she says, at a time when we urgently need carbon-free power sources. But Ravichandran and other researchers in the Bucci laboratory are starting to fill in some large gaps in our understanding.

They first performed experiments to determine how quickly bubbles form when water hits a hot surface, how big the bubbles get, how long they grow, and how the surface temperature changes. “A typical experiment took two minutes, but it took more than three weeks to identify each bubble that formed and to track its growth and development,” explains Ravichandran.

To streamline this process, she and Bucci implement a machine learning approach based on neural network technology. Neural networks are good at recognizing patterns, including those associated with bubble germs. “These networks are hungry for data,” says Ravichandran. “The more data they get, the better they are.” The networks were trained on experimental results regarding bubble formation on different surfaces; the nets were then tested on surfaces for which the NSE researchers had no data and did not know what to expect.

After the results of the machine learning models have been experimentally validated, the team is now trying to get these models to make reliable predictions about when the bladder crisis itself will occur. The ultimate goal is a fully autonomous system that can not only predict the boiling crisis but also show why it is happening and automatically turn off experiments before it goes too far and the lab equipment starts to melt.

Meanwhile, Ravichandran and Bucci have made some important theoretical advances, which they report in a recent article for Applied Physics Letters. There has been a debate in the nuclear community as to whether the boiling crisis is caused by bubbles covering the surface of the fuel rod or bubbles growing on top of each other and extending outward from the surface. Ravichandran and Bucci found it to be a surface phenomenon. Additionally, they identified the three main factors that are triggering the boiling crisis. First, there is the number of bubbles that form on a given surface, and second, there is the average bubble size. The third factor is the product of the bubble frequency (the number of bubbles that form in a given location in one second) and the time it takes for a bubble to reach its full size.

Ravichandran is pleased to have shed new light on this subject, but admits that there is still much to be done. Although her research agenda is ambitious and almost complete, she never forgets where she is from and the isolation she felt while studying engineering. On her own initiative, she supervises female engineering students in India and offers both research and career advice.

“I sometimes feel that there was a reason I went through these early troubles,” says Ravichandran. “That made me want to become a kindergarten teacher.” She is also grateful for the opportunities that have opened up since joining MIT. As a recipient of a MathWorks Engineering Fellowship 2021-22, she says, “Now it feels like the only limits for me are those that I have set for myself.”

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More information:
Madhumitha Ravichandran et al., Deciphering the Boiling Crisis through Data-Driven Investigation of High-Resolution Infrared Thermometric Measurements, Applied Physics Letters (2021). DOI: 10.1063 / 5.0048391 Provided by the Massachusetts Institute of Technology

Quote: The boiling crisis and how to avoid it (2021, August 25), accessed August 25, 2021 from https://phys.org/news/2021-08-crisis.html

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