Graduate, postdoctoral

Subterms

    More stories

    • 113 Shares159 Views

      in Energy

      The boiling crisis — and how to avoid it

      by

      It’s rare for a pre-teen to become enamored with thermodynamics, but those consumed by such a passion may consider themselves lucky to end up at a place like MIT. Madhumitha Ravichandran certainly does. A PhD student in Nuclear Science and Engineering (NSE), Ravichandran first encountered the laws of thermodynamics as a middle school student in Chennai, India. “They made complete sense to me,” she says. “While looking at the refrigerator at home, I wondered if I might someday build energy systems that utilized these same principles. That’s how it started, and I’ve sustained that interest ever since.”

      She’s now drawing on her knowledge of thermodynamics in research carried out in the laboratory of NSE Assistant Professor Matteo Bucci, her doctoral supervisor. Ravichandran and Bucci are gaining key insights into the “boiling crisis” — a problem that has long saddled the energy industry.

      Ravichandran was well prepared for this work by the time she arrived at MIT in 2017. As an undergraduate at India’s Sastra University, she pursued research on “two-phase flows,” examining the transitions water undergoes between its liquid and gaseous forms. She continued to study droplet evaporation and related phenomena during an internship in early 2017 in the Bucci Lab. That was an eye-opening experience, Ravichandran explains. “Back at my university in India, only 2 to 3 percent of the mechanical engineering students were women, and there were no women on the faculty. It was the first time I had faced social inequities because of my gender, and I went through some struggles, to say the least.”

      MIT offered a welcome contrast. “The amount of freedom I was given made me extremely happy,” she says. “I was always encouraged to explore my ideas, and I always felt included.” She was doubly happy because, midway through the internship, she learned that she’d been accepted to MIT’s graduate program.

      As a PhD student, her research has followed a similar path. She continues to study boiling and heat transfer, but Bucci gave this work some added urgency. They’re now investigating the aforementioned boiling crisis, which affects nuclear reactors and other kinds of power plants that rely on steam generation to drive turbines. In a light water nuclear reactor, water is heated by fuel rods in which nuclear fission has occurred. Heat removal is most efficient when the water circulating past the rods boils. However, if too many bubbles form on the surface, enveloping the fuel rods in a layer of vapor, heat transfer is greatly reduced. That’s not only diminishes power generation, it can also be dangerous because the fuel rods must be continuously cooled to avoid a dreaded meltdown accident.

      Nuclear plants operate at low power ratings to provide an ample safety margin and thereby prevent such a scenario from occurring. Ravichandran believes these standards may be overly cautious, owing to the fact that people aren’t yet sure of the conditions that bring about the boiling crisis. This hurts the economic viability of nuclear power, she says, at a time when we desperately need carbon-free power sources. But Ravichandran and other researchers in the Bucci Lab are starting to fill some major gaps in our understanding.

      They initially ran 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 lasted two minutes, but it took more than three weeks to pick out every bubble that formed and track its growth and evolution,” Ravichandran explains.

      To streamline this process, she and Bucci are implementing a machine learning approach, based on neural network technology. Neural networks are good at recognizing patterns, including those associated with bubble nucleation. “These networks are data hungry,” Ravichandran says. “The more data they’re fed, the better they perform.” The networks were trained on experimental results pertaining to bubble formation on different surfaces; the networks were then tested on surfaces for which the NSE researchers had no data and didn’t know what to expect.

      After gaining experimental validation of the output from the machine learning models, the team is now trying to get these models to make reliable predictions as to when the bubble crisis, itself, will occur. The ultimate goal is to have a fully autonomous system that can not only predict the boiling crisis, but also show why it happens and automatically shut down experiments before things go too far and lab equipment starts melting.

      In the meantime, Ravichandran and Bucci have made some important theoretical advances, which they report on in a recently published paper for Applied Physics Letters. There had been a debate in the nuclear engineering community as to whether the boiling crisis is caused by bubbles covering the fuel rod surface or due to bubbles growing on top of each other, extending outward from the surface. Ravichandran and Bucci determined that it is a surface-level phenomenon. In addition, they’ve identified the three main factors that trigger the boiling crisis. First, there’s the number of bubbles that form over a given surface area and, second, the average bubble size. The third factor is the product of the bubble frequency (the number of bubbles forming within a second at a given site) and the time it takes for a bubble to reach its full size.

      Ravichandran is happy to have shed some new light on this issue but acknowledges that there’s still much work to be done. Although her research agenda is ambitious and nearly all consuming, she never forgets where she came from and the sense of isolation she felt while studying engineering as an undergraduate. She has, on her own initiative, been mentoring female engineering students in India, providing both research guidance and career advice.

      “I sometimes feel there was a reason I went through those early hardships,” Ravichandran says. “That’s what made me decide that I want to be an educator.” She’s also grateful for the opportunities that have opened up for her since coming to MIT. A recipient of a 2021-22 MathWorks Engineering Fellowship, she says, “now it feels like the only limits on me are those that I’ve placed on myself.” More

    • 38 Shares189 Views

      in Energy

      A material difference

      by

      Eesha Khare has always seen a world of matter. The daughter of a hardware engineer and a biologist, she has an insatiable interest in what substances — both synthetic and biological — have in common. Not surprisingly, that perspective led her to the study of materials.

      “I recognized early on that everything around me is a material,” she says. “How our phones respond to touches, how trees in nature to give us both structural wood and foldable paper, or how we are able to make high skyscrapers with steel and glass, it all comes down to the fundamentals: This is materials science and engineering.”

      As a rising fourth-year PhD student in the MIT Department of Materials Science and Engineering (DMSE), Khare now studies the metal-coordination bonds that allow mussels to bind to rocks along turbulent coastlines. But Khare’s scientific enthusiasm has also led to expansive interests from science policy to climate advocacy and entrepreneurship.

      A material world

      A Silicon Valley native, Khare recalls vividly how excited she was about science as a young girl, both at school and at myriad science fairs and high school laboratory internships. One such internship at the University of California at Santa Cruz introduced her to the study of nanomaterials, or materials that are smaller than a single human cell. The project piqued her interest in how research could lead to energy-storage applications, and she began to ponder the connections between materials, science policy, and the environment.

      As an undergraduate at Harvard University, Khare pursued a degree in engineering sciences and chemistry while also working at the Harvard Kennedy School Institute of Politics. There, she grew fascinated by environmental advocacy in the policy space, working for then-professor Gina McCarthy, who is currently serving in the Biden administration as the first-ever White House climate advisor.

      Following her academic explorations in college, Khare wanted to consider science in a new light before pursuing her doctorate in materials science and engineering. She deferred her program acceptance at MIT in order to attend Cambridge University in the U.K., where she earned a master’s degree in the history and philosophy of science. “Especially in a PhD program, it can often feel like your head is deep in the science as you push new research frontiers, but I wanted take a step back and be inspired by how scientists in the past made their discoveries,” she says.

      Her experience at Cambridge was both challenging and informative, but Khare quickly found that her mechanistic curiosity remained persistent — a realization that came in the form of a biological material.

      “My very first master’s research project was about environmental pollution indicators in the U.K., and I was looking specifically at lichen to understand the social and political reasons why they were adopted by the public as pollution indicators,” Khare explains. “But I found myself wondering more about how lichen can act as pollution indicators. And I found that to be quite similar for most of my research projects: I was more interested in how the technology or discovery actually worked.”

      Enthusiasm for innovation

      Fittingly, these bioindicators confirmed for her that studying materials at MIT was the right course. Now Khare works on a different organism altogether, conducting research on the metal-coordination chemical interactions of a biopolymer secreted by mussels.

      “Mussels secrete this thread and can adhere to ocean walls. So, when ocean waves come, mussels don’t get dislodged that easily,” Khare says. “This is partly because of how metal ions in this material bind to different amino acids in the protein. There’s no input from the mussel itself to control anything there; all the magic is in this biological material that is not only very sticky, but also doesn’t break very readily, and if you cut it, it can re-heal that interface as well! If we could better understand and replicate this biological material in our own world, we could have materials self-heal and never break and thus eliminate so much waste.”

      To study this natural material, Khare combines computational and experimental techniques, experimentally synthesizing her own biopolymers and studying their properties with in silico molecular dynamics. Her co-advisors — Markus Buehler, the Jerry McAfee Professor of Engineering in Civil and Environmental Engineering, and Niels Holten-Andersen, professor of materials science and engineering — have embraced this dual-approach to her project, as well as her abundant enthusiasm for innovation.

      Khare likes to take one exploratory course per semester, and a recent offering in the MIT Sloan School of Management inspired her to pursue entrepreneurship. These days she is spending much of her free time on a startup called Taxie, formed with fellow MIT students after taking the course 15.390 (New Enterprises). Taxie attempts to electrify the rideshare business by making electric rental cars available to rideshare drivers. Khare hopes this project will initiate some small first steps in making the ridesharing industry environmentally cleaner — and in democratizing access to electric vehicles for rideshare drivers, who often hail from lower-income or immigrant backgrounds.

      “There are a lot of goals thrown around for reducing emissions or helping our environment. But we are slowly getting physical things on the road, physical things to real people, and I like to think that we are helping to accelerate the electric transition,” Khare says. “These small steps are helpful for learning, at the very least, how we can make a transition to electric or to a cleaner industry.”

      Alongside her startup work, Khare has pursued a number of other extracurricular activities at MIT, including co-organizing her department’s Student Application Assistance Program and serving on DMSE’s Diversity, Equity, and Inclusion Council. Her varied interests also have led to a diverse group of friends, which suits her well, because she is a self-described “people-person.”

      In a year where maintaining connections has been more challenging than usual, Khare has focused on the positive, spending her spring semester with family in California and practicing Bharatanatyam, a form of Indian classical dance, over Zoom. As she looks to the future, Khare hopes to bring even more of her interests together, like materials science and climate.

      “I want to understand the energy and environmental sector at large to identify the most pressing technology gaps and how can I use my knowledge to contribute. My goal is to figure out where can I personally make a difference and where it can have a bigger impact to help our climate,” she says. “I like being outside of my comfort zone.” More