The Burning Questions

Peering into nooks and corners no one else has yet explored can yield significant returns for researchers. After all, says Jagannath Jayachandran, assistant professor of aerospace engineering, “That’s where you see new and potentially unexplained phenomena, which makes it exciting.”

Since joining the WPI faculty six years ago, Jayachandran has focused his attention on what he calls “relatively less explored combustion phenomena.” That work, which has ranged across such topics as refrigeration, explosions, and aircraft propulsion, has produced often unexpected and sometimes paradigm-challenging results, with implications for safety, sustainability, and climate change.

Through a combination of laboratory experiments and computational modelling, he and his team of graduate students and undergraduates (the Flamelets, as they call themselves) have crafted a host of innovative ways to view complex combustion phenomena through new eyes. In the process, they’ve shown that commonly held beliefs about how things burn and explode can sometimes be wrong, opening the door to potential new breakthroughs and technologies.

One area where this has proven to be true is the study of the combustion behavior of refrigerants.

When modern mechanical refrigeration technology emerged in the 1920s, a surprising variety of chemicals (everything from ammonia to methyl chloride) were put to use as refrigerants, the working fluid in a refrigerator or air conditioner that pulls heat from the air as it changes from a liquid to a gas. But most of these compounds were highly flammable, highly toxic, or both. In the 1930s, chemists developed a new class of refrigerants, called chlorofluorocarbons, or CFCs, that were non-toxic and non-flammable. CFCs were the workhouses of the refrigeration industry (and were also used as fire suppressants and aerosol propellants) for about half a century.

By the late 1970s, though, it became clear that CFCs were migrating to the stratosphere, where they were slowly breaking down to produce chlorine, which destroys ozone. As research revealed the rapid thinning of the stratospheric ozone layer, which shields the planet from harmful ultraviolet radiation, the United States and many other nations signed onto an international treaty, the Montreal Protocol on Substances that Deplete the Ozone Layer, which ultimately led to the phase-out of CFCs in virtually all applications.

Fortunately, there was another group of chemicals, called hydrofluorocarbons, or HFCs, that made perfectly fine refrigerants and didn’t harm the ozone layer. But by the law of unintended consequences, HFCs would prove to have their own serious drawback: They are greenhouse gases, and because they linger in the atmosphere for years, they are several times more potent than carbon dioxide. So, while the ozone layer slowly recovered, this new class of chemicals was threatening to accelerate climate change.

But not all HFCs are greenhouse gases. A small group are more reactive, chemically, so they break down more quickly in the air. But their reactivity may also make them more flammable. Just how flammable is a question Jayachandran is working to determine with a four-year, $450,000 award from the National Science Foundation.

One key measure of flammability is flame speed: the rate at which a flame expands after ignition. Measuring flame speed has been an interest of Jayachandran’s since he was pursuing his PhD in mechanical engineering at the University of Southern California, where he also served as a postdoctoral scholar and research associate in the Combustion and Fuels Research Laboratory.

To account for heat loss, Jayachandran and his team turned to computer models. When existing models seemed unable to handle the complexity of the HFC flame behavior, they developed their own radiation heat loss model that can accurately predict the inward movement of the cooling ball of flame in most cases.

To get around the effects of gravity, Jayachandran simply took gravity out of the picture. Most methods of studying physical phenomena in the absence of gravity (like sending experiments to the International Space Station or booking flights on special airplanes that create brief periods of gravity by flying in parabolic arcs) are expensive. The Jayachandran lab found a way to produce the brief period of weightlessness they needed with a homemade device that cost just $5,000.

The device is a benchtop drop tower, initially designed by a team of undergraduates and refined and put into action by PhD candidate Joel Mathew. A small platform with a combustion chamber and a high-speed camera is held at the top of the tower by electromagnets. At the same instant, the HFC in the chamber is ignited and the magnets are turned off. The platform falls just over four feet into a layer of cushioning foam, producing a half second of zero gravity—enough time for the flame to expand to the edges of the chamber.

“We think this is the most accurate method for measuring HFC flame speed,” Jayachandran says. “What we saw is that if you do the experiment using a static set-up the results are quite different from what we observed with the drop tower. Still, compared to traditional fuels, the flame speed is still quite low, even without gravity.”

By confirming the slow flame speed of the more reactive HFCs, the Jayachandran team would seem to have confirmed that these compounds do not pose a serious flammability risk. Therefore, it should be possible to safely substitute them for HFCs with low reactivity but high greenhouse gas potential. But is that really the case? Earlier large-scale experiments at the Raytheon Research Center in Virginia found that even the less reactive compounds were problematic. “When they filled a chamber with a mixture of air and these compounds, which are non-reactive, by design,” Jayachandran says, “they found that they posed a significant explosion, or overpressure, risk.”

How can a gas that should be difficult to burn cause an explosion? The answer, Jayachandran says, may lie in a type of instability seen in such violent events as volcanic eruptions and supernovae. “It’s called the Rayleigh-Taylor instability and it’s been explored a lot for liquids,” he says. “If you place a layer of a dense fluid, like salt water, on top of a less dense liquid, like fresh water, the denser liquid wants to end up on the bottom because it is heavier. When this happens, you will see perturbations grow at the interface.”

These Rayleigh-Taylor instabilities can also affect a flame as it moves through the more-dense air in its path, against gravity, he notes. It doesn’t happen with a highly flammable liquid like natural gas, because the flame moves too fast to be affected by the instability. “But when the velocities are slower, these instabilities start to grow,” he says. “We’ve done simulations that show that as these flames propagate, they form shapes at their boundaries that look like wrinkles.”

Surprisingly, instability-caused explosions are more likely to be of concern for non-reactive HFCs already in widespread use, rather than the more-reactive compounds that will likely replace them, Jayachandran says. “But it is likely that even the more reactive HFCs will prove to be more explosive than would be expected,” he says.   

That is information he would like to share with regulators, he says. “Right now, there are no safety regulations that account for instabilities. It is important for these folks to know about these effects.”

Turbulence may lie at the heart of another common phenomenon: explosions. Since the invention of gunpowder in the 9th century, people have been blowing things up. But when it comes to gases, like hydrogen, the factors that lead to the transition from flames to a detonation are not well understood. “Understanding how these detonations form is a 100-year-old mystery,” Jayachandran says. “This is something I took up as a challenge when I joined WPI.”

He says his particular focus is hydrogen, a gas that could play a key role in a sustainable future because it can be combined with oxygen or burned to produce energy, with water as the only byproduct. A better understanding of how hydrogen explodes should point to safer ways to store, transport, and use hydrogen. “Historically, we know that hydrogen has a tendency to detonate,” he says, “but we still don’t know the fundamental physics behind the formation of detonations.”

To learn more, Jayachandran and PhD candidate Nolan Dexter-Brown fill a long polycarbonate tube with a combustible mixture. They ignite the gas at one end of the tube and watch as the flame propagates toward the other end at about 1 kilometer per second. At some point, the flame suddenly transitions to a detonation, which propagates at 3-to-4 kilometers per second.

The current understanding is that the shock wave created by a rapidly moving flame compresses the gas into superheated hot spots that spontaneously detonate even before the flame reaches them. But that’s not what Jayachandran and Dexter-Brown observed.

As a faculty member in WPI’s new master’s program in explosion protection engineering, which includes researchers from five engineering programs, including fire protection engineering, Jayachandran says he hopes to continue to uncover the causes of—and potential ways to prevent—hydrogen detonations.

The study of how and why things detonate has taken Jayachandran’s team to interesting places, perhaps none more exotic than the interior of scramjet (supersonic ramjet) engines, which are designed to propel aircraft many times the speed of sound. These engines operate by compressing a supersonic stream of air until it achieves high temperatures. When fuel is sprayed into this superheated air, it bursts into flames.

Since the air is moving so quickly, it spends very little time inside the combustion chamber. In fact, Jayachandran says, “its residence time scales are comparable to the reaction time scales. If the reaction times scales are too long, combustion won’t be complete and the flames will be blown out the back of the engine before they can do any work.”

With a fellowship from the Department of Defense, which would like to use scramjets in future military planes, PhD candidate Amelia Kokernak is seeking to quantify just how flames work in this unusual environment. She and Jayachandran believe the first step is making better measurements of the flame speeds of these volatile mixtures. But achieving flames as hot as those inside a scramjet (2,800 degrees Kelvin, or 4,580 Fahrenheit) in the lab is nearly impossible. To get around this challenge, they removed the nitrogen, an inert gas that merely soaks up heat, from the air and achieved the proper temperatures without changing the chemistry of the flames.

The flame speeds they measured did not agree with the results of conventional models. To help understand why, they collaborated with quantum chemists at Argonne National Laboratory. “With their help, we figured out that there is some very interesting, exotic chemistry happening in these flames. Unless you take that chemistry into account, you can’t predict the reactivity correctly,” Jayachandran says.

Once again, by taking on a difficult, little-explored challenge, Jayachandran and his research team have emerged with unexpected results that may topple the status quo and ignite new thinking and new technologies. But that’s what comes from a burning desire to take the research road less traveled.