Go Smaller to Go Faster

(left) John A. Clark and Edward T. Crossan Chair Professor, Nikhil Koratkar with Professor and Associate Dean for Academic Affairs School of Engineering, Matthew Oehlschlaeger
What would it take to fly from New York to Los Angeles in less than an hour—or send a cruise missile across oceans in a few minutes?

The aerospace industry has dreamed of hypersonic propulsion (i.e., propulsion at five times the speed of sound or more) for decades. Researchers have devoted whole careers to every aspect of making it possible.

One of those aspects is a fuel with extraordinary energy density and an enhanced ability to absorb heat from the airframe.

“In hypersonic flight, the aircraft undergoes extreme heating due to the compression of air against its surfaces,” explained Rensselaer Professor Matt Oehlschlaeger. “That requires fuels that are not only energy-dense but also act as a heat sink for cooling the aircraft.”

The aerospace industry has dreamed of hypersonic propulsion (i.e., propulsion at five times the speed of sound or more) for decades.

In this regard, nanofluid fuels look promising. They consist of stable colloidal suspensions of energy-dense nanoparticles in a base fluid, in this case liquid hydrocarbon fuel. But little is known about how the particles affect the all-important combustion process—and thus whether the nanofluids will, or will not, fulfill their promise.

Oehlschlaeger, who specializes in the relationship of fuel properties and combustion, sought to explore the nanoparticles’ influence on the evaporation rate of fuel droplets, which relates directly to combustion efficiency. As part of that exploration, he needed to understand the nanoparticles themselves—or, rather, find someone who did.

He didn’t have to go far.

Professor Nikhil Koratkar, Oehlschlaeger’s colleague in MANE, has spent most of his career on the nano level. His research interests lie in the development and characterization of advanced nanostructured materials and devices.

“Nikhil’s lab was crucial to helping us characterize materials and understand nanoparticle aggregation, the properties of those aggregates, and the influence on droplet evaporation,” Oehlschlaeger said. “His students, using state-of-the-art nanoscale imaging and optical experiments, helped quantify the size and shape of the nanoparticle aggregates.”

The aggregation aspect was key. Oehlschlaeger’s lab conducted experiments to quantify the influence of added particles and their aggregation on droplet evaporation. As it turned out, raising the percentage of particles in the nanofluids from 1 percent to 3 percent sparked a dramatic decrease in the evaporation rate. Subsequent modeling revealed the cause: the nanoparticles aggregated at the surface of the droplet, “blocking” the fluid molecules from evaporating and thus slowing the energy release required for propulsion.

Follow-on experiments have since confirmed these findings: nanoparticles increase the energy content of liquid fuels but modify the behavior of combusting sprays.

Now that the joint team has an idea of the problem, it can aim more effectively at a solution. Next steps include investigating the possibility of coating the nanoparticles to hinder the aggregation process. If the investigation succeeds, it will represent one more step in maximizing the energy benefits of nanofluids while minimizing their drawbacks.

According to Oehlschlaeger, the findings confirm his long-held convictions about the power of experimentation. “My work always starts with experiment,” he explained. “Combustion is complicated, so the experiments often yield interesting and unexplained observations. From there, we consider theories, mechanisms, and models to explain the observations. That, we hope, leads to new knowledge that enables the design of new systems and the development of new technologies.”

In this case, it might help to make hypersonic propulsion a reality. Stay tuned.