Alec Houpt, who earned his doctorate from the Department of Aerospace and Mechanical Engineering (AME) at the University of Notre Dame in 2020, will collaborate with Sergey Leonov, research professor in AME, to pursue the development of a supersonic nozzle designed to freeze and capture carbon dioxide and water from industrial exhaust.
Houpt and Leonov, who previously served as Houpt’s doctoral advisor, received Phase I funding from the National Science Foundation’s Small Business Tech Transfer (STTR) program to advance work already underway at Houpt’s startup, Inertial. The project aims to freeze carbon dioxide and water out of industrial exhaust streams by pushing the exhaust through an axisymmetric supersonic nozzle, while swirling the stream to separate the heavier and denser frozen particles from the surrounding gas. The captured carbon, which has been prevented from being emitted into the atmosphere, would then be sold to a downstream customer looking to turn it into a value-added product, like fuel or building materials.
The funds provided in the STTR Phase I grant will bring Houpt’s initial nozzle designs into the supersonic wind tunnels at Notre Dame’s White Field Research Laboratory. Here, Leonov’s research team will test various geometries and swirling rates to optimize nozzle design for the freezing and separation of water and CO2.
Collaborating with Leonov at White Field brings the duo around full circle to the earliest origins of Houpt’s idea for Inertial.
Back in graduate school, Houpt was running an experiment with a Mach-6 supersonic nozzle on the High Enthalpy Arc Tunnel (ACT1) at White Field. When air or any gas moves through a supersonic nozzle, the flow drops in temperature and pressure as the internal energy of the gas is converted into kinetic energy. To prevent freezing on the backend of the nozzle, the temperature of the whole wind tunnel is generally raised with a heating system.
One time, however, the heater didn’t kick on at all and the nozzle flow condensed. Ten years after the fact, Houpt has now returned to his experience with the foggy equipment fluke. Last year, he got together with Leonov and a current postdoctoral scholar, Philip Lax, whose doctoral scholarship focused on condensation in supersonic and hypersonic wind tunnels, to entertain the possibility of a specialized nozzle that is intentionally designed to freeze and capture CO2.
“That’s not uncommon for supersonic, hypersonic labs in general, to unintentionally freeze carbon dioxide or whatever their flow gas is,” Houpt explained. “It’s never the goal, it’s always an error, but we’d like to flip the script on that.”
Wind tunnels are large tube-like structures used to study the aerodynamic properties of objects, such as aircraft and spacecraft, and their components, including engines and nosecones. The tunnel generates a uniform flow of air around a stationary object, which simulates flight in a controlled environment. Researchers can then analyze how air moves in and around an object and optimize designs accordingly.
The characteristics of the industrial exhaust systems Houpt is looking to retrofit are not so different from those of wind tunnels, whose high pressure and uniformity mimic the flow he’s hoping to process from industrial partners.
To control how fast the air flows into the test section of a supersonic wind tunnel, a nozzle contracts the flow until it is moving at the speed of sound, or Mach 1, then expands the stream to accelerate it beyond Mach 1. The nozzles on research wind tunnels like those at White Field are designed to provide the most uniform flow possible. Part of maintaining a predictable, repeatable, uniform flow is avoiding condensation, which can happen if the temperature of the flow drops too drastically during expansion.
The nozzle Houpt is designing will become the centerpiece of a wind tunnel technology retrofitted to the exhaust ducts of power plants and similar industrial facilities. The apparatus will route the flue gas output from plant operations into a compressor, which will increase the pressure, and then push it through the nozzle, first contracting the gas, then expanding it, like a research wind tunnel.
But instead of trying to optimize the gas’ expansion for uniformity, Houpt is seeking to optimize it for phase change. The optimal nozzle will facilitate a conversion from heat energy to kinetic energy so significant that the carbon dioxide and water within the flue gas reliably freezes, a phenomenon which Leonov and Lax documented while developing the Phase I proposal.
Though proven, freezing is only half the story. Limited experimental work related to actually removing the frozen particles from the larger flow has been performed.
“Right now, the frozen flow just stays in one line once it leaves the nozzle,” Houpt said. “We’re swirling everything and trying to get the frozen solids to migrate to the outside, so we can essentially isolate them while letting the rest of the exhaust flow by.”
Houpt’s swirling technique aims to take advantage of the fact that frozen particles are heavier and have more momentum than the surrounding gas. When the mixture is spun, everything inside wants to keep moving in a straight line, but the spinning forces them into a circle. This creates a push toward the center of the spin. The heavier particles, however, have more resistance to this inward push, so they end up getting thrown outward to the edges of the spinning flow.
Upcoming experiments will test and refine the geometry of Houpt’s nozzle design, as well as investigate swirling rates, to optimize the inertial separation. Once the first small prototype is perfected, the team can scale up—literally.
“One of the great things about aerospace engineering is that everything’s based on ratios,” Houpt said. “So you can perfect the nozzle on a small scale, then just make the area of ratios bigger, and it functions the same.”
While Houpt is only at the threshold of a lengthy technology maturation process, his choice to reconnect with Leonov’s research team and collaborate with Notre Dame engineers reflects his long-term aspirations.
In order for Inertial’s technology to be considered ready for commercial operations, the nozzle must be capable of capturing 1,000 metric tons of carbon dioxide per year. To reach this benchmark, Houpt’s design will be scaled up to at least three times larger than the initial prototype.
“The initial stages of developing, testing, and transitioning from the lab to industry, like in the case of Alec’s system, are critical but challenging to sustain,” Leonov said. “We’re very grateful to have received the funding for collaborative testing with Inertial. Financial support like this is invaluable for pursuing the development of novel technologies to their fullest extent, especially in such a critical area as the mitigation of carbon emissions.”
Leonov added, “One of the great additional benefits of this project is the exposure Notre Dame students will receive to exciting areas of engineering, which will, in turn, be the catalyst for a new generation of highly-skilled engineers to enter the workforce.”
After proving the nozzle design capable at White Field in Phase I—a subscale design which can capture roughly 50 tons of CO2 per year—Houpt will seek Phase II funding from the NSF and a partnership with engineers at Notre Dame Power & Propulsion (ND P&P).
“White Field’s testing capabilities will get Inertial started with the fundamental science and development,” said Joshua Szczudlak, senior associate director at ND P&P. “Our role at P&P is to help Alec move his concept from the benchtop experiment into a larger, practical system. We’re able to leverage both our deep knowledge of turbomachinery and our broader experience in technology transition to support that process, while staying within the Notre Dame ecosystem.”
To learn more about Notre Dame Power & Propulsion’s research, please visit ND P&P’s website.
Originally published at research.nd.edu by Erin Fennessy on December 8, 2025.