Under pressure: How droplets inform high-pressure engine combustion in airplanes

The UW, led by ME Assistant Professor John Palmore Jr, is part of a collaborative NASA center working to better understand aerospace propulsion. Aerospace engine manufacturers want to know if higher operating pressures could lead to improved efficiency. However, the processes of liquid fuel injection and combustion under high pressure are not well understood.

The NASA SPARC (SpaceIgnite Center for Advanced Research-Education in Combustion) center is a collaboration between the UW, Clemson University and Cal State Fullerton. In 2025, the UW received a five-year grant of more than $400,000 as part of the center.

“In aerospace propulsion, thrust is generated through the combustion of liquid fuels in an engine,” says Palmore, an expert in fluid dynamics and combustion. “To predict liquid fuel combustion, you need to be able to predict atomization and evaporation.”

Atomization, the process by which a liquid stream breaks down into small droplets, often occurs in combustion-based engines, including those used in aerospace, automotive and marine systems. The researchers are focused on the physics of atomization under high pressure, which is different from normal circumstances.

At high temperatures and pressures, fuel transitions into an exotic “supercritical” fluid state with both liquid-like and gas-like properties. Transcritical describes the situation where a fluid is on the boundary between supercritical and ordinary states. Because of the differences between supercritical and ordinary fluid states, the processes of fuel atomization and evaporation under these conditions is different than a conventional engine, and that will affect combustion.

Palmore’s lab, the Combustion, Atomization, Multiphase, & Particulate Physics Research & Education (CAMP-PhyRE), develops numerical simulation methods to study atomization, droplet motion and evaporation. Using these methods, they can investigate and characterize these complex fluid flows, generating knowledge that informs the design of high-pressure engines.

Droplet shape affects combustion

Palmore’s past work has investigated how to predict fuel droplet evaporation and combustion. “Since most of the fuel evaporation happens after atomization into droplets, studying droplets is key,” he says.

Common droplet models treat liquid droplets the same as rigid, solid spheres. But these models aren’t accurate in certain environments, such as aerospace engines with higher operating pressures. CAMP-PhyRE researchers found that when the droplets enter the high-pressure airflow, the air flow stresses and stretches the droplet.

“Droplets typically form into an oblate shape similar to a lentil,” Palmore says. “This causes the droplets to experience different forces than a sphere, and ultimately to have different trajectory and evaporation rates in the engine.”

Pavan Kancharla (MS ‘25) worked to develop a library of droplet shapes that result from atomizing fuels through detailed numerical simulations and machine learning methods. ME Ph.D. student Yushu Lin is investigating how real droplets disperse differently than rigid spheres after atomization.

“This is important to predict fuel combustion, since combustion can only occur where the fuel is located,” Palmore says.

The team has presented their project in several spaces. In 2025, Pavan’s droplet library was presented at the American Institute of Aeronautics and Astronautics (AIAA) SciTech Forum, and Yushu Lin presented his work on droplet dispersion at the American Physical Society (APS) Division of Fluid Dynamics Conference. Natnael Haile, a Cal State Fullerton undergraduate student working with Palmore’s lab, also presented at the APS conference and received an award for a project poster.

“It was a great experience to recognize the giants of my field tuning into my presentation,” Palmore says. “The conference also led to some great conversations that could lead to new collaborations.”

A piece of the puzzle

Moving forward, the lab will study the atomization process of transcritical liquid streams.

The UW lab’s work is an important piece of the NASA project, which Palmore compares to a jigsaw puzzle. Each university collaborator is working on their own parts, and later they will fit them together to understand transcritical fuel atomization. Palmore’s lab does simulations, while Clemson University, led by Professor Yuhao Xu, and Cal State Fullerton, led by Professor Haowei Wang, do experiments.

“Simulations are usually better than experiments at generating certain types of data and vice versa, so having both gives a more comprehensive picture of what’s going on,” Palmore says.

For example, Clemson researchers previously did experiments on the International Space Station to investigate the burning of an bio-derived jet fuel. These experiments generated global data, like the droplet’s average temperature and volume. Using simulation, Palmore’s team extracted detailed information about the instantaneous droplet shape and generated localized field data for temperature and vapor fraction. This helps to understand how these quantities change in space and to explain how combustion starts.

SPARC is supported through the NASA program Minority University Research and Education Project Institutional Research Opportunity (MIRO). Palmore says the program creates unique opportunities for collaboration between students and faculty at different types of universities  such as the UW and Clemson (both R1 research universities) with Cal State Fullerton, a primarily undergraduate Hispanic-Serving Institution (HSI).