PORTLAND, Ore. — In small rooms, body heat may draw particles to all the wrong places. Thermal plumes radiating off a person can waft microbes, pollen and dust into breathing range, a study presented March 16 at a meeting of the American Physical Society finds.
Understanding how body heat affects particle motion may help engineers design airflow systems that minimize particle exposure, said study coauthor John McLaughlin of Clarkson University in Potsdam, N.Y.
“One of the conventional ideas about thermal plumes was that hot air rising up over a body will protect you from having particles fall down on top of you,” McLaughlin says. “But in small rooms, it’s exactly the opposite.”
The researchers created a computer simulation of a human form sitting in the middle of a 4.8-square-meter room as 1,000 particles flooded in through an air vent. Particles were 10 micrometers in diameter — the size of some flu-carrying saliva droplets that launch out of a sneeze or cough. Air left the room through a ceiling vent above and behind the virtual body.
When the body’s surface temperature was 25 degrees Celsius — about the same as a resting person’s clothes — some particles whooshed up to the 2.4-meter-high ceiling, bounced back and then collected over its head, pinned in place by a thermal plume. When the body was at room temperature, though, fewer particles hovered directly above it. At the end of the simulation, 29 particles had landed on the heated body, but only one had set down on the unheated body. The researchers got similar results with smaller 2-micrometer-diameter particles, McLaughlin said. Like the warm air above a radiator, McLaughlin said, body heat “has a big effect on the motion of the particles in the room.”
McLaughlin cautioned that the findings are preliminary, and more detailed simulations are needed to verify the result. He and his colleagues are developing more realistic models that incorporate breathing, because hot air expelled into a room might change the particles’ motion significantly.
Different researchers at Syracuse University are currently conducting experiments to study particle motion around bodies in small spaces, McLaughlin said. Ultimately, his simulations will be compared to those experimental results, he said.
Simple numerical simulations like these are important for understanding how particles move in more complex situations, commented fluid dynamics expert Marcel Ilie of the University of Central Florida in Orlando. “Modeling plays a key role in understanding the physics. We start with the small scale and then extrapolate to the large scale,” he said.
