“There are sets of conditions where we found clearly there’s a problem, and when you look at the predicted concentration of aerosols around other people in the room, in some cases it was much higher than what the (standard) models would say,” said Leon Glicksman, an MIT architecture and engineering professor.
‘Given ceiling ventilation, then, the aim is to create upward vertical air movement to cycle air out of the room, while limiting horizontal air movement, which spreads aerosols among seated students.’
For the study, appearing online in the journal Building and Environment, the researchers used computational fluid dynamics — sophisticated simulations of air flow — to examine 14 different classroom ventilation scenarios, nine involving HVAC systems and five involving open windows. The research team also compared their modelling to past experimental results.
One ideal scenario involves fresh air entering a classroom near ground level and moving steadily higher, until it exits the room through ceiling vents. This process is aided by the fact that hot air rises, and people’s body warmth naturally generates rising “heat plumes,” which carry air toward ceiling vents, at the rate of about 0.15 meters per second.
This is why wearing masks indoors makes sense, Glicksman said. Masks limit the horizontal speed of exhaled aerosols, keeping those particles near heat plumes so the aerosols rise vertically, as the researchers observed in their simulations. Normal exhaling creates aerosol speeds of 1 metre per second, and coughing creates still higher speeds — but masks keep that speed low.
But, with closed windows and HVAC use, airflow problems emerged in a simulated classroom in winter, with cold windows on the side. In this case, because the cold air near the windows naturally sinks, it disrupts the overall upward flow of classroom air, despite people’s heat plumes.
In this scenario, someone infected with Covid-19 sitting near a window would be particularly likely to spread their aerosols around. But there are fixes for this problem: Among other things, placing heaters near cold windows limits their impact on classroom airflow, Glicksman said.
Source: IANS
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