STBE Researcher Q&A

Controlling Airborne Contaminant Exposure in Passenger Aircraft Cabins

From ASHRAE Journal Newsletter, November 10, 2020

Passenger aircraft may be the most effective way to spread infectious germs worldwide, says a University of Guelph researcher.

Two recent articles published in Science and Technology for the Built Environment discuss airborne contaminants inside commercial aircraft cabins.

In a Q&A with ASHRAE Journal, researcher Amir Abbas Aliabadi discusses the two articles: “Airflow Design and Source Control Strategies for Reducing Airborne Contaminant Exposure in Passenger Aircraft Cabins during the Climb Leg” and “Normal and Extreme Aircraft Accelerations and the Effects on Exposure to Expiratory Airborne Contaminant Inside Commercial Aircraft Cabins.”

1. What is the significance of this research?

Passenger aircraft may be the most effective way to spread infectious germs worldwide. Using aircraft, people are displaced thousands of kilometers in a matter of hours. Further, the cabin space may provide a fertile ground for cross human-human infections via the surface contact and/or airborne routes.

Throughout our studies we realized that aircraft cabin HVAC systems are not designed to mitigate the risk of airborne infection spread effectively. The ventilation approach promoted by the aircraft ventilation code in most cabins is based on a high dilution of the air using the well-mixed air ventilation strategy, where the cabin air is circulated and refreshed at a high rate to remove the contaminant.

Our studies, however, showed that the high mixing of air in the cabin can result in dispersion of contaminant throughout the space within minutes, exposing passengers to infection. On the other hand, we identified a need in aircraft ventilation design to understand detailed air movement and adaptation of control measures to minimize dispersion of airborne contaminant.

2. How does this research further the industry's knowledge on this topic?

Using computational fluid dynamics (CFD) analysis, for instance, we could predict movement of air in the cabin in connection to the HVAC system design parameters. Various control measures could be implemented by aircraft ventilation designers:

• The layout of the cabin, the proximity of the seats and aerosol barriers could be adapted to minimize dispersion of airborne contaminant;
• Various modes of ventilation strategies such as mixing ventilation, displacement ventilation, and personalized ventilation could be optimized;
•  The air exchange rate could be adjusted;
• Air diffuser and exhaust layouts could be optimized;
• Air inlet velocity and injection orientation could be tuned;
• Various filtration solutions could be considered, such as the use high efficiency particulate arrestance (HEPA) filters and careful recirculation of air in the cabin; and 
• Positioning of an infected person with symptoms in the cabin could be carefully planned to reduce airborne infection risk.

3. What lessons, facts and/or guidance can an engineer working in the field take away from this research?

Engineers studying our work can realize that the simple bulk measures (assuming air flow, and contaminant properties are the same everywhere) in HVAC design for aircraft cabins, such as the well-mixed ventilation strategy, may not always work. Engineers should carefully study detailed airflow in cabin spaces and adapt their designs accordingly to mitigate the risk of airborne infection spread.

Further, a variety of aircraft cabin designs exist, given the airliner fleet of aircraft. This is true even within the same aircraft: first class, premium economy and economy seats, each of which demand separate analysis and optimization for HVAC design.

Another area of investigation inspired by our research was the effect of body forces due to aircraft acceleration on contaminant dispersion. We found that when aircraft accelerate during climb or descent conditions, the air circulation patterns in the cabin could change in comparison to steady flight with no acceleration, with implications on airborne infection risk, which need to be considered and mitigated by HVAC design.

4. Were there any surprises or unforeseen challenges for you when preparing this research?

The most interesting surprise and challenge for our study was building a CFD model for the cabin HVAC system that represented reality. Of most significance were proper choices for the heat transfer model, turbulence model, contaminant dispersion model, and boundary condition treatment. We had to iterate through many model options to evaluate our model performance against experimental measurements and show good agreement between the model predictions and the experiments.

5. What are the next steps to further this research?

Various next steps are required to operationalize this research for real aircraft HVAC designs.

As stated earlier, an abundance of aircraft models and cabin designs exist. The HVAC system in each of the configurations should be optimized to mitigate airborne infection risk. This requires separate analysis for each design.

Our simulations simplified the contaminant dispersion in the cabin by assuming that the contaminant was in the gaseous phase, while the real contaminant is in the particulate phase with different sizes and densities, which trigger different dispersion behavior driven by gravitational settling, turbulent diffusion and other mechanisms. Future work should simulate particle dispersion to understand the dispersion forces and the physics behind dispersion in more detail.

Of most importance is experimental measurements of HVAC performance in real conditions. It is imperative to release and measure surrogate contaminant in aircraft cabins while flying under operational conditions. The HVAC designs can be further tuned and optimized using such ground-truth measurements.