ASHRAE Journal - July 2022 - 12

TECHNICAL FEATURE
FIGURE 2 Cough particle size distribution on a semi-log scale. Adapted
from Zayas.14
100
10
1
0.1
0.01
0.001
0.0001
0.00001
0.1
1
Initial Diameter (µm)
may not have been well represented even in the original
studies.
Riley9 discusses the mix of recirculated to outdoor airflow
varying from 28.6% to 100% of total flow supplied to
each room depending on the outdoor temperature for
the school building studied. The ventilation in an ICS
may also be on or off depending on airflow and temperature
requirements. The filtration used for recirculated
air was relatively ineffective for viruses-particularly
measles. The filters in the study only had an efficiency
between 12%-30% (depending on school wing). In addition,
the Wells-Riley approach assumes a transmission
from an index occupant to a susceptible occupant to
have taken place in the school, while ignoring other
potential transmission environments, such as the school
bus, and is applied retrospectively.
While studies have attempted to use the Wells-Riley
model, there are a few issues with the applicability of
any well-mixed model assumption for the airplane environment,
particularly with respect to airflow and filtration.
Since the airflow in an aircraft is predominantly
downward, with each row of seats having multiple air
inlets near the ceiling, and multiple air outlets near
the floor, as seen in Figure 1, the aircraft is more representative
of a mixed-flow, advection-dispersion model
(advection: airflow driven; dispersion: concentration
gradient-driven) than a well-mixed model. In a typical
commercial building environment, where people
are present and aerosols are not driven in an overall
downward direction, an instantaneous and well-mixed
assumption is more reasonable than in the airplane
cabin environment, but still an imperfect assumption
due to stagnation points.
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An example advection-dispersion model is below,
where D is the dispersion coefficient, v = velocity and C =
concentration:
∂
∂
C
t
=


DD D
2
xx yy z
∂
∂
C
2 − v
x
∂
∂
C
x


+


∂
∂
2
C
2 − v
y
∂
∂
C
y


+


∂
∂
2
C
2 − vvz
z
∂
∂
C
z


For an airplane cabin, the deterministic term is the
velocity in the z direction.
The assumptions for outdoor and recirculated air
studied by Riley cannot be applied to the outdoor and
recirculated air in an aircraft due to effective filtration
in an aircraft. In an ICS, air recirculated through low
MERV-rated filters can spread contaminants throughout
the indoor space. In an aircraft, the effectiveness of the
HEPA filters prevents the recirculation of particulate
contaminants such as viruses.10 As a result, the lower
an indoor space's filtration efficiency, the more a wellmixed
model assumption might apply as the ventilation
systems, designed to maintain even temperature distribution,
recirculate particulates not filtered out.
Commonly Used Models for Aircrafts
In all environments, but especially the aircraft cabin
environment, there is a significant difference between
the behaviors of gases and particulates. HEPA filters
provide essentially particulate-free air, while gases (e.g.,
CO2) pass directly through the HEPA filter to be recirculated
into the cabin. In addition, airflow does not separate
gases. The study of the behavior of gases in the aircraft
interior are thus not applicable to the study of particles.
The concentrations of gases emitted by passengers
may be higher than those of particulates because gases
are not removed by HEPA filters. The continuous downward
airflow gradient (unlike the intermittent airflow in
some buildings) keeps the time required to remove particles
within the breathing zone of aircraft passengers
short, and consequently there is no long-lasting, high
concentration of aerosol particles in that zone.
Methods
CFD Overview
To compare the lifespan and movement of particles
within an airplane cabin11 to an ICS, computational
fluid dynamics (CFD) studies were performed tracking
particles released from a single cough and from sinusoidal
tidal breathing. Both the cough and breathing were
Particle Count (millions)
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ASHRAE Journal - July 2022

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