ASHRAE Journal - July 2021 - 16

TECHNICAL FEATURE
of about 1.5 million hexahedral cells was created by placing
fine mesh near the strategic locations. The ventilation
effectiveness of each configuration was evaluated
using the spread index as described later. The release of
contaminant from an infected individual is simulated as
a passive source of a hypothetical gaseous component.
The probability of infection is evaluated using the WellsRiley
correlation.
The Eulerian approach, which assumes contaminants
as gaseous components, was used to model the transport
of contaminants, and the rate of release was computed
using the input for the Wells-Riley equation as described
later. The probability of infection and its distribution
in the space is evaluated at the breathing level of 4.25 ft
(1.3 m) from the floor.
These analyses were performed by keeping the supply
airflow rate and the contaminant release rate
(60 quanta/h) the same for both cases. A steady-state
analysis performed in this study represents the worstcase
scenario of constant release of contaminants from
an infected individual. The supply air is assumed to be
clean and free of any infectious aerosols. A commercial
software (Ansys Fluent) was used to perform the
computations.
Spread Index
Spread index is a CFD-based ventilation effectiveness
metric. It is a ratio of the space volume occupied
by the contaminated air at or above a certain threshold
concentration to the total volume of the space.3 Ideally,
the airflow patterns formed by the HVAC layout should
minimize the spread of contaminants and reduce the
probability of infection everywhere in a space. Assuming
the target concentration (TC) is a safe exposure limit
for a certain contaminant, ideally the spread index SITC
should be close to zero everywhere and every time in
a space. The safe level of concentration can depend on
several risk factors including the type of contaminants
or pathogens in a space and their safe exposure limits.
Therefore, for each space, SITC can be evaluated for various
levels of target concentrations (TC) and infection
levels based on the exposure risk.
The design of an HVAC layout and the resulting flow
path of airborne contaminants can play an important
role in determining the SITC levels in a space.2,3 The
spread index thus provides a normalized metric to compare
the ventilation effectiveness of various HVAC layouts.
16
ASHRAE JOURNAL ashrae.org J U LY 2021
In this study, the spread index of the probability of infection
is set arbitrarily at 10%; hence, SI10 is evaluated.
Probability of Infection
Infection risk assessment is performed using the
Wells-Riley model, which has been extensively used for
quantitative infection risk assessment of respiratory
infectious diseases in indoor spaces.5,6 This model as
stated in Equation 1 considers the intake dose of airborne
pathogens in terms of the number of quanta to evaluate
the probability of infection.
P
I = =− −

C
S
1 exp
Iqpt
Q


where
PI = Probability of infection, which is a ratio of the
number of infection cases (C) to the number of
susceptible (S)
I = Number of infectors
p = Pulmonary ventilation rate of a person
q = Quanta generation rate
t = Exposure time interval
Q = Room ventilation rate with clean air.
Unlike contaminant dilution theory, this equation
evaluates the statistical probability of infection; hence,
the exponential term is not a dimensionless number.
This equation assumes the supply air is evenly distributed
in the entire space, and, thus, predicts a single
number for the infection probability for each space.
In a real situation, the spatial and temporal variations
of airflow patterns in a space can result in a nonuniform
airflow distribution, which in turn can yield a nonuniform
distribution of infection probability. This study
demonstrates that the risk of infection depends on the
location of each individual and the HVAC configuration
of the space.
In the present study, the number of infectors (I) is
assumed to be a single person, and the exposure time (t)
is assumed to be one hour. The quanta generation rate
for influenza varies from 2 to 128 quanta/h. The q value
of 60 quanta/h is assumed, which is frequently used in
the ventilation analysis.5,6
Results and Discussion
Case 1: Single Supply and Single Return
Figure 2 shows the airflow patterns at two vertical
planes in the space passing through the center of a
(1)
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ASHRAE Journal - July 2021

Table of Contents for the Digital Edition of ASHRAE Journal - July 2021

Contents
ASHRAE Journal - July 2021 - Intro
ASHRAE Journal - July 2021 - Cover1
ASHRAE Journal - July 2021 - Cover2
ASHRAE Journal - July 2021 - 1
ASHRAE Journal - July 2021 - Contents
ASHRAE Journal - July 2021 - 3
ASHRAE Journal - July 2021 - 4
ASHRAE Journal - July 2021 - 5
ASHRAE Journal - July 2021 - 6
ASHRAE Journal - July 2021 - 7
ASHRAE Journal - July 2021 - 8
ASHRAE Journal - July 2021 - 9
ASHRAE Journal - July 2021 - 10
ASHRAE Journal - July 2021 - 11
ASHRAE Journal - July 2021 - 12
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ASHRAE Journal - July 2021 - 14
ASHRAE Journal - July 2021 - 15
ASHRAE Journal - July 2021 - 16
ASHRAE Journal - July 2021 - 17
ASHRAE Journal - July 2021 - 18
ASHRAE Journal - July 2021 - 19
ASHRAE Journal - July 2021 - 20
ASHRAE Journal - July 2021 - 21
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