ASHRAE Journal - June 2023 - 52

TECHNICAL FEATURE
TABLE 1 Summary of results.
PARAMETER
Total UV-C output (W) from UV fixtures
Volumetric UV-C input (mW/m3)
Space average UV-C irradiance (µW/cm2)
Average UV-C irradiance at the central plane of UV fixtures (µW/cm2)
Average UV-C irradiance at the breathing plane (µW/cm2)
Average mean age of air (s) at the central plane of UV fixture
Average UV-C dose at the central plane of UV fixtures (J/m2)
Space average UV-C dose (J/m2)
Average probability of infection at the breathing plane (%)
Minimum probability of infection at the breathing plane (%)
Spread Index (%)
Survival fraction at the breathing plane
UV-C effectiveness at the breathing plane
Equivalent Air Changes per Hour (ACH, 1/h)
a space. The present study demonstrates that the risk
of infection depends on the location of occupants and
HVAC configuration of the space.
In the present work, the number of infectors (I)
was assumed to be one and the exposure time (t) was
assumed to be one hour. The quanta generation rate
for influenza varies from 2 to 128 quanta per hour.
The q value of 60 quanta per hour is assumed, which
is most frequently used in the ventilation analysis.5,10
The release of quanta is assumed to be continuous,
and transient events like coughing or sneezing are not
part of these analyses.
Results and Discussion
Airflow Patterns
Figure 2a shows the airflow patterns in the space in
two orthogonal vertical planes passing through the
center of a supply diffuser. The air exiting the diffuser
travels along the ceiling, descends along the walls and
moves inward along the floor away from the walls.
Such airflow patterns create large recirculation loops
along and across the room. These large recirculation
zones form inside the cubicles between the room
walls and the dividing partitions in the occupied
52
ASHRAE JOURNAL ashrae.o rg
J U N E 2023
No UV
1045
11.3
7.1
49.4
1
3.0
0.35
4.6
0.3
2.0
0.075
1045
20.9
3.04
8.3
4.9
16.7
0.79
0.21
4.1
0.7
9.2
0.6
4.0
0.15
1045
41.8
6.08
6.9
4.0
12.0
0.64
0.36
5.4
1
13.1
0.86
5.7
0.21
1045
59.8
8.7
6.1
3.4
10.0
0.56
0.44
6.6
1.5
19.6
1.3
8.6
0.32
1045
89.7
13.0
5.0
2.7
7.3
0.45
0.55
9.0
zone. Three-dimensional airflow patterns (not shown
here) are quite complex and promote mixing in the
entire space as expected from such diffusers.
Figure 2b shows the resulting distribution of the
mean age of air [s] at the center plane. Under ideal
conditions for a single pass flow, for the 3 air changes
per hour (h-1) supply airflow rate, the average mean
age of air should be 1200 s. However, recirculation
zones, which also promote stagnant zones, cause nonuniform
distribution of the mean age of air. The large
air recirculation zones in the second cubicle, which
was close to the door, form a stagnation zone in which
the mean age of air is higher than the average value.
Higher values of the mean age of air can increase the
residence time of the airborne pathogens, which in
turn can result in a higher dose of UV-C.
Distribution of UV-C Intensity and Dose
Figure 3 shows the distribution of UV-C irradiance
at the central plane along the long side of the room
for various UV-C outputs. As shown in Figure 3, the
highest UV-C intensity occurred close to the fixtures
and gradually reduced toward the center of the room.
Since only 5% of the UV-C radiation was reflected
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