ASHRAE Journal - October 2021 - 34

TECHNICAL FEATURE
placed at the center of each dining table, which drew
the room air from the entire round side of the unit and
exhausted the cleaned air from the top.
This study focused on talking-the main concern during
dining and ballroom events. Studies showed that
talking may release 2,600 droplets per second at a speed
of 1 m/s to 5 m/s (197 fpm to 984 fpm).15 The range of the
total airflow rate from a mouth when speaking is about
284 cm3/s to 759 cm3/s (0.60 cfm to 1.61 cfm).16 Using
the average 500 cm3/s (1.06 cfm) and assuming mouth
opening area at 1.8 cm2 (0.28 in.2) leads to an average
talking airflow speed of 2.77 m/s (545 fpm), which meets
the particle image velocimetry test result at the order of
3.1 m/s (610 fpm).17
The study simulated multiple talking persons at different
locations, while this article only
presents the results from one " pollutant
source " as shown in Figure 3. Table 3 summarizes
the simulated virus particle details.
The number of virus particles released
from the mouth was assumed as 5,000 to
ensure that the deviation of the particle
statistical results is less than 1%.18
Particles larger than 10 micron tend to
drop quickly, while smaller particles tend
to flow with air, and 3 micron represents a
mean airborne particle size during talking.19
Sensitivity studies were conducted in CFD,
which indicates that particles with sizes
less than 10 micron present good airborne
behaviors; thus 3 micron particles were modeled.
Y
17.98 m
Z
X
Floor Air Purifier
(8 Total)
Pollution Source
Performance Analysis of Different Ventilation Conditions
Indoor airflow patterns in public spaces such as restaurants
are extremely complicated, influenced by many
factors such as HVAC supply and return locations and
conditions as well as indoor object layouts and adjacent
space conditions. Occupant movement provides additional
disturbance to the air distribution, which was not
considered here-typical for system design/evaluation
purposes. Note that occupant movements (e.g., the head
direction swing during talking) can play an important
role in actual contaminant dissipation.
The selected results illustrate the different performances
under three ventilation scenarios: (1) central
air-conditioning system (CA), (2) central air-conditioning
system with floor air purifiers (CAF) and (3) central
34
ASHRAE JOURNAL ashrae.o rg O CTO B E R 2021
air-conditioning system with both floor and table air
purifiers (CAFT). Figure 4 shows the predicted air velocity
vectors at the height of the breathing zone (Z = 1.1 m
[3.61 ft] above the table with the virus carrier. The installation
of portable air purifiers changes the flow directions
as expected and mitigates the cross-table airflow
that may cause the cross-infection. The table unit displays
explicit and favorable air inflow toward the purifier.
As a result, most of the particles released from the
source mouth flow with the air and enter the purifiers
(Figure 5). The strong upward outflow from the purifiers
also helps move the particles toward the ceiling where
the central HVAC exhausts are located.
Table 4 quantifies the ventilation performance by comparing
the destination of particles. Numbers of particles
that were, respectively, discharged from the central
exhausts, or portable purifiers, or deposited on different
surfaces were counted and computed in percentage (out
Occupant
(177 Total)
TABLE 2 Purifier specifications.
UNIT
Dimensions
Inlet Size
Outlet Size
Clean Air Delivery Rate
FLOOR AIR PURIFIER
0.56 m × 0.33 m × 0.61 m
0.1291 m2
0.0452 m2
0.156 m3/s
TABLE 3 Particle release conditions.
OBJECT
Open Area of Talking Mouth16
Airflow Rate From Talking Mouth20
Temperature of Airflow From Talking Mouth
Aerodynamic Diameter of Particle21
Density of Particle22
Number of Particles Released From Talking Mouth18
FIGURE 4 Computer model of a restaurant.
Diffuser Inlet
(20 Total)
Exhaust Outlet
(8 Total)
8.08 m
3.05 m
1.52 m
VALUE
1.8 cm2
500 cm3/s
27°C
3 mm
600 kg/m3
5,000
TABLE AIR PURIFIER
0.42 m × 0.2 m × 0.2 m
0.1935 m2
0.0316 m2
0.056 m3/s
29.26 m
http://ashrae.org
ASHRAE Journal - October 2021

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