ASHRAE Journal - October 2021 - 32

TECHNICAL FEATURE
FIGURE 2 Distribution of particles with no portable air purifier. The yellow figure
represents the infected spreader.
FIGURE 3 Distribution of particles with portable air purifier located adjacent to the
seating area. The yellow figure represents the infected spreader.
or are pulled into the HVAC return air vent.
2. Based on the airflow patterns, HVAC supply and
return vent locations, the shape of the room and other
factors, there will generally be locations where the air
is not well mixed or moved, creating a potential for
areas with a higher concentration of fluid particles
(Figure 1, page 29).
3. The height of the air purifier also has an impact on
efficacy. When the air purifier is less obstructed, the
device inlet can treat more air in a set amount of time.
Experimentation shows that a purifier at tabletop level
is more effective than one on the floor level.
At a steady state, a well-designed and well positioned,
efficient air purifier can eliminate 99.97% of particulate
matter of 0.3 μm particles within a space in less than an
hour. When modeling a sneeze with an air purifier on
the tabletop next to the subject, fluid analysis shows the
air purifier is 80% effective in containing all the small
and medium particles, while the remaining large particles
were captured on the surface of furniture or pulled
down to the carpet due to gravity within a relatively tight
radius 1.8 m to 2.7 m (6 ft to 9 ft) of the subject (Figure 2).
Case Studies by Computational Fluid Dynamics Modeling
This study used a commercial computational fluid
dynamics (CFD) tool to predict the airflow pattern and
virus-carrying particle transport in a typical restaurant
dining area within a hotel. Since indoor airflow is
highly chaotic, a proper turbulence modeling method
and model are critical for accurate flow prediction. This
study used the steady-state RANS simulation method
with the RNG k-epsilon turbulence model,12 which
were proven to be effective and suitable for indoor
airflow simulation.13 The transient trajectories of particles
released from sources (e.g., mouths) were tracked
using the software's discrete particle model, with the
32
ASHRAE JOURNAL ashrae.o rg O CTO B E R 2021
TABLE 1 Restaurant specifications.
OBJECT
Diffuser Inlet
Dining Table
Exhaust Outlet
Occupant
Ceiling Light
Total Air Change Rate
Supply Air Velocity
DIMENSIONS
1.22 m × 0.30 m
0.87 m × 0.87 m × 0.75 m
0.61 m × 0.61 m
0.30 m × 0.43 m × 1.30 m
-
-
-
CONDITIONS
Tin = 17.6°C
Adiabatic
Tex = 20°C
T = 27°C
10.7 W/m2
5/h
0.3429 m/s
assumption of monodispersed noninteracting spherical
particles.
Due to the scarceness of virus particles in the space, the
momentum impact from the particles to the air turbulence
is negligible.14 Multiple numerical grids were tested
to ensure sufficient numerical accuracy and reasonable
computing cost. Approximately 650,000 numerical cells
were used for the CFD model, among which finer grids
were allocated for the occupants and purifiers.
Case Description
A typical separate restaurant dining area in a hotel
was modeled with the full dining capacity (177 people)
and the designed HVAC conditions (locations, sizes and
capacities) (as shown in Figure 3 and summarized in
Table 1).
A set of portable air purifiers were tested in the simulation
with various combinations of model, capacity,
number and layout. This article presents the results of
using two types of commercially available air purifiers
to demonstrate the influence of portable air purifiers
(Table 2). Eight floor air purifiers (FAP) were arranged at
both ends of the restaurant (five at one end and three
at the other end) (Figure 3), which drew the room air
from one side of the purifier and exhausted cleaned air
from the top of the unit. One table air purifier (TAP) was
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