ASHRAE Journal - June 2024 - 48

TECHNICAL FEATURE
FIGURE 2 Variation of concentration in the exhaust duct with and without DCV shows initiation and termination time of DCV
for a total of 12.5 minutes for 12 ach.
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2
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2
Without DCV
With DCV
6 ach
12 ach
6 ach
the four-way diffusers was
adjusted to obtain 3 ft (0.9 m)
throw for a terminal velocity
of 150 fpm (0.76 m/s). The
room was operated at slightly
negative pressure and the
makeup air was supplied
through the gap under the
room door. The makeup airflow
rate was maintained at 100 cfm
(47.2 L/s).
High ach -12.5 minutes
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Time (minutes)
(PT)TC are introduced to evaluate the effectiveness of DCV
systems. The impact of DCV is analyzed by comparing
the CFD results with and without DCV under identical
design and operating conditions.
Virtual Laboratory Setup
A three-dimensional, transient, isothermal
computational fluid dynamics (CFD) model of a
laboratory was developed for this study. The laboratory
has about 660 ft2 (61.3 m2) of floor area (30 ft × 22 ft
[9 m × 6 m]) with 9.5 ft (2.9 m) ceiling height. As shown
in Figure 1 (page 47), the virtual laboratory has three rows
of workbenches and three occupants located at various
locations. The outdoor air was supplied through a set
of three four-way overhead diffusers located over the
work benches. It is also common to use radial diffusers
in laboratories. The diffuser type can affect the airflow
patterns. This setup is similar to the previous field test
laboratory setup.5 Traditionally this type of diffuser
is used to enhance the mixing. The room air was
exhausted through the two ceiling grilles. The chemical
spill was simulated as a source of a hypothetical
chemical vapor with a constant release rate of 7 mL/s
(7 × 10-6 m3/s) for 12 minutes. This release rate is similar
to the previous field tests conducted in a laboratory.5
The vapors are assumed to be neutrally buoyant. As
shown in Figure 1, the source of the contaminant is placed
on the bench in front of Person 2.
The transient analysis was performed for a total
duration of 30 minutes with the normal supply airflow
rate of 6 ach (590 cfm [278 L/s]). The discharge area of
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Virtual sensors were placed
in the model in front of each
occupant's face to monitor
time-varying concentrations
at each location. The chemical
exposure (dose) for each occupant was computed by
the cumulative product of the chemical concentration
(ppm) at each sensor location and the duration of the
exposure (min). The standard k-e turbulence model
was used to compute the turbulent viscosity of the air. A
computational mesh of 1.2 million hexahedral mesh was
created by placing fine mesh near the strategic locations.
The present study was performed for isothermal
conditions. The strength and location of heat sources
can impact the airflow patterns.
During the analysis, the average concentration at
the two exhaust grilles was monitored at every time
step. This concentration was assumed to represent
the concentration in the exhaust duct. As shown in
Figure 2, when the exhaust duct concentration exceeded
5 ppm (an arbitrary value), the supply airflow rate was
increased from 590 cfm (278 L/s, 6 ach) to 1,180 cfm
(557 L/s, 12 ach). The supply airflow rate remained at
12 ach until the exhaust duct concentration reduced to
the 5 ppm level. At this time the supply airflow rate was
reduced back to 590 cfm (278 L/s, 6 ach).
As shown in Figure 2 during the DCV phase, the lab
was operated at a high supply airflow rate of 12 ach for
12.5 minutes. The delay in ramping up and ramping
down the fan speed can occur due to several factors
including the control system lag and the type, location
and number of sensors, which in turn can affect the
performance of DCV systems. Such delays are not
considered in this analysis.
Breathing Zone Spread Index (SI)BZTC. This is a
CFD-based metric developed to analyze and quantify
Concentration (ppm)

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ASHRAE Journal - June 2024

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