ASHRAE Journal - May 2020 - 16

TECHNICAL FEATURE

was selected based on the
FIGURE 2 CFD models for O.R.: a) MV design, b) UD design; 1: supply air (inlet); 2: return air (outlet); 3: operation table (wall);
opening-closing door pro4: medical staff (wall); 5: anesthesiologist; 6: equipment (wall); 7: door gap (opening); 8: contaminant source (volume source).
cedure of ASHRAE RP-1431,8
1
related to controlled envi1
ronments. A value of 16
4
seconds, close to this value,
6
2
6
3
was also used for one open3
2
ing-closing door cycle in a
similar study.15
2
5
5
4
The contaminant source
8
8
Y
Z
Z
Y
was sized to produce a sufX
X
7
7
A.
B.
ficient concentration for
the numerical simulations,
given the restrictions on CONTAM software precision.
Multizone models use the assumption that contamiThe purging performance of the two ventilation stratenant concentrations are homogeneously distributed
gies (UD and MV) was then assessed by a time-averaged
inside the room ("well-mixing" assumption), restricting
evaluation of the contaminant concentration on a region its applicability for this analysis. The use of the coupled
located at h = 0.1 m (4 in.) over the operation table. The
CFD-multizone model overcomes those limitations.
higher the result of this calculation, named here as max- One zone of the building may be modeled in detail
imum exposure dose (Dmax), the higher the probability
(CFD), while the others use the "well-mixing" assumpof an infection on the patient's wound caused by the
tion in this coupled solution. In this case, each interface
opening-closing door procedure. The transient simulabetween the CFD zone and other zones is represented as
tions were performed over 600 seconds for each case.
a node.
The comparative performance was then assessed by a
Information on airflow, pressure, temperature and
calculation of the relative maximum dose (Drelative), from contaminant concentration, stored at that node after
one scenario to the other, according to Equation 1.
calculation by the multizone model, are used as a
boundary condition for the CFD module. After solvD
Drelative = max,UD
(1)
ing the detailed flow field, the CFD module returns
Dmax, MV
the updated variables to the node interface with the
The same contaminant source was adopted for both
multizone model. An iterative solution is performed,
the cases. Parametric tests were conducted to ensure
where information of airflow rates, concentrations and
that the relative dose is independent of the contaminant pressures at the boundaries are exchanged between
source that was used, as long as both cases use the same
CONTAM and its CFD module in each step until a consource, and the related concentration does not modify
vergence criterion is reached. Reference 16 provides
the flow field. This methodology was adopted for this
detailed information on this method. For this analysis,
case study since the purpose of the analysis is not to pre- the coupled simulation was used (CONTAM -> CFD ->
dict the exposure dose, but rather to assess the relative
CONTAM option).
performance of two ventilation strategies on contamiOne of the ORs was selected for detailed calculanant purging. The selected time cycle of 10 seconds for
tion, and its CFD model is depicted in Figure 2. General
the contaminant burst is consistent with the time-scale
dimensions are 6 m × 4 m × 3 m (20 ft × 13.3 ft × 10 ft).
of the opening-closing door procedure of References 8
The UD configuration complies with ASHRAE Standard
and 15.
170-2017 and provides at least 305 mm (12 in.) of overlap
The numerical simulations were performed using a fil- beyond the table. The CFD simulations were performed
ter efficiency of 100% for the contaminant. This premise with non-isothermal conditions.
was modeled to focus on the room airflow purging perMesh Selection. A grid with 256,932 elements was
formance and given the high efficiency of HEPA filters
chosen since it provided no substantial variation in the
for particles the size of interest.
results and low computational time, a key factor for
16

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ASHRAE Journal - May 2020

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