ASHRAE Journal - June 2019 - 38

TECHNICAL FEATURE

These calculations indicate that in both cases the buoyant force would influence the airflow patterns in the OR.
However, previous CFD analysis of the operating room
indicated that increasing the supply airflow rates (discharge velocity) helps in reducing the thermal gradients
between the sterile and non-sterile zone, which in turn,
reduces the acceleration in the discharge velocity and
the potential entrainment of air from the non-sterile
zone into the sterile zone.
The Ar values for the various non-isothermal cases
analyzed in this study are presented in Table 1. Increasing
the discharge velocity (increasing the mass flow rate
of the supply air) reduces theoretical temperature difference between supply and return temperature (DT),
which in turn results in lowering the Ar. As evident from
the above non-isothermal analyses at low Ar (200 cfm
[94.4 L/s]) the flow field, temperature distribution, and
entrainment were dominated by the hot plumes due
to the heat sources. Only when the Ar number was less
than 1 such as in the cases of 600 (283.2 L/s) and 800 cfm
(377.6 L/s), the downward supply air jets could overcome
the buoyant forces. Please note in these cases cooling
capacities of the supply air were three and four times
that of the respective cooling loads. It is also evident
from these analyses that predominantly unidirectional
flows can be obtained when the Ar number is smaller
than 0.2.
As shown in the above example, a calculation of
Archimedes number during early stages of HVAC design
can provide an estimate about the directionality of the
downward flow, especially when most of the sensible
heat sources are located within the breathing zone of 4
to 6 ft (1.2 to 1.8 m) above the floor. It should be noted
that such calculation of Ar assumes the flow is wellmixed and the temperature distribution in the space is
uniform. However, this analysis indicate that local thermal gradients can significantly affect the directionality
and entrainment of the unidirectional flow. The layout
of the room and locations of the exhaust grille can affect
these thermal gradients. Previous analysis of a hospital
operating room demonstrated that the thermal gradients and the acceleration in the centerline velocity can
be reduced by replacing the low wall exhaust grilles by
the ceiling grilles.6 In such cases in-depth CFD analysis
can provide valuable insights into the flow pattern and
directionality of the supply air jet.

38

ASHRAE JOURNAL

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FIGURE 10 Variation of normalized centerline velocity along the normalized dis-

tance from the laminar diffuser.

a) Isothermal

b) Non-Isothermal Heat Source on Table

c) Non-Isothermal Heat Source Near Ceiling
200 cfm (94.4 L/s)
600 cfm (283.2 L/s)

Summary

400 cfm (188.8 L/s)
800 cfm (377.6 L/s)

Unidirectional ("laminar") flows are often employed
in cleanrooms, hospital operating rooms, laboratories,
and similar other spaces where high level of cleanliness
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ASHRAE Journal - June 2019

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