ASHRAE Journal - June 2019 - 4

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EDITORIAL
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Jay Scott
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ASHRAE JOURNAL

Jay Scott

Studying Unidirectional Airﬂow
Unidirectional airﬂow (often referred
to as "laminar" airﬂow) is used in
cleanrooms, hospital operating rooms,
laboratories, and elsewhere where a
high level of cleanliness is desired.
Perfectly unidirectional ﬂows are
rare in HVAC applications, according to a technical feature this month.
Obstructions such as occupants, furniture, and equipment along with the
upward buoyant plumes from heat
sources can disrupt the directionality
of these airﬂows and form recirculation zones. Such turbulent ﬂows promote mixing and entrainment of the
surrounding air, which can compromise the sterility of a space.
THE AUTHOR SYSTEMATICALLY
analyzed the behavior of unidirectional airﬂow with what he calls a
rather simple setup of a test chamber.
The CFD study evaluated the impact of
supply air velocity on the overall airﬂow patterns, temperature and velocity distribution under isothermal and
non-isothermal conditions. Also, the
entrainment of the surrounding air
was evaluated by studying the acceleration in the centerline velocity of the
supply air jet under these conditions.
Under isothermal conditions when
the cooling capacity of the supply air is
much higher than the sensible cooling
load in the space, such as in cleanrooms with high air change rates, the
supply airﬂow rate (discharge velocity)
has little impact on the directionality
of the laminar ﬂow.

When the ﬂow rate or the cooling
capacity of the supply air is much
lower than the sensible cooling load,
the discharge air cannot sustain the
downward unidirectional motion due
to upward resistance of the buoyant
plume from the heat sources. However,
such airﬂow patterns can form a protective layer of hot air surrounding the
heat sources and help create nearly
isothermal conditions in the space, the
author states,
WHEN THE COOLING CAPACITY
of the supply air exceeds about three
times the cooling load, the air jet can
sustain the downward unidirectional
motion. However, non-isothermal
conditions surrounding the supply
diffuser, created by signiﬁcant lighting load near the ceiling or thermal
stratiﬁcation in the space, can cause
entrainment and acceleration of the
supply air jet.
This study found that with ceilingmounted laminar diffusers the local
thermal gradients can signiﬁcantly
affect the directionality of laminar ﬂow
and entrainment from the surrounding air. The room layout can affect the
airﬂow patterns and temperature distribution in the space, which in turn
affects the performance of unidirectional ﬂows. In-depth CFD analysis can
predict airﬂow patterns, temperature
distribution, and potential entrainment from the surrounding air, the
author concludes.
Enjoy the issue.

Mission Statement: ASHRAE Journal reviews current HVAC&R technology of broad interest through publication of application-oriented articles. ASHRAE Journal's editorial content ranges from back-to-basics features
to reviews of emerging technologies, covering the entire spectrum of professional interest from design and
construction practices to commissioning and the service life of HVAC&R environmental systems.
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ASHRAE Journal - June 2019

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