ASHRAE Journal - June 2019 - 20

TECHNICAL FEATURE

Dynamics of
Unidirectional Airflow
BY KISHOR KHANKARI, PH.D., FELLOW ASHRAE

Unidirectional airflow (often referred to as "laminar" airflow) is employed in cleanrooms, hospital operating rooms, laboratories, and in other applications where a high
level of cleanliness is desired. In such cases, laminar flow diffusers (Group E outlets1)
in combination with high-efficiency particulate air (HEPA) filters are used for air
distribution. Reduced free area (less than 35%) of these diffusers helps in maintaining
uniform discharge velocity across the face of the diffuser with a unidirectional downward flow for the ceiling mounted units.2 Such unidirectional flow should ideally
maintain its directionality as it moves downward without significant recirculation,
mixing, and entrainment from the surrounding air. Such mixing airflows are usually
referred to as "turbulent" airflows. Perfectly unidirectional flows are rare in HVAC
applications. Nearly unidirectional flow can be obtained only under limited situations
in which air is supplied at high flow rate through the entire ceiling of a small chamber
with restraining barriers surrounding the array of diffusers such as in a mini-environment or a glove box chamber.3
However, in practice, perfectly unidirectional flows
are difficult to maintain for large spaces. Airflow
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 cleanliness and sterility of a
space.
High sensible heat loads such as in the case of hospital
operating room can create non-isothermal conditions
with large temperature difference between the cold
supply air and the surrounding hot air. Such thermal
gradients can cause entrainment of the surrounding air
from the contaminated zone into the clean sterile zone
Kishor Khankari, Ph.D., is the president at AnSight LLC in Ann Arbor, Mich.
20

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which can further result in the acceleration of the cold
air jet.4 Unlike cleanrooms in a hospital operating room,
the air is supplied at relatively low velocities (25 to 35
fpm [0.13 to 0.18 m/s]) and only through a part of the
ceiling over the sterile zone. Previous studies indicated
that the unidirectional flow can be maintained mostly
in the core of the air jet; whereas, in the other locations
recirculating airflow patterns are formed, including at
the periphery of the supply air jet.5,6 This "semi-unidirectional" downward air jet from the ceiling mounted
laminar diffusers accelerates as it approaches the surgical table. Previous studies indicate that increasing the
supply airflow rate can significantly reduce the thermal
gradients which in turn reduces relative acceleration of
the centerline velocity of the air jet.5 A subsequent study


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ASHRAE Journal - June 2019

Table of Contents for the Digital Edition of ASHRAE Journal - June 2019

Contents
ASHRAE Journal - June 2019 - Intro
ASHRAE Journal - June 2019 - Cover1
ASHRAE Journal - June 2019 - Cover2
ASHRAE Journal - June 2019 - 1
ASHRAE Journal - June 2019 - Contents
ASHRAE Journal - June 2019 - 3
ASHRAE Journal - June 2019 - 4
ASHRAE Journal - June 2019 - 5
ASHRAE Journal - June 2019 - 6
ASHRAE Journal - June 2019 - 7
ASHRAE Journal - June 2019 - 8
ASHRAE Journal - June 2019 - 9
ASHRAE Journal - June 2019 - 10
ASHRAE Journal - June 2019 - 11
ASHRAE Journal - June 2019 - 12
ASHRAE Journal - June 2019 - 13
ASHRAE Journal - June 2019 - 14
ASHRAE Journal - June 2019 - 15
ASHRAE Journal - June 2019 - 16
ASHRAE Journal - June 2019 - 17
ASHRAE Journal - June 2019 - 18
ASHRAE Journal - June 2019 - 19
ASHRAE Journal - June 2019 - 20
ASHRAE Journal - June 2019 - 21
ASHRAE Journal - June 2019 - 22
ASHRAE Journal - June 2019 - 23
ASHRAE Journal - June 2019 - 24
ASHRAE Journal - June 2019 - 25
ASHRAE Journal - June 2019 - 26
ASHRAE Journal - June 2019 - 27
ASHRAE Journal - June 2019 - 28
ASHRAE Journal - June 2019 - 29
ASHRAE Journal - June 2019 - 30
ASHRAE Journal - June 2019 - 31
ASHRAE Journal - June 2019 - 32
ASHRAE Journal - June 2019 - 33
ASHRAE Journal - June 2019 - 34
ASHRAE Journal - June 2019 - 35
ASHRAE Journal - June 2019 - 36
ASHRAE Journal - June 2019 - 37
ASHRAE Journal - June 2019 - 38
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ASHRAE Journal - June 2019 - 40
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ASHRAE Journal - June 2019 - Cover3
ASHRAE Journal - June 2019 - Cover4
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