ASHRAE Journal - June 2019 - 24

TECHNICAL FEATURE

FIGURE 3 Contours of normalized air velocity at the central plane for isothermal conditions show velocity distribution is nearly independent of the supply airflow rate.

a) 200 cfm (94.4 L/s)

c) 600 cfm (283.2 L/s)

at much lower velocity (Figure 2). Such stagnant zones
can promote accumulation of contaminants with high
concentrations.
Nearly isothermal conditions can occur in the case of
high supply airflow rate (air change rate) cleanrooms
with relatively small sensible cooling loads. In such
situation perhaps the room layout including the height
of the space, height and locations of obstacles and the
HVAC configuration including the size, type, and locations of exhaust grilles as well as the number and locations of supply diffusers can impact the airflow patterns.
However, it seems for a certain layout of HVAC configuration the supply airflow rate would have a little impact
on the overall airflow patterns and the directionality of
the supply jet.

Non-Isothermal Analysis-Heat Source On The Table
The non-isothermal analysis was performed by placing
a heat source at the center on the table for four different
supply airflow rates. It indicates that the supply airflow
24

ASHRAE JOURNAL

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J U N E 2 0 19

b) 400 cfm (188.8 L/s)

d) 800 cfm (377.6 L/s)

rate or discharge velocity can significantly impact the
performance of a unidirectional air jet. Analysis results
are presented in Figures 4, 5, and 6 for the airflow patterns, velocity, and temperature distribution, respectively at the central plane location. These analyses show
significantly two different flow patterns: one for the
supply airflow rates up to 600 cfm (283.2 L/s) and the
other for the supply airflow rate for 800 cfm (377.6 L/s).
When the heat source is placed on the table the downward moving cold air encounters a resistance from the
upward moving hot buoyant plume. At the same time
the air surrounding the heat source gets entrained into
the upward moving hot plume and lowers its temperature. As this hot plume gets colder it cannot sustain the
upward motion and starts falling sideward. In the case
of 200 cfm (94.4 L/s) supply flow rate, the weak supply air jet cannot sustain the downward unidirectional
motion and starts spreading sideward right after the discharge. It is evident from the temperature distribution
as shown in Figure 6a that the zone of low temperature


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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
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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
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ASHRAE Journal - June 2019 - 24
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ASHRAE Journal - June 2019 - 32
ASHRAE Journal - June 2019 - 33
ASHRAE Journal - June 2019 - 34
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ASHRAE Journal - June 2019 - 36
ASHRAE Journal - June 2019 - 37
ASHRAE Journal - June 2019 - 38
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ASHRAE Journal - June 2019 - Cover3
ASHRAE Journal - June 2019 - Cover4
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