ASHRAE Journal - June 2019 - 32

TECHNICAL FEATURE

FIGURE 7 Airflow patterns at the central plane for non-isothermal conditions with heat source near the ceiling show increasing supply airflow rates set unidirectional motion
for the discharge jet and form recirculation patterns.

a) 200 cfm (94.4 L/s)

c) 600 cfm (283.2 L/s)

downward unidirectional motion cannot sustain (Figure
10b). It shows that up to 600 cfm (283.2 L/s) supply flow
rate, the centerline velocity decreases until about 70%
of the distance from the diffuser where the downward
jet meets the upward plume from the heat source. This
reversal in the flow direction is indicated by negative
values of the centerline velocity. It should be noted
such point of intersection moves downward with
increasing discharge velocity. When the supply airflow rate is increased to 800 cfm (377.6 L/s) such trend
reverses entirely. Note in this case significant thermal
gradient is established between the supply airstream
and the surrounding air (Figure 6d) and the zone of high
velocity detaches from the ceiling (Figure 5d). The centerline velocity starts increasing as the jet moves downward and reaches a peak value of 1.38 (38%) at a travel
distance of about 67% from the diffuser.
Thereafter, the centerline velocity shows sharp
decline until it reaches the top of the table. This
32

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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)

indicates entrainment from the surrounding air during the initial descend and loss of directionality after
67% of the travel as the cold airstream approaches the
table.
Similar trend in the acceleration of the centerline
velocity is observed when the heat sources are located
surrounding the supply diffuser (Figure 10c). Note in
these situations also a significant thermal gradient is
established between the supply airstream and the surrounding air (Figure 9). However, with increase in the
supply airflow rate (increase in the cooling capacity)
thermal gradient in the room decrease which results
in reducing the acceleration of the centerline velocity (Figure 10c). In the case of 400 cfm (188.8 L/s) supply
airflow rate, the peak centerline velocity reaches about
2.2 times (120%) of the initial discharge velocity at about
60% distance from the ceiling.
Whereas in the cases of 600 and 800 cfm (283.2 and
377.6 L/s), the peak centerline velocity reaches about


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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
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ASHRAE Journal - June 2019 - 14
ASHRAE Journal - June 2019 - 15
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ASHRAE Journal - June 2019 - 19
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ASHRAE Journal - June 2019 - 21
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ASHRAE Journal - June 2019 - 31
ASHRAE Journal - June 2019 - 32
ASHRAE Journal - June 2019 - 33
ASHRAE Journal - June 2019 - 34
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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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