ASHRAE Journal - June 2019 - 34

TECHNICAL FEATURE

FIGURE 8 Contours of normalized air velocity at the central plane for non-isothermal conditions with heat source near the ceiling show high velocity zone moves downward
with increasing supply airflow rate

a) 200 cfm (94.4 L/s)

c) 600 cfm (283.2 L/s)

1.84 (84%) and 1.49 (49%), respectively. These peak
velocities occur at about 67% and 62% distance. In general, the location of peak velocity (zone of high velocity)
shifts downward with increasing flow rates. It indicates
with increase in the supply airflow rate the directionality of the downward flow sustains for longer distance.
However, the case of low supply airflow rate of 200 cfm
(94.4 L/s) is an exception. It does not follow this trend.
Otherwise, it would have shown the highest increase in
the centerline velocity. As stated before, the low velocity of the discharge air cannot sustain the downward
motion and spreads sideward which results in forming
uniform temperature distribution in the space. Thus,
the reduced thermal gradient results in reduced acceleration in the centerline velocity. Unlike the other cases,
in this case the centerline velocity increases during the
initial 20% travel, and then, gradually decreases as it
approaches the table.
34

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)

Archimedes Number (Ar)
Archimedes Number (Ar) as defined in Equation 1 is
a non-dimensional parameter which determines the
relative strength of upward buoyant force due to sensible heat sources over the downward inertial force of a
discharge jet. The Ar value much greater than 1 indicates
buoyancy dominated flow in which case the upward
motion of the plume can dominate the flow and disrupt
the downward unidirectional motion of the supply jet.
Whereas the Ar value much smaller than 1 indicates the
downward jet can remain unidirectional by overcoming
the upward resistance from the buoyant plume.
Equation 1 is a modified form of the equation stated
in the ASHRAE Handbook.1 This form incorporates generally known parameters during the design process such
as cooling load, discharge velocity, supply air temperature, and dimensions of a laminar diffuser array. Also
the known constants in Equation 1a are grouped in single


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ASHRAE Journal - June 2019

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ASHRAE Journal - June 2019 - Cover1
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