ASHRAE Journal - June 2019 - 30

TECHNICAL FEATURE

FIGURE 6 Contours of temperature at the central plane with the heat source on the table show that when the cooling capacity of the discharge jet reaches four times the cooling load the downward discharge air can overcome the resistance of the upward buoyant plume.

a) 200 cfm (94.4 L/s)

b) 400 cfm (188.8 L/s)

c) 600 cfm (283.2 (L/s)

d) 800 cfm (377.6 L/s)

Variations in the Centerline Velocity
Unidirectional flow should ideally maintain consistent
downward motion without any significant entrainment
from the surrounding air. There are no direct measures
to quantify the directionality of the unidirectional flows
and entrainment from the surrounding air. However,
variations in the magnitude of centerline velocity
can indirectly help in estimating these parameters.
Acceleration in the centerline velocity indicates entrainment from the surrounding air whereas the deceleration
indicates lateral spread of the downward air jet and loss
of air mass to the surrounding.
Figure 10 shows variation of normalized velocity-a
ratio of centerline velocity at a specific distance along
the vertical centerline to the respective discharge
velocity. This variation is plotted against the nondimensional vertical distance-a ratio of a vertical distance for the supply diffuser along the centerline to the
height of the supply diffuser from the top surface of the
table. Thus, a non-dimensional height of 0.0 is located
30

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J U N E 2 0 19

at the supply diffuser where the value of non-dimensional centerline velocity is 1 and the non-dimensional
height of 1.0 is located at the surface of the table where
the value of non-dimensional centerline velocity is 0.
The extent of acceleration in the centerline velocity depends on the thermal gradients in the space.
Under isothermal conditions, however, as shown in
Figure 10a, a slight increase in the centerline velocity
indicates a small entrainment from the surrounding
air. This can be attributed mainly to the momentum
of the fast moving downward jet. Also as shown in this
figure the centerline velocity starts decelerating at
about 55% of the downward distance indicating loss of
unidirectional motion, which is evident by the airflow
patterns and velocity distribution as shown in Figures
2 and 3.
Under non-isothermal conditions when the sensible
heat sources are present at the table level, the centerline velocity shows consistent decline as the air moves
downward indicating that under such situations the


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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
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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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