ASHRAE Journal - December 2020 - 26

TECHNICAL FEATURE

FIGURE 5 CFD simulation results with mesh fineness of eight.

Temperature (°F)
70

FIGURE 6 Temperature distribution with same size dampers.

Temperature (°F)
70
65
60
55
50
45
40
35
30

65
60
55
50
45
40
35
30

Since the mesh fineness of eight yielded results within
the desired accuracy of 10% of the test results and
increased fineness increased computational time without improving the results, it was decided to use the fineness of eight for the rest of the simulations. The behavior
of the temperature distribution with all mesh settings
was similar between each other, but with isolated low
temperature points that were causing the increased
error using mesh fineness seven. In all cases, a hot air
layer was found at the bottom of the air mixing plenum
and colder/mixed air at the top. Low temperature spots
were at the top, left and right sections, which correspond to Probes 2, 3, 8 and 9.

CFD Results
Once the CFD model was validated with the published
report as described above, the first change was to make
the RA damper the same size as the OA damper, while
keeping everything else constant. Figure 6 shows the temperature distribution. This change decreased the mixing
effectiveness considerably by creating low temperatures
at Probe 1, 2, 9 and 10.
The second change was changing the plenum size to
90 in. × 28 in. (2286 mm × 711 mm) from 117 in. × 38 in.
(2972 mm × 965 mm), while keeping the damper sizes at
their original size. Results are like the previous change
with the temperature distribution following the same
behavior. This change decreased the mixing effectiveness considerably by creating low temperatures at Probe
1, 2, 9 and 10.
The third case was making both the plenum size
90 in. × 28 in. (2286 mm × 711 mm) and making RA
dampers the same size as the OA damper. In this configuration the temperature distribution changed
26

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FIGURE 7 Plenum 90 in. × 28 in. with return air damper same size as outdoor air

damper.
Temperature (°F)
70
65
60
55
50
45
40
35
30

compared to all previous scenarios, as cold air coming
from the OA damper tends to go to the bottom and sides
of the plenum, while the hot air stayed in the middle of
the plenum (Figure 7).
Range and statistical mixing effectiveness were
reduced to 26.3% and 68.9%, respectively. It seems the
reduction of incoming return air speed with the increase
of RA damper area and the increase of mixed air speed
with the reduction of mixed air plenum area led to
increased stratification. The pressure drop increased
from the original of 0.22 in. w.c. to 0.27 in. w.c. (55 Pa to
67 Pa).
To finalize this study, the damper sizes were changed
to 24 in. × 88 in. (610 mm × 2235 mm) to match the
dimensions of the mixing box of Figure 3. This mixing
box was analyzed at 10,000 cfm (4719 L/s) total flow
rate, 30% of which is coming through the OA damper.
The mixing effectiveness was reduced even further as
shown in Figure 8, consequently being the configuration
with the most potential to cause freezing of the coils in
the air-handling unit due to its low temperature on the
bottom of the plenum. However, this configuration had

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ASHRAE Journal - December 2020

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