ASHRAE Journal - December 2020 - 25

TECHNICAL FEATURE

It was assumed that blades opened at 90 degrees
equals a 100% airﬂow through that damper. It was estimated that a 30% airﬂow would occur with the blade
angle at 27 degrees. The exact location of the blades is
also unknown, which might be another factor increasing error between the CFD and the real test. According
to ASHRAE Guideline 16-2018,9 the return dampers are
usually low value and will behave more linearly for the
control response as parallel dampers. Also according
to Guideline 16-2018, the outdoor air damper does not
control airﬂow, so it could be parallel or opposed blade.
In this study both dampers are modeled as parallel blade
dampers.

FIGURE 4 Probe grid layout for 117 in. × 38 in. and 90 in. × 28 in. plenum.

117 in.×38 in. Plenum
P5
P6
P7

P11

P12

P13

P14

P15

P16

P21

P22

P23

P24

P25

P26

90 in.×28 in. Plenum

P10

P17

P18

P19

P20

P27

P28

P29

P30

P12

P19

P22

P29

CFD Preparation
A full-cloud computer-aided engineering (CAE)
software with various simulation types was used to
perform the simulations of the mixing box CAD models. In this case a convective heat transfer simulation was selected with a turbulent ﬂow algorithm. To
select the appropriate turbulent model, the Reynolds
number was calculated assuming the full ﬂow rate of
16,000 cfm (7551 L/s) and is ﬂowing in the direction
shown in Figure 2 with an average air temperature of
90°F (32.2°C).
The Reynolds number was calculated as 244,300,
which deﬁnes a turbulent ﬂow. As recommended by
CFD literature, a K-Omega SST model was selected.10
This model is a two-equation model, combining turbulent kinetic energy and dissipation.
Boundary conditions were set as published in the
ASHRAE report5 for Conﬁguration 4 and Test 7 using the
normalized temperature rather than test temperatures:
* OA Inlet: 4,800 cfm (2265 L/s) and 30°F (-1.1°C);
* RA Inlet: 11,200 cfm (5286 L/s) and 70°F (21.1°C); and
* Mixed Air Outlet: 1 atm.
As previously mentioned, the range mixing effectiveness and the statistical mixing effectiveness were used
to validate this CFD simulation to the published test
results. To achieve this, it was required to read the temperature values in the same position as the test probes
in the test report. As shown in Figure 2, a probe grid was
used with 12 in. (305 mm) spacing between each probe,
4.5 in. (114 mm) from the side wall and 7 in. (178 mm)
from top and bottom walls. This produces three rows
of 10 probe points each. Note that these probe points,
which are physically visible in Figure 2 and initially

modeled as features, were replaced with virtual probe
points within the software to reduce mesh size and computational time.
This probe grid is located using coordinates with the
origin in the centroid of the model. As the plenum size
is reduced in the study, the probes remain in the same
location with relation to the center of the model, making
probe points 1, 10, 11, 20, 21 and 30 no longer inside the
control volume being analyzed, as shown in the bottom
portion of Figure 4.

Mesh
The model was run with different mesh settings to test
the sensitivity of the results and how convergence was
affected by the mesh setting. The software allows for the
adjustment of mesh size using a " ﬁneness " value from
one to 10, with 10 being the ﬁnest mesh. Simulations
were run with mesh sizes from six to 10. The simulation
failed with mesh ﬁneness at six. With the ﬁneness at
seven, the simulation was successful and convergence
reasonable, but with a high mixing effectiveness error
compared to the published test report. When reﬁning the mesh to a ﬁneness of eight, convergence was
improved, and the range and statistical mixing effectiveness values were within 2% and 7% of the test data,
respectively.
Figure 5 shows a cut plot showing the temperature distribution using a mesh ﬁneness of eight, which yields
range and statistical mixing effectiveness of 61.0% and
83.9% compared to 60.3% and 87.2% of the test data,
respectively. Pressure drop was calculated to be about
0.22 in. w.c. (55 Pa).
D ECEM BER 2020

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ASHRAE Journal - December 2020

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