ASHRAE Journal - June 2020 - 59

COLUMN DIGITAL LABORATORY

FIGURE 4 CFD model of ﬁrst ﬂoor operations center section.

FIGURE 5 Temperature 42 in. above the ﬂoor in the ofﬁce with an overhead mixing system: (a) with thermal radiation calculation; (b) without thermal radiation
calculation.

A. With Thermal Radiation Calculation

transfer, there is less heat incident on the ﬂoor and more
heat buildup near the ceiling of the ofﬁce (which is then
conducted into the ﬂoor above). The net effect of removing the thermal radiation calculation is the heat ﬂow
from the ofﬁce into the plenum decreasing and the heat
ﬂow from the ﬂoor below increasing.

Ofﬁce Space with Overhead Mixing System
The next example is a CFD simulation of a 9,800

ft2 (910 m2) section of ofﬁce from a two-story opera-

tions center building (170,000 ft2 [15 794 m2] total size)
project in Texas. A snapshot of the CFD model of the
ﬁrst ﬂoor is shown in Figure 4, while the design details
and CFD modeling parameters are shown in Table 2.
The CFD model includes the occupied space, as well as
models of the occupants, the furniture and the computers. The mixing diffusers are modeled using the
"momentum method" outlined in ASHRAE RP-1009
titled "Simpliﬁed Diffuser Boundary Conditions for
Numerical Room Airﬂow Models." This study is simpler
than the underﬂoor ofﬁce, since only the ofﬁce space
itself is modeled.
Before delving into the results, it's worth noting that
the design of this ofﬁce space is quite different from the
previous example. This ofﬁce has a higher ceiling and
uses an overhead air distribution system, so we would
expect the air speed in the space (especially at the ceiling and the walls) to be higher on average than in the
previous example and therefore have a less pronounced
thermal radiation effect. The second major difference
is the windows and the solar load; the window size relative to the footprint of the space is much smaller in this

B. Without Thermal Radiation Calculation

example, and we would therefore expect the effect of the
solar load (which is 10 times smaller on a per-squarefoot basis) to be less signiﬁcant.
The average air temperature (Figures 5a and 5b) in both
cases (with and without radiation model) is almost identical, as one might expect, but there are some nuances
that are instructive to point out. First, localized hot
spots are in the left portion of the ofﬁce due to thermal
radiation from the lighting load, which does not project
into the occupied zone in the case without a thermal
radiation model. The head-to-foot temperature difference, which is small in both cases, is doubled in the case
without thermal radiation, similarly to the previous
example.
It is also worth examining the ceiling temperature
in this model; since the lighting load is applied to the
entire ceiling (lighting layout was not available at the
time of simulation) and due to the method of air distribution, it is a surface exposed to a high air speed. The
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ASHRAE Journal - June 2020

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