ASHRAE Journal - June 2020 - 60

COLUMN DIGITAL LABORATORY

FIGURE 6 Temperature 1in. below ceiling in the ofﬁce with an overhead mixing system: (a) with thermal radiation calculation; (b) without thermal radiation
calculation.

A. With Thermal Radiation Calculation

TABLE 4 Overhead mixing office example-averaged results.
VARIABLE

UNITS

WITH RADIATION
MODEL

WITHOUT RADIATION
MODEL

T42"

°F (°C)

73.5 (23)

74.3 (23.5)

Thf

R (K)

0.8 (0.5)

1.6 (0.9)

Twindow

°F (°C)

78.3 (25.7)

160.9 (71.6)

Tsurf

°F (°C)

77 (25)

76.2 (24.6)

What this means is that if we were to calculate Standard
55-2017's PMV for both cases (with and without radiation), it would be quite similar. If we focus on a particular surface, like the window, however, we see a drastic
82°F (46°C) difference in the predicted surface temperature (Table 4), despite having diffusers that "wash" the
glass with cold air.

What Does It All Mean?

B. Without Thermal Radiation Calculation

ceiling temperatures shown in Figures 6a and 6b reveal
that locally the temperature distributions do differ by
up to 10°F (5.6°C) in spots where the local air speed
(from the ceiling diffusers) is lower. This is due to the
lighting load being 100% convective in the case with no
thermal radiation, which makes it more difﬁcult for
this energy to diffuse through the space.
Since it's not possible to calculate a proper mean radiant temperature in the simulation without a thermal
radiation model (CFD uses the local radiation intensity
to do this) and to keep a fair comparison, the areaweighted average of all the surface temperatures in the
space will be used as a proxy and is shown in Table 4. As
one might expect, the values for both cases are almost
identical since the windows in this space are small and
most of the large surfaces in the space (like the ceiling and walls) experience a high air speed due to the
inherent nature of an overhead air distribution system.
60

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J U N E 2020

Despite the signiﬁcant differences between the size
and design of the two spaces, some common conclusions can be drawn from both. One common theme is
an overestimation of the surface temperature of heat
sources, which is most pronounced with windows even
when they are "washed" with diffusers from above or
below. Secondly, the head-to-foot temperature difference is signiﬁcantly higher when thermal radiation
is neglected, regardless of the air distribution system.
Comparing the two examples, one can also conclude
that the accuracy penalty of neglecting thermal radiation heat transfer in systems that have higher air speeds
is smaller compared to stratiﬁed and semi-stratiﬁed
systems (especially underﬂoor) where the air speeds are
lower on average.
As a ﬁnal note, the examples shown here are picked
on purpose to demonstrate the full range of effects
that thermal radiation heat transfer (and the choice
of neglecting it in a CFD simulation) has on the accuracy of the CFD results. Most real-life applications will
generally fall somewhere between these two cases.
Since most modern buildings have relatively large solar
loads (due to generous window-to-wall
ratios), this implies that accounting for
thermal radiation in CFD simulations
of the indoor environment is important
regardless of the air distribution system https://bit.ly/2YUre1l
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being used in the space.

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ASHRAE Journal - June 2020

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