ASHRAE Journal - June 2020 - 55

COLUMN DIGITAL LABORATORY

(Table 1) portion of the total heat ﬂow. In addition, even
in a system where air speeds are higher, not every single
surface in that space will see those high air speeds, and
the radiant heat transfer effect may still be important.
Doesn't the solar load stay at the perimeter? This is a
regularly used assumption that isn't valid even if shading is used. The way solar load enters the space is a complicated process given the prevalence of double- and triple-pane windows and low emissivity (low-e) coatings.
Shading devices further complicate this process. In very
simple terms, a solar load that is incident on a window
is split into three components: the reﬂected portion,
the transmitted portion and the absorbed portion. Only
the transmitted and absorbed portions of the solar load
affect the heat balance in the space, while the absorbed
portion has a strong effect on the surface temperature of
the glass.
The transmitted load is the critical one because it
affects all the surfaces in an indoor environment (to
varying degrees) through absorption and reﬂection.
Even if an interior shade is used, unless it's 100% reﬂective, the solar load transmitted through a window heats
up the shade, which in turn reradiates that heat back
into the space. Even though a high performance window and shading system can do a great job reducing the
transmitted solar load (by reﬂecting it back outdoors) it
does not eliminate it nor does it "lock" it at the window.
What about thermal comfort? In addition to indirectly heating the air in an indoor space, thermal radiation directly affects comfort via three thermal comfort
metrics outlined in ASHRAE Standard 55-2017, Thermal
Environmental Conditions for Human Occupancy. One of those
is the predicted mean vote (PMV) index (Section §5.3.2),
which is a measure of whole body comfort for a representative occupant. This index is directly tied to thermal
radiation through the mean radiant temperature in the
space. The mean radiant temperature is a measure of
the thermal radiation heat transfer from all the surfaces
in an indoor space to the human body. It is essentially
an aggregate temperature that a human body "sees" as
a result of the radiant heat transfer in the space from
internal and external (solar) sources.
The second comfort index affected by thermal radiation is the radiant asymmetry in the space (Standard
55-2017, Section §5.3.4.2), which is a measure of how the
difference in surface temperatures in the indoor environment affects thermal comfort. Finally, the vertical air

TABLE 1 Radiant heat transfer fraction for various indoor heat sources.
HEAT
SOURCE

RADIANT
FRACTION (%)

NOTES

2017 ASHRAE HANDBOOKFUNDAMENTALS, CH. 18

Occupant

27 to 60

Seated, Very Light Work

Table 1

38 to 58

Moderately Active Ofﬁce
Work

35 to 49

Moderate Dancing

Lighting

Equipment

55 to 60

Linear Pendant LED

61 to 73

Recessed Fluorescent
Luminaire with Lens

Table 3

15 to 18

Recessed LED Downlight

Desktop

Table 8A

Laptop

Table 8B

Monitor

Table 8D

Printer

Table 9

temperature difference in the space (Standard 55-2017,
Section §5.3.4.3) is affected by thermal radiation heat
transfer from a warm ceiling to a cold ﬂoor and vice
versa, attenuating this quantity in practice.
We will explore the comparative effects of thermal
radiation in air-based HVAC systems by exploring CFD
results from two ofﬁce building projects: one with an
underﬂoor system and one with an overhead mixing
system. A summary of the design parameters for both
examples is shown in Table 2. We will focus on a simple
comparison of the CFD results with and without a thermal radiation model for both of these projects. The total
thermal loads (heat sources) will be kept identical in
both cases. When the thermal radiation model is active,
the thermal loads will be split according to a recommended radiant fraction taken from the 2017 ASHRAE
Handbook-Fundamentals.

Ofﬁce Building with Underﬂoor System
This example is a CFD simulation of a 5,000 ft2
(465 m2) section of ofﬁce from a ﬁve-story ofﬁce
building (290,000 ft2 [26 942 m2] total size) project
in Delaware. A snapshot of the CFD model is shown
in Figure 1, while the design details and CFD modeling parameters are shown in Table 2. The CFD model
includes the occupied space, the underﬂoor plenum,
the ceiling plenum and the ceiling and ﬂoor concrete
slabs. The various pieces of the model are interfaced
to allow heat transfer between them based on the heat
gain in the ofﬁce. The model also includes the effects of
heat gain/loss from adjacent ﬂoors (above and below) by
J U N E 2020

ashrae.org

ASHRAE JOURNAL

https://www.ashrae.org/

ASHRAE Journal - June 2020

Table of Contents for the Digital Edition of ASHRAE Journal - June 2020