ASHRAE Journal - June 2020 - 54

DIGITAL LABORATORY
COLUMN

Mike Koupriyanov

Can CFD Models Ignore
Thermal Radiation
Heat Transfer?
BY MIKE KOUPRIYANOV, P.ENG., ASSOCIATE MEMBER ASHRAE

Computational fluid dynamics (CFD) is a powerful tool for verifying comfort and air
- in the indoor environment and for validating the sizing of mechanical systems
quality
serving that environment. As with any analysis tool, the term "garbage in, garbage
out" applies heavily to the process of CFD modeling. There seems to be a misconception in the industry that, unless a radiant system (high or low mass) is being modeled,
thermal radiation heat transfer can be safely ignored. Taking one glance at the 2017
ASHRAE Handbook - Fundamentals (Ch. 18 Tables 1 through 9) shows this is simply not
the case; between 15% and 73% of the thermal load from most indoor heat sources
such as people, lights and computers shows up as thermal radiation. If we average the
range, that works out to 44%.
Common Misconceptions
About Thermal Radiation Heat Transfer
Aren't the surfaces in the space the same temperature as the air? The surface temperature of the walls
in a space depends on the instantaneous heat transfer
in that space (hourly heat gain/loss) and the thermal
storage of the building materials. If a specific building assembly has a lot of thermal mass (like concrete),
a good chunk of the radiant load will be absorbed by
the thermal mass in the early part of a day. As the day
progresses, the thermal mass will no longer act as a heat
sink, at which point the instantaneous radiant heat
transfer will heat up the air in the room. This means
that, in general, assuming the surfaces in the space are
54

ASHRAE JOURNAL

ashrae.org

J U N E 2020

at the same temperature as the air is not correct, if a
peak design condition is being investigated.
But what about air speed? Air speed is an important parameter, as it tends to increase convective heat
transfer from any heat source and, to varying degrees,
displaces radiant heat transfer. Systems with high air
speeds like those with ceiling fans or data centers tend
to be convection dominant, which reduces the effect of
thermal radiation. However, in most occupied spaces air
speed is limited on purpose due to thermal comfort concerns, so radiant heat transfer tends to be a significant
Mike Koupriyanov, P.Eng., is the manager of Predict by Price in Winnipeg, Manitoba,
Canada. He was the chair of SPC 200 and TC 4.10 and is a voting member of TC 5.3
and SSPC 55.



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ASHRAE Journal - June 2020

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

Contents
ASHRAE Journal - June 2020 - Intro
ASHRAE Journal - June 2020 - Cover1
ASHRAE Journal - June 2020 - Cover2
ASHRAE Journal - June 2020 - 1
ASHRAE Journal - June 2020 - Contents
ASHRAE Journal - June 2020 - 3
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ASHRAE Journal - June 2020 - Cover3
ASHRAE Journal - June 2020 - Cover4
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