ASHRAE Journal Supplement - AMCA InMotion - November 2021 - 28

With a jet-fan system, strategically placed arrays of
jet fans direct supply air from air-supply points toward
extraction points, ventilating all parts of the parking
garage in the process. Supply air can be delivered in a
number of ways: via vehicle entrance or exit ramps, freshair
openings (grilles), or supply-air fans.
A key advantage of jet fans is that they can be positioned
between building structural elements. By not requiring
ductwork, they lower height requirements and allow for
easier maintenance.
Advantages of Jet Fans
Jet fans are small and can be attached directly to or
suspended from a ceiling. A jet fan moves air at both
ceiling level and at lower levels by means of entrainment.
Figure 2 illustrates 4,500-cfm (2 m3/s) fl ow through a
jet fan. The red jet is the air leaving the jet fan. The black
arrows illustrate what is called the " multiplier effect, "
whereby the jet entrains air in its wake, resulting in up
to 20,000 cfm (9 m3/s) of fl ow moving downstream of
the jet fan. It is this multiplier effect that makes jet
fans so effective at moving air around a parking garage.
Only about one-quarter of the air that needs to be moved
Velocity
5.00
4.00
3.00
2.00
1.00
0.50
0.20
0.00
4,500 cfm
Up to 20,000 cfm
3.5 m
2.5 m
2.5 m
1.5 m
1.5 m
0 m
3.5 m
0 m 5 m 10 m
15 m
20 m
25 m
30 m 35 m
40 m
45 m 50 m
FIGURE 2. Airfl ow through a jet fan. Image courtesy of
Woods Air Movement Ltd.
actually needs to pass through the jet fan itself. The
remainder is motivated to move by the jet-fan jet.
Figure 2 is from a computational-fl uid-dynamics (CFD)
simulation. CFD is the process of modeling a physical
phenomenon involving fl uid fl ow mathematically and
solving it numerically. In practice, CFD may be thought
of as building and testing jet fans in a virtual parking
garage, as opposed to a real one.
Over the last 10 years, CFD code development has
focused on mesh optimization and the user interface, a
process that has moved CFD out of the research laboratory
and into the engineering offi ce, where it remains an
advanced, but nevertheless routinely used, engineering tool.
Figures 3 and 4 are from a CFD simulation used to
assess the ability of strategically placed jet fans to move
air from supply-air points through a parking garage to
extractor fans, with the airfl ow splitting as it travels
around the central space. In Figure 3, we are looking
down at a plane halfway between the parking garage's
fl oor and ceiling, with blue representing low velocity
and red representing high velocity. These results were
processed into average velocities from the supply-air
points toward the air-extraction point to show they were
above 10 fps (3 m/s).
Measuring air velocity in a parking garage is impractical,
as airfl ow is driven by the heat released by fi re. Thus,
a parking garage literally would have to be set ablaze for
air to move as it would in the event of a fi re. Strengthening
the case for CFD simulation is that, as parking garages
get larger, with multi-layer interaction between levels,
air movement becomes progressively harder to predict
using any other method.
FIGURE 3. CFD simulation of the fl ow induced by 11
separate jet fans in a parking garage. The jet out of
each fan is colored red, with slower-moving air
colored blue. This was the fi nal simulation in a series
of simulations intended to refi ne the design until it
met code requirements for both normal and
emergency operating modes. Image courtesy of Woods Air
Movement Ltd.
28
2021 AMCA inmot ion
FIGURE 4. CFD simulation of a two-level parking
garage with interconnected air-supply and extraction
systems. Image courtesy of Woods Air Movement Ltd.
www.amca.org
http://www.amca.org
ASHRAE Journal Supplement - AMCA InMotion - November 2021

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