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Courtesy of TRW Occupant Safety Systems
TRW Occupant Safety Systems, the U.S.based inflatable restraint, seat
belt, and steering systems manufacturer, is one of the biggest companies
of its kind in the world. TRW has played a leading role in the development
and production of multistage airbags, which deploy at different levels
depending on the severity of the crash and positions of the occupants.
They have also pioneered the development of heated gas inflators (HGIs)
that perform as well as traditional airbags, but are simpler and more
environmentally friendly. This inflator is basically a sealed aluminum
cylinder (bottle) at the base of the airbag filled with a stable, combustible
gas mixture ignited by a pyrotechnic squib. It contains and produces no
toxic components.
Simulation showing contours of water vapor mass fraction in a deployed
HGI
In the past, experimental testing of HGIs yielded little that was useful
for improving their design. While the test rig could determine if a given
design would meet its application requirements, it could neither illuminate
the complex processes nor explain unexpected results. Recently, however,
TRW has used CFD to better understand the workings of these devices. From
a modeling perspective, an HGI poses difficult challenges, since it involves
a highly compressible and transient flow process with chemical reactions
involving dozens of steps. Using FLUENT, TRW engineers implemented several
combustion mechanisms for an HGI, ranging from 1-step to 38-step models,
and compared the predictions. From the transient simulations, the engineers
computed the velocity of detonation waves (where the reaction front approaches
the speed of sound), and found good agreement with measured values for
most reaction schemes tested. Based on the success of these studies, TRW
engineers felt confident about applying the technique to their design
process. Rather than build prototypes for each design concept, engineers
now reach their final design goal more quickly by cycling through design
concepts using CFD.
Typical crash dummy analysis of an airbag
Courtesy of the Insurance Institute for Highway Safety
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