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By K.C. Adiga, NanoMist Systems LLC, Warner Robins, GA
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Using the DPM, the entrainment of surrounding water mist into the
firebase is shown by stochastic droplet trajectories
In the 1960s, Halon 1301 was hailed as
an effective fire suppression chemical.
Shortly thereafter, it was identified as an
ozone-depleting environmental hazard, and
a search for newer and safer fire suppression
agents was renewed. The chemical
HFC-227ea, a safer alternative to Halon 1301,
is the most popular agent in use today. It
has some limitations, however, because of
by-products formed during its use. Water
in some form would be an ideal permanent
solution.
Throughout the years, the concept of
using an ultra-fine water mist (with droplet
diameters < 10 mm) for fire suppression has
been generally ignored, based on the notion
that such extremely small droplets would
not have enough momentum to reach the fire, and would vaporize before contact.
This notion is changing, however, as a patented
ultra-fine water mist technology with
the trade name NanoMist has recently attracted
a great deal of attention. Using droplets
less than 10 mm in diameter, the technology
has demonstrated efficient fire suppression
capability in various configurations. With
the help of CFD and laboratory tests, scientists
from NanoMist Systems have
demonstrated that this ultra-fine mist may
be a potential alternative to HFC-227ea for
meeting both government and industrial
fire protection needs, pending full-scale testing.
The CFD results have yielded an improved
understanding of how to generate, scale,
and deliver fine mist clouds into a fire location,
while addressing concerns about the
premature loss of liquid droplets.
The new technology has some very
attractive features, including 1) near self-entrainment
of gas-like mist into the firebase;
2) considerably less water required
compared to conventional water sprays;
and 3) the relatively non-wetting nature
of the mist on nearby surfaces because of
fast vaporization. The amount of water
required to put out a fire is far less compared
to larger droplets because of the
increased total surface area of the smaller
droplets, the rapid increase in heat removal
from the combustion zone, and the dilution
of oxygen due to expansion as vaporization
occurs. While the process technology
required to produce such a fine mist and
deliver it to the fire area on a commercial
scale has been difficult to develop,
FLUENT has helped make this aspect of
the development and evaluation quick and
affordable.
In the CFD study, a medium scale fire
was generated using a volumetric heat generation
source term. The discrete-phase
model (DPM) was used to simulate the mist.
Stochastic particle trajectories, influenced
by turbulent fluctuations, were computed.
The coupled solution predicted the rate
of vaporization of water droplets subjected
to the fire, and the subsequent cooling
of the local gas field. The predicted
centerline peak temperatures were used
to judge the fire cooling capacity of the
mist. CFD results vividly show the entrainment
of water mist droplets into the firebase
as seen from DPM droplet trajectories.
The firebase pulls the mist from its surroundings.
The behavior predicted by the
CFD simulations was reproduced closely
by tests of mist deployed to the base of
heptane pool fires. These fires were put
out in 10 seconds or less.
NanoMist Systems is now collaborating
with the U.S. Naval Research Laboratory
(NRL) and Hughes Associates, Inc. (HAI)
in evaluating NanoMist fire suppression
technology for electronics applications.
Preliminary results are very promising.
NanoMist local
flooding
experiments show
the entrainment of
surrounding mist
into the firebase,
extinguishing the
heptane pool fire
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