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Fire in microgravity



 
 
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  #1  
Old May 18th 05, 10:38 AM
JotaCe
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Default Fire in microgravity

I have been thinking about the way the fire behaves in microgravity. I
have found little information on this in the internet, so I am mostly
just wondering, and these thoughts are what I want to share with you
for discussion:

In absence of gravity, the flames create a sort of sphere around the
core of the fire (of course, this sphere will be limitated by the
object in fire; depending on its shape, we will have an almost entire
ball for a match, or just a semi-sphere in the case of a fire on a flat
surface). Well, if the core of the fire is surrounded by flames
(incandescent gases), how can the core be feed by fresh oxygen to keep
the fire burning? Maybe this creates a sort of pulsating effect on the
spherical flame, contracting it when the oxygen is running out, and
allowing thus the flux of more oxygen into the core, which feeds again
the flames to its previous size?

If someone knows where can I find more information on this matter, I
would be very grateful. Best regards,

Javier Casado
Space stuff in spanish in: http://es.geocities.com/fjcasadop

  #2  
Old May 18th 05, 07:16 PM
snidely
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JotaCe wrote:
I have been thinking about the way the fire behaves in microgravity.

I
have found little information on this in the internet, so I am mostly
just wondering, and these thoughts are what I want to share with you
for discussion:

[...]
If someone knows where can I find more information on this matter, I
would be very grateful. Best regards,


Well, SpaceHab/SpaceLab/ISS experiments on flames are relatively
recent, but there should be something published by now (Rusty?). In
addition to the NASA servers, you might check sites with abstracts of
journal submissions. You might also look up some mission status
reports to find Principal Investigator names, and do a search for their
papers.

The largest flame experiment was done on Mir, but the documentation was
a bit sketchy. Check Jim Oberg's book for a summary ;-}

/dps

  #3  
Old May 18th 05, 11:02 PM
Gene P.
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On 18 May 2005, JotaCe wrote:

I have been thinking about the way the fire behaves in microgravity. I
have found little information on this in the internet, so I am mostly
just wondering, and these thoughts are what I want to share with you
for discussion:

In absence of gravity, the flames create a sort of sphere around the
core of the fire [...]

[snip]

Actually, no.

The traditional incandescent flame mostly doesn't happen in zero g fires.

The problem with zero g combustion is that the heat creates no
convection... as a result both the combustion products (heat, ash, co2,
water vapor, etc) all stay more or less right where they were produced.

Now most of this is actually a good thing since it retards further
combustion... But because the heat stays put too, and will continue to
build so long as the immediate supply of either the oxidizer or fuel are
not exhausted, that heat can grow to *VERY* intense levels.

It's usually the oxidizer that is the limit. And that's ordinary air.

So the moment an astronaut or cabin fan (or something else) causes
airflow, the fire spreads... First by allowing more oxygen to get to the
hot fuel and secondly by transporting *extremely hot* exhaust gasses
away... which can transfer their heat and trigger other combustion.

There will only be a visible "flame" for as long as there's fuel and
oxygen actively mixing... but the hazardous heat bubble can persist for a
very long time since air is a poor conductor of heat without convection to
assist it.

So the danger is this: A fire starts and quickly chokes itself, but in
the process triggers an alarm... The astronaut comes to investigate, and
as he approaches to have a look causes eddy currents in the air which feed
the fire... and worse yet, could stumble into a pocket of superheated
combustion products that aren't visible but are more than hot enough to
set *him* on fire...

It's *not* pretty.

And to the extent that materials *do* get hot enough to glow... that's
heat radiation. It spreads the heat around. Perhaps enough so to trigger
other fires nearby.

If you still the air (or remove it), generally zero g fires go out on
their own... very slowly, as the heat dissipates below the ignition
threshold.

(All bets are off though if the environment contains mixtures of
hypergolics... just mix and watch your fire re-ignite!)

Gene P.
Slidell LA

--
Alcore Nilth - The Mad Alchemist of Gevbeck



  #4  
Old May 18th 05, 11:07 PM
David Summers
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I don't really know, but I bet some sort of diffusion keeps the oxygen
coming in. The hot gas surrounding the match or whatever would be less
dense, so oxygen atoms would slip in between the other gasses' atoms.

  #5  
Old May 19th 05, 01:22 AM
Jorge R. Frank
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"JotaCe" wrote in
oups.com:

I have been thinking about the way the fire behaves in microgravity. I
have found little information on this in the internet, so I am mostly
just wondering, and these thoughts are what I want to share with you
for discussion:


If someone knows where can I find more information on this matter, I
would be very grateful. Best regards,


No problem:

http://exploration.grc.nasa.gov/combustion/

(Just how hard did you look, anyway...?)
--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.
  #6  
Old May 19th 05, 12:22 PM
Michael Smith
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"Maybe this creates a sort of pulsating effect on thespherical flame,
contracting it when the oxygen is running out, and allowing thus the
flux of more oxygen into the core, which feeds againthe flames to its
previous size?"

In perfectly still air I would expect the core of the fire to gradually
lose heat by radiation and go out. This will take much longer than heat
loss by convection, which can't happen without gravity.

If there is any movement of air I would expect combustion to continue
to the extent that oxygen can reach the core of the fire.

I wonder if you could make a fire retardent bag for use in
microgravity. You would enclose the fire in the bag and seal it. Oxygen
in the bag would be used up quickly and the mess inside would slowly
cool.

Of course, if a fire starts on the ISS, it is unlikely to cooperate to
this extent.

Michael Smith
...who wishes he new how to quote in google groups

  #7  
Old May 19th 05, 12:54 PM
Greg D. Moore \(Strider\)
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"JotaCe" wrote in message
oups.com...
I have been thinking about the way the fire behaves in microgravity. I
have found little information on this in the internet, so I am mostly
just wondering, and these thoughts are what I want to share with you
for discussion:

In absence of gravity, the flames create a sort of sphere around the
core of the fire (of course, this sphere will be limitated by the
object in fire; depending on its shape, we will have an almost entire
ball for a match, or just a semi-sphere in the case of a fire on a flat
surface). Well, if the core of the fire is surrounded by flames
(incandescent gases), how can the core be feed by fresh oxygen to keep
the fire burning? Maybe this creates a sort of pulsating effect on the
spherical flame, contracting it when the oxygen is running out, and
allowing thus the flux of more oxygen into the core, which feeds again
the flames to its previous size?


I'm sure NASA has some sort of article on this as they've done several
experiments on-board the shuttle with fire.

Ibeleive the pulsing effect is exactly what happened with a "candle" they
were testing.

Of course then there is the Mir fire, where the "candle" WAS the oxygen
source and as such had no trouble burning.


If someone knows where can I find more information on this matter, I
would be very grateful. Best regards,

Javier Casado
Space stuff in spanish in: http://es.geocities.com/fjcasadop


  #8  
Old May 20th 05, 12:31 AM
Nog
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Default


"JotaCe" wrote in message
oups.com...
I have been thinking about the way the fire behaves in microgravity. I
have found little information on this in the internet, so I am mostly
just wondering, and these thoughts are what I want to share with you
for discussion:

In absence of gravity, the flames create a sort of sphere around the
core of the fire (of course, this sphere will be limitated by the
object in fire; depending on its shape, we will have an almost entire
ball for a match, or just a semi-sphere in the case of a fire on a flat
surface). Well, if the core of the fire is surrounded by flames
(incandescent gases), how can the core be feed by fresh oxygen to keep
the fire burning? Maybe this creates a sort of pulsating effect on the
spherical flame, contracting it when the oxygen is running out, and
allowing thus the flux of more oxygen into the core, which feeds again
the flames to its previous size?

If someone knows where can I find more information on this matter, I
would be very grateful. Best regards,

Javier Casado
Space stuff in spanish in: http://es.geocities.com/fjcasadop


If heat cannot rise and draw in cool oxygenated air from the bottom it seems
to me that the fire would tend to smother itself.


  #9  
Old May 20th 05, 12:34 AM
Anthony Bachler
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The flame itself is much smaller, and oxygen can easily permeat the flame
zone. The rate of permeation is what limits the size of the flame. The
lack of convection allows the reduced combuston rate to sustain the
temperatures necessary to maintain the reaction. The flame itself can burn
much hotter as a result of the lack of convection, since the primary loss of
heat is limited to radiation.

"JotaCe" wrote in message
oups.com...
I have been thinking about the way the fire behaves in microgravity. I
have found little information on this in the internet, so I am mostly
just wondering, and these thoughts are what I want to share with you
for discussion:

In absence of gravity, the flames create a sort of sphere around the
core of the fire (of course, this sphere will be limitated by the
object in fire; depending on its shape, we will have an almost entire
ball for a match, or just a semi-sphere in the case of a fire on a flat
surface). Well, if the core of the fire is surrounded by flames
(incandescent gases), how can the core be feed by fresh oxygen to keep
the fire burning? Maybe this creates a sort of pulsating effect on the
spherical flame, contracting it when the oxygen is running out, and
allowing thus the flux of more oxygen into the core, which feeds again
the flames to its previous size?

If someone knows where can I find more information on this matter, I
would be very grateful. Best regards,

Javier Casado
Space stuff in spanish in: http://es.geocities.com/fjcasadop



  #10  
Old May 24th 05, 07:31 PM
Derek Lyons
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"Greg D. Moore \(Strider\)" wrote:

Of course then there is the Mir fire, where the "candle" WAS the oxygen
source and as such had no trouble burning.


It wasn't really a 'candle' (per se), but a source of pressurized
oxidiser and fuel. (I.E. the published descriptions sound to me like
the O2 candle was acting as a (small) SRM.)

D.
--
Touch-twice life. Eat. Drink. Laugh.

-Resolved: To be more temperate in my postings.
Oct 5th, 2004 JDL
 




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