Space Forum / Space Flight / June 2005

[...]

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

[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

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.

No problem:

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

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

"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

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

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.