Fire in microgravity

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