[LINK] FW: home emergencies

[Karl Auer]

On Fri, 2009-02-20 at 09:41 +1100, Danny Yee wrote:
> Ash Nallawalla wrote:
> > > Mr Ropar, sobbing at times, said it took just two minutes for the 
> > > flames to travel 20km, roar up the Sherwin Ranges and explode over the
> > > top. 
> > Which was calculated by Karl to be 600 km/h.
> 
> Is that completely impossible?  With a crown fire spreading by radiant
> heat, I don't see why the wind speed is necessarily a limitation.

We probably need an actual fire expert at this point. Joan Webster
writes:

"Exorbitant bushfire-speed claims by the media are fantasy. [...] The
fastest rate at which forest fire fronts have been measured is 15kph in
forest at Cockatoo and Mt Macedon and 20kph in grassland [in] South
Australia on Ash Wednesday". That was with winds of about 100kph.

She also writes (regarding grass fire): "...as the wind speed increases,
so does the rate of spread of the fire. But the rate of spread of the
fire can *never* exceed the wind speed".

The *heat* from the fire can travel at the speed of light but is subject
to the inverse square law. Heated air does travel as fast as the wind;
this doesn't move the firefront any faster, because the heat is not
sufficient to actually ignite anything. Flames travel at the same speed
as the wind, but because they are burning gas they don't travel very far
before they burn out; again, they only move the firefront faster if the
front is moving up hill.

Embers can also travel at the speed of the wind (well, slightly slower),
which means they can ignite new fires well ahead of the fire front -
many kilometres. However, such ignitions do not flash instantly into
huge fires; they take time to get established. Most people only see them
when they get big enough and are well established and aren't aware of
how long the ignition site smouldered. To the casual observer, the fire
seems to spring out of nowhere.

Assuming a 100kph wind, even an ember would take at least twelve minutes
to travel 20 kilometres. Upon landing, it would typically be many
minutes before a good blaze got going.

> It's probably not possible in practice for the heat pulse from an
> exploding crown to set off nearby trees all by itself, but I can't
> see a basic physics proof of that.

The inverse square law - it's quite hard for radiant heat alone to
actually start something burning, unless combustion is happening at much
greater temperatures than required for ignition.

If an object being irradiated gives off (say) combustible vapour at
lower than combustible temperatures, the vapour can form a "fuse" to the
object. The "flashover" from crown to crown is probably (i.e., I'm
guessing) burning eucalyptus vapour carrying the combustion, heat from
below reaching the crowns, or actual contact between crowns.

Regards, K.

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Karl Auer (kauer at biplane.com.au)                   +61-2-64957160 (h)
http://www.biplane.com.au/~kauer/                  +61-428-957160 (mob)

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