Thursday, August 16, 2007

Firestorms

[crossposted from WAAGNFNP]

If you try to light a match under micro-gravity conditions (we all got used to “zero-g” so some smarty pants had to go and call it “micro-gravity”) and just hold it in one place, it will self-extinguish. The match will use up enough of the oxygen in its surrounding volume of air to extinguish the flame. It doesn’t have to use up all the oxygen, either; most flames go out in air that still has enough O2 in it for people to breathe—barely.

Depending on the fuel, (e.g. hydrogen needs less oxygen to burn than methane does), the usual figure given is that 14%-16% oxygen is needed to sustain a fire. People can manage on a bit less; Biosphere II dropped below 14% before they pumped in some additional O2, but they didn’t have to contend with elevated CO2 levels; in fact, what they’d been losing was CO2, by absorption into their nice new concrete structure, with bacteria converting soil organics and O2 into CO2. They’d had a bit of a “slow burn.”

Your basic candle flame is fed fresh air by gravity, specifically, the air coming in to replace the hot gases that have become lighter than air in the hot flame. That’s called the “fire draft” and fireplaces exist to direct the fire draft upwards, so the smoke doesn’t choke the people warming themselves by the fire. The chimney/flue of the fireplace also accelerates the fire draft if you build it right, and both Ben Franklin and Benjamin Thompson, (Count Rumford), invented some tricks that are still in use.

So fires always produce an updraft. In truly big fires, the question becomes how the updraft interacts with the local weather. If the local winds are stronger than the updraft, and the fire is big, uncontrolled, and uncontained, you have a conflagration. If the fire creates its own winds, you have a firestorm.

Neither is anything you or I want to be near. A running wildfire can exceed 70 mile per hour under upslope flow conditions, where the fire draft adds to the natural winds. Firestorms generate their own weather, their own winds, and can create small tornadoes, “dust devils” made out of flaming gases that light everything they touch.

Oakland Hills Firestorm 1991

The heat in the interior of a firestorm pyrolizes everything within its boundaries, but the fuel produced exceeds the air available. So the hot mass rises as a fireball, sucking more air into it, maintaining its heat even as it expands, because there is still plenty of fuel gas left to burn. A firestorm spreads as much by thermal radiation as by flaming contact, sometimes triggering fires at a distance, like across a valley.

Kurt Vonnegut lived through the firebombing of Dresden, and wrote about it, so more people know about the 35,000 people who died there than in Operation Gomorrah, which killed a larger number (50,000 est.) in Hamburg, or the 120,000 who died in the Tokyo fire raids. I’d never even heard of the raids on Kassel, Braunschweig, Darmstadt, Heilbronn, Pforzheim, and W├╝rzburg until I looked them up for this essay.

Aerial view of Tokyo firebombing

But fires are tricky to set with conventional incendiaries. Most of the WWII fire raids were duds, or semi-duds, producing some fires, but nothing like a real firestorm. There were four attempts on Hamburg before they hit the jackpot.

Hamburg: Operation Gomorrah

But Hiroshima was a jackpot; what the first blast didn’t do, the subsequent firestorm did, and 4 square miles of the city just went away, nothing left, not even steel, much less teeth and bones.

Hiroshima firestorm aftermath

The Nagasaki bomb was bigger, but the targeting wasn’t as good, and the city had the good fortune of having a lot of hills, which shielded some from the blast, the heat, the radiation. Moreover, the hills altered the wind field, and the resulting fires are only classed as a conflagration. Still, 40,000 people died quick, and maybe four times that number died slow, from injuries, from radiation, from the long term illnesses that go with radiation, from trauma, and grief.

In 1944, the most powerful bomb used in warfare was the British Grand Slam on the order of 10 tons of TNT, although the U.S. developed (but never used) one that was twice as big. In 1945, of course, nuclear weapons increased bomb yields by three orders of magnitude.

It’s hard to develop a sense of scale once you start dealing in factors of a thousand. People think “nuke” and think “Hiroshima and Nagasaki.” But those bombs were measured in kilotons.

Thermonuclear weapons are measured in megatons, another factor of a thousand, so we got a factor of one million increase in about half a decade. Go look for pictures of atmospheric bomb tests. See that one near the mountain? You can see the billows in the clouds and dust that it shakes up the near field.

Fifteen kiloton airburst

That’s a bomb that’s in the kiloton range. There have been industrial accidents that can be measured in kilotons, like the Pacific Engineering Company plant in Henderson, Nevada, where over a thousand tons of ammonium perchlorate blew up. That was a kiloton explosion.

Pepcon Explosion

Now go find some photos of the Pacific island H-Bomb tests. Google on “Castle Bravo” for example, the biggest miscalculation in the history of nuclear weapons. It was supposed to produce 6 megatons; instead they got 15. “Castle Romeo” was part of the same mistake. They expected 4, but got 11.

Castle Bravo - 15 megatons

See those shapes in the sky above the mushroom cloud? That’s the stratosphere.

The thermal radiation effects of nuclear devices loom larger as the energy release increases. In Hiroshima, the firestorm was likely caused by the blast itself, in the same way that an earthquake causes fires, by turning building into kindling, by releasing natural gas, by rupturing fuel tanks. The heat from the bomb itself probably lit only a few of the fires.

But megaton blasts, perhaps over grasslands, forests, farms? The experiment has yet to be preformed. And calculations, simulations, and estimates are so very, very unsatisfying, aren’t they?

1 comment:

Anonymous said...

thank you for this article :) came across it by googling when debating elsewhere
As I've pointed out to folk lately, there's a huge flaw in the calculations, since, the amount of goods people have in their houses has vaslty increased from prior times
ie, My grandparents were poor, like most folk, so their houses had little to burn as they couldn't affor dmuch
Today, people have much, much more goods, which can burn...
And firestorms can grow if fuel exists.

Ugly stuff, but hey, needs dealt with. We need vast reductions in nuclear arsenals.

silver@steve14.freeserve.co.uk