# Temperatures Required for Explosive Expansion of Air?

1. Aug 5, 2012

### dannystone

Hello, I was wondering what would be the minimum temperature required for a (powerful enough to be somewhat damaging to most unarmoured and/or lightly armoured objects) explosion in air to be generated via the air expanding to cool itself, similiar to how a nuclear weapon works.

The best answers I have been able to figure out are;

around 20,000 Kelvin, this is roughly the temperature in a bolt of lightning, generating thunder, and in some cases a small shockwave has done somewhat negligable damage to nearby objects, although of course, most lightning damage is due to the enormous current and temperatures involved.

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Several Million Kelvin, the temperatures in a modern nuclear weapon, of course, these temperatures are overkill, and definitely not the minimum I'm trying to find.

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300,000 Kelvin, roughly the temperature in the fatman and little boy bombs, due to their relative inefficiency and low power, but still plenty damaging enough by any means.

So it would seem between 20,000 Kelvin and 300,000 kelvin, would be about right, of course the amount of energy is also important, and probably other factors (for the sake of the simplicity, I'm assuming the air is at standard atmospheric temperature and pressure at sea level). so what are they, and how significant are those factors?

Also, all of these examples are very brief, what would happen if I heated it slowly, but not slowly enough for the heat to dissipate? And would happen if I heated it just as rapidly as a lightning bolt or an atomic weapon does, but then continued to heat it? Would it generate a sustained overpressure?

2. Aug 5, 2012

### Danger

I certainly hope that you're not actually going to try this.
Anyhow, it's impossible to have a sustained overpressure. One of the first and most destructive aspects of an explosion is the vacuum created at ground zero because all of the air was expelled in the first instant.

3. Aug 5, 2012

### voko

Temperature is energy, or, more correctly, density of energy. If you heat a given volume of gas with a particular amount of energy, you get a very particular temperature. And a very particular pressure, too. The destructiveness of exploding gas is usually expressed in its overpressure (diff with normal atmospheric pressure), 1 bar considered enough to cause serious damage. Obviously, as the blast wave advances, its overpressure decreases, so what is important is the overpressure at a given radius from the epicenter. Given that, one can compute how much energy must be released initially.

Continuous release of energy would not be effective. If the flow is too small, then the blast wave will not form. If it is high, it will just "blow off" the atmosphere from the vicinity of the release point, so very soon there will be nothing to create the blast wave from.