5 replies to this topic

[frosty90]

Hi,

I just recently asked the question in the general pyrotechnics section "why do whistle comps whistle" and several answers I got were generally along the lines of: The whistle comp strobes at audio frequency etc. This makes perfect sense. I unsderstand fully, but now (can you sense the question coming?).....so at a more fundamental level: why do strobing comps do so? why do some (i.e. whistle mixes) strobe faster than others? I know some chemistry, and a fair bit of physics, so if anyone knows in depth, I love to know whyy things work...


Cheers,
Jesse

[Creepin_pyro]

Clive Jennings-White covers the Shimizu strobe hypothesis in detail in the book 'pyrotechnics chemistry'. The abstract gives a good overview:

"Pyrotechnic strobe compositions burn in an oscillatory manner such that there is a marked variation of emitted light intensity as a function of time. The most common theoretical explanation for the phenomenon is that such compositions contain within them sub-compositions that may be regarded as a smoulder composition and a flash composition. The smoulder reaction would have a lower activation energy and low heat output. The flash reaction would have a high activation energy and high heat output. The actual chemical components of strobe compositions are extremely diverse, confounding a unified chemistry of the phenomenon. Nevertheless the majority of strobe compositions can in fact be reasonably deconstructed into smoulder and flash compositions. However, there appear to be a few strobe compositions that defy such explanation; although it is possible that the author lacks the wit to comprehend the chemistry. It is also possible that the prevailing theory applies to some, but not to all, strobe compositions; and it is yet possible that a different theory will provide a more encompassing explanation with better predictive power."

Taken from JPyro:
http://www.jpyro.com...ue_20/index.htm

[frosty90]

Clive Jennings-White covers the Shimizu strobe hypothesis in detail in the book 'pyrotechnics chemistry'. The abstract gives a good overview:

"Pyrotechnic strobe compositions burn in an oscillatory manner such that there is a marked variation of emitted light intensity as a function of time. The most common theoretical explanation for the phenomenon is that such compositions contain within them sub-compositions that may be regarded as a smoulder composition and a flash composition. The smoulder reaction would have a lower activation energy and low heat output. The flash reaction would have a high activation energy and high heat output. The actual chemical components of strobe compositions are extremely diverse, confounding a unified chemistry of the phenomenon. Nevertheless the majority of strobe compositions can in fact be reasonably deconstructed into smoulder and flash compositions. However, there appear to be a few strobe compositions that defy such explanation; although it is possible that the author lacks the wit to comprehend the chemistry. It is also possible that the prevailing theory applies to some, but not to all, strobe compositions; and it is yet possible that a different theory will provide a more encompassing explanation with better predictive power."

Taken from JPyro:
http://www.jpyro.com...ue_20/index.htm

Hi,

Makes sense I suppose, but once the 'flash' component has reached a high enough temperature, why should it not be self sustaining? why does it then go out and go back to the smoulder state? And this definitely doesnt really explain the something like the strobing of a whistle formula. There must be some fairly complicated chemistry going on in the flame, to do twith the benzene ring that all whistle forumlas seem to use. (Now I wish I took chemistry at uni instead of mathematics....I used to think chemistry was boring...) Thinking of something like glitter formula, if you burn a small pile on the ground, it burns up, then there is a delay and the 'glitter' bit ignites, but it all tends to ignite at once. Why in strobes should the brightly burning component go out so regularly and reliably? Are they simply mixed in such a way that thy are too course to be self sustaining? This has thinking, a pile of fairly course aluminium and sulphur will ingite and burn a bit with when you put a torch to it, but it usually wont sustain itself, it will go out and splutter quite a bit. Could perhaps a strobe be achieved by using something like this with a more evenly burning comp?
Im gonna try that now!

Cheers,
Jesse

[Mumbles]

The way I've always had it explained to me was that the "flash" phase was so violent that it blows itself out. If you have ever seen a dragon egg burn, it is like that in fast forward. The composition smolders for the great part of it's duration, only once a critical temperature/concentration/magic is reached does the whole thing explode. This expels the whole smolder layer, which is required for the flash layer, so it must start over again.

The same think happens to an extent with aluminum/sulfur. The mixture smoulders forming a sulfide melt around the aluminum granules. After a while, the aluminum and sulfur react explosively at once. With glitters these melts, called spritzels, are blown off and go through their durations while falling in the air. On the ground you will notice fewer but more flashes because the melt particles presumably are larger.

[Vic]

Eric Hunkins had a interesting answer to the phenomena posted on Passfire.


Re: what makes whistle fuel whistle?

The short answer

Whistles whistle because of oscillatory burning which happens to be in the hearing range of humans.

The long answer

Whistle fuels are simply benzene rings with a metal hanging off it, such as potassium or sodium. The only fuel available for burning in the chemical is in the benzene ring itself. A lot of energy is required to break a benzene ring so it can release it's fuel, but when it is broken, it also releases lots of energy.

In the first phase of burning, the mass takes a lot of heat\energy to start breaking some benzene rings. The mass needs time to absorb the thermal energy created in the last phase to break more benzene rings to make more fuel. The oxidizer is giving off oxygen, but no fuel is available and the flame front becomes over oxidized and slows.

The second phase the mass has had time to absorb the energy from the previous phase and starts breaking benzene rings. The rings release a large amounts of carbon (fuel) and energy. With all of the new available fuel, the flame front grows exponentially setting off a sound wave.

This exponential burning, burns up all of the available fuel and the flame front returns to being over oxidized.

This on again off again burning happens somewhere between 10,000 and 20,000 times per second, which puts it in human hearing range.

Thermodynamics and construction of the fuel molecule are very important. With bigger molecules that have more fuel hanging off of them, the oscillatory burning is much less pronounced because there is more fuel available during the off cycle. They go into more of a straight burn. Metals like copper and aluminum have better thermodynamics than sodium or potassium. These compounds can absorb heat faster and the oscillation therefore is much faster, putting them out of the human hearing range.

[frosty90]

Thermodynamics and construction of the fuel molecule are very important. With bigger molecules that have more fuel hanging off of them, the oscillatory burning is much less pronounced because there is more fuel available during the off cycle. They go into more of a straight burn. Metals like copper and aluminum have better thermodynamics than sodium or potassium. These compounds can absorb heat faster and the oscillation therefore is much faster, putting them out of the human hearing range.

Hi,

So are there anyother molecules appart from benzene rings which could exhibit this behaviour?

Cheers,
Jesse