Flame Academy

I heard on the radio this morning from that nice Mr Cowan that today is the anniversary of the start of the Great Fire of London which burned for four days in 1666. That provides for a bit of delayed synchronicity with yesterday’s post about the dreadful fires in the outskirts of Los Angeles and a similar conflagration in Athens (which now thankfully appears to be under control).

Fires are of course terrifying phenomena, and it must be among most people’s nightmares to be caught in one. The cambridge physicist Steve Gull experienced this at first hand when his boat exploded and caught fire recently. I’ll take this opportunity to wish him a speedy recovery from his injuries.

But frightening as such happenings are, a flame (the visible, light emitting part of a fire) can also be a very beautiful and fascinating spectacle. Flames are stable long-lived phenomena involving combustion in which a “fuel”, often some kind of hydrocarbon, reacts with an oxidizing element which, in the case of natural wildfires at any rate, is usually oxygen. However, along the way, many intermediate radicals are generated and the self-sustaining nature of the flame is maintained by intricate reaction kinetics.

The shape and colour of a flame is determined not just by its temperature but also, in a complicated way, by diffusion, convection and gravity. In a diffusion flame, the fuel and the oxidizing agent diffuse into each other and the rate of diffusion consequently limits the rate at which the flame spreads. Usually combustion takes place only at the edge of the flame: the interior contains unburnt fuel. A candle flame is usually relatively quiescent because the flow of material in it is predominantly laminar. However, at higher speeds you can find turbulent flames, like in the picture below!

Sometimes convection carries some of the combustion products away from the source of the flame. In a candle flame, for example, incomplete combustion forms soot particles which are convected upwards and then incandesce inside the flame giving it a yellow colour. Gravity limits the motion of heavier products away from the source. In a microgravity environment, flames look very different!

All this stuff about flames also gives me the opportunity to mention the great Russian physicist Yakov Borisovich Zel’dovich. To us cosmologists he is best known for his work on the large-scale structure of the Universe, but he only started to work on that subject relatively late in his career during the 1960s.  He in fact began his career as a physical chemist and arguably his greatest contribution to science was that he developed the first completely physically based theory of flame propagation (together with Frank-Kamenetskii). No doubt he used insights gained from this work, together with his studies of detonation and shock waves, in the Soviet nuclear bomb programme in which he was a central figure.

But one thing even Zel’dovich couldn’t explain is why fires are such fascinating things to look at. I remember years ago having a fire in my back garden to get rid of garden rubbish. The more it burned the more things  I wanted to throw on it,  to see how well they would burn rather than to get rid of them. I ended up spending hours finding things to burn, building up a huge inferno, before finally retiring indoors, blackened with soot.

I let the fire die down, but it smouldered for three days.

4 Responses to “Flame Academy”

  1. Anton Garrett Says:

    Warwick also suffered a great fire on this day in 1694; autumn was always a likely time for a fire to catch, especially after a hot summer.

  2. telescoper Says:

    It’s not likely that anything in Cardiff will catch fire today. It’s been tipping down with rain all day.

  3. Bryn Jones Says:

    Yes, Rob Cowan is my favourite DJ too.

    The Great Fire of London, of course, had some scientific and astronomical consequences. Gresham College was spared, but offered a temporary home to other organisations, and as a result the Royal Society moved out temporarily to find somewhere less busy. Robert Hooke, an experimental demonstrator for the Royal Society, was appointed as a surveyor following the fire to prepare for the reconstruction. Christopher Wren was the Savilian Professor of Astronomy at the University of Oxford at the time of the fire. He had done some work in surveying and a little in architecture before the fire, but it was the rebuilding after the fire that led to him being appointed Surveyor of the King’s Works. This led him to his new career as an architect. It is said that he designed one of the towers of his new St. Paul’s Cathedral so that it could be used as an astronomical observatory (I don’t think is was ever used for that). Wren and Hooke designed the Monument to the Great Fire with the intention of using the hollow inside of the tower to hold a vertical telescope to attempt the first measurement of stellar parallax (no parallax was found).

  4. “But one thing even Zel’dovich couldn’t explain is why fires are such fascinating things to look at.”

    I think the reason is obvious: they were a Good Thing back in the caveman days. Similar feelings often occur with respect to caves and other places of shelter. John Barrow (I’m sure you’ve heard of the Famous Writer), in his book THE ARTFUL UNIVERSE, shows some examples of simulated landscapes which look real but somehow not pleasing. It turns out the reason is the lack of shelter, which is rather rare in a real landscape but often present in paintings etc (think of stuff from Italy or the Netherlands from the 1500s).

    Light has a similar fascination: it probably jibes with our urge to be aware of what is going on around us. Many of the artists I admire (Vermeer, Bergman and his cameraman Nyquist etc) are masters of light composition.

    Is mindlessly watching the telly a substitute for the fire at the mouth of the cave? (One can actually buy video tapes or, these days, DVDs of fires burning!)

    Of course, nude women are also interesting to watch and are also often the subject of art because they were also good to have back in the caveman days.

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