Naked Science Forum
General Science => General Science => Topic started by: another_someone on 01/08/2007 02:29:46
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I was just wondering, given the number of forest fires currently raging in southern Europe, whether it would not be possible to use vacuum bombs (thermobaric bombs) both to help clear the fire front of fuel, but also to starve it of air (if only temporarily, but long enough to stop the flame front).
These devices could ofcourse only be used in areas well away from population.
Just idle speculation.
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If they could knock the trees over that would probably help as a pile of wood on the ground burns much less well than a vertical forest, but I am not sure they would starve the fire of oxygen long enough for it to cool down below its flash point, which is what you would need to put a fire out.
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As you say, even simply knocking the trees down would help starve them of oxygen.
But how much of the wood is actually above flash point temperature? I would have thought that any wood that is above flash point temperature, but has not yet ignited, would probably be turned to charcoal. The process of turning the tree into charcoal (evaporating water, and breaking down complex chemicals) should itself hep to keep the core of the tree cool; so I would have though the tree could survive a long time with combustion only happening at the surface, and burning from the surface inwards. To the extent that this is the case (clearly, in the core of a very intense fire, this may not be always so), then I would have thought it would be fairly quick to cool the thin layer of wood on the surface to a temperature below flash point (I am not talking about cooling it to normal ambient temperatures, only to below flash point).
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I think that there are other solutions too. One would be to have controlled burnings of areas of forest, to remove the layer of fuel on the ground. Another is to "high prune" trees (remove the lower branches), so that these lower limbs don't transfer the fire from the ground to the canopy. In many ecosystems, forest fire is a benefit, and is a natural element of the forest cycle.
You need to consider the species too. Some species like the Eucalypts have highly resinous leaves and bark, and go up like a torch. However, the main part of the tree survives and self-coppices. In California, some of the trees are very thickly barked as a fire protection. But the cones need fire in order to open and cast their seeds.
The problem (and it's only a problem as far as humans are concerned) is that forest fires are extremely hot and move very quickly - they are a kind of flash fire. There is a great deal of open structure, and preventing oxygen from getting in would be a real challenge. I'm not familiar with thermobaric bombs or the area under which they are effective, but I suspect they would have to be very rapidly deployed to take effect, and of not small dimension. Something I have seen in Canada was the use of explosives around trees to remove them and create a fire breach.
Charcoals are still at a very high temperature, and even the smallest amount of oxygen will spark combustion in any reasonable size pile. Thus rekindling of fires would still be a problem, though could perhaps be doused aerially.
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I think that there are other solutions too. One would be to have controlled burnings of areas of forest, to remove the layer of fuel on the ground. Another is to "high prune" trees (remove the lower branches), so that these lower limbs don't transfer the fire from the ground to the canopy.
These are all valid prevention tools, but they don't fix the problem once it has happened.
In many ecosystems, forest fire is a benefit, and is a natural element of the forest cycle.
You need to consider the species too. Some species like the Eucalypts have highly resinous leaves and bark, and go up like a torch. However, the main part of the tree survives and self-coppices. In California, some of the trees are very thickly barked as a fire protection. But the cones need fire in order to open and cast their seeds.
Agreed - I was talking about how forest fires might be quenched, not whether they should be.
Ofcourse, forest fires also provide a paradox is the forest is considered as a a long term sink to store environmental carbon.
The problem (and it's only a problem as far as humans are concerned) is that forest fires are extremely hot and move very quickly - they are a kind of flash fire. There is a great deal of open structure, and preventing oxygen from getting in would be a real challenge.
As you say, the problem is human - but that is not an insignificant issue (as we just happen to be humans).
I'm not familiar with thermobaric bombs or the area under which they are effective,
http://en.wikipedia.org/wiki/Thermobaric_weapon#Calculations
The overpressure within the detonation can reach 430 lbf/in² (3 MPa) and the temperature can be 4500 to 5400 °F (2500 to 3000 °C). Outside the cloud the blast wave travels at over 2 mi/s (3 km/s). Following the initial blast (compression) is a phase in which the pressure drops below atmospheric pressure (rarefaction) creating an airflow back to the center of the explosion strong enough to lift and throw a human. It draws in the unexploded burning fuel to create almost complete penetration of all non-airtight objects within the blast radius, which are then incinerated. Asphyxiation and internal damage can also occur to personnel outside the highest blast effect zone, e.g. in deeper tunnels, as a result of the blast wave, the heat, or the following air draw.
http://en.wikipedia.org/wiki/GBU-43_Massive_Ordnance_Air_Blast_bomb#Description
Blast radius is 450 feet (137.61m, 150 yards), though the massive shockwave created by the air burst is said to be able to destroy an area as large as nine city blocks.