Just watched Crawl (2019) and got me thinking.
Edit: i should clarify, why do they not extinguish like a normal flame.
In: Technology
Anything which can sustain combustion underwater is able to do so because the three requirements for fire, namely a fuel, and oxidizer, and adequate heat, are present. For many fires, water is a good choice to extinguish the fire because it disrupts both the oxidizer and heat production. However, it is possible to create mixtures of substances which have adequate contact between fuel and oxidizer despite the presence of water and generate enough heat to continue the combustion reaction even though much of the heat is going towards boiling water.
A typical flare that works underwater will have a finely powdered metal like calcium, aluminum or magnesium mixed with an oxidizer like a sulfate or nitrate compound, all bound together with some glue-type mixture. These reactions release a lot of heat and the flare has the oxidizer and fuel in physical contact and therefore the flare can burn underwater.
There are also underwater cutting torches which work through a different mechanism, namely that they release high pressure shielding gas around the oxidizer and fuel gases to prevent the water from coming in direct contact with the flame so that the flame is not extinguished.
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Simpler:
Although the fires you’re used to generally consume air, you can create fires that don’t need air because they use a solid mixture of fuel and oxidizer. Those fires can burn underwater if they can produce enough heat to not be extinguished by the cold water.
Probably use a self oxidizing chemical reaction. Once you supply an ignition you need heat fuel and oxygen for fire to burn so as long as you have all three you can have
A fire needs 3 things to burn:
1. Heat
2. Fuel
3. Oxygen
Normally there’s plenty of oxygen just hanging out in the air, so that one is a given. If you want an underwater fire though you’ve got to bring your own oxygen. So underwater flares are usually a combination fuel and oxydizer in a solid stick. Give it a good shot with a friction striker and you have everything you need for a fire!
We’re used to seeing fires that get their oxygen out of the air, but that’s not the only way to do it. If you heat up a material with a lot of oxygen in it, like a sulfate, a phosphate or a nitrate, it can break down and release oxygen gas. So a mixture of, say, magnesium metal and some kind of nitrate, could keep itself going underwater. The nitrate breaks down and releases oxygen, then the oxygen helps burn the magnesium.
Probably not THE answer but something similar:
Liquid Aluminum https://melscience.com/US-en/articles/characteristics-aluminum-and-combustion-reaction-m/
Edit: forgot this was eli5.. uhm..
To add on to the other replies, it’s a similar reason to why rockets can still fire their engines in the vacuum of space; they bring their own oxygen with them.
I just want to thank you for asking this. My question has always been ‘how can you weld underwater.’ This has now been answered. Thank you.
Flares don’t have to specifically be meant for use underwater. Just as an example, standard road flares will burn underwater…won’t light underwater but they’ll burn if lit and then submerged.
The solid propellant contains the oxygen needed to burn as well as the fuel.
For a different reason but explosives contain all their own oxygen too! In that case it’s because you want them to “burn” so fast that there wouldn’t be enough time for enough oxygen to drift in from the air. So you use something like TNT that has -NO3 groups hanging all over it so there’s lots of oxygen readily available pre-mixed throughout the fuel. Boom!
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