The Physics Behind Fanning The Flames ?

[Erwin Schrodinger]

The Physics Behind "Fanning The Flames"?

So no one in class knew the answer and, of course, I don't know the answer (that's why I come to the geniuses on this board) . The question was:

Explain why blowing on a fire may make it burn better.


I'm pretty sure it has something to do with increasing the reaction rate of the fire so that oxygen combines with carbon and hydrogen at a greater rate. But why does blowing on a fire cause this to happen? This is where I'm stuck. Any help would be appreciated.

 

[Astronuc]

I usually associate it with getting the air (oxidant) to the fuel (wood, coal, etc). The air displaces the combustion products like CO and CO₂ which interfere with the combustion process.

Edit: Some matters Erwin Schrodinger and I discussed by PM.

Blowing the ash away would help, but getting the air to the fuel is what increases combustion. CO forms when insufficient oxygen is present, and CO₂, carbon dioxide is the more stable compound forms when oxygen is abundant. CO, carbon monoxide and CO₂ are gases. The ash is the non-combustable residue including oxides and carbonates of metals like potassium, magnesium, calcium, etc (http://en.wikipedia.org/wiki/Ash_(analytical_chemistry) , http://en.wikipedia.org/wiki/Fire) .


In wood, the heat breaks down the wood fiber or cellulose (http://en.wikipedia.org/wiki/Cellulose) by pyrolytic decomposition, and in some cases, the wood may contain flammable compounds like pine pitch or resins (http://en.wikipedia.org/wiki/Resin) , some of which contain terpenes (http://en.wikipedia.org/wiki/Terpene) . The are complex hydrocarbons and can burn directly or are broken down. Air combines with hydrocarbons (C, H) and forms CO, CO₂, and H₂O.

See also - http://www.iit.edu/~smile/ch96m3.html
http://www.iit.edu/~smart/pawebar/lesson3.html

 

[kyle8921]

I can provide a response to the question of why blowing on a fire may make it burn better. The phenomenon of fanning the flames is based on the principles of fluid dynamics and heat transfer.

When we blow on a fire, we are essentially increasing the flow of air towards the fire. This causes the fire to receive a greater supply of oxygen, which is necessary for combustion to occur. Oxygen is one of the reactants in the fire triangle, along with heat and fuel. By increasing the supply of oxygen, we are essentially increasing the reaction rate of the fire.

In addition to increasing the supply of oxygen, blowing on a fire also helps to remove the products of combustion, such as carbon dioxide and water vapor. These products can form a layer around the fire, hindering the supply of oxygen and slowing down the reaction rate. By blowing on the fire, we are removing this layer and allowing fresh oxygen to reach the fire, promoting a more efficient and intense burn.

Furthermore, blowing on a fire also helps to increase the transfer of heat from the fire to its surroundings. When we blow on a fire, we are essentially increasing the velocity of the air around the fire. This causes the fire to release more heat into the surrounding air, creating a feedback loop where the fire receives more oxygen and fuel, leading to a more intense burn.

In conclusion, the physics behind fanning the flames can be explained by the principles of fluid dynamics and heat transfer. By increasing the supply of oxygen, removing the products of combustion, and promoting heat transfer, blowing on a fire can make it burn better.