Naked Science Forum

General Science => General Science => Topic started by: neilep on 01/11/2006 20:33:36

Title: Why does a candle go out when you blow on it?
Post by: neilep on 01/11/2006 20:33:36

My son asked me to ask you why candles go out when blown upon ?

After all, you are not depriving them of air to burn in....

I said it was the speed of the ' blowing ' that stops the flame from burning but is there a more concise answer ?

Title: Re: Why does a candle go out when you blow on it?
Post by: another_someone on 01/11/2006 21:03:44

I think it is more to do with the cooling effect of blowing - the current of air draws the hot gasses away from the flame so fast that the temperature of the flame drops, causing the temperature to drop below that required for combustion of the wick.

Incidentally, on a grander scale, jet engines have been used to put out fires in oil wells by blowing huge amounts of air at the flame, and thus cooling it down (despite the heat of the jet exhaust).

Title: Re: Why does a candle go out when you blow on it?
Post by: neilep on 01/11/2006 21:09:00

Thanks George,

I was going to ask you a secondary question but I think you may have answered it too...I was going to say , what then if you use hot air to blow it out ?...but..I suppose even hot air is a magnitude in temperature cooler than a candle flame.

thanks

Title: Re: Why does a candle go out when you blow on it?
Post by: daveshorts on 01/11/2006 21:48:04

It depends on the type of jet engine. Turbofans basically have a jet engine in the middle which drives a big fan on the front which just blows lots of air back - like a propellor. Modern ones have a bypass ratio (air around the side:air in turbojet) of up to 9:1 in which case the average temperature of the air blowing out the back would be quite low.

Title: Re: Why does a candle go out when you blow on it?
Post by: lightarrow on 01/11/2006 22:27:04

I don't think the cooling effect is the most important here. I believe it has more to do with removing the fuel vapors and moving away the flame from the fuel vapors: they need a certain time to mix with air and then enough heat to be able to burn; so if the vapours are suddenly removed and the flame is moved far away, the new vapours cannot burn immediately because they have the time to mix with air first, but the flame is not there anymore; that is: I think that moving away the flame and removing the vapours, puts out the flame for an instant, and then the time delay I was mentioning prevents the flame to light again.

Title: Re: Why does a candle go out when you blow on it?
Post by: another_someone on 01/11/2006 23:11:24

Quote from: lightarrow on 01/11/2006 22:27:04

I don't think the cooling effect is the most important here. I believe it has more to do with removing the fuel vapors and moving away the flame from the fuel vapors: they need a certain time to mix with air and then enough heat to be able to burn; so if the vapours are suddenly removed and the flame is moved far away, the new vapours cannot burn immediately because they have the time to mix with air first, but the flame is not there anymore; that is: I think that moving away the flame and removing the vapours, puts out the flame for an instant, and then the time delay I was mentioning prevents the flame to light again.

In a sense, this too is a cooling effect, since the fuel, when it mixes with the air around it would still burn if the air is hot enough, but that it has been removed from the source of heat by the time it is ready for combustion is what prevents if from burning.

Title: Re: Why does a candle go out when you blow on it?
Post by: eric l on 02/11/2006 16:02:27

I think it is related to what is called the "speed of the flame front".  If you put fire on a combustible surface, the flame will progress with a more or less constant speed.
If the speed of the air you blow over the candle is higher than this speed of the flame front, you blow the flame away from the wick.
This does not happen when you blow into a coal or wood fire :  the surface is too important to blow the flamefront away from it.