How can I blow out a candle

Why can candles be blown out?

They have once again gilded the Christmas season for us ... In the end, a short blow was enough for the candle flames to turn into thin wisps of smoke. As is well known, they can be blown out without any problems, while it looks completely different with a fireplace or campfire: Here a draft of air really makes the flames blaze. Why does fire react so differently and why can a candle or a match be blown out at all?

Bernd Binninger from the Institute for Technical Combustion at the University of Aachen knows the answer: "When you blow, the flame is blown so far away from the wick that the flame goes out when it cools down and the fuel supply is cut off," he explains. Because for it to burn at all, three things are always necessary: ​​fuel, oxygen and heat. In the case of a candle, the fuel is the vaporized wax, and the oxygen is provided by the air. In order to start a combustion process, all that is missing is the ignition temperature of around 250 degrees. If the candle burns, it then generates the necessary heat itself.

The wick, on the other hand, does not play a direct role in the burning process: it only serves to guide the liquid wax. When burning, a gas mixture is created around the wick, which, however, only meets the conditions for combustion after a certain distance. “What we see as a flame near the wick is a thin, luminous layer, the reaction zone, which spreads at a certain burning rate,” says Binninger. Blowing creates a current away from the wick, which the flame has nothing to oppose. In addition, the proportion of fuel and the temperature decrease, so that the burning rate also drops. "The flame has no way of getting near the wick and goes out."

Large flames, such as those from a campfire, are too big to blow out. “Our breath is not enough to create the effect of blowing out in enough places at the same time,” explains Binninger. In this case, a flow of air has the opposite effect: it supplies the flames with more oxygen, which starts the combustion. "In principle, however, any flame can be blown out - all that is required is a correspondingly strong and spatially extended air flow," says Binninger.

© Wissenschaft.de - Martin Vieweg / dapd
December 30, 2014