Eli5: Why blowing make campfire go bigger but blowing candle make fire disappear?
In: 0
When you blow on a fire you bring cold oxygen to it. The fires tends to be starved for oxygen so bringing in more oxygen will make the fire burn faster. However fires also need to maintain its heat so bringing inn cold air will cool it down. So there is a balance of bringing inn enough oxygen to make the fire bigger but not so much that it cools down the fire. For a small fire like a candle you cool it down too much when you blow on it. But for a larger fire you help bring oxygen to it.
Fire needs three things. Fuel, an oxidizer (commonly oxygen) and heat. When you blow out a candle, a relatively small fire, what you’re doing is cooling it down, removing enough heat that the reaction can’t proceed.
When you blow on a much larger fire you can’t cool it down sufficiently with just your blowing. Instead all you’re doing is providing more oxygen for it to react with.
Everything is about the correct proportions: just as too much or too little oxygen would kill us so too much or too little airflow will kill a fire (without an oxidizer present).
Other than scale candles and log fires do work on slightly different principals. When it comes to the candle you will note that neither the wax nor the wick are themselves on fire. Instead the combustion is occurring slightly above the candle. After you light the wick it does burn for a bit but it liquifies and then evaporates the hydrocarbons that form the wax of the candle. The wick provides a channel for the melted wax to be drawn up to the top where it can then evaporate. These hydrocarbons are then escaping in an upward stream and are then being combusted by the chain reaction of the flame once they’ve mixed with a little air containing O2 (and relatively low CO2 levels). When you blow on the candle you are moving and dispersing that stream of hydrocarbons and at some point it is no longer aligned with the combustion chain reaction and it is no longer dense enough or hot enough for those vapors to keep igniting. At the same time you are moving away the heat source that triggers the melting and evaporation of further hydrocarbon fuel.
With a log fire the fuel for the combustion reaction is coming from the breakdown of the cellulous fibers which is the main component of the wood. This cellulous doesn’t melt but skips straight to a gaseous state and has to fight its way to gaseous pockets in the wood until these gaseous pockets build up enough pressure that they can eventually erupt from the wood. It takes some time and a lot of heat to get this process going. Left to it’s own devices the combustion of these gases occur at various levels above the logs, as and when, they are able to find enough oxygen to react with. As the fire matures the burning penetrates the surface of the wood and you get a coal reaction occurring with the other residual materials. When you are blowing on the log fire you are bringing in fresh oxygen right to the surface of the wood and therefore you are allowing the combustion reaction to occur much closer to and within the surface of the logs/coals. This means that the logs/coals can be better heated by the both the combustion of the volatile gases being released from the lumber and combustion of the remaining dry material in the coals. This leads to quicker breakdown of the remaining cellulous material.
On the camp fire, you are not blowing hard enough.
If I remember my firefighter training well enough, we were told of expensive systems that use air pressure to put out small fires. It’s also plausible for me that you could put out fires with something like a jet engine.
But as these systems are expensive, they are less wide spread – even for firefighters – than the good old water pump, hoses and nozzles.
It’s really simple guys. Big fire breath gives oxygen which fire needs, small fire goes out because of the brief vacuum you create when blow air completely overtop of it.
First, fire hot. Second, fire need air, but too much air bad for fire. Fire need air proportional to size fire.
What are 2 things a fire needs? Fuel and air…
Campfire you blowing adds air to the amount of wood (fuel).
Candle has little amount of fuel so blowing on it overwhelms it.
Latest Answers