bunsen burners

[GiTS]

Why doesnt the flame recede into the tube of a bunsen burner? It premixes with air so the flame of a bunsen burner should travel all the way down to the were the gas comes out of the base. But instead it stays at the top, why?

[Gokul43201]

Why doesnt the flame recede into the tube of a bunsen burner? It premixes with air so the flame of a bunsen burner should travel all the way down to the were the gas comes out of the base. But instead it stays at the top, why?Good question. Have you looked at the slots at the bottom that control the airflow into the tube? If you had to guess, how much parts of air would you say got mixed with each part of gas?

[GiTS]

depends on how open the valve is. idealy, equimolar.
edit: this isnt for homework, i am just curious. especially after reading in an article that bunsen burners will have flashbacks if the tube isnt the right width and height.

[Gokul43201]

Okay, let's go with that - when fully open, say they draw up as much air as there's gas flowing through the pipe. Now remember that air itself is only about 20% oxygen. So the ratio of gas : O2 in the tube is at best 5:1. Now depending on whether you are burning methane or butane or some natural gas mixture that lies in between, you need roughly about 2 to 6 moles of O2 for every mole of gas. So, clearly there's not sufficient oxygen inside the burner tube. It's only when the mixture reaches the top and sees all the additional air that combustion happens.

Some burners, however, let you draw up a lot more air (it depends on the geometry of the intake and flow rates involved), and by doing so you could accidentally burn through the air in the tube and extinguish the flame.