Why Does a Bunsen Burner Flame Stay at the Top?”

  • Context: High School 
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Discussion Overview

The discussion centers on the behavior of the flame in a Bunsen burner, specifically why the flame remains at the top of the burner rather than receding into the tube. Participants explore the mixing of gas and air, combustion conditions, and the design of the burner.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions why the flame does not recede into the tube, suggesting that since the gas premixes with air, it should travel down to the base.
  • Another participant points out the importance of the airflow control slots at the bottom of the burner and asks about the air-to-gas mixing ratio.
  • A different participant notes that the mixing ratio depends on how open the valve is, suggesting that ideally it would be equimolar.
  • One participant elaborates that when the valve is fully open, the air drawn in is only about 20% oxygen, leading to a gas-to-oxygen ratio that may not support combustion within the tube.
  • This participant also mentions that different types of gas (methane, butane, or mixtures) require varying amounts of oxygen for combustion, indicating that the flame only ignites when the mixture reaches the top where more air is available.
  • Additionally, they caution that some burners can draw in excessive air, potentially leading to extinguishing the flame if the air supply exceeds the gas flow.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding the mixing of air and gas, the combustion process, and the design of the burner. No consensus is reached, and multiple competing explanations are presented.

Contextual Notes

Participants discuss the implications of burner design, such as the geometry of the intake and flow rates, which may affect combustion efficiency and flame stability. There is an acknowledgment of the complexity involved in the combustion process, including the specific gas used.

GiTS
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Why doesn't 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?
 
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GiTS said:
Why doesn't 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?
 
depends on how open the valve is. idealy, equimolar.
edit: this isn't for homework, i am just curious. especially after reading in an article that bunsen burners will have flashbacks if the tube isn't the right width and height.
 
Last edited:
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.
 

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