What would happen if propane was well-mixed with air?

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Discussion Overview

The discussion revolves around the hypothetical scenario of what would occur if propane were well-mixed with air prior to ignition. Participants explore the implications for combustion characteristics, particularly the color of the flame produced, and the nature of the resulting explosion. The conversation touches on both theoretical and practical aspects of combustion, including comparisons to real-world experiences with propane and other fuels.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that a well-mixed propane-air mixture would result in a blue flame due to complete combustion.
  • Others note that propane flames are typically blue when burned properly, indicating a complete combustion process.
  • One participant expresses uncertainty about whether well-mixed propane would burn invisibly, indicating confusion about combustion characteristics.
  • A participant mentions that adjusting the air-to-fuel ratio in gas appliances can change the flame color from yellow to blue.
  • There is a discussion about the visibility of flames, with references to bunsen burners and the characteristics of different gas mixtures, including methane and propane.
  • Some participants argue that a fireball may not occur, suggesting instead a bright light from the combustion.
  • Another participant explains that the color of the flame relates to the air-to-fuel ratio, with blue flames indicating a lean mixture and yellow flames indicating incomplete combustion.
  • There is a clarification that soot and smoke relate to rich mixtures and are not indicative of flame color.

Areas of Agreement / Disagreement

Participants express a mix of agreement and disagreement regarding the combustion characteristics of well-mixed propane. While some assert that it would burn with a blue flame, others question the visibility of the flame and the nature of the resulting explosion. The discussion remains unresolved with multiple competing views on the topic.

Contextual Notes

Participants reference practical experiences with propane and other fuels, but there are limitations in the assumptions made about combustion conditions and the specific ratios of air to fuel. The discussion does not resolve the complexities of combustion dynamics in this hypothetical scenario.

DaveC426913
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Here in Toronto we were rocked by a giant propane blast. It was so big that some thought it was nuclear dawn; it had both the characteristic mushroom cloud and a visible, expanding shockwave. Seeing the shockwave was awesome.

Despite being located in a residential area (:bugeye:), the only known casualties are a missing employee and a firefighter.

Anyway. That has nothing to do with my question.

The explosion was a giant yellow-orange fireball, just like gasoline or any other. Yellow-orange fire and black smoke is an indication of incomplete combustion. Natrually, this is because the propane was being ignited as it was mixing with air.


I was wondering what might happen in a hypothetical situation, if the air were first well-mixed with a large amount of propane. Would the fireball be blue? Invisible?
 
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I would say it should be blue because the air well mized with fuel would cause a complete combustion.
 
russ_watters said:
I have propane for my house. The flame is blue.
Yes, I know it is on a small scale. On the other hand, isn't it set burning as it's mixed?

I guess I've gotten two things confused here. I was thinking that well-mixed propane burned invisibly. Now I'm not so sure.
 
Well-mixed propane burns with a blue flame. If you have to tune up a grill or a gas stove (we have both) there is generally a damper on each burner's gas feed tube that is open to the air. This allows the gas to entrain air as it flows to the burner head Close the damper off and the flame will be yellow. Open the damper gradually until you get a clean-burning blue flame. That's the setting you want.
 
We used to get nearly invisible flames with bunsen burners, but that's primarily NG (methane).

I think propane and gas flames have near stoichiometric mix and are blue, which indicates a hot flame. I used to adjust acetylene flames to get a bright blue 'feather' in the middle of a dark blue flame, which was almost invisible.

I think thermobaric bombs release a clear cloud fuel (butane), which when ignited, burns bright orange or yellow. LPG is usually a blend of propane and butane.
 
DaveC426913 said:
Yes, I know it is on a small scale. On the other hand, isn't it set burning as it's mixed?
No, just like with a bunsen burner, a stove or grill mixes air with the gas via induction before it is burned.
 
I don't think there would be a fireball... more like a firebang with a bight light (probably bluish white).
 
burning propane is not invisible to humans. The color you see is a black body radiation from the hot molecules. If it's blue, you're seeing the radiation from the CO2/H20 molecules that are very hot. If it's yellow or orange you're seeing glowing carbon atoms, which indicates a rich mixtures (incomplete combustion due to lack of oxygen).

A stoiciometric or leaner mixture will contain only blue flames.

----

Earl Stirling
Swiss Army Knife
http://exploratorium.edu
 
  • #10
does not the color of the flame directly relate to the air to fuel ratio? like when diesel trucks billow black smoke when they floor the accelerator?
 
  • #11
The color only changes when there's not enough oxygen (burning rich). Those diesel particles are soot, which are clumps of carbon molecules... (and you're talking about smoke, not flame). The flames in those conditions would be orange.
 
  • #12
sorry, should have specified. but does that not also lend to the same idea?
 
  • #13
nope
 

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