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Unsure if this is more chemistry or physics, but here goes...

Consider a Bunsen burner and the flame it produces when the air inlets are closed vs open. According to the description on the linked Wikipedia page, combustion is incomplete when the air inlets are closed, but essentially complete when the air inlets are suitably adjusted.

This raises a number of questions about what is going on in each scenario.

Why is combustion incomplete when the inlets are closed? Wouldn't gases mix as they rise up allowing most or all of the fuel to burn? Where exactly is combustion occurring relative to the incandescence? Obviously no lower than the tip of the burner where the fuel gas meets air, but how far up the visible flame? According to the answer to this question, combustion is only occurring at the outer shell, but how does fuel remain unexposed to air until it is too cool to react?

It would seem that even when the inlets are open, combustion only starts at the tip of the burner, not inside the barrel... why? What prevents the flame from migrating down inside the barrel where fuel and air are already mixing?

Motivating my question is a desire to understand how my gas fireplace can make a pretty yellow flame while the flames of a gas cooktop or gas barbecue are blue.

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    $\begingroup$ This is a remarkably complicated set of inter-related questions! There is a large literature on flames, their chemistries and properties. Michael Faraday even wrote the classic book on candle flames. I am upvoting, but you really should go for one tight question at a time! By the way, having the flame propagate down the barrel can lead to a memorably loud event called a flashback. The burner heads in flame atomic absorption spectrometers are tied down to prevent burner head launching if a flashback happens in the pre-mixed acetylene and air combustion mixture. $\endgroup$
    – Ed V
    Dec 1 '21 at 2:34
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    $\begingroup$ Does this answer your question? Why are hydrocarbon flames yellow or blue? $\endgroup$
    – Mithoron
    Dec 1 '21 at 20:23
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    $\begingroup$ Considering that there are already very high quality answers here specific to the historically important invention of the Bunsen burner, it's principle of operation and observed behaviors, I think serves the site and future readers better to keep this question open rather than close as duplicate and direct views away from it. Therefore I'm voting to leave open Related in meta and so far unanswered: Have there been analyses to see if views of well-received answers are reduced by closure as duplicate which try to control for other factors? $\endgroup$
    – uhoh
    Dec 2 '21 at 0:55
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    $\begingroup$ Not sure if the fireplace is supposed to look like that, but the yellow / blue colour can indicate an incomplete combustion (eventually too much CO, not only soot). If the fireplace is something related to camping, then it can be also because you burn butane instead of methane. $\endgroup$
    – Alchimista
    Dec 2 '21 at 12:02
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    $\begingroup$ I say leave open, vote other question as duplicate. $\endgroup$
    – A.K.
    Dec 6 '21 at 22:04
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The yellow emission has a continuous spectrum. So it is not due to an atom or of a molecule. It is due to a solid black body, hot enough to emit light (about $1000$°C). The solid stuff is solid carbon $\ce{C_n}$ with an unknown but rather high $n$ value. The presence of solid carbon may be demonstrated by inserting a cold glass tube for a while in the yellow flame. It soon becomes covered by black soot.

The blue flame has been analyzed by Herzberg in $1939$. Its spectrum is made of thin lines belonging to the unstable molecule $\ce{C2}$. The distance between the lines is typical of the vibration spectrum of the diatomic molecule $\ce{C2}$.

So when burning, the scenario is the following : The methane (or any other alcane) molecule begins by loosing its $\ce{H}$ atoms, which burn to make $\ce{H2O}$. Then the remaining $\ce{C}$ atoms start polymerization to produce first $\ce{C2}$ which emits a blue light. Then if enough oxygen is available, the $\ce{C2}$ molecules are oxidized into $\ce{CO2}$. But if Oxygen is lacking, the polymerization goes on to produce a black body (soot) which is warm enough to emit a continuous light. At the end, when the hot gas gets out of the flame, this hot carbon polymer is oxidized by outer air into $\ce{CO2}$ without producing light.

Of course, if oxygen is seriously lacking, the flame may produce black deposit of soot, but this never happens in a Bunsen burner.

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    $\begingroup$ This answer is an excellent compliment to M. Farooq's answer, providing the chemical reaction details that suppliment M. Farooq's fluid dynamics analysis. $\endgroup$ Dec 1 '21 at 12:34
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    $\begingroup$ @electronpusher Entirely agree with you and I upvoted both answers and the question. Now maybe someone will ask about the reverse flames mentioned by M.Farooq or ask about the flame experiments that were conducted on the ISS! $\endgroup$
    – Ed V
    Dec 1 '21 at 14:10
  • $\begingroup$ This is a good answer about what happens in flames, but it misses the actual questions posed. $\endgroup$
    – matt_black
    Dec 4 '21 at 0:49
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    $\begingroup$ @matt black. The question asked was : What is going on in and below the yellow and the blue gas flame ? I have given the answer : The blue flame is made of $\ce{C2}$ molecules. The yellow flame is made of solid carbon polymer $\ce{Cn}$, with $n$ is a huge number. $\endgroup$
    – Maurice
    Dec 4 '21 at 13:14
  • $\begingroup$ @Maurice But the context given in the full question also pointed out "a desire to understand how my gas fireplace can make a pretty yellow flame" and asked "Why is combustion incomplete when the inlets are closed?" $\endgroup$
    – matt_black
    Dec 5 '21 at 14:10
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As Ed V has told you, there are plenty of mixed questions, but very interesting ones. I will try to address your key question.

Consider a Bunsen burner and the flame it produces when the air inlets are closed vs open. According to the description on the linked Wikipedia page, combustion is incomplete when the air inlets are closed, but essentially complete when the air inlets are suitably adjusted.

The key point is the Bernoulli's principle in action in a Bunsen burner. If look at the base of the burner's, fuel enters the vertical tube via a very narrow orfice (see part "a" in the picture). Therefore, the velocity of the fuel is pretty high at the base of the burner. This also creates vacuum at the base of the burner. Depending on the size of the air opening at the base (adjustable in modern burners), air is drawn in.

Now the diameter of the Burner tube is not very narrow so there is no turbulence (my own speculation) rather the gases (fuel and oxidant) have a laminar flow. In perfect laminar flow, fluids should not mix at all. The blue cone in the flame reflects the shape of laminar flow of the gases. Fastest in the center of the tube and slower near the tube periphery. The amount of oxygen entering the orifice is still not sufficient to burn all the fuel.

You have to rely on the diffusion of oxygen in the air to burn the remaining fuel.

Important point: In a pure blue flame, there is no incandescence. This blue color is molecular emission.

When the inlet is closed, air supply is definitely limited. The flame is relatively yellowish. The cone vanishes and now you have incandescence from soot particles forming from incomplete combustion.

What is rather interesting, and I saw a video long time ago on YouTube was the question, what if you supply oxygen and burn the flame in the atmosphere of the fuel gas. All these flame structures vanish in a so-called reverse flame.

Picture taken from J. Chem. Educ. 2000, 77, 5, 558 Burner

EDIT: A reader asks the evidence of laminar flow in the comment. Think of the diameter of the Bunsen burner tube, and think of the gas velocities needed to create an Reynold's number greater than 2000. With turbulene, you will not get a beautiful blue cone (indicative of laminar flow). See the example here Turbulence in flames

enter image description here

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    $\begingroup$ I think your speculation on turbulence is exactly wrong. The setup in the bunsen encourages effective fuel/air mixing and that, most likely, involves turbulence (unless you have some evidence otherwise). $\endgroup$
    – matt_black
    Dec 5 '21 at 14:14
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    $\begingroup$ Yes turbulence in flames exist but not in normal conditions. You need very high gas velocities. Added pictures. If you have flame spectroscopy with nitrous oxide, you will see the flame is crazily turbulent with an eerie sunset like glow! $\endgroup$
    – M. Farooq
    Dec 5 '21 at 14:30
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    $\begingroup$ The issue isn't about what the flow in the flame looks like: it is about mixing gases in the bunsen tube far below the flame. The flow at the exit if far slower and less turbulent than the flow around the gas nozzle at the bottom. $\endgroup$
    – matt_black
    Dec 5 '21 at 14:37
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    $\begingroup$ Did you read the linked paper in the edited section? There is no turbulence in ordinary flame or in the Bunsen burner tube! In a Bunsen burner tube which is about 1 cm wide, can you calculate the gas velocities required to generate a Reynold's number of 2000? $\endgroup$
    – M. Farooq
    Dec 5 '21 at 15:31
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    $\begingroup$ The abstract talks about flames, I can't tell if it talks about what happens at the bottom of the tube. The fuel in a bunsen in emitted from a very narrow orifice and is, therefore, moving a great deal faster than the (mixed) gas at the top of the tube. (ratios are maybe 100 sq mm at the top to perhaps 1/100 sq mm at the orifice). $\endgroup$
    – matt_black
    Dec 5 '21 at 21:43
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Bunsen Burners are specifically designed for the flame not to backpropagate into the tube

The other answers give good descriptions of what is happening in the flame under different conditions. But what is missing is that Bunsen Burners are designed to achieve the properties you observe.

For a "blue" flame where the air flows into the burner at the bottom and is well mixed with the fuel (which is what the gas nozzle and the tube are designed to achieve), the rate of gas flow is part of the design. And that rate is intended to produce a hot, well mixed flame where the gas flows fast enough to prevent the flame propagating down the tube.

When the air inlet is closed, the gas in the tube is mostly fuel and mixing only occurs above the top of the tube and is much less efficient, hence the yellow flame resulting from only partial combustion. The flame can't propagate down the tube as there is no air to enable the fuel to burn. The mixing is less efficient, by the way, because it is limited by the slower speed of the gas at the top of the tube compared to the fast speed of the fuel emerging from the small aperture in the fuel inlet which causes turbulent mixing inside the tube with air when the bottom inlet is open.

Most burners in cookers and fires are designed to work for a specific fuel and create the ideal well-mixed fuel/air flow at the outlet of their burners (and the mix depends on the fuel which is why different burners are needed for different fuels–as when UK gas cookers had to be retrofitted with new burners when goal gas (a mix of hydrogen and CO) was replaced by natural gas (mostly methane) in the 1960s).

So, if you want to adjust a flame in your fire to be yellow for a pleasant effect, you will have to fiddle with the burner heads to adjust the incoming flow of air in those burner heads. This might be very hard if the burners are not designed to be adjustable and will have the (possibly undesired) effect of greatly reducing the amount of heat given out.

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    $\begingroup$ It still leaves a bit of mystery as to why well-mixed fuel and air starts burning exactly at the very tip of the Bunsen tube and not over a region inside or above. Same goes for any other clean-burning gas appliance. I guess the question might be how does a flame holder so neatly hold the flame of already-well-mixed fuel and air that one might think could already start burning somewhere else? $\endgroup$
    – Anthony X
    Dec 3 '21 at 23:40
  • $\begingroup$ @AnthonyX It isn't that much of a mystery if you recognise that these devices are designed. A generic burner can have all sorts of pathologies including the flame back propagating down the tube. But a Bunsen burner or cooker burner is designed to get the flow rates right so the gas flow balances the speed of flame propagation so the edge of the flame stays close to the tip of the burner. $\endgroup$
    – matt_black
    Dec 4 '21 at 0:47

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