Yellow flame vs Blue flame

[SpyXynt]

So in this picture, the temperature gap between a yellow flame and a blue flame is significantly large. Why is that? 

[exchemist]

41 minutes ago, SpyXynt said:

So in this picture, the temperature gap between a yellow flame and a blue flame is significantly large. Why is that? 

The yellow flame is what you get without premixing of air and gas. This gives less efficient combustion, as shown by the yellow colour which is due, if I recall correctly, to incandescent unburnt carbon particles, formed by thermal cracking of the hydrocarbon fuel before it has a chance to burn. These flames have to burn from the outside in, as air reaches progressively further into the gas stream. 

My understanding is that the blue colour in the (far hotter) premixed flame is not just due to black body emission but to chemiluminescence of some of the species generated during combustion, which are formed in excited states and emit light as they drop down to the ground state.  There is a reference to CH. and OH. radicals, which do this, here:

https://ncbi.nlm.nih.gov/pmc/articles/PMC7345390/

But flames are very  complex systems, chemically speaking, involving branched chain radical reactions in which a large range of molecular fragments take part. 

 

 

Edited January 9 by exchemist

[John Cuthber]

The biggest difference is between the first flame and the subsequent ones.
That's largely because when gas burns in air it has to heat up the  nitrogen that comes along with the oxygen.
Feeding the flame with pure oxygen avoids that.
 

[exchemist]

2 hours ago, John Cuthber said:

The biggest difference is between the first flame and the subsequent ones.
That's largely because when gas burns in air it has to heat up the  nitrogen that comes along with the oxygen.
Feeding the flame with pure oxygen avoids that.
 

Yes fair enough I was thinking more about the colours than the temperatures. A roaring Bunsen flame is also blue but nowhere near as hot as oxyacetylene.

[John Cuthber]

The big difference in colour is caused by soot in the first flame. Because there's not much oxygen, some acetylene decomposes to carbon which is heated to incandescence.

The blue colour of the other flames is due to emissions from hot C2 molecules (if I recall correctly).
A Bunsen flame would be a lot hotter if you could run it with acetylene.
It's a fuel with a higher energy density.