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.