Diffusion flame

[2] Diffusion flames tend to burn slower and to produce more soot than premixed flames because there may not be sufficient oxidizer for the reaction to go to completion, although there are some exceptions to the rule.

Diffusion flames are often studied in counter flow (also called opposed jet) burners.

[citation needed][3] Their interest is due to possible application in the flamelet model for turbulent combustion.

These are also known under the name of "edge flames", characterized by a local extinction on their axis because of the high strain rates in the vicinity of the stagnation point.

There is no convection to carry the hot combustion products away from the fuel source, which results in a spherical flame front, such as in the candle seen here.

In a diffusion flame , combustion takes place at the flame surface only, where the fuel meets oxygen in the right concentration - the interior of the flame contains unburnt fuel. This is opposite to combustion in a premixed flame . The fire breather's spurting of fuel (likely kerosene ), combined with strong convection flows due to intense heat gives a turbulent diffusion flame.
The common flame of a candle is a classic example of a diffusion flame. The yellow color of the flame is due to the large number of incandescent soot particles in the incomplete combustion reaction of the flame.