Diffusion and Premixed Flames

Diffusion flames are not as efficient as premixed flames. The fuel and oxidiser are not well mixed when they react, so oxidation reactions are incomplete and lead to the formation of CO and soot, both primary components of smoke.

Heated soot particles emit energy as visible light, giving diffusion flames their characteristic orange-yellow colour.

Different combustion efficiencies in the two pool fires shown in Figure 32 lead to different flame plumes and burning characteristics. On the left is a methanol-fuelled fire and on the right is an acetone-fuelled one; both are in the same size of dish. The methanol flame is an example of a pre-mixed flame where the fuel and air are able to mix prior to combustion. The acetone flame is an example of a diffusion flame where flaming occurs as the fuel and oxygen mix in the flame plume. The pale blue flame of the methanol fire indicates better combustion efficiency when compared to the taller, brighter, yellow-orange acetone flame.

Laminar and Turbulent Flames

Figure 33: Laminar and turbulent flames

Turbulent flames or flowing gases appear rough and crinkled around the edges and appear to be made up of many smaller cells. Considerable mixing between the fuel vapour and air takes place within and along the edges of turbulent flames, both where the flame is observed and where no flame is present.