Mixing of air and gaseous fuel

In diffusion burner air and gaseous fuel are supplied separately in the furnace. In most combustion systems mass of air is at least 8 to 9 times that of fuel. When air and fuel pass through the burner, the momentum flux of air is several times greater than fuel. Some fraction of total air is mixed with the fuel and this air is known as primary air. Rest amount of air, known as secondary air is supplied in the furnace through appropriate locations.  Mixing and combustion take place simultaneously.

When a mixture of air and gaseous fuel passes through the burner, a free jet is produced downstream the burner. If the air fuel mixture is discharged in the furnace then a confined jet is produced because of the furnace walls. The difference between the free jet and confined jet is that in the former the amount of surrounding is unlimited whereas in the later the amount is limited. Mixing of secondary air in the jet is important for complete and efficient combustion. In the absence of mixing the chances of  formation can not be ruled out.