Role of primary air

Thus in the design of burner for gaseous fuel it is important to design the primary air depending on the requirement. Since the amount of air is many times greater than the fuel, momentum flux within the jet is controlled by the primary air.  The primary air

• controls the fuel /air mixing rate
• assists in stabilizing the jet and to control recirculation.

In designing a gaseous fuel burner total mass flow rat of air is subdivided into primary, secondary and tertiary air. Whereas primary air is mixed with fuel, secondary and tertiary amounts of air are introduced through the pots in the furnace. 

Degree of recirculation

Recirculation sets in when the secondary air is mixed completely with the fuel. Degree of recirculation indicates complete mixing of fuel with air.

In confined jets, absence of recirculation results in a tendency for the flame to expand until it impinges into the furnace walls of load. Hot gases will be in direct contact with the refractory brick which may result in failure. The recirculating gases provide a “cushion” of cooler inert flue gases which prevents direct impingement of flame.  A high momentum recirculartory jet will also produce a stable flame which is comfortable.