Definition
In a turbine engine combustion chamber, the portion of compressor discharge air that bypasses the immediate combustion zone and is used to cool the combustion liner and to mix with and cool the hot combustion gases before they reach the turbine section.
Plain English
It is the part of the air coming out of the compressor that is not burned with the fuel. Instead, it flows around and through holes in the combustion chamber to keep the metal cool and to bring the very hot gases down to a temperature the turbine blades can survive.
Context Anchor
Seen in turbine engine discussions, especially when explaining how a turbofan splits incoming air into air that goes through the engine core and air that flows around it.
Derivation
Secondary means second in order or importance. The primary air stream is the smaller portion that mixes directly with fuel and burns. The secondary stream is the larger portion that does the cooling and dilution work afterward, hence its position as the second-named stream.
Why Pilots Care
In high-bypass engines the secondary air stream produces most of the total thrust while improving fuel efficiency and lowering noise.
Grounding Statement
Picture the fan at the front of a jet engine pushing some air around the hot center of the engine, then sending that air out the back to help move the airplane.
Intuition Check
Secondary does not mean unimportant here. It means the second path of air: air that goes around the engine core instead of passing through the hot gas-producing section.
Example Sentence 1
About 75 percent of the compressor discharge air becomes the secondary air stream, cooling the combustion liner and diluting the hot gases before they enter the turbine.
Example Sentence 2
During cruise the secondary air stream contributes to propulsive efficiency by moving a greater mass of air at lower velocity.