Which term describes air externally supplied to the flame at a point of combustion?

• A. Primary air
• B. Secondary air
• C. Processed air
• D. Outside air

Answer: (B)

Air streams in a gas burner are divided into primary air and secondary air. Primary air mixes with the gas before it enters the burner to set the fuel–air ratio. Secondary air is supplied at the flame zone to complete combustion and stabilize the flame. The description “air externally supplied to the flame at a point of combustion” fits secondary air because it is introduced directly into the flame area, not mixed with gas beforehand. Outside air and processed air refer to different concepts and don’t specifically describe air added at the flame itself.