In aircraft gas-turbine (jet) propulsion, a very large excess of air is supplied for cooling and complete combustion. For typical operating conditions, what overall air–fuel ratio (by mass) is most representative for a turbojet/turbofan combustion chamber?

Introduction / Context:
In gas-turbine (jet) engines, the combustor uses much more air than the chemically required amount. The question assesses fundamental understanding of why overall air–fuel ratio (AFR) in jet engines is very high compared to the stoichiometric value for hydrocarbon fuels (about 15:1 by mass) and asks for a representative numerical value used in practice.

Given Data / Assumptions:

• Overall AFR refers to total mass of air entering the combustor compared to total fuel, not just the primary zone.
• Stoichiometric AFR for typical aviation kerosene is around 14.7–15:1, but excess air is routed for dilution and liner cooling.
• We are looking for a typical range used at cruise or rated conditions, not a transient or idle point.

Concept / Approach:
Jet engines require high turbine inlet temperature limits and uniform exit temperature profiles. To protect hardware and ensure complete combustion with low emissions, only part of the air participates in initial burning; the rest is added downstream for dilution and wall cooling. Therefore, overall AFR is many times the stoichiometric AFR, commonly 50:1 to 80:1, with about 60:1 being a widely cited representative figure for design estimates.

Step-by-Step Solution:

Verification / Alternative check:
Back-of-envelope: If only ~25–30% of compressor air is used for primary combustion and the rest is for dilution/cooling, then overall AFR ≈ (air total) / fuel ≈ 3 * (stoich AFR) to 4 * (stoich AFR) ≈ 45:1 to 60:1, consistent with 60:1 being a reasonable pick.

Why Other Options Are Wrong:

• 30:1 or 40:1: Too low for typical cruise/full-load overall AFR; would overheat turbine components.
• 50:1: Plausible but slightly on the lower side of typical representative values.
• 80:1: Possible at some conditions, but not commonly quoted as the representative value.

Common Pitfalls:
Confusing stoichiometric AFR with overall combustor AFR, or assuming all air takes part in the initial chemical reaction. Also, mixing local zone AFR with overall system AFR leads to underestimation.

Final Answer:
60:1