Gas turbines: effect of a high air–fuel ratio on performance and temperatures Consider a gas turbine operating with a high (lean) air–fuel ratio relative to stoichiometric conditions. Choose the correct outcome that most directly follows from this condition.

Given data

• Operating condition: high air–fuel ratio (lean mixture) in a gas turbine.

Concept/ApproachFor a fixed fuel flow, adding more air (i.e., making the mixture leaner) lowers the adiabatic flame temperature and consequently the turbine inlet and exhaust temperatures. Lower temperatures also reduce blade thermal stress, which is beneficial for durability.

Step-by-step reasoning1) High A/F ⇒ excess air relative to stoichiometric requirement.2) Excess air absorbs heat ⇒ lower gas temperature at turbine inlet and outlet.3) Power output primarily follows turbine inlet temperature; with leaner mixture at the same fuel flow, power typically decreases rather than increases.4) Thermal efficiency has an optimum; too lean can lower efficiency due to reduced mean temperature levels and increased compressor work fraction.

Common pitfallsAssuming that adding more air always increases power is incorrect unless matched with additional fuel to hold turbine inlet temperature constant.

Final AnswerReduces exhaust temperature