Gas turbines – effects of a high air–fuel ratio on performance and hardware In a gas-turbine combustor, if the overall air–fuel ratio is made high (i.e., the mixture is lean with much more air than stoichiometric), which outcome is most characteristic of this condition?

Introduction / Context:
A key knob in gas-turbine operation is the overall air–fuel ratio. Most practical engines run very lean (large air–fuel ratio) to control turbine inlet temperature, meet emissions, and protect turbine blades. Understanding how this ratio influences temperatures, power, and efficiency is essential for gas-turbine performance analysis.

Given Data / Assumptions:

• Simple Brayton cycle viewpoint with combustion at (approximately) constant pressure.
• High air–fuel ratio means excess air relative to stoichiometric requirements.
• Comparisons are qualitative for a fixed fuel flow unless noted.

Concept / Approach:
At constant fuel flow, adding more air dilutes the combustion products, lowering flame temperature and consequently reducing turbine inlet and exhaust temperatures. Lower gas temperatures at turbine exit also reduce available exhaust enthalpy. While component maps and control schedules complicate details, the first-order effect of going leaner is cooler gas at the turbine and in the exhaust.

Step-by-Step Solution:
Model combustion as a mixing/heat-release process at nearly constant pressure.Increase total mass flow of air for the same fuel → mixture temperature drops because the released chemical energy is shared by more mass.Lower turbine inlet temperature leads to lower exhaust temperature after expansion.Therefore, the outcome most consistently associated with a high air–fuel ratio is reduced exhaust temperature.

Verification / Alternative check:
Engine test data commonly show exhaust gas temperature (EGT) decreasing as mixture is leaned (within stable-combustion limits). Control systems often schedule extra air for blade life and emissions, aware of the EGT impact.

Why Other Options Are Wrong:

• Improves thermal efficiency: not necessarily; efficiency depends on pressure ratio, component efficiencies, and firing temperature. Leaner mixtures alone do not guarantee higher cycle efficiency.
• Do not damage turbine blades: the intent is blade protection, but the statement is not a guaranteed outcome; damage depends on temperature, cooling, and materials.
• Increases power output / raises turbine inlet temperature: the opposite is typical when only air is increased for the same fuel flow.

Common Pitfalls:
Assuming any change in air always improves efficiency; in reality, cycle design and permissible firing temperature dominate efficiency trends.

Final Answer:
reduces exhaust temperature