In an aircraft gas turbine engine combustor, the primary zone equivalence ratio is typically maintained close to stoichiometric mainly to:
Options:
(A) Minimize turbine inlet temperature
(B) Prevent compressor surge
(C) Ensure flame stability and high combustion efficiency
(D) Eliminate NOx emissions completely
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Detailed Step-by-Step Solution (GATE AE 2018 Q.28)**
**1. Intuitive Physical Setup & Given Parameters**:
- In gas turbine combustors, air from the compressor is split into primary, secondary, and dilution zones.
- Primary zone receives approximately 15–20% of the total airflow to mix with 100% of the injected fuel.
**2. Step-by-Step Physical Derivation**:
1. **Flammability Limits & Reaction Rates**:
- Hydrocarbon fuel burning rates reach their maximum near stoichiometric conditions ($\phi \approx 1.0$).
- High temperatures in the primary zone accelerate chemical kinetics, ensuring rapid ignition, robust flame stabilization, and preventing blow-off.
2. **Dilution Cooling**:
- Downstream secondary and dilution air then reduce the temperature to the allowable Turbine Inlet Temperature (TIT).
**3. Comprehensive Options & Distractor Analysis**:
- **(A)** (minimize TIT): ❌ Incorrect. Dilution air controls TIT; stoichiometric burning maximizes temperature.
- **(B)** (prevent surge): ❌ Incorrect. Surge is an aerodynamic stall phenomenon of the compressor.
- **(C)** (ensure flame stability and high efficiency): ✅ **CORRECT**. Near-stoichiometric primary zone ensures rapid chemical kinetics and anchoring of the flame.
- **(D)** (eliminate NOx): ❌ Incorrect. Near-stoichiometric temperatures actually increase thermal NOx.
**4. Official Master Key & Exam Takeaway**:
- **Official Answer**: **C**
- **Exam Rule**: Primary zone = flame stabilization ($\\phi \approx 1$); Dilution zone = temperature profiling for turbine.
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