Which of the following statements regarding combustion in gas turbine combustors and reheat afterburners are **CORRECT**?

(A) Combustion in the primary zone of a conventional gas turbine combustor operates close to stoichiometric conditions to ensure high flame speed and combustion stability

(B) Secondary and dilution air are introduced downstream to cool the combustion gases down to the allowable turbine entry temperature (TET)

(C) In an afterburner (reheat system), the stagnation pressure increases across the flame holder due to Rayleigh heat addition in subsonic flow

(D) Rayleigh heat addition to a subsonic flow inside an afterburner causes an increase in flow Mach number and a decrease in total pressure

Answer Key:

Verified in official key

### 1. Strategy & Theoretical Principles 1. **Gas Turbine Combustor Air Distribution:** - Overall air-fuel ratio in a gas turbine is highly lean (e.g., $50:1$ to $80:1$) to avoid exceeding materials limits at the turbine entry. - However, a mixture that lean cannot sustain a stable flame (lean flammability limit). - Therefore, the combustor is zoned: only $15-20\%$ of the air enters the **primary zone** where combustion occurs near stoichiometric (equivalence ratio $\phi \approx 1.0$), ensuring peak flame temperatures, fast chemical kinetics, and flame stability. - Dilution air is subsequently mixed in to lower the gas temperature to the metallurgical limit ($T_{04}$). 2. **Rayleigh Heat Addition Mechanics:** - Rayleigh line flow governs 1D flow with heat addition in a constant area duct. - For **subsonic flow** ($M < 1$): - Heat addition causes Mach number to *increase* ($M \to 1$). - Total temperature $T_0$ increases. - Stagnation pressure $p_0$ *always decreases* ($dp_0 < 0$) due to thermal choking effects (fundamental thermodynamic irreversibility). ### 2. Statement Evaluation - **Statement A:** **CORRECT.** Primary zone runs near-stoichiometric for maximum reaction rate and flame stability. - **Statement B:** **CORRECT.** Dilution holes tailor the temperature profile to protect turbine rotor blades. - **Statement C:** **INCORRECT.** Heat addition to a subsonic stream *decreases* stagnation pressure ($dp_0 < 0$), it never increases it. - **Statement D:** **CORRECT.** By 1D compressible flow relations, heat addition in subsonic flow accelerates the flow ($M \uparrow$) and drops total pressure ($p_0 \downarrow$). ### 3. Exam Trap & Standard Reference - **Trap Alert:** A frequent misconception is believing that adding heat increases total pressure. In flowing fluids (Rayleigh flow), total pressure drops when heat is added. - **Standard Reference:** Mattingly, J.D., *Elements of Propulsion*, AIAA Education Series, Chapter 2 & 7.

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