GATE AE 2018 • Q26 MCQ • 1 Mark Propulsion

Propulsion

Compressors and Turbines

Subtopic: Axial Flow Gas Turbines & Blade Cooling

Question Statement:

In an ideal Brayton cycle for a gas turbine engine, which of the following processes correctly represents the heat addition and rejection?

Options:

  • (A) Heat addition at constant volume, heat rejection at constant pressure
  • (B) Heat addition at constant pressure, heat rejection at constant pressure
  • (C) Heat addition at constant volume, heat rejection at constant volume
  • (D) Heat addition at constant temperature, heat rejection at constant temperature

Official Answer Key:

B

Step-by-Step Solution & Explanation

### **Detailed Step-by-Step Solution (GATE AE 2018 Q.26)** **1. Intuitive Physical Setup & Given Parameters**: - The standard air-standard Brayton cycle models ideal gas turbine and jet engine operation. **2. Step-by-Step Thermodynamic Derivation**: 1. **Four Ideal Processes of Brayton Cycle**: - $1 \to 2$: Isentropic (reversible adiabatic) compression in the compressor. - $2 \to 3$: Isobaric (constant pressure) heat addition in the combustor ($P_2 = P_3$). - $3 \to 4$: Isentropic expansion in the turbine and nozzle. - $4 \to 1$: Isobaric (constant pressure) heat rejection to the atmosphere ($P_4 = P_1$). 2. Both heat addition and heat rejection occur at constant pressure. **3. Comprehensive Options & Distractor Analysis**: - **(A)** (constant volume addition): ❌ Incorrect. That describes the Otto or Dual cycle. - **(B)** (constant pressure addition and rejection): ✅ **CORRECT**. Defines the isobaric combustor and exhaust of the Brayton cycle. - **(C)** (constant volume addition and rejection): ❌ Incorrect. That is the Otto cycle. - **(D)** (isothermal addition and rejection): ❌ Incorrect. That is the Carnot or Ericsson cycle. **4. Official Master Key & Exam Takeaway**: - **Official Answer**: **B** - **Exam Rule**: Brayton cycle = 2 Isentropics + 2 Isobars.
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