Gas Turbine Engine Fundamentals and Thermodynamics
Domain focus: This section tests the thermodynamic and mechanical principles that explain how a gas turbine produces thrust or shaft power, and how pressure, temperature and velocity change through each module.
Core rules
- The engine operates on the Brayton cycle: compression, constant-pressure heat addition, expansion and exhaust. Ideal thermal efficiency depends mainly on pressure ratio; specific work depends on turbine entry temperature and component efficiencies.
- Station numbering is used in manuals and calculations: 1 inlet, 2 compressor face, 3 HP compressor delivery, 4 combustor exit, 5 turbine exit, 8 exhaust. Use total (stagnation) values when calculating compressor and turbine work.
- A diffuser converts velocity into pressure; a nozzle converts pressure into velocity. Thrust is the momentum change of the airflow plus any pressure thrust: F = ṁ(Vj − V0) + (pj − p0)Aj.
- Turbine work must balance compressor work, accessory load and any power offtake; the surplus in a turbojet appears as high-velocity exhaust, while a turboprop or turboshaft uses a free turbine or reduction gear.
Worked example
An engine has pressure ratio 12, inlet total temperature 288 K and compressor isentropic efficiency 0.85. Find compressor delivery temperature. For air, (γ−1)/γ = 0.2857, so 12^0.2857 = 2.034. Isentropic temperature rise = 288(2.034 − 1) = 297.8 K. Actual rise = 297.8/0.85 = 350.4 K. Delivery total temperature = 288 + 350.4 = 638 K (about 365 °C). The reasoning: pressure ratio fixes the ideal temperature rise; inefficiency adds extra heat, so the real delivery temperature is higher. This is why compressor efficiency matters for fuel burn and turbine cooling.
Traps
Do not use Carnot efficiency for a gas turbine; do not confuse pressure ratio with compression ratio; do not ignore bleed air and power offtake; do not treat gauge pressure as absolute; do not assume exhaust velocity alone gives thrust when pressure thrust is present.