1. Gas turbine cycles for aircraft propulsion In shaft power cycles, power is in form of generated power. In air craft cycles, whole power is in the form of thrust. Propulsion units include turbojets, turbofans and turboprops In turbojets and turbofans, the whole thrust is generated in propelling nozzles. In turboprops, most of the thrust is produced by a propeller with only a small contribution from exhaust nozzle.

2. Gas turbine cycles for aircraft propulsion

3. Turbojet The turbine is designed to produce just enough power to drive the compressor. The gas leaving the turbine at high pressure and temperature is expanded to atmospheric pressure in a propelling nozzle to produce high velocity jet. The propelling nozzle refers to the component in which the working fluid is expanded to give a high velocity jet.

4. Gas turbine cycles for aircraft propulsion Turbojet

5. Gas turbine parts

7. Compressor and turbine of a Gas turbine

8. Gas turbine cycles for aircraft propulsion Turbojet

10. Turbojet Operation

11. Temperature and pressure distributions

12. Thrust

13. Turbofan Part of the air delivered by an LP compressor or fan bypasses the core of the engine (HP compressor, combustion and turbines) to form an annular propulsive jet or cooler air surrounding the hot jet. This results in a jet of lower mean velocity resulting in better propulsive efficiency and reduced noise.

14. Turbofan

15. Flow in a turbofan

16. Turbofan Thrust

17. Turboprop For lower speed, a combination of propeller and exhaust jet provides the best propulsive efficiency. It has two stage compressor and ‘can-type’ combustion chamber. Turboprops are also designed with a free turbine driving the propeller or propeller plus LP compressor (called free-turbine turboprop).

18. Turboprop

19. Flow in a turboprop

20. Turboprop

21. Comparison

22. Performance Criteria The net momentum thrust is due to the rate of change of momentum Thus, the total thrust is Ca is the velocity of air at inlet relative to engine Cj Velocity of air at exit relative to engine. The net pressure thrust is

23. The propulsion efficiency Propulsive efficiency is a measure of the effectiveness with which the propulsive dust is being used for propelling the aircraft but it is not the efficiency of energy conversion.

24. The propulsion efficiency Energy conversion efficiency Overall efficiency

25. The propulsion efficiency Specific fuel combustion: fuel consumption per unit thrust, i.e. kg/h N = 0.12 Specific thrust, Fs

26. Thermodynamics of air craft engines Diffuser: Velocity decreases in diffuser while pressure increases Nozzle: Velocity increases in nozzle while pressure decreases

27. Thermodynamics of air craft engines Isentropic efficiency of a diffuser

28. Thermodynamics of air craft engines The rest of the components ( compressor, turbine combustion chamber) are treated before. The ram efficiency is Propelling nozzle Propelling nozzle is the component in which the working fluid is expanded to give a high velocity jet. Nozzle Efficiency for adiabatic flow

29. Thermodynamics of air craft engines for unchoked nozzle (Mj<1); P5=Pa For choked nozzle ( Max. rate is reached) M=1, P5=Pc To check if it is choked or not

30. Thermodynamics of air craft engines for choked condition M=1 But isentropic efficiency is

31. Thermodynamics of air craft engines Pc is calculated as substituting for

32. Thermodynamics of air craft engines To calculate A5 of nozzle

33. Thermodynamics of air craft engines Example Simple turbojet cycle

34. Thermodynamics of air craft engines

39. Example:2: Turbofan Analysis Overall pressure ratio given sea level P a =1 bar T a =288 K

40. Thermodynamics of air craft engines State 1 is sea level since Ca=0.0 Required: sfc, Fs

41. Thermodynamics of air craft engines

42. check for choking of both nozzles ( hot and cold)

43. Thermodynamics of air craft engines

44. C7= 476 m/s for cold nozzle ( do same) note: Nozzles are independent of each other regarding choking.

45. Thermodynamics of air craft engines Notes: a8=344.2; M8<1

46. Thermodynamics of air craft engines