Aircraft Jet Engine Power Cycles Optimization: Technology Trends

1. Lecture 5 Shaft power cycles Cycle selection Technology trends Aircraft engine performance Thrust and propulsion efficiency Intakes and engine installation Theory 5.1 and 5.2 Problem 3.1

2. Simple ideal cycle – max. efficiency • Maximum thermal efficiency when compressor exit temp. = max allowed turbine inlet temp. • T3 = 1500 => η = 81% efficiency attained at rc = 320 • In practice titanium alloy compressor rotors withstand around 870 K and nickel alloys around 990 K • T3 = 990 K => η = 71% efficiency attained at rc = 75 T2 => Tmax

3. Simple real cycle – max. efficiency Conservative assumptions (old technology) • Setting: • T3 = 1500 => η∞,c= 87%, η∞,t= 85%, 5% burner pressure drop and 99% mechanical efficiency => no power delivered at rc = 143 (far above allowable t2). • For real cycle maximum efficiency obtained for (with data above) • rc = 38 at which a cycle efficiency of η = 43.8% is obtained.

4. Curve is fairly flat around optimum • Lower pressure ratio may be taken with small perfor-mance penalty • Lower pressure ratio is cheaper to manufacture and maintain

5. Suitable compromise in this case is: • rc = 20 • Selecting a lower rc also • Reduces the number of required turbomachinerystages • Allows more efficientcooling (Tc low) Between 0.6-0.7 is current state of the art.

6. GTX 100 π = 19.2 T3 = 1570 K

7. Technology improvements – permissible T3 • T3 is increasing with 8 K/year

8. Technology improvements – permissible T3

9. Cycle Efficiency • Optimal pressure ratioincrease with t3. • Gain in efficiency becomes marginal as T3 increases

10. Specific output • Considerable increasein specific outputwith increasing t3. • Trend: • Increase T3 to increasespecific output • Follow with rc to obtain high efficiency

11. Heat exchange cycle • Recall: • Heat exchanger useful when: • Optimum for r > 1in real cycle. • Optimum rc increasewith t3. • Gain in efficiency with t3 is greater than for simple cycle • Power output curves about the same • Cooling simplified – t2 low

12. 131 Heat exchanger versus simple cycle • Heat exchanger cycle: • IRA (intercooled recuperatedpromises increased fuelefficiency). • More efficient cooling • Heavy and bulky

13. Which cycle has the highest ideal efficiency? Theoretically the same!

14. The performance of the jet engine Aircraft aerodynamics Covered by the Henrik Ekstrand material Characteristics of the atmosphere What are the fundamentals of flight?

15. Aircraft propulsion – thrust generation

16. Jet engine – principles of thrust generation

17. Efficiency considerations How much of the power in the jet is transformed to thrust ?:

18. ηp is at maximum when Cj=Ca but then the thrust is zero. Make difference as small as possible, still obtaining the necessary thrust =>classes of engines!!! Efficiency considerations

19. Further efficiency considerations Note that:

20. Decrease T3 => Decrease jet velocity Poor cycle efficiency Poor specific output => high engine weight What if we could: Use high T3 and rc cycle and still obtain an average low Cj, optimized for the aircraft speed Ca !? How should we design the engine

21. Propulsion engines – families The turbofan: BPR 0+-10

22. Propulsion engines – families The turboprop: BPR typically around 25-30

23. High speed flight requires high specific thrust RM12 engine powering the Swedish GRIPEN fighter – Military turbofan (low bpr)

24. Very high speed flight requires very high specific thrust Variable intake optimize aerodynamic performance of “shock-compression” system

25. Approximation to conditions averaged over location and season Deviations largest at sea level Deviations largest in temperature The International Standard Atmosphere (ISA)

26. The International Standard Atmosphere (ISA) • Temperature drops with 6.5K per 1000 meters • At 11000 m variation stops and T remains constant up to 20000 meters • Pressure variation can be computed by simple integration of hydrostatic effects.

27. Hydrostatic integration....

28. Intakes • Adiabatic duct used to recover kinetic energy in air at minimal pressure loss, i.e. we have Another example of the first law for open systems with no heat or work exchange (same idea as for the nozzle)

29. Intake efficiencies • The available stagnation temperature is: T´01is the stagnation temperature that would have been necessary to achieve P01 under isentropic conditions, i.e.: Some algebra gives (as well as definition of Mach number and a relation for cp):

30. Intakes • Design criteria: • Minimize inlet compressor inlet distortion • Distortion may lead to surge => flame out or mechanical damages

31. Static conditions, very low aircraft speeds Intake acts as a nozzle Cruise – normal forward speeds Intake performs as diffuser Supersonic operation System of shock waves followed by a subsonic diffusion section Functionality

32. Supersonic intakes • Pressure recovery factor is used: where: A rough rule of thumb published by the Department of Defense is:

33. SR71 – intake ram pressure ratio

34. SR71 – intake ram pressure ratio The ram pressure rise is estimated using the following expression: where the second factor in the left hand expression is obtained from: Includes both shock and viscous losses the first factor is obtained from:

35. SR71 – intake ram pressure ratio The subsonic part is calculated from from our “universal” assumption of ηi=0.93, i.e: The shock pressure recovery factor is estimated by the crude formula stated by the Department of Defence (assuming a cruise Mach number of 3.0):

36. SR71 – intake ram pressure ratio and thus the pressure recovery factor is: The pressure ratio over the intake can finally be estimated to: This is a very crude approximation methodology, but it gives a demonstration of the considerable pressure ratio that a successfully designed inlet may give. It also illustrates why the ram jet engine provides a thermodynamically attractive cycle at very high speeds.

37. Some examples of engine installation

38. Wing mounted pod installation (attached to the wing by pylons): Engine installation examples • Third engine buried in the tail fuselage

39. Theory 5.1 – Stagnation pressure for isentropic compression We have already introduced the stagnation temperature as: and shown that (revision task): The specific heat ratio γ is defined: The Mach number is defined as:

40. Theory 5.1 – Stagnation pressure for isentropic compression Thus: but we have: which directly gives:

41. Theory 5.2 - Continuity in stagnation property form

42. Theory 5.2 - Continuity in stagnation property form Thus: extremely powerful.

43. Have an understanding for the propulsive efficiency concept and how it: relates to the total efficiency relates to the different jet engine types available Have a quantitative understanding of how real cycle effects impact cycle efficiency and choice of design conditions Have a basic understanding of how intakes work and know how engines can be integrated in aircraft Learning goals