1. Subject Name: AIRCRAFT PROPULSION Subject Code: 10AE55
Prepared By : DEEPA M S
Department : AERONAUTICAL
Date :
11/10/2018

2. CONTENTS Internal flow and Stall in subsonic inlets
Boundary layer separation
Major features of external flow near a subsonic inlet
Relation between minimum area ratio and external deceleration ratio
Diffuser performance
Supersonic inlets
Starting problem on supersonic inlets
Shock swallowing by area variation
External declaration
Models of inlet operation.
11/10/2018

3. INTRODUCTION
Inlets are very important to the overall jet engine performance & will greatly influence jet engine thrust output.
The faster the airplane goes the more critical the inlet duct design becomes.
Engine thrust will be high only if the inlet duct supplies the engine with the required airflow at the highest possible pressure.

4. The nacelle/duct must allow the engine to operate with minimum stall/surge tendencies & permit wide variation in angle of attack & yaw of the aircraft.
For subsonic aircraft, the nacelle shouldn’t produce strong shock waves or flow separations & should be of minimum weight for both subsonic & supersonic designs.
For certain military applications, the radar cross sectional control or radar reflectance is a crucial design requirements.

5. Inlet ducts add to parasite drag skin friction+ viscous drag) & interference drag.
It must operate from static ground run up to high aircraft Mach number with high duct efficiency at all altitude, attitudes & flight speeds.
It should be as straight & smooth as possible & designed such a way that Boundary layer to be minimum.
It should deliver pressure distribution evenly to the compressor.

6. Spring loaded , blow-in or such-in-doors are sometimes placed around the side of the inlet to provide enough air to the engine at high engine rpm & low aircraft speed.
It must be shaped such a way that ram velocity is slowly & smoothly decreases while the ram pressure is slowly & smoothly increases.

7. DUCT EFFICIENCY
The duct pressure efficiency ratio is defined as the ability of the duct to convert the kinetic or dynamic pressure energy at the inlet of the duct to the static pressure energy at the inlet of the compressor without a loss in total pressure . It is in order of 98% if there is less friction loss.

8. RAM RECOVERY POINT
The ram recovery point is that aircraft speed at which the ram pressure rise is equal to the friction pressure losses or that aircraft speed at which the compressor inlet total pressure is equal to the outside ambient air pressure.
A good subsonic duct has km/h.

9. SINGLE ENTRANCE DUCT

10. SUBSONIC DUCTS

11. VARIABLE GEOMETRY DUCT FOR SUPERSONIC A/C

12. NORMAL SHOCK RELATION

13. OBLIQUE SHOCK RELATIONS

14. BOUNDARY LAYER

15. Gas Turbine Engine Components
inlet, compressor, combustor, turbine, nozzle
Inlet
An inlet reduces the entering air velocity to a level suitable for the compressor.
The design and operation of the inlet depend on whether the air entering the duct is subsonic or supersonic.
Subsonic inlet:
The subsonic inlet can be a divergent duct, as shown

16. Supersonic inlet
Because shock waves will occur in supersonic flow, the geometry of supersonic inlets is designed to obtain the most efficient compression.
If the velocity is reduced from a supersonic speed to a subsonic speed with one normal shock wave, compression process is inefficient.
If several oblique shock waves are employed to reduce the velocity, the compression process is more efficient.

17. OVERVIEW: INLETS AND DIFFUSERS
Purpose:
Capture incoming stream tube (mass flow)
Condition flow for entrance into compressor (and/or fan) over full flight range
At take-off (M0~0), accelerate flow to 0.4 < M2 < 0.7
At cruise (M0~0.85), slow down flow to 0.4 < M2 < 0.7
Remain as insensitive as possible to angle of attack, cross-flow, etc.
Requirements
Bring inlet flow to engine with high possible stagnation pressure
Measured by inlet pressure recovery, pd = Pt2/Pt1
Provide required engine mass flow
May be limited by choking of inlet
Provide compressor (and/or fan) with uniform flow

18. EFFECT OF MASS FLOW ON THRUST VARIATION
Mass flow into compressor = mass flow entering engine
Re-write to eliminate density and velocity
Connect to stagnation conditions at station 2
Connect to ambient conditions
Resulting expression for thrust
Shows dependence on atmospheric pressure and cross-sectional area at compressor or fan entrance
Valid for any gas turbine

19. NON-DIMENSIONAL THRUST FOR A2 AND P0
Thrust at fixed altitude is nearly constant up to Mach 1
Thrust then increases rapidly, need A2 to get smaller

20. CONTROL VOLUME ANALYSIS: SECTION 6.2
Aerodynamic force is always favorable for thrust production

21. OPERATIONAL OVERVIEW High Thrust for take-off Low Speed, M0 ~ 0
High Mass Flow
Stream Tube Accelerates
Lower Thrust for cruise
High Speed, M0 ~ 0.8
Low Mass Flow
Stream Tube Decelerates
Aerodynamic force is always favorable for thrust production

22. OPERATIONAL OVERVIEW: FIGURE 6.1
Low Thrust
High Speed
Low Mass Flow
Stream Tube Decelerates
High Thrust
Low Speed
High Mass Flow
Stream Tube Accelerates

24. Minimize internal and external losses
DESIGN COMMENTS
Minimize internal and external losses
Rounded lip to avoid flow separation (both internal and external (drag))
Well designed inlet pd ~ 0.97 at design condition
Design to minimize external acceleration during take-off so that external deceleration occurs during level cruise

25. INLETS OVERVIEW: SUPERSONIC INLETS
At supersonic cruise, large pressure and temperature rise within inlet
Compressor (and burner) still requires subsonic conditions
For best hthermal, desire as reversible (isentropic) inlet as possible
Some losses are inevitable

26. SUPERSONIC INLETS
Normal Shock Diffuser
Oblique Shock Diffuser

27. NORMAL SHOCK TOTAL PRESSURE LOSSES
Example: Supersonic Propulsion System
Engine thrust increases with higher incoming total pressure which enables higher pressure increase across compressor
Modern compressors desire entrance Mach numbers of around 0.5 to 0.8, so flow must be decelerated from supersonic flight speed
Process is accomplished much more efficiently (less total pressure loss) by using series of multiple oblique shocks, rather than a single normal shock wave
As M1 ↑ p02/p01 ↓ very rapidly
Total pressure is indicator of how much useful work can be done by a flow
Higher p0 → more useful work extracted from flow
Loss of total pressure are measure of efficiency of flow process

28. REPRESENTATIVE VALUES OF INLET/DIFFUSER STAGNATION PRESSURE RECOVERY AS A FUNCTION OF FLIGHT MACH NUMBER

29. C-D NOZZLE IN REVERSE OPERATION (AS A DIFFUSER)
Not a practical approach!

30. C-D NOZZLE IN REVERSE OPERATION (AS A DIFFUSER)

31. 11/10/2018