1. MAE 4261: AIR-BREATHING ENGINES
Subsonic and Supersonic Inlets
Mechanical and Aerospace Engineering Department
Florida Institute of Technology
D. R. Kirk

2. 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

3. 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

4. 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

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

6. 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

7. 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

9. 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

10. 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

11. SUPERSONIC INLETS
Normal Shock Diffuser
Oblique Shock Diffuser

12. 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

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

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

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