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

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

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

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

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

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

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

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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

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

SUPERSONIC INLETS Normal Shock Diffuser Oblique Shock Diffuser

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

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

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

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