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

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

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.

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.

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.

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.

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.

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 257.4 km/h.

SINGLE ENTRANCE DUCT

SUBSONIC DUCTS

VARIABLE GEOMETRY DUCT FOR SUPERSONIC A/C

NORMAL SHOCK RELATION

OBLIQUE SHOCK RELATIONS

BOUNDARY LAYER

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

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.

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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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

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)

11/10/2018