Transcript Inlets - My FIT (my.fit.edu) - Florida Institute of Technology

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:
1. Capture incoming stream tube (mass flow)
2. 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
1. Bring inlet flow to engine with high possible stagnation pressure
• Measured by inlet pressure recovery, pd = Pt2/Pt1
2. Provide required engine mass flow
• May be limited by choking of inlet
3. Provide compressor (and/or fan) with uniform flow
EFFECT OF MASS FLOW ON THRUST VARIATION
m 2   2U 2 A2
•
m 2
P

  2U 2  2 M 2 RT2  P2
M2
A2
RT2
RT2
•
m 2

 Pt 2
A2
RT2
m 0
P0

A2
RT0
M2
  1 2 
M2 
1 
2


  1 2 
M2 
1 
2


 T  m 0 
T

a0
 

A2 P0  m0 a0  A2 P0 
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
 1
2  1
 1
2  1
M2
  1 2 
M2 
1 
2


 1
2  1
NON-DIMENSIONAL THRUST FOR A2 AND P0
Plot of Thrust Normalized by Compressor Inlet Area and Ambient Pressure
vs. Flight Mach Number for Compressor Inlet Mach Number, M 2=0.5
30
Theta_t=6
Theta_t=9
Thrust / (A2 P0)
25
20
15
10
5
0
0
0.5
1
1.5
Flight Mach Number
•
•
Thrust at fixed altitude is nearly constant up to Mach 1
Thrust then increases rapidly, need A2 to get smaller
2
2.5
3
CONTROL VOLUME ANALYSIS: SECTION 6.2
 Ui 
T

 1 
1 U 2 A  U 0 
0 i
2
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
2
 Ui 
T

 1 
1 U 2 A  U 0 
0 i
Aerodynamic force is always favorable for thrust production
2
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
http://www.globemaster.de/airextreme/jets.html
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
1
0.9
0.8
M2, P02/P01
0.7
0.6
0.5
0.4
0.3
0.2
Downstream Mach Number, M2
Total Pressure Ratio, P02/P01
0.1
0
1
1.5
2
2.5
3
3.5
Upstream Mach Number, M1
4
4.5
5
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)