The document discusses axial flow compressors and their off-design performance. It covers topics such as compressor maps, surge lines, efficiency islands, best efficiency points, operating lines, surge margins, effects of inlet pressure distortion, Reynolds number, Mach number, and tip clearance on compressor performance. The conclusion summarizes that changes in total temperature, total pressure, Mach number, and density can affect compressor operation and efficiency.

1. Aeropropulsion
Unit
Axial Flow Compressors-off Design Performance
2005 - 2010
International School of Engineering, Chulalongkorn University
Regular Program and International Double Degree Program, Kasetsart University
Assist. Prof. Anurak Atthasit, Ph.D.

2. Aeropropulsion
Unit
2
A. ATTHASIT
Kasetsart University
Typical Compressor Map
Surge:
Instability that could cause mechanical failure
Maintaining the correct speed fixed
Does not necessarily involve the physical speed but can also be effected by changing the inlet total temperature
Efficiency Islands
The efficiency is constant
BEP:
Best Efficiency Point
Operating line:
The different conditions at which an engine typically operates

3. Aeropropulsion
Unit
3
A. ATTHASIT
Kasetsart University
Typical Compressor Map
Surge Margin:
A small surge margin is undesirable because the compressor could surge as the result of a small perturbation
- Typical surge margins for modern engines range from 15 to 20 percent.
Efficiency Islands
The efficiency is constant
BEP:
Best Efficiency Point
Operating line:
The different conditions at which an engine typically operates

4. Aeropropulsion
Unit
Kasetsart University A. ATTHASIT 4
Compressor off-design Operation
The compressor operating region is
limited by the surge line, which
represents a state that the
compressor is facing mechanical
failure.
This is the reason behind the added
emphasis in this class on the
compressor off-design operation,
including the phase of start-up.

5. Aeropropulsion
Unit
5
A. ATTHASIT
Kasetsart University
Real Flow Effect
•Effect to the incidence angle
•Effect of the Reynolds Number
•Effect of the Mach Number
•Tip clearance effect

6. Aeropropulsion
Unit
6
A. ATTHASIT
Kasetsart University
Effect to The Incidence Angle
40
30
20
0.075
0.050
0.025
wmin
2wmin
Stalling Point
w
e
Incidence Angle, i (deg)
Total Pressure
Loss Coefficient
Flow Deflection Angle
i
e
Stalling is defined as the state at which the total- pressure loss coefficient is twice its minimum value.
2112tP V w   

7. Aeropropulsion
Unit
7
A. ATTHASIT
Kasetsart University
Inlet Pressure and Temperature Distortion
•Inlet distortion: spatial variation of inlet pressure or temperature, can significantly affect the overall compressor map
•The most important effect is a reduction in the surge line.

8. Aeropropulsion
Unit
8
A. ATTHASIT
Kasetsart University
Inlet Pressure and Temperature Distortion
•DC60 coefficient: the difference between the average total pressures in the most distorted 60° sector and the full 360° intake, divided by the average inlet dynamic head.
•-0.2 for a civil subsonic transport

9. Aeropropulsion
Unit
9
A. ATTHASIT
Kasetsart University
Inlet Pressure and Temperature Distortion
B
A
Total pressure for the rest Is slightly higher than The circumferential average
60 deg. Sector with
The lowest inlet
Total pressure
Working line for lowest pressure sector A
Average working line
Working line for lowest pressure sector B

10. Aeropropulsion
Unit
10
A. ATTHASIT
Kasetsart University
Effect of Inlet Flow Angle - VIGVs
•Variable Inlet Guide Vane are mainly required to allow a compressor to have an acceptable low speed surge line with all the stages on one shaft
 impact on compressor efficiency

11. Aeropropulsion
Unit
11
A. ATTHASIT
Kasetsart University
Effect of The Reynolds Number
•The critical Re is partially influenced by the level of turbulence at the cascade inlet station.
•Re<2x105  high profile losses (dominated by viscosity rather than inertia)
ReinVC  

12. Aeropropulsion
Unit
12
A. ATTHASIT
Kasetsart University
Effect of the Mach Number
•Increase in the inlet Mach number will cause a notably rapid increase in the drag coefficient.
•Choking in the flow passage

13. Aeropropulsion
Unit
13
A. ATTHASIT
Kasetsart University
Tip Clearance Effect
•Direct tip leakage: flow stream is in the direction opposite to the primary flow
•Indirect tip leakage is produced by the secondary flow
•The secondary flow is capable of producing more aerodynamic damage by comparison

14. Aeropropulsion
Unit
14
A. ATTHASIT
Kasetsart University
Tip Clearance Effect
•Tip clearance is the radial gap between the rotor blades and casing and is usually in the range 1-2% rms steady state
•More effect on small compressors
•1% increase in rms tip clearance reduces efficiency by approximately 1-2%

15. Aeropropulsion
Unit
15
A. ATTHASIT
Kasetsart University
Tip Clearance Effect
•The exchange rate will be in the range of a 1% increase in rms tip clearance reducing the surge line by between 2% and 15% of surge margin

16. Aeropropulsion
Unit
Kasetsart University A. ATTHASIT 16
, ,
0.07
1.0 10.0
cos cos t av t av
c  c

  
 
    
 
 
Tip Clearance Effect
 2
p t C T
r
u
r

w

w



The ratio of the tip clearance
to the average blade height
Blade aspect ratio
(height/average true chord)
The tip-radius
average swirl angle
Stage work
coefficient
Flow
coefficient
Stage loading
And flow
coefficient

17. Aeropropulsion
Unit
17
A. ATTHASIT
Kasetsart University
Tip Clearance Effect 0.80.61.5      

18. Aeropropulsion
Unit
Kasetsart University A. ATTHASIT 18
Conclusion
*
2
1
*
2
1
1
*
*
2
*
1
2( 1)
2
*
1
2
1
1
2
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2
1
1
2
1
2
1
1
2
1
1
1 2
1
2
T
T
M
P
P
M
P
P
T
M
T
P
m AV AM
R T
M
A
A M



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  
 
   
           
 
    
    
    
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 
 
2
0
0 t
dA d du
A u
udu dP
dh dh udu
dP d dT
P T
a
P







  
 
  
 

P dP
T dT
d
A dA
u du
 





P
T
A
u

dx
2
dP
P 
Cabin Crew!
Prepare for
take-off!