Axial Flow Compressors. Axial Flow Compressors. Elementary theory. Axial Flow Compressors. Axial Flow Compressors. Comparison of typical forms of turbine and compressor rotor blades. Axial Flow Compressors. Axial Flow Compressors Stage= S+R S: stator (stationary blade)
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Comparison of typical forms of turbine and compressor rotor blades
Axial Flow Compressors
S: stator (stationary blade)
R: rotor (rotating blade)
First row of the stationary blades is called guide vanes
Assume mid plane is constant r1=r2, u1=u2
assume Ca=const, in the direction of u.
, in the direction of u.
Inside the rotor, all power is consumed.
Stator only changes K.E.P static, To2=To3
Increase in stagnation pressure is done in the rotor.
Stagnation pressure drops due to friction loss in the stator:
C1: velocity of air approaching the rotor.
: angle of approach of rotor.
u: blade speed.
V1: the velocity relative t the rotor at inlet at an angle 1 from the axial direction.
V2: relative velocity at exit rotor at angle 2 determined from the rotor blade outlet angle.
2: angle of exit of rotor.
Ca: axial velocity.
Two dimensional analysis:
Only axial ( Ca) and tangential (Cw). no radial component
from velocity triangles
the power input to stage
or in terms of the axial velocity
From equation (a)
pressure ratio at a stage
Degree of reaction
is the ratio of static enthalpy in rotor to static enthalpy rise in stage
For incompressible isentropic flow Tds=dh-vdp
h=p/ ( constant )
Thus enthalpy rise could be replaced by static pressure rise ( in the definition of )
but generally choose =0.5 at mid-plane of the stage.
=0: all pressure rise only in stator
=1: all pressure rise in only in rotor
=0.5: half of pressure rise only in rotor and half is in stator. ( recommend design)
=0 ( impulse type rotor)
from equation 3
1=-2 , velocities skewed left, h1=h2, T1=T2
=1.0 (impulse type stator from equation 1)
velocities skewed right, C1=C2, h2=h3T2=T3
Three dimensional flow
1. the effects due to radial movement of the fluid are ignored.
2. It is justified for hub-trip ratio>0.8
3. This occurs at later stages of compressor.
3-D are valid due to
1. due to difference in hub-trip ratio from inlet stages to later-stages, the annulus will have a substantial taper. Thus radial velocity occurs.
2. due to whirl component, pressure increase with radius.
Is ok. Discussed later. Results r-t=0.2262, r-r=0.1131, r-m=0.1697 m
- normally To5 is small at first stage
de haller criterion V2/V1 >0.72
- work factor can be taken as 0.98, 0.93, 0.88 for 1st, 2nd, 3 rd stage and 0.83 for rest of the stages.
check de Haller