499
13.5 Performance Characteristics and Operating Range
to Fig. 13.25, the sense of propagation within the relative frame for a rotor blade
row is opposite to the running sense. The propagation speed is lower than the blade
speed, causing the stall cells to move in the rotation sense within the absolute frame.
With the same figure one sees that the propagation sense of cells in stator vanes is
in the running sense of the rotor. Typically, rotating stall cells form in a number of
consecutive stator vane rows and rotor blade rows.
13.5.3 Choking
By reduction of the backpressure of a subsonic flow compressor, the flow rate increases and all velocities increase until sonic speed is attained in some through-flow
section. With further reduction of the backpressure, the flow rate then stays blocked.
This phenomenon is called choking. The choking flow rate depends on the rotational speed, both for choking within the rotor blade passages as for choking in the stator vane passages. This may be understood by the following, simplified, derivation.
Within the relative system it applies that
At the inlet of the rotor, without pre-swirl,
(13.18)
2
2
1/2
1/2
constant.
I h
w
u
= +
−
=
2
2
2
2
1
1
1
1
1
1
01
, so that
1/2
.
w
u v
I I h
v
h
= +
= = +
=
2
2
M 1 is attained for w
c
( 1) , so that then
g
g
=
=
=
=
−
RT
h
,
2
2
01
h
h 1/ 2(
1 )h 1/ 2u
1/2(
1 )h 1/ 2u
g
g
=
+
−
−
=
+
−
2
2
01
2
01
01
01
1/2
2
2
1
1
.
1
1
2
h
u
h
u
h
h
c
g
g
g
+
−
=
=
+
+
+
Fig. 13.25 Rotating stall propagation mechanism
13.5 Performance Characteristics and Operating Range
to Fig. 13.25, the sense of propagation within the relative frame for a rotor blade
row is opposite to the running sense. The propagation speed is lower than the blade
speed, causing the stall cells to move in the rotation sense within the absolute frame.
With the same figure one sees that the propagation sense of cells in stator vanes is
in the running sense of the rotor. Typically, rotating stall cells form in a number of
consecutive stator vane rows and rotor blade rows.
13.5.3 Choking
By reduction of the backpressure of a subsonic flow compressor, the flow rate increases and all velocities increase until sonic speed is attained in some through-flow
section. With further reduction of the backpressure, the flow rate then stays blocked.
This phenomenon is called choking. The choking flow rate depends on the rotational speed, both for choking within the rotor blade passages as for choking in the stator vane passages. This may be understood by the following, simplified, derivation.
Within the relative system it applies that
At the inlet of the rotor, without pre-swirl,
(13.18)
2
2
1/2
1/2
constant.
I h
w
u
= +
−
=
2
2
2
2
1
1
1
1
1
1
01
, so that
1/2
.
w
u v
I I h
v
h
= +
= = +
=
2
2
M 1 is attained for w
c
( 1) , so that then
g
g
=
=
=
=
−
RT
h
,
2
2
01
h
h 1/ 2(
1 )h 1/ 2u
1/2(
1 )h 1/ 2u
g
g
=
+
−
−
=
+
−
2
2
01
2
01
01
01
1/2
2
2
1
1
.
1
1
2
h
u
h
u
h
h
c
g
g
g
+
−
=
=
+
+
+
Fig. 13.25 Rotating stall propagation mechanism