525
14.5 Diffusers
with
The contribution of the blade loading in the diffusion factor is 0.383/σ for β 1 = -50º
as mean value at the inlet. A general deceleration is not required for inducer functioning. So, the magnitudes of 1
w' and w 1 might even be equal. Further, the diffusion factor may exceed that of an axial compressor due to absence of a trailing
edge. For equal velocity magnitudes and DF = 0.6 follows σ = 0.64, which is a low
value. This implies that a general deceleration is possible (
)
1
1
w' < w . Even for a
rather strong deceleration, e.g.
=
1
1
w' 0.7 w , solidity still does not have to be very
high. An inducer may thus be realised with much fewer blades than needed for the
radial part. Application of splitter blades therefore is customary. This means that
the inducer has half the number of blades of the radial rotor part (Figs. 14.2 and
14.7). With such an inducer, the displacement by the blades within the suction eye
is lower, which increases the maximum possible flow rate.
14.5 Diffusers
14.5.1 Flow Non-homogeneity at Rotor Outlet
Within the radial part of a rotor with high work, the flow splits, due to turbulence
segregation, into a high-velocity zone, called the jet flow and a low-velocity zone,
called the wake flow. Within the turbulent motion, there are fast and slow fluid parts.
The fast parts experience a larger Coriolis force than the slow ones and migrate to
the pressure side in a blade channel. The slow parts assemble at the suction side.
Splitting into high-energy and low-energy flow parts leads to an almost lossless jet
flow, while the wake flow systematically accumulates flow losses. A segregated
flow thus resembles a separated one. The jet flow is not subjected to diffusion, as
the space made available by the widening of a blade channel is filled by the wake.
This is once more analogous to a separated flow. The deflection from axial to radial
direction within the inducer generates a centrifugal force, which causes segregation
as well. The jet flow thus comes into the zone formed by the pressure side of a blade
and the rotor disc. The wake flow sits in the corner formed by the suction side and
the shroud. Within each zone, a velocity gradient due to Coriolis force, as sketched
in Fig. 14.9, occurs. The consequence is that the outflow of the impeller is very
inhomogeneous, which leads to a significant downstream mixing loss. The segregation phenomenon is less intense the more the impeller has backward sweep, since
flow curvature compensates partly the effect of the Coriolis force. So, backward
sweep reduces the irregularity of the flow leaving the impeller. The phenomenon
D
s
= −
+
u
1
1
1
w
w'
DF 1
,
w
2 w
1
u
1
1
1
1
u
w u ;
sin
0.766; (
50 ).
w
D
b
b
=
=
≈
≈ −
14.5 Diffusers
with
The contribution of the blade loading in the diffusion factor is 0.383/σ for β 1 = -50º
as mean value at the inlet. A general deceleration is not required for inducer functioning. So, the magnitudes of 1
w' and w 1 might even be equal. Further, the diffusion factor may exceed that of an axial compressor due to absence of a trailing
edge. For equal velocity magnitudes and DF = 0.6 follows σ = 0.64, which is a low
value. This implies that a general deceleration is possible (
)
1
1
w' < w . Even for a
rather strong deceleration, e.g.
=
1
1
w' 0.7 w , solidity still does not have to be very
high. An inducer may thus be realised with much fewer blades than needed for the
radial part. Application of splitter blades therefore is customary. This means that
the inducer has half the number of blades of the radial rotor part (Figs. 14.2 and
14.7). With such an inducer, the displacement by the blades within the suction eye
is lower, which increases the maximum possible flow rate.
14.5 Diffusers
14.5.1 Flow Non-homogeneity at Rotor Outlet
Within the radial part of a rotor with high work, the flow splits, due to turbulence
segregation, into a high-velocity zone, called the jet flow and a low-velocity zone,
called the wake flow. Within the turbulent motion, there are fast and slow fluid parts.
The fast parts experience a larger Coriolis force than the slow ones and migrate to
the pressure side in a blade channel. The slow parts assemble at the suction side.
Splitting into high-energy and low-energy flow parts leads to an almost lossless jet
flow, while the wake flow systematically accumulates flow losses. A segregated
flow thus resembles a separated one. The jet flow is not subjected to diffusion, as
the space made available by the widening of a blade channel is filled by the wake.
This is once more analogous to a separated flow. The deflection from axial to radial
direction within the inducer generates a centrifugal force, which causes segregation
as well. The jet flow thus comes into the zone formed by the pressure side of a blade
and the rotor disc. The wake flow sits in the corner formed by the suction side and
the shroud. Within each zone, a velocity gradient due to Coriolis force, as sketched
in Fig. 14.9, occurs. The consequence is that the outflow of the impeller is very
inhomogeneous, which leads to a significant downstream mixing loss. The segregation phenomenon is less intense the more the impeller has backward sweep, since
flow curvature compensates partly the effect of the Coriolis force. So, backward
sweep reduces the irregularity of the flow leaving the impeller. The phenomenon
D
s
= −
+
u
1
1
1
w
w'
DF 1
,
w
2 w
1
u
1
1
1
1
u
w u ;
sin
0.766; (
50 ).
w
D
b
b
=
=
≈
≈ −