119
From the velocity triangle at rotor outlet in Fig. 3.5, we further derive
The flow coefficient and work coefficient are
The Pfleiderer moment coefficient may also be written as a function of the kinematic parameters:
The results are given in Table 3.1. The angle α 2 is set to 70º and C M is set to 1.4.
In the last column are given the numbers typically used in practice. For the rotor
with radial end blades, the safety margin on the number of blades may seem small,
but in reality the rotor has an axial part at the inlet (the inducer), which contributes
to the moment solidity (see Chap. 14: radial compressors). The number of blades
might be larger than given in the last column, depending on the margin that the
σ
π
M
Z
r
r
=
−






2
1
1
2
.
2
2
2
2
2
2
2
2
2
2 2
2
2
,
,
or
(
) .
u
u
u
u
r
r
r
v
w
tg
tg
v
u w
u
tg
tg v
v
v
a
b
a
b
=
=
= +
=
−
2
2
2
2
2
2
2
2
2
1
and
.
u
r
v
v
tg
u
tg
tg
u
tg
tg
a
f
y
a
b
a
b
=
=
=
=
−
−
C M
M
=
+ −
φψ
σ φ
ψ
[
(
) ]
.
2
2
1
Fig. 3.14 Centrifugal rotor blade shapes: backward curved, straight backswept, radial end (inducer
not drawn), forward curved
3.3 Radial Fan Analysis for Lossless Two-Dimensional Flow …