552
15 Axial and Radial Turbines for Gases
The degree of reaction follows from
The work coefficient is
1 1u
2
1
u v
0.8.
u
y
s
=
= ≈
Regarding work, a radial turbine is not very advantageous, as the work coefficient
is lower than with a typical axial turbine, for which it may even reach values above
2. The peripheral speeds are comparable, namely about 400 m/s. The observation
that there is no strict limitation to the flow turning of an accelerating flow implies
that the work coefficient is not better. Within an axial turbine, the flow turning in
the rotor may amount to about 140°. There is a limitation to about 70° with a radial
turbine.
It is typical for a radial turbine that backpressure is given. This implies that the
total-to-static efficiency has to be maximised. When defining speed ratio, the totalto-static enthalpy drop should be used. The rotor work is
With the total-to-static efficiency of about 0.8, it follows that
2
2
1
1
1
2
2
2
1 1u
v
v
1 R
.
u v
−
− =
With
:
2
1u
1u
1
1
2
1r
2
2
1 1u
1
v
v
v
v
R 1
1
1
0.6.
u v
u
2
s
≈
≈ −
= −
= − ≈
2
1
ts
s
W
u
h .
D
s
h D
=
=
thus
2
2
1
1
s
s
s
u
1
u
h v 2
0.7.
v
2
D
l
≈
=
=
=
≈
Fig. 15.11 h-s diagram for
expansion in a centripetal
turbine