5
1.3 Mean Line Analysis
energy transfer is partly noticeable by the kinetic energy increase in the absolute
frame ( v
v
2
2
1
2
2
2
/
/
>
) and partly by the kinetic energy decrease in the relative
frame ( w
w
2
2
1
2
2
2
/
/
<
) with a corresponding pressure increase.
•  Diffuser: a stationary ring of vanes downstream of the rotor turns the entering
velocity v 2 into the axial direction (tangential component of the velocity becomes
zero) while the axial velocity component is reduced by increased through-flow
area. Kinetic energy in the flow decreases ( v
v
3
2
2
2
2
2
/
/
<
), causing pressure
increase. The stator part downstream of the rotor converts kinetic energy into
pressure. Therefore, this component is termed a diffuser.
The rotating component of a pump is sometimes called the impeller. The meaning
of the term is to impart motion to the fluid. As such, this is a general term well describing the role of the rotor in a power receiving machine, but it is typically only
used with radial pumps and radial compressors. The term runner, mentioned in the
previous section is, as such, also a general term completely equivalent to the term
rotor, but it is typically only used with hydraulic turbines.
1.3 Mean Line Analysis
The fundamental analysis of a turbomachine applies the average flow concept. For
a machine with periodicity in the circumferential direction (axial, radial, mixedflow), the flow is calculated by averaging quantities in the circumferential direction
L
u
u
u
1
v
2
v
3
v
2
w
rotor
diffuser
1
w
Fig. 1.3 Axial pump (courtesy Flygt, a Xylem company); cylindrical section of a rotor blade and
the diffuser ring