Handbook of Turbomachinery Second Edition Revised - Ventech!

etween the velocity diagram and the physical geometry of the turbine. The
flow leaves the stator at an angle of a1 from the axial direction. The velocity
vector V ! 1 can be broken into two components, Vx1 in the axial direction
and Vy1 in the tangential direction. Note that the turbine work is controlled
by the tangential component, while the turbine flow rate is controlled by the
axial component (for an axial-flow turbine). The vector V ! 1 is measured in
an absolute, nonrotating reference frame and is referred to as the absolute
rotor inlet velocity. Likewise, the angle a1 is called the absolute flow angle at
rotor inlet. A rotating reference frame can also be fixed to the rotor.
Velocities in this reference are determined by subtracting the rotor velocity
from the absolute velocity. Defining the relative velocity vector at the inlet
to be W ƒ! 1, we can write
ƒ!
W 1 ¼ V ! 1 U1 ð10Þ
The vector notation is not used for the rotor velocity U1 as it is always in the
tangential direction. The relative velocity vector is also shown in Fig. 8(a).
The relative flow angle b 1 is defined as the angle between the relative velocity
vector and the axial direction. Inspection of the diagram of Fig. 8(a) reveals
Figure 8 Velocity diagrams for an axial-flow turbine: (a) rotor inlet velocity
diagram, (b) rotor exit velocity diagram.
Copyright © 2003 Marcel Dekker, Inc.