234
6 Steam Turbines
1.80 m with a 3.00 m hub diameter (turbines at 1500 rpm: nuclear). The triangles in
Fig. 6.18 are drawn with a constant axial velocity in the rotor. In reality, axial velocity increases from inlet to outlet because of the density drop. At the stage outlet, the
axial velocity is about constant over the radius. At the rotor inlet, the axial velocity
is higher at the hub and smaller at the casing (for realistic triangles, see Fig. 6.38 in
the exercise section). Figure 6.18 (also Fig. 6.38) demonstrates that the rotor blade
profile strongly changes from hub to tip, with a strong turning at the root and a very
weak one at the tip. Figure 6.33 sketches the variation of the flow angles at stator
outlet and rotor inlet and the Mach number at rotor inlet for the final stage of a large
steam turbine with straight stacked vanes and blades (data provided by Alstom). A
rotor blade is also shown in Fig. 6.33.
The problem with very long blades is that the Mach number at rotor inlet is large
at the hub and at the casing, and may approach unity. This generates shock waves
(see fluid mechanics), with associated shock loss. Therefore, a final stage in an LP
part is adapted in order to minimise shock loss. The other stages in the LP part and
the stages in the HP and IP parts also show variation of the degree of reaction with
the height. Therefore rotor blades are twisted (see Fig. 6.25). This variation does not
cause high Mach number problems at the hub and the casing, however. There only
arises a problem in the final stage of an LP section.
Three adaptations reduce the rotor inlet Mach number at the hub and the casing.
The traditional one is setting the stator blade outlet more axial at the hub (opening the vanes) and setting it more tangential at the casing (closing the vanes). The
Fig. 6.33 Flow angles and Mach number for the final stage of an LP part of a large steam turbine
with straight stacking of blade sections ( angles to tangential direction); right: rotor blade. (Courtesy Alstom)
6 Steam Turbines
1.80 m with a 3.00 m hub diameter (turbines at 1500 rpm: nuclear). The triangles in
Fig. 6.18 are drawn with a constant axial velocity in the rotor. In reality, axial velocity increases from inlet to outlet because of the density drop. At the stage outlet, the
axial velocity is about constant over the radius. At the rotor inlet, the axial velocity
is higher at the hub and smaller at the casing (for realistic triangles, see Fig. 6.38 in
the exercise section). Figure 6.18 (also Fig. 6.38) demonstrates that the rotor blade
profile strongly changes from hub to tip, with a strong turning at the root and a very
weak one at the tip. Figure 6.33 sketches the variation of the flow angles at stator
outlet and rotor inlet and the Mach number at rotor inlet for the final stage of a large
steam turbine with straight stacked vanes and blades (data provided by Alstom). A
rotor blade is also shown in Fig. 6.33.
The problem with very long blades is that the Mach number at rotor inlet is large
at the hub and at the casing, and may approach unity. This generates shock waves
(see fluid mechanics), with associated shock loss. Therefore, a final stage in an LP
part is adapted in order to minimise shock loss. The other stages in the LP part and
the stages in the HP and IP parts also show variation of the degree of reaction with
the height. Therefore rotor blades are twisted (see Fig. 6.25). This variation does not
cause high Mach number problems at the hub and the casing, however. There only
arises a problem in the final stage of an LP section.
Three adaptations reduce the rotor inlet Mach number at the hub and the casing.
The traditional one is setting the stator blade outlet more axial at the hub (opening the vanes) and setting it more tangential at the casing (closing the vanes). The
Fig. 6.33 Flow angles and Mach number for the final stage of an LP part of a large steam turbine
with straight stacking of blade sections ( angles to tangential direction); right: rotor blade. (Courtesy Alstom)