480
13 Axial Compressors
13.2.3 Corner Vortices
In principle, corner vortices may form, driven by the passage vortex, in the corners
between the hub and a blade and between the casing and a blade. According to
Fig. 13.10, four corner vortices might be expected in each blade passage. These
corner vortices interact however with other vortices, as discussed below.
13.2.4 Horseshoe Vortex
Figure 13.13 demonstrates that the stagnation pressure at the blade leading edge
is higher within the core flow than in the end wall boundary layer (the static pressure is constant on a line perpendicular to parallel streamlines). As a consequence
thereof, a vortex is generated that wraps around the leading edge, with forming of
a pressure side leg and a suction side leg. This vortex is denominated horseshoe
vortex, because of its shape.
Figure 13.14, left, demonstrates how the passage vortex drives the horseshoe
vortex leg at the pressure side to the suction side of the neighbouring blade (a rotor
hub zone is shown). The rotation sense of this horseshoe vortex leg is the same as
that of the passage vortex. Both vortices merge, making the difference between both
of them mostly invisible in the outlet plane of the cascade. The horseshoe vortex leg
at the suction side of the blade is driven by the passage vortex into the corner between the blade and the end wall. The rotation sense of this horseshoe vortex leg is
opposite to the rotation sense of the passage vortex. The two vortices do not merge,
but reinforce each other. The horseshoe vortex leg may be directly in the corner,
but a corner vortex may also form underneath the horseshoe vortex, as suggested in
Fig. 13.14. In particular, this may happen with separated flow in the corner between
the suction side of the blade and the end wall. Typically, no corner vortex forms in
the corner at the pressure side, as no stall occurs. Analogous horseshoe vortex legs
and corner vortices are generated at the stator vanes near the casing. The same occurs when there is no clearance at the hub (typically: cover bands; see Fig. 13.11).
In case of a clearance, a leakage vortex and a scraping vortex are generated, as
discussed next.
13.2.5 Leakage Vortex and Scraping Vortex
With rotor blades, there is a clearance at the casing. The leakage flow from the
pressure side to the suction side forms a leakage vortex and the motion of the blade
with respect to the casing causes a scraping vortex at the pressure side (Fig. 13.14,
left). The leakage vortex and the scraping vortex have the same sense of rotation,
opposite to the rotation sense of the passage vortex. Downstream of the blade passage, the leakage and scraping vortices have the tendency to merge. The result is
13 Axial Compressors
13.2.3 Corner Vortices
In principle, corner vortices may form, driven by the passage vortex, in the corners
between the hub and a blade and between the casing and a blade. According to
Fig. 13.10, four corner vortices might be expected in each blade passage. These
corner vortices interact however with other vortices, as discussed below.
13.2.4 Horseshoe Vortex
Figure 13.13 demonstrates that the stagnation pressure at the blade leading edge
is higher within the core flow than in the end wall boundary layer (the static pressure is constant on a line perpendicular to parallel streamlines). As a consequence
thereof, a vortex is generated that wraps around the leading edge, with forming of
a pressure side leg and a suction side leg. This vortex is denominated horseshoe
vortex, because of its shape.
Figure 13.14, left, demonstrates how the passage vortex drives the horseshoe
vortex leg at the pressure side to the suction side of the neighbouring blade (a rotor
hub zone is shown). The rotation sense of this horseshoe vortex leg is the same as
that of the passage vortex. Both vortices merge, making the difference between both
of them mostly invisible in the outlet plane of the cascade. The horseshoe vortex leg
at the suction side of the blade is driven by the passage vortex into the corner between the blade and the end wall. The rotation sense of this horseshoe vortex leg is
opposite to the rotation sense of the passage vortex. The two vortices do not merge,
but reinforce each other. The horseshoe vortex leg may be directly in the corner,
but a corner vortex may also form underneath the horseshoe vortex, as suggested in
Fig. 13.14. In particular, this may happen with separated flow in the corner between
the suction side of the blade and the end wall. Typically, no corner vortex forms in
the corner at the pressure side, as no stall occurs. Analogous horseshoe vortex legs
and corner vortices are generated at the stator vanes near the casing. The same occurs when there is no clearance at the hub (typically: cover bands; see Fig. 13.11).
In case of a clearance, a leakage vortex and a scraping vortex are generated, as
discussed next.
13.2.5 Leakage Vortex and Scraping Vortex
With rotor blades, there is a clearance at the casing. The leakage flow from the
pressure side to the suction side forms a leakage vortex and the motion of the blade
with respect to the casing causes a scraping vortex at the pressure side (Fig. 13.14,
left). The leakage vortex and the scraping vortex have the same sense of rotation,
opposite to the rotation sense of the passage vortex. Downstream of the blade passage, the leakage and scraping vortices have the tendency to merge. The result is