Shroud band for an axial-flow turbine

In a device for sealing the gap between the moving blades and the stator, designed with a conical contour, of a turbomachine, the moving blades are provided at the blade end with encircling shroud plates. These shroud plates project into a cavity in the stator and, while forming radial gaps, make a seal against the stator, which is provided with sealing strips. The cavity at the labyrinth inlet is subdivided in its radial extent into at least two axially staggered cavities. The shroud plate is of stepped design with at least two choke points with respect to the stator, the sealing strips acting on one step each while enclosing a vortex chamber. A curved sealing strip which runs at least approximately horizontally preferably acts on each step of the shroud plate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a device for sealing the gap between the moving 
blades and the casing, designed with a conical contour, of a turbomachine, 
the moving blades being provided with encircling shroud plates which, 
while forming radial gaps, make a seal against the casing, which is 
provided with sealing strips. 
2. Discussion of Background 
Such devices are known. They form a smooth or a stepped half labyrinth 
having entirely radial gaps. Such a seal is shown in FIG. 2, which is to 
be described later. 
As a result of the better efficiency and the greater reliability, this type 
of gap seal is in the meantime already being used for the moving blades of 
the penultimate stage of condensing steam turbines. The mechanical 
requirements here, at circumferential speeds of 450 m/sec, are quite high, 
whereas the thermal conditions, at about 90.degree. C., are modest. The 
geometrical requirements are problematic: on the one hand, on account of 
the pronounced conicity, which leads to deep cavities of the known sealing 
device in the casing wall; on the other hand, on account of the large 
differential expansions between rotor and casing, which lead to wide 
cavities with the abovementioned half labyrinths. 
The large cavity formed in this case in the inlet region of the seal 
produces an unfavorable cross exchange of flow material with the main flow 
in the blade duct. This cross exchange is encouraged by the exceptionally 
large fluctuation of the pressure difference between two adjacent blades 
in the plane of the leading blade edge. In addition, a pronounced vortex 
is stimulated in this region by the main flow and the side wall of the 
shroud band. 
Less effective is the half labyrinth having the sealing strips with which 
the casing is provided and which make a seal against the encircling shroud 
band. This is because, under the existing conditions, the operating 
clearance must have a size of about 1/3 of the free chamber height. Even a 
plurality of sealing strips are therefore not much more effective than a 
single sealing strip. 
Finally, the large cavity in the outlet region of the seal also permits an 
undesirable cross exchange with the main flow in the blade duct, since 
here, too, the pressure difference between the adjacent blade tips is 
subjected to large fluctuations. In addition, the guidance of the main 
flow is completely lost in this region. 
In addition, the large vortex space which is formed behind the outer 
sealing strip and produces considerable dissipation of the outlet-side gap 
flow is of disadvantage in the case of these seals. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the invention, in the case of blades of the type 
mentioned at the beginning, is to provide by means of a novel shroud-band 
geometry a seal which, while fulfilling all the boundary conditions, leads 
to better efficiency. 
The advantage of the invention may be seen, inter alia, in the fact that 
only small gap quantities occur in the case of the novel seal. In 
addition, the gap flow is effectively directed into the main flow.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, only the 
elements essential for the understanding of the invention are shown, and 
the direction of flow of the working medium is designated by arrows, the 
three center stages of low-pressure blading, which each consist of a guide 
row Le and a moving row La, are shown according to FIG. 1. In this case, 
the stage Le3/La3 corresponds to the penultimate stage. The moving blades 
La, which are inserted with their roots 21 into turned grooves of the 
rotor 9, are provided with shroud plates 16 at their blade ends. The 
radially outer contours of the shroud plates are of stepped design. While 
forming labyrinths 15, they make a seal with their steps against sealing 
strips which are arranged in the stator 8 in a suitable manner. The guide 
blades Le, which are inserted with their roots 13 into turned grooves of 
the stator 8, are provided with shroud plates 20 at their blade ends. 
While forming labyrinths 19, they also make a seal against sealing strips 
which are arranged in the rotor 9 in a suitable manner. 
As an initial situation, the duct 50 through which flow occurs has the 
conically running outer contour 51 at the stator and the cylindrically 
running inner contour 11 at the rotor. However, neither is absolutely 
necessary. Irrespective of the actual profile of the walls, the outer, 
flow-limiting contour 10 in the region of the moving-blade body is always 
formed by the shroud plate 16, which faces the duct, of the moving blades 
La. Located directly upstream of the shroud plates 16, 20 are axial gaps 
18 which constitute the labyrinth inlets 40. Located directly downstream 
of these shroud plates 16, 20 are radial gaps 26 which constitute the 
labyrinth outlets 42. As a rule, said gaps are defined on the other side 
by stator parts, which perform the function of directing the flow in the 
planes where there are no blades. 
The shroud-plate seal of the moving row La3, as corresponds to the prior 
art mentioned at the beginning, is shown in FIG. 2. It essentially 
comprises the shroud plate 16A, which extends over the entire blade width 
and, with its outside diameter and the four sealing strips 17A caulked in 
place in the stator 8A, forms a half labyrinth having entirely radial 
gaps. The labyrinth inlet 40A of large area and the labyrinth outlet 42A 
of unfavorable configuration can be recognized. The duct wall is 
designated by 54 if it leads into a bleed. 
As shown in FIG. 3, both the geometry of the shroud band and its embedding 
in the stator is now improved in a three-fold manner according to the 
invention. 
In order to reduce the cross exchange of flow material and the vortex 
intensity, the radially directed cavity at the labyrinth inlet is 
subdivided in its radial extent into two axially staggered cavities, i.e. 
is of zigzag configuration in the example. To this end, the contour of the 
turned groove in the stator first of all runs inward into the material, 
then outward in the axial direction while forming a tooth 41 projecting 
into the cavity. The shroud plate 16 is configured in a corresponding 
manner. It is provided with a recess 43, which is adapted to the shape of 
the tooth. The axially running part of the recess is dimensioned in its 
diameter in such a way that shroud plate and stator do not come into 
contact with each other during the assembly and during operating 
transients. A comparison with FIG. 2 shows that, in the operating 
position, a substantially smaller through-gap 18 appears between stator 
and shroud plate. The gap mass flow is therefore considerably reduced by 
the novel measure. 
Furthermore, the known half labyrinth is replaced by a full labyrinth. To 
this end, the outside diameter of the shroud plate is stepped and provided 
with only two choke points. Two sealing strips 17, which are calked in 
place in the stator and in each case act on a step, define a vortex 
chamber 22 which functions effectively. The choke points, due to their 
radial offset, do not influence one another. With this full labyrinth, a 
further reduction in the gap mass flow is achieved. 
A third measure serves to improve the inflow of the labyrinth mass flow 
into the main duct again. To this end, the cavity at the labyrinth outlet 
42 is reduced in the radial direction to a permissible minimum size. The 
gap flow is immediately received by a stator wall bent outward relative to 
the general conicity. The harmful cross exchange of flow material can thus 
be substantially reduced and the unnecessary dissipation of the highly 
energetic gap flow can be largely avoided. In addition, the total-pressure 
profile of the main flow is favorably influenced by the bent stator wall. 
For this purpose, the flow-limiting wall of the duct 50 is provided with a 
kink angle A directly at the outlet of the moving blades La3. This kink 
angle is dimensioned in such a way that the outflow from the moving blades 
is homogenized with regard to total pressure and outflow angle. In the 
example, this means that the angle A shown is defined as positive. The 
bent wall part runs radially outward, i.e. it is directed away from the 
machine axis (not shown). The cross exchange of flow material, which is 
induced by the pressure zone, which depends on the spacing, is reduced by 
this design. This is because this cross exchange may be the cause of 
separation at the especially sensitive suction side of the blades. 
The selection of the kink angle is based on the following considerations: 
there is a divergent flow, with associated swirl at the cylinder, at the 
outlet of the moving blades. At least the flow in the radially outer zone 
has substantially higher energy than in the radially inner rotor zone, a 
factor which is manifested in the form of substantially higher total 
pressures in the radially outer zone. With the kink-angle idea, it is now 
necessary to achieve the lowest possible total-pressure and outflow-angle 
inhomogeneity over the blade height. The equation for the radial 
equilibrium teaches that this can be achieved primarily via the meridian 
curvature of the flow lines. This must therefore be influenced primarily 
by adaptation of the kink angle. A homogeneous total-pressure distribution 
at the outer boundary wall can only be achieved if the corresponding kink 
angle A relative to the conical contour of the duct always opens outward. 
In this case, the desired total-pressure reduction in this region is 
achieved. 
Accurate directing of the flow over a certain region is required in order 
to fully realize this kink-angle idea. This is done from the knowledge 
that the flow inhomogeneities originating from the blade circulation 
slowly disappear only at a distance which corresponds to half the distance 
between moving-blade outlet and guide-blade inlet divided by the blade 
spacing. 
The wall further upstream, at least approximately in the inlet region of 
the guide blades of the following stage (not shown), is expediently 
provided with a kink angle B directed radially inward. 
The wall provided with this kink angle B, in the root region of the guide 
blade situated upstream, runs radially inward again following the opposite 
kink angle, so that the resulting flow-limiting wall, which is interrupted 
between guide-blade root and subsequent moving-blade shroud plate by the 
axial gap 18, has a common point P with the original straight duct contour 
at least approximately in the plane of the moving-blade inlet of this 
following stage. These facts are illustrated in FIG. 3 with reference to 
that wall which is located upstream of the cavity and which may possibly 
be the flow-limiting part of the guide-blade root situated at the front. 
The opposite kink angle at the upstream wall increases the negative 
pressure or reduces the positive pressure over the downstream labyrinth, a 
factor which leads to a further reduction in the gap mass flow. 
In the exemplary embodiments explained below, the elements having the same 
function are provided with the same reference numerals as in FIG. 3. 
FIG. 4 shows a solution in which the shroud band has the same conicity of 
about 25.degree. as that in FIGS. 2 and 3. The cavity at the labyrinth 
inlet is subdivided in its radial extent into three axially staggered 
cavities 40a, 40b and 40c. Three sealing strips 17 calked in place in the 
stator are arranged at the labyrinth outlet. 
Here, too, in order to improve the inflow of the labyrinth mass flow into 
the main duct again, the cavity at the labyrinth outlet 42, directly 
behind the last sealing strip, is reduced in the radial direction to a 
permissible minimum size. As a rule, this minimum size is also provided in 
the front cavities. To this end, the shroud plate 16 is of stepped design. 
The individual cavities are sealed with sealing strips 52 which run 
approximately horizontally in their first section and are then curved. 
These sealing strips 52 are preferably caulked in place with their 
horizontally running section in the axially running casing parts. It goes 
without saying that other fastening methods and geometries are also 
possible. 
FIG. 4 shows the shroud plate in the normal operating position. The front 
sealing strips 52 act on the front edges of the horizontally directed 
shroud-plate steps. The rear sealing strips 17 act on the last 
horizontally directed shroud-plate step. 
In FIG. 5, on a somewhat reduced scale, the shroud plate is shown in its 
extreme positions, namely during transients, as occur during the start-up 
and shutdown of the machine. It can be seen that, in the position shown by 
chain-dotted lines, the sealing strips 52 engage at the intersection 
between axially and radially directed step parts. In order to facilitate 
this, inter alia, the radial step part is designed to slope against the 
direction of flow. In addition, the curvature of the sealing strips 
permits problem-free escape in the event of the shroud plate assuming an 
even more extreme position. Furthermore, in this position, the frontmost 
sealing strip 17 makes a seal against the horizontally directed, rear 
shroud-plate part. In the position shown by dashes the sealing strips 52 
are no longer in engagement. Here, only the last sealing strip 17 makes a 
seal and thus prevents working medium from flowing through the gap 42 in 
an uncontrolled manner. 
FIG. 6 shows the novel solution in the case of a shroud plate having a 
conicity of only about 10.degree., as is used in front stages of 
low-pressure parts of steam turbines. Here, the cavity is subdivided into 
two sectional cavities 40a and 40c. These sectional cavities are separated 
by a sealing strip 52 which runs approximately horizontally in its first 
section and is then curved. This strip acts on a shroud plate 16 which has 
a single step. The other sealing strips 17 are arranged in such a way that 
at least one of the strips 52 or 17 is effective even in extreme 
positions. 
Finally, FIG. 7 shows the novel solution in the case of a shroud plate 
having a conicity of about 45.o slashed., as is used in the rear 
low-pressure stages of steam turbines. It can be seen here that, even in 
the case of such extreme duct openings, the solution according to FIG. 4 
can be readily applied. In addition, this solution offers the advantage 
that the above-described kink angle B at the inlet, which kink angle B is 
directed radially inward and is fluidically harmful per se, can be 
avoided. That is to say, the shroud-band contour corresponds here to the 
duct contour predetermined overall. 
Compared with the prior art, all the solutions shown and described thus far 
have the advantage that, as a result of the stepped arrangement and in 
particular the sloping radial parts, a substantially increased sealing 
length is available. In addition, at least the shroud plates according to 
FIGS. 4, 6 and 7 also have smaller shroud-plate masses. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that, within the scope of the appended claims, the invention 
may be practiced otherwise than as specifically described herein.