U.S. patent application number 12/596244 was filed with the patent office on 2010-09-09 for diffuser arrangement.
Invention is credited to Sascha Becker, Marc Broker, Ralf Hoffacker, Mario Koebe, Stefan Mahlmann, Ulrich Stanka.
Application Number | 20100226767 12/596244 |
Document ID | / |
Family ID | 38329988 |
Filed Date | 2010-09-09 |
United States Patent
Application |
20100226767 |
Kind Code |
A1 |
Becker; Sascha ; et
al. |
September 9, 2010 |
DIFFUSER ARRANGEMENT
Abstract
A diffuser arrangement through which a fluid may flow is
provided. The diffuser includes an outer diffuser comprising an
inner surface, and a flow-guiding device, which is configured such
that at least part of the boundary layer flow forming on the inner
surface of the outer diffuser can be accelerated in the main flow
direction, so that a flow separation is prevented on the inner
surface of the outer diffuser. Also provided are an exhaust steam
plenum of a steam turbine and an exhaust gas plenum of a gas
turbine, both including a diffuser arrangement.
Inventors: |
Becker; Sascha; (Abu Dhabi,
AE) ; Broker; Marc; (Dinslaken, DE) ;
Hoffacker; Ralf; (Krefeld, DE) ; Koebe; Mario;
(Mulheim an der Ruhr, DE) ; Mahlmann; Stefan;
(Buhren, DE) ; Stanka; Ulrich; (Essen,
DE) |
Correspondence
Address: |
SIEMENS CORPORATION;INTELLECTUAL PROPERTY DEPARTMENT
170 WOOD AVENUE SOUTH
ISELIN
NJ
08830
US
|
Family ID: |
38329988 |
Appl. No.: |
12/596244 |
Filed: |
February 25, 2008 |
PCT Filed: |
February 25, 2008 |
PCT NO: |
PCT/EP2008/052222 |
371 Date: |
April 9, 2010 |
Current U.S.
Class: |
415/207 |
Current CPC
Class: |
F01D 25/30 20130101;
F15D 1/0005 20130101; F15D 1/0025 20130101; F01D 9/02 20130101;
F05D 2220/32 20130101; F05D 2250/232 20130101; F05D 2250/323
20130101; F15D 1/06 20130101; F05D 2220/31 20130101; F05D 2250/292
20130101 |
Class at
Publication: |
415/207 |
International
Class: |
F04D 29/44 20060101
F04D029/44 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 13, 2007 |
EP |
07005175.0 |
Claims
1.-12. (canceled)
13. A diffuser arrangement which is exposed to a throughflow by a
fluid, comprising: an outer diffuser including an inner surface;
and a stationary flow-guiding device, the flow-guiding device
extending inside the outer diffuser, wherein a first length of the
flow-guiding device lies in a range between 5% to 40% of a second
length of the outer diffuser in the main flow direction, and
wherein the flow-guiding device may be used as a flow-accelerating
device whereby a first outer surface of the flow-guiding device,
which faces a second inner surface of the outer diffuser, and a
section of the second inner surface foams a nozzle passage with the
result that at least some of a boundary layer flow which develops
on the second inner surface may be accelerated in the main flow
direction so that a flow separation on the second inner surface is
prevented.
14. The diffuser arrangement as claimed in claim 13, wherein the
flow-guiding device includes a leading edge which is exposed to an
inflow of the fluid and a trailing edge which lies opposite the
leading edge and on which the fluid flows off, wherein an inlet
cross section is located between the second inner surface and the
leading edge, and an outlet cross section is located between the
second inner surface and the trailing edge, and wherein a first
cross-sectional area of the inlet cross section is greater than a
second cross-sectional area of the outlet cross section.
15. The diffuser arrangement as claimed in claim 13, wherein the
flow-guiding device includes a first inner surface, facing away
from the first outer surface, and by the first inner surface an
inner diffuser is formed through which the fluid flows and may be
decelerated in the main flow direction.
16. The diffuser arrangement as claimed in claim 13, wherein the
outer diffuser and the flow-guiding device are axially
symmetrically formed and are concentrically arranged around a
common symmetry axis.
17. The diffuser arrangement as claimed in claim 13, wherein the
nozzle passage is formed as an annular passage.
18. The diffuser arrangement as claimed in claim 13, wherein the
flow-guiding device is formed as a straight guide plate which is
oblong in a longitudinal section or the flow-guiding device is
aerodynamically profiled.
19. The diffuser arrangement as claimed in claim 13, wherein the
flow-guiding device is arranged in the range between 80% to 100% of
a passage radius of the outer diffuser.
20. The diffuser arrangement as claimed in claim 13, wherein the
flow-guiding device is arranged in a region of the inlet cross
section of the outer diffuser.
21. The diffuser arrangement as claimed in claim 13, wherein the
outer diffuser is formed as an essentially straight diffuser.
22. The diffuser arrangement as claimed in claim 13, wherein the
flow-guiding device is pivotably mounted relative to the main flow
direction.
23. An exhaust steam plenum of a steam turbine, comprising: a
diffuser arrangement, comprising: an outer diffuser including an
inner surface, and a stationary flow-guiding device, the
flow-guiding device extending inside the outer diffuser, wherein a
first length of the flow-guiding device lies in a range between 5%
to 40% of a second length of the outer diffuser in the main flow
direction, and wherein the flow-guiding device may be used as a
flow-accelerating device whereby a first outer surface of the
flow-guiding device, which faces a second inner surface of the
outer diffuser, and a section of the second inner surface fauns a
nozzle passage with the result that at least some of a boundary
layer flow which develops on the second inner surface may be
accelerated in the main flow direction so that a flow separation on
the second inner surface is prevented.
24. The exhaust steam plenum of a steam turbine as claimed in claim
23, wherein the flow-guiding device includes a leading edge which
is exposed to an inflow of the fluid and a trailing edge which lies
opposite the leading edge and on which the fluid flows off, wherein
an inlet cross section is located between the second inner surface
and the leading edge, and an outlet cross section is located
between the second inner surface and the trailing edge, and wherein
a first cross-sectional area of the inlet cross section is greater
than a second cross-sectional area of the outlet cross section.
25. The exhaust steam plenum of a steam turbine as claimed in claim
23, wherein the flow-guiding device includes a first inner surface,
facing away from the first outer surface, and by the first inner
surface an inner diffuser is formed through which the fluid flows
and may be decelerated in the main flow direction.
26. The exhaust steam plenum of a steam turbine as claimed in claim
23, wherein the outer diffuser and the flow-guiding device are
axially symmetrically formed and are concentrically arranged around
a common symmetry axis.
27. The exhaust steam plenum of a steam turbine as claimed in claim
23, wherein the nozzle passage is foamed as an annular passage.
28. An exhaust gas plenum of a gas turbine, comprising: a diffuser
arrangement, comprising: an outer diffuser including an inner
surface, and a stationary flow-guiding device, the flow-guiding
device extending inside the outer diffuser, wherein a first length
of the flow-guiding device lies in a range between 5% to 40% of a
second length of the outer diffuser in the main flow direction, and
wherein the flow-guiding device may be used as a flow-accelerating
device whereby a first outer surface of the flow-guiding device,
which faces a second inner surface of the outer diffuser, and a
section of the second inner surface forms a nozzle passage with the
result that at least some of a boundary layer flow which develops
on the second inner surface may be accelerated in the main flow
direction so that a flow separation on the second inner surface is
prevented.
29. The exhaust gas plenum of a gas turbine as claimed in claim 28,
wherein the flow-guiding device includes a leading edge which is
exposed to an inflow of the fluid and a trailing edge which lies
opposite the leading edge and on which the fluid flows off, wherein
an inlet cross section is located between the second inner surface
and the leading edge, and an outlet cross section is located
between the second inner surface and the trailing edge, and wherein
a first cross-sectional area of the inlet cross section is greater
than a second cross-sectional area of the outlet cross section.
30. The exhaust gas plenum of a gas turbine as claimed in claim 28,
wherein the flow-guiding device includes a first inner surface,
facing away from the first outer surface, and by the first inner
surface an inner diffuser is formed through which the fluid flows
and may be decelerated in the main flow direction.
31. The exhaust gas plenum of a gas turbine as claimed in claim 28,
wherein the outer diffuser and the flow-guiding device are axially
symmetrically formed and are concentrically arranged around a
common symmetry axis.
32. The exhaust gas plenum of a gas turbine as claimed in claim 28,
wherein the nozzle passage is formed as an annular passage.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is the US National Stage of International
Application No. PCT/EP2008/052222, filed Feb. 25, 2008 and claims
the benefit thereof. The International Application claims the
benefits of European Patent Office application No. 07005175.0 EP
filed Mar. 13, 2007, both of the applications are incorporated by
reference herein in their entirety.
FIELD OF INVENTION
[0002] The invention refers to a diffuser arrangement and
especially to an exhaust steam plenum of a steam turbine or an
exhaust gas plenum of a gas turbine with the diffuser
arrangement.
BACKGROUND OF INVENTION
[0003] A diffuser is a passage which is exposable to throughflow by
fluid and which in the case of separation-free throughflow
decelerates the fluid by means of cross-sectional widening and, in
accordance with Bernoulli's theorem, reduces the kinetic pressure
of the fluid to the benefit of the static pressure.
[0004] The quality of the diffuser is described by means of the
pressure recovery coefficient, which is defined by
C.sub.p=(p.sub.out-p.sub.in)/(p.sub.total,in-p.sub.in)
[0005] and the overall or total pressure p.sub.total,in, the static
pressure p.sub.in at the diffuser inlet and the static pressure
p.sub.out at the diffuser outlet.
[0006] Diffusers are used for example in pipelines for pressure
recovery or for constant bridging of cross-sectional widenings
(transition diffuser). In the case of pipelines with circular cross
section the diffusers are axially symmetrically formed. In FIG. 4,
a longitudinal section of an axially symmetrical diffuser 101 is
shown, and schematically shows the flow which typically occurs
within it. The diffuser 101 has an inlet cross section 102 and an
outlet cross section 103, the area ratio of which is greater than
one. Upstream of the diffuser 101, a cylindrical inflow pipe is
arranged, through which flows an inflow 108, and downstream of the
diffuser 101 a cylindrical outflow pipe is arranged, through which
flows an outflow 109.
[0007] On account of the adherence of the fluid on the diffuser
wall, a boundary layer develops in the flow close to the wall. In
the lower half of the diffuser 101 which is shown in FIG. 4, five
characteristic velocity profiles 110 to 114 are shown along the
main flow direction, wherein the first of the two velocity profiles
110, 111 show the flow close to the wall in the inflow pipe and the
three velocity profiles 112 to 114 which follow upstream show the
flow close to the wall in the diffuser 101.
[0008] Since the flow in the diffuser 101 is decelerated, the flow
velocity of the main flow decreases in the flow direction, as a
result of which by fulfilling the first main theorem of
thermodynamics the static pressure of the flow correspondingly
increases in the flow direction. According to Prandtl's boundary
layer theory, the static pressure in the boundary layer is constant
transversely to the flow direction.
[0009] On account of the deceleration effect of the diffuser 101
and the adherence condition on the diffuser wall, the flow velocity
of the flow close to the wall decreases. After negotiating a
specific flow path the gradient of the flow velocity transversely
to, and on, the diffuser wall is zero. This position is a
separation point 105 of the flow, which is shown in the boundary
layer profile 113.
[0010] At the separation point 105, the flow moves away from the
diffuser wall towards the middle of the diffuser 101, wherein
downstream of the separation point 105 in the wall proximity a
backflow develops which forms a separation bubble 106. The
separation bubble 106 brings about a narrowing of the cross section
of the diffuser 101 which is effectively exposed to throughflow so
that the main flow in the region of the separation bubble 106 is
accelerated. As a result, in the main flow the kinetic energy is
increased and the flow is reapplied in the outlet pipe at a
reapplication point 107.
[0011] The degree of opening of the diffuser 101 which is shown in
FIG. 4 substantially determines the shape and the size of the
separation bubble 106 and the position of the separation point 105
and of the reapplication point 107 which possibly occurs. The
higher the degree of opening of the diffuser 101, the further
upstream the separation point 105 lies.
[0012] On account of the narrowing of the effective cross section
of the diffuser 101, the separation bubble 106 reduces the pressure
recovery effect of the diffuser 101 compared with a diffuser in
which the flow is fully applied.
[0013] In order to create a laminar boundary layer flow in a
diffuser, a bladed wheel which is seated on a hub, the blades of
which, being shrouded by a diffuser plate, are connected on the
blade tip side by means of a ring, is known from laid-open
specification DE 1 628 337. A ring of stator blades is arranged on
the ring in such a way that this widens the jet flow which flows
off the bladed wheel while maintaining the boundary flow which is
guided by the diffuser plate. In addition to the stator blades,
this is especially achieved by the ring having a corresponding
cross-sectional shape which, moreover, benefits the course of the
entrained flow filaments and blows these out at higher
velocity.
[0014] Furthermore, for avoiding flow separations in a diffuser, a
pipe which is arranged parallel to the diffuser wall and which
extends along the flow direction, is known from JP 08 260905. On
account of the diverging cross section of the diffuser and of the
correspondingly diverging pipe which is parallel to it, the flow
cross section of the annular passage which is formed between
diffuser wall and the pipe is increased so that medium flowing in
the annular passage is decelerated.
[0015] A steam turbine or a gas turbine is run at partial load,
base load and overload. In the construction and design of the steam
turbine or gas turbine, their individual components can be
geometrically designed in an optimized manner only at a single
operating point for example with regard to efficiency or
aerodynamic or thermodynamic effectiveness. This has the result
that at other operating points, which are not identical to the
design operating point, the components cannot operate in an optimum
manner.
[0016] This state also applies to an exhaust steam plenum of the
steam turbine or to an exhaust gas plenum of the gas turbine. The
exhaust steam plenum or the exhaust gas plenum is conventionally
constructed as an axial diffuser.
[0017] As a rule, the axial diffuser is geometrically designed in
an optimized manner with regard to the base load so that at partial
load and overload the axial diffuser cannot be operated in an
optimized manner.
[0018] At the inlet of the axial diffuser, in the case of optimum
design, a lower static pressure exists than at the outlet. As a
result of lowering the pressure at the diffuser inlet, which at the
same time represents the exit of the blading, the last rotor blade
ring is brought to a higher power output.
[0019] The mass flow of the flow which flows through the axial
diffuser is lower in the partial load range than in the base load
range, as a result of which the average flow velocity in the axial
diffuser in the base load range is higher than in the partial load
range. As a result, the flow in the axial diffuser in the partial
load range is more prone to separation than the flow which occurs
in the axial diffuser at base load.
[0020] Therefore, the pressure recovery in the axial diffuser at
partial load is lower compared with the pressure recovery at base
load. This has the result that at partial load the turbine output
is lowered compared with the turbine output at base load. The
influence of an improvement in the pressure recovery of a gas
turbine diffuser of c.sub.p=0.1 was estimated by Farohki at 0.8% of
the delivered turbine output. This connection is similarly
applicable to axially exhausting steam turbines.
[0021] A reduction of the degree of opening of the axial diffuser
could provide a remedy in this case since the flow is decelerated
less sharply as a result and is therefore prone to separation to a
lesser degree. However, the overall length of the axial diffuser is
consequently extended, as a result of which the total overall
length of the steam turbine or gas turbine is disadvantageously
increased.
SUMMARY OF INVENTION
[0022] It is the object of the invention to create a diffuser
arrangement, the pressure recovery of which is high and its overall
length short.
[0023] The diffuser arrangement according to the invention is
exposable to throughflow by fluid and has an outer diffuser which
has an inner surface, and a flow-accelerating device which is
installed in such a way that at least some of the boundary layer
flow which develops on the inner surface of the outer diffuser can
be accelerated in the main flow direction so that a flow separation
on the inner surface of the outer diffuser is prevented.
[0024] If the fluid flows through the outer diffuser, then it is
decelerated in the main flow direction, as a result of which the
boundary layer flow which develops on the inner surface of the
outer diffuser is principally prone to separation. The separation
would emanate from a point at which the kinetic energy of the flow
is zero.
[0025] By means of the flow-accelerating device according to the
invention at least some of the flow close to the wall is
accelerated so that the kinetic energy of the flow close to the
wall is increased. As a result, the effect of the kinetic energy of
the flow close to the wall not being zero at any point is
prevented, as a result of which a flow separation on the inner
surface of the outer diffuser is prevented. Therefore, the diffuser
arrangement has a high pressure recovery.
[0026] Furthermore, the outer diffuser of the diffuser arrangement
can have a large degree of opening without a flow separation
occurring in it. Consequently, the outer diffuser and therefore the
diffuser arrangement has a shorter overall length.
[0027] The flow-accelerating device has a flow guiding device which
extends inside the outer diffuser, and by its outer surface, which
faces the inner surface of the outer diffuser, and a section of the
inner surface of the outer diffuser, forms a nozzle passage through
which the part of the boundary layer flow can flow.
[0028] Therefore, the flow-accelerating device is formed by the
nozzle passage which is defined by the flow guiding device
interacting with the inner wall of the outer diffuser. As a result,
the effect is achieved of the flow close to the wall, i.e. just the
flow portion with otherwise low kinetic energy, being accelerated
directly on the inner surface of the outer diffuser. Consequently,
a separation in the diffuser arrangement is effectively
prevented.
[0029] Furthermore, the extent of the flow-guiding device in the
main flow direction lies in the region of 5% to 40% of the extent
in the main flow direction of the outer diffuser. As a result, the
flow-guiding device is arranged entirely inside the outer diffuser
and can be accurately placed on any section on the inner wall of
the outer diffuser on which a separation of the fluid flow is to be
expected. Therefore, the flow-guiding device can be purposefully
arranged on a section where separation is a risk, as a result of
which an effective prevention of flow separation is achieved and
therefore the disturbance of the main flow by the flow-guiding
device is low.
[0030] It is preferred that the flow-guiding device by its inner
surface which faces away from the outer surface forms an inner
diffuser through which the fluid flow can flow and in so doing can
be decelerated in the main flow direction.
[0031] Therefore, in addition to the nozzle effect in the outer
region the flow-guiding device also has a diffuser effect in the
inner region so that the flow through the diffuser arrangement is
sharply decelerated. As a result, the effect is achieved of the
pressure recovery of the diffuser arrangement according to the
invention being high.
[0032] It is preferred that the outer diffuser and the flow-guiding
device are axially symmetrically formed and are concentrically
arranged around a common symmetry axis.
[0033] Furthermore, it is preferred that the nozzle passage is
formed as an annular passage.
[0034] From this, the diffuser arrangement is advantageously
created as an arrangement of a plurality of diffusers and a nozzle.
This arrangement is formed by a series-connecting of the three
diffusers, specifically the region of the outer diffuser upstream
of the flow-guiding device, the inner diffuser of the flow-guiding
device, and the region of the outer diffuser downstream of the
flow-guiding device, and a parallel-connecting of the nozzle
passage to the inner diffuser of the flow-guiding device. As a
result, a compact, simple and effectively operating division of the
outer diffuser is achieved, wherein the diffuser arrangement has a
compact type of construction.
[0035] The flow-guiding device is preferably formed as a straight
guide plate.
[0036] As a result, the guide plate can advantageously be
cost-effectively produced.
[0037] Alternatively to this, it is preferred that the flow-guiding
device is aerodynamically profiled. Consequently, the flow-guiding
device has a low flow resistance.
[0038] Furthermore, it is preferred that the flow-guiding device is
arranged in the region of 80% to 100% of the passage height
(radius) of the outer diffuser.
[0039] Consequently, the flow-guiding device is advantageously
effectively placed in the flow close to the wall and, as a result,
is aerodynamically effectively placed.
[0040] Furthermore, the flow-guiding device is preferably arranged
in the region of the inlet cross section of the outer diffuser.
[0041] As a result, it is advantageously made possible for the
inlet flow into the outer diffuser from the flow-guiding device to
already have an accelerated flow in the boundary layer region,
which accelerated flow over the course along the inner surface of
the outer diffuser is not therefore prone to separation.
[0042] Furthermore, it is preferred that the flow-guiding device is
pivotably mounted relative to the main flow.
[0043] In this way, the effect is advantageously achieved of the
flow-guiding device being able to be individually adjusted by
pivoting with regard to the respective flow conditions inside the
outer diffuser in such a way that the flow-guiding device is
aerodynamically effective.
[0044] An exhaust steam plenum of a steam turbine or an exhaust gas
plenum of a gas turbine preferably features the diffuser
arrangement according to the invention.
[0045] Furthermore, it is preferred that the flow-accelerating
device is arranged on the inner surface of the outer diffuser in
the region of its inlet.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] Preferred exemplary embodiments of a diffuser arrangement
according to the invention, with reference to the attached
schematic drawings, are explained in the following text. In the
drawing:
[0047] FIG. 1 shows a longitudinal section through a first
exemplary embodiment of the diffuser arrangement,
[0048] FIG. 2 shows a longitudinal section through a second
exemplary embodiment of the diffuser arrangement,
[0049] FIG. 3 shows a longitudinal section through a third
exemplary embodiment of the diffuser arrangement, and
[0050] FIG. 4 shows a longitudinal section of a diffuser with
schematic representation of the flow conditions.
DETAILED DESCRIPTION OF INVENTION
[0051] As is apparent from FIG. 1, a diffuser arrangement 1 has an
outer diffuser 2 which is axially symmetrically formed around its
symmetry axis 3. An inlet cross section 4 of the outer diffuser 2,
through which an inflow 5 flows into the outer diffuser 2, lies in
a plane which is perpendicular to the symmetry axis 3, and its
outlet cross section 6, from which an outflow 7 discharges from the
outer diffuser 2, lies in another plane which is perpendicular to
the symmetry axis 3 of the outer diffuser 2. This outer diffuser
has an inner surface 8 which delimits the inside space of the said
outer diffuser 2.
[0052] The outer diffuser 2 is formed as a straight diffuser, i.e.
the inner surface 8 of the outer diffuser 2 forms a truncated cone,
wherein the cross-sectional area at the inlet cross section 4 is
smaller than the cross-sectional area at the outlet cross section
6.
[0053] A flow-guiding device 9 is arranged inside the outer
diffuser 2. The flow-guiding device 9 is formed as a guide plate
which is oblong in longitudinal section and which,
axially-symmetrically arranged around the symmetry axis 3 of the
outer diffuser 2 concentrically with the outer diffuser 2, delimits
a truncated cone-shaped annular passage which narrows in the flow
direction.
[0054] On its outer periphery the flow-guiding device 9 has an
outer surface 10 which with regard to the inner surface 8 of the
outer diffuser 2 is inclined in such a way that the annulus cross
section decreases in the flow direction in a plane which is
perpendicular to the symmetry axis 3 and formed between the
flow-guiding device 9 and the outer diffuser 2.
[0055] That is to say, the outer surface 10 of the flow-guiding
device 9 interacts with a section of the inner surface 8 of the
outer diffuser 2 which lies opposite it in such a way that the
annular passage, which lies between the flow-guiding device 9 and
the outer diffuser 2, forms a nozzle passage 11. Therefore, the
section of the inner surface 8 of the outer diffuser 2 which faces
the outer surface 10 of the flow-guiding device 9 is an inner
surface 12 of the nozzle passage 11.
[0056] Upstream, the flow-guiding device 9 is delimited by its
leading edge 13 and downstream is delimited by its trailing edge
14. An inlet cross section 15 of the nozzle passage 11 is located
in the region from the leading edge 13 of the flow-guiding device 9
up to the inner surface 8 of the outer diffuser 2, and the outlet
cross section 16 of the nozzle passage 11 is located in the region
of the trailing edge 14 of the flow-guiding device 9 up to the
inner surface 8 of the outer diffuser 2, wherein the
cross-sectional area of the inlet cross section 15 is greater than
the cross-sectional area of the outlet cross section 16.
[0057] Facing away from the outer surface 10 of the flow-guiding
device 9, this has an inner surface 17 which forms an inner
diffuser 18. The leading edge 13 of the flow-guiding device 9 is
arranged in a plane which is perpendicular to the symmetry axis 3
and forms an inlet cross section 19 of the inner diffuser 18, and
the trailing edge 14 of the flow-guiding device 9 is arranged in a
plane which is perpendicular to the symmetry axis 3 and forms an
outlet cross section 20 of the inner diffuser 18, wherein the inlet
cross section 19 is smaller than the outlet cross section 20.
[0058] From FIG. 2, the aerodynamic effectiveness of the
flow-guiding device 9 is evident. According to FIG. 2, the
flow-guiding device 9 is formed as a profiled annular guide
plate.
[0059] For representing the flow conditions in the diffuser
arrangement 1, in FIG. 2 flow lines 21 are drawn in the region of
the flow-guiding device 9, and a velocity profile 22 upstream of
the flow-guiding device 9, a velocity profile 23 at the trailing
edge 14 of the flow-guiding device 9, and also a velocity profile
24 downstream of the flow-guiding device 9 are shown.
[0060] The flow lines 21 have a converging path in the main flow
direction, as a result of which the flow acceleration which is
induced by means of the flow-guiding device 9 is indicated. The
velocity gradient, which is normal to the wall, on the inner
surface 8 of the outer diffuser 2 is flatter in the case of the
velocity profile 22 upstream of the flow-guiding device 9 than in
the case of the velocity profile 23 at the trailing edge 14 of the
flow-guiding device 9, which is flatter than the velocity gradient,
which is normal to the wall, of the velocity profile 24 downstream
of the flow-guiding device 9.
[0061] Consequently, it is shown that the flow, which is guided by
the flow-guiding device 9 through the nozzle passage 11, is
accelerated (energized). Therefore, the flow-guiding device 9
locally increases the velocity of the flow in the proximity of the
inner surface 12 of the outer diffuser 2. In the process,
high-energy flow material from the core flow is deflected in the
direction towards the inner surface 12 of the outer diffuser 2 and
therefore is added to the boundary layer on the inner surface 12 of
the outer diffuser 2. As a result of this energizing, the boundary
layer on the inner surface 12 of the outer diffuser 2 can overcome
greater positive pressure gradients in the main flow direction
without being separated from the inner surface 12 of the outer
diffuser 2 in the process.
[0062] As a result, the outer diffuser 2 reacts kindly to premature
separation phenomena. Therefore, by provision of the flow-guiding
device 9 in the outer diffuser 2 a higher pressure recovery of the
outer diffuser 2 is achieved.
[0063] FIG. 3 shows an exhaust gas plenum of a gas turbine, which
is formed as the outer diffuser 2. The outer diffuser 2 is arranged
downstream of a turbine rotor 25 and guides away the outflow, which
issues from the turbine rotor 25, from the inlet cross section 4 of
the outer diffuser 2 to the outlet cross section 6 of the outer
diffuser 2, recovering pressure.
[0064] The turbine rotor 25 has a turbine rotor hub 26 which is
continued by a cylindrical outer diffuser hub 27 with the turbine
rotor hub 26.
[0065] The turbine rotor 25 has a multiplicity of turbine rotor
blades 28 which on their radial outer ends have a blade tip 29. The
turbine rotor 25 is enclosed by a turbine casing 30. During
operation of the turbine rotor 25 this rotates around its
rotational axis (not shown), while the turbine casing 30 remains
stationary. Therefore, a gap 31 is provided between the turbine
rotor blade tip 29 and the turbine casing 30 so that the turbine
rotor blade tip 29 does not rub on the turbine casing 30 during
operation of the turbine rotor 25.
[0066] In order to avoid rubbing of the rotor blades on the turbine
casing 30 and to thereby avoid damage, a minimum distance as a gap
31, the so-called clearance, is necessary between rotor blade 28
and casing 30. Some of the mass flow can flow through this gap
without power yield to the rotor blade 28 and leads to energizing
of the boundary layer. Depending upon the configuration of this gap
31, with or without sealing, mass flow can flow through to a
greater or lesser extent. In order to avoid, or greatly delay, a
subsequent separation of the flow in the diffuser, a further
energizing of the boundary layer by means of the flow-guiding
device 9 is desired.
[0067] According to FIG. 3, a remedy is provided by arranging the
flow-guiding device 9 close to the inner surface 8 of the outer
diffuser 2 in the region of the inlet cross section of the outer
diffuser 4. The boundary layer which is disturbed by the leakage
flow is accelerated in the main flow direction by the flow-guiding
device 9 on the inner surface of the outer diffuser 2 so that the
kinetic energy in this flow region is increased. As a result, the
effect is achieved of the flow not separating in the outer diffuser
2 on the inner surface 8 of the outer diffuser 2. Therefore, the
flow losses in the outer diffuser 2 are low and the pressure
recovery of the outer diffuser 2 is high.
* * * * *