Low pressure turbine exhaust diffuser with turbulators   

Abstract: A turbine exhaust diffuser for use adjacent a last stage row of buckets fixed to a turbine rotor includes an annular inner diffuser ring and an annular outer diffuser ring defining a flow path for steam exiting a last stage row of buckets in a first substantially axial direction at a diffuser inlet, and turning substantially ninety degrees to a diffuser outlet. The outer diffuser ring has a curved steam guide surface extending between the diffuser inlet and the diffuser outlet including a first portion extending away from the diffuser inlet and a second portion extending to said diffuser outlet, the second portion shaped to extend beyond vertical and back toward the diffuser inlet so as to establish a flow component in a second opposite axial direction. The second portion is also provided with a plurality of turbulators to minimize flow separation along the steam guide surface.

BACKGROUND OF THE INVENTION

This invention relates to steam turbine technology in general, and to an axial-to-radial flow, low pressure, steam turbine exhaust gas diffuser in particular.

A steam turbine low pressure (LP) section typically includes an inlet domain, multiple turbine stages and an exhaust gas diffuser (sometimes referenced to as an exhaust hood). The exhaust gas diffuser is typically located at the last row of rotating blades or buckets, and is formed to include a steam flow guide between an axial flow inlet and a radial flow outlet. Flow diffusion takes place in the initial section of the diffuser, formed by the diffuser steam flow guide while the remainder of the diffuser features collect the gas flow in a chamber and guide it to the condenser.

One of the main functions of the diffuser or exhaust hood is to recover static pressure as it guides the exhaust gas flow from the last stage row of buckets into the condenser. In fact, diffusers are typically designed with respect to optimized turbine performance which may be measured in terms of maximum possible static pressure recovery.

The degree of static pressure recovery in the low pressure exhaust diffuser depends to a large extent on the Area Ratio formed by steam guide profile and on the last stage bucket exit profile, including bucket tip clearance. Generally, maximum pressure recovery comes at the end of the steam guide, but after that, pressure losses occur due to improper area scheduling.

On the other hand, in order to reduce production costs, it is desirable to shorten the length of the turbine rotor or shaft. A reduced shaft length results in reduced available Area Ratio at the end of the steam guide. To compensate for loss of area due to reduced shaft length, it has been proposed to design the steam guide section of the diffuser more aggressively, to increase the area at the diffuser outlet. The consequence of such an aggressive design, however, is flow separation along the steam guide, i.e., along the outer steam guide wall portion closest to the last stage bucket tips, and particularly at the outlet end of the outer steam guide wall portion.

There remains a need, therefore, for a solution to the problem of achieving maximum pressure recovery with an aggressive steam guide design, but without incurring flow separation along the steam guide surface of the diffuser.

In accordance with a first exemplary but nonlimiting aspect, the invention provides a turbine exhaust diffuser for use adjacent a last stage row of buckets fixed to a turbine rotor comprising an annular inner diffuser ring and an annular outer diffuser ring defining a flow path for steam exiting a last stage row of buckets in a first substantially axial direction at a diffuser inlet, and turning substantially ninety degrees to a diffuser outlet; the outer diffuser ring having a curved steam guide surface extending between the diffuser inlet and the diffuser outlet and having a first portion extending away from the diffuser inlet and a second portion, extending to the diffuser outlet, the second portion shaped to extend beyond vertical and back toward the diffuser inlet so as to establish a flow component in a second opposite axial direction; wherein the second portion is provided with a plurality of turbulators.

In accordance with another exemplary but nonlimiting aspect, there is provided a turbine exhaust diffuser adjacent a last stage row of buckets fixed to a turbine rotor comprising an annular inner diffuser ring and an annular outer diffuser ring defining a flow path for steam exiting a last stage row of buckets in a first substantially axial direction at a diffuser inlet, and turning substantially ninety degrees to a diffuser outlet; the outer diffuser ring having a curved steam guide surface extending between the diffuser inlet and the diffuser outlet and having a first portion extending away from the diffuser inlet and a second portion extending to the diffuser outlet, the second portion shaped to extend beyond vertical and back toward the diffuser inlet so as to establish a flow component in a second opposite axial direction; wherein the second portion is provided with a plurality of turbulators arranged in at least two radially-spaced, circumferential rows.

In accordance with still another exemplary embodiment, there is provided a method of method of increasing an outlet area of a turbine exhaust diffuser and minimizing flow separation along an outer wall portion of said turbine exhaust diffuser comprising shaping the outer wall portion extending between an axially-oriented diffuser inlet to a substantially radially oriented diffuser outlet to extend beyond vertical and back toward the diffuser inlet so as to establish a flow component in an upstream direction; and providing plural turbulators on the outer wall portion to generate localized vortices to minimize flow separation along the outer wall portion.

The invention will now be described in detail in connection with the drawings identified below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section through an upper or radially outer portion of a gas turbine exhaust diffuser in accordance with an exemplary but nonlimiting embodiment of the invention;

FIG. 2 is a perspective view of the diffuser portion shown in FIG. 1;

FIG. 3 is an enlarged detail taken from FIG. 2;

FIG. 4 illustrates the degree of flow separation in an example diffuser in accordance with the invention, but without turbulators or vortex generators; and

FIG. 5 illustrates a diffuser as shown in FIG. 4 but illustrating a reduction in flow separation achieved with the addition of turbulators or vortex generators in accordance with the invention.

DETAILED DESCRIPTION

With reference initially to FIG. 1, there is shown a radially outer portion of an exhaust hood or diffuser 10 that may be part of a low pressure steam turbine. The diffuser 10 guides the turbine engine exhaust flow from the last stage blades or buckets 12 into an exhaust steam housing (not shown) and then to a condenser (also, not shown). The diffuser is generally shaped as a hollow toroid, with only a profile or cross section of the radially outer portion of the diffuser shown in FIG. 1. The diffuser 10 is generally formed by an outer ring 14 and an inner ring 16 that are joined to create the hollow toroidal shape, with an inlet 18 to the diffuser 10 closely adjacent the last row of blades or buckets, represented by the single blade 12. Reference to the outer ring 14 and inner ring 16 is primarily for convenience. These rings may be of substantially channel-shape in cross section, and joined by welding or other suitable means, to form the annular, hollow toroid. The construction and assembly details, however, are not part of this invention, except as otherwise described herein. The diffuser 10 thus forms a steam guide or flow path for steam exiting the last stage row of blades or buckets that extends initially in a substantially axial direction at the last stage 12 and then turns substantially ninety degrees, terminating at the diffuser outlet 20. Adjacent the outlet 20 it can be seen that an outer ring portion (also referred to as the steam guide portion) of a conventional diffuser closest to the blade tips extends substantially vertically, i.e., substantially perpendicular to the turbine rotor axis (that extends substantially parallel to the diffuser surface 23). This steam guide portion of the diffuser is shown at 22 in dotted line-format in FIG. 1. In other words, at the outlet 20, both the inner and outer rings are substantially vertically-oriented.

In order to compensate for reduced shaft length and maintain maximum pressure recovery, the area ratio at the outlet 20, along diffuser steam guide wall portion or surface 24, is increased by turning the inner ring beyond vertical, back toward the turbine exit stage, thus providing a flow component in a second axial or upstream direction, e.g., by about 15° (See FIG. 1 and compare the dotted line 22 with the solid line 24). As noted above, this approach has a tendency to cause undesirable boundary layer separation along the steam guide wall surface 24, particularly that portion closest to the outlet 20, as shown in FIG. 4.

It has been determined that the addition of turbulators or vortex generators 26 to the interior of steam guide wall surface 24 helps the exhaust flow to remain attached along that section of the diffuser wall. With reference also to FIGS. 2 and 3, the turbulators or vortex generators 26 are disclosed as hemispherical projections with two, radially-spaced, circumferential rows arranged about the inner ring 14, specifically about that portion of steam guide wall surface 24 that is shaped to extend beyond vertical and back toward the last stage buckets. It will be appreciated, however, that the invention is not limited to turbulators or vortex generators of hemispherical shape, but also contemplates hemispherical dimples or recesses. The shape may also vary from round to oval to rectangular, etc. and the height or depth may also vary as will be appreciated by those skilled in the art. It is also within the scope of the invention to vary the number of rows of turbulators or vortex generators, and to have the turbulators or vortex generators in adjacent rows circumferentially aligned with, or staggered relative to adjacent rows.

The described use of turbulators or vortex generators 26 on the inside surface 24 of the diffuser outlet has been shown to increase pressure recovery to a significant degree. Specifically, the turbulators or vortex generators 26 energize the boundary layer due to increased turbulence or localized vortices, helping to keep the flow attached to the surface 24 and thus increasing the static pressure recovery. In one example, where the clearance between the last stage turbine blade shroud or tip and the surrounding casing is 110 mils, the pressure recovery improved by a factor of 0.07 (where 100% pressure recovery is given a value of 1.0, the pressure recovery improved from 0.64 to 0.71). This difference is illustrated diagrammatically in FIGS. 4 and 5 with flow separation indicated by cross-hatching along the steam guide surface 24.

It will be understood that specific pressure recovery enhancements will depend on turbine configuration, bucket tip clearances and the like.

Thus, the invention provides for highly effective flow diffusion which yields a reduction of the so-called backpressure for the turbine, allowing the turbine to have an increased overall pressure ratio for the same temperature reservoir of the thermodynamiccycle, or to deliver the same output at a higher efficiency (i.e., for a reduced fuel input).