Patent Application
20160215625
Embodiments of the invention relate generally to rotary machines and, more particularly, to the control of wheel space purge air in gas turbines.
As is known in the art, gas turbines employ rows of buckets on the wheels/disks of a rotor assembly, which alternate with rows of stationary vanes on a stator or nozzle assembly. These alternating rows extend axially along the rotor and stator and allow combustion gasses to turn the rotor as the combustion gasses flow therethrough.
Axial/radial openings at the interface between rotating buckets and stationary nozzles can allow hot combustion gasses to exit the hot gas path and radially enter the intervening wheelspace between bucket rows. To limit such incursion of hot gasses, the bucket structures typically employ axially-projecting angel wings, which cooperate with discourager members extending axially from an adjacent stator or nozzle. These angel wings and discourager members overlap but do not touch, and serve to restrict incursion of hot gasses into the wheelspace.
In addition, cooling air or “purge air” is often introduced into the wheelspace between bucket rows. This purge air serves to cool components and spaces within the wheelspaces and other regions radially inward from the buckets as well as providing a counter flow of cooling air to further restrict incursion of hot gasses into the wheelspace. Angel wing seals therefore are further designed to restrict escape of purge air into the hot gas flowpath.
Nevertheless, most gas turbines exhibit a significant amount of purge air escape into the hot gas flowpath. For example, this purge air escape may be between 0.1% and 3.0% at the first and second stage wheelspaces. The consequent mixing of cooler purge air with the hot gas flowpath results in large mixing losses, due not only to the differences in temperature but also to the differences in flow direction or swirl of the purge air and hot gasses.
In one embodiment, the invention provides a turbine bucket comprising: a platform portion; an airfoil extending radially outward from the platform portion; a shank portion extending radially inward from the platform portion; at least one angel wing extending axially from a face of the shank portion, the at least one angel wing including an angel wing rim extending radially upward toward the airfoil; and a plurality of voids disposed along the angel wing rim.
In another embodiment, the invention provides a turbine bucket comprising: a platform portion; an airfoil extending radially outward from the platform portion; a shank portion extending radially inward from the platform portion; at least one angel wing extending axially from a face of the shank portion, the at least one angel wing including an angel wing rim extending radially upward toward the airfoil; a plurality of voids disposed along the angel wing rim; and a plurality of dam members extending axially inward from the angel wing rim toward the face of the shank portion.
In yet another embodiment, the invention provides a gas turbine comprising: at least one turbine bucket extending radially outward from a rotating shaft, the at least one turbine bucket including: an airfoil extending radially outward from a platform; a shank portion extending radially inward from the platform portion; and at least one angel wing seal extending axially from a face of the shank portion, the at least one angel wing seal having an angel wing rim extending radially upward toward the airfoil; and a nozzle surface disposed radially outward from the at least one angel wing seal, the nozzle surface having a radially inwardly facing erodible portion adjacent the angel wing rim, wherein, in operation, the angel wing rim erodes a groove into the erodible portion of the nozzle surface.
In yet another embodiment, the invention provides a gas turbine comprising: a diffuser; and a last stage turbine bucket adjacent the diffuser, the last stage turbine bucket comprising: an airfoil extending radially outward from a platform portion; a shank portion extending radially inward from the platform portion; at least one angel wing extending axially from a face of the shank portion, the at least one angel wing including an angel wing rim extending radially upward toward the airfoil; and a plurality of voids disposed along the angel wing rim.
These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the invention, in which:
It is noted that the drawings of the invention are not to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements among the drawings.
Turning now to the drawings,
Shank portion 60 includes a pair of angel wing seals 70, 72 extending axially outward toward first stage nozzle 20 and an angel wing seal 74 extending axially outward toward second stage nozzle 22. It should be understood that differing numbers and arrangements of angel wing seals are possible and within the scope of the invention. The number and arrangement of angel wing seals described herein are provided merely for purposes of illustration.
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In operation, purge air 180 passes into groove 131 of nozzle surface 130 and then downward between dam members 177, toward face 162. Purge air 180 then flows circumferentially within gap 164, adjacent face 162, as turbine bucket 140 rotates, providing increased swirl to purge air 180.
The increases in turbine efficiencies achieved using embodiments of the invention can be attributed to a number of factors. First, as noted above, increases in swirl velocity reduce the escape of purge air into hot gas flowpath 28, increases in swirl reduce the mixing losses attributable to any purge air that does so escape, and the curtaining effect induced by voids according to the invention reduce or prevent the incursion of hot gas 95 into wheelspace 26. Each of these contributes to the increased efficiencies observed.
In addition, the overall quantity of purge air needed is reduced for at least two reasons. First, a reduction in escaping purge air necessarily reduces the purge air that must be replaced, which has a direct, favorable effect on turbine efficiency. Second, a reduction in the incursion of hot gas 95 into wheelspace 26 reduces the temperature rise within wheelspace 26 and the attendant need to reduce the temperature through the introduction of additional purge air. Each of these reductions to the total purge air required reduces the demand on the other system components, such as the compressor from which the purge air is provided.
While reference above is made to the ability of angel wing rim voids to change the swirl velocity of purge air within a wheelspace, and particularly within a wheelspace adjacent early stage turbine buckets, it should be noted that angel wing rim voids may be employed on turbine buckets of any stage with similar changes to purge air swirl velocity and angle. In fact, Applicants have noted a very favorable result when angel wing rim voids are employed in the last stage bucket (LSB).
Spikes in total pressure (PT) and swirl profiles at the inner radius region of the diffuser inlet are a consequence of a mismatch between the hot gas flow and the swirl of purge air exiting the wheelspace adjacent the LSB. Applicants have found that angel wing rim voids according to various embodiments of the invention are capable of both increasing PT spikes at a diffuser inlet close to the inner radius while at the same time decreasing swirl spikes at or near the same location. Each of these improves diffuser performance. Angel wing rim voids, for example, have been found to change the swirl angle of purge air exiting the LSB wheelspace by 1-3 degrees while also increasing PT spikes by 15-30%.
The principle of operation of the voids described above may also be applied to the operation of steam turbines. For example,
Steam turbines employing embodiments of the invention such as those described herein will typically realize improvements in efficiency of between 0.1% and 5%, depending, for example, on the leakage flow and the stage at which the features are employed.
As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any related or incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.
