Bucket platform cooling scheme and related method

This invention was made with Government support under Contract No. DE-FC21-95MC31176 awarded by the Department of Energy. The Government has certain rights in this invention.

This invention relates to the cooling of gas turbine components and, more specifically, to the cooling of platform areas of gas turbine buckets.

Turbine buckets include an airfoil region and a hollow base or shank portion radially between the airfoil and an assembly end such as a dovetail by which the bucket is secured to a turbine rotor wheel. A relatively flat platform lies at the base of the airfoil and forms the top surface or wall of the hollow shank portion.

The airfoil has leading and trailing edges, and pressure and suction sides. The airfoil is exposed to the hot combustion gases, and internal cooling circuits within the airfoil itself are commonly employed, but are not part of this invention. Here, it is cooling of the bucket platform that is of concern.

Low Cycle Fatigue (LCF) is one of the failure mechanisms common to all gas turbine high-pressure buckets. Low cycle fatigue is a function of both stress and temperature. The stress may arise from the mechanical loading, or it may be thermally induced. Diminishing the thermal gradients in order to increase LCF life of the component, by incorporating optimal cooling schemes, is a challenge encountered by gas turbine component designers.

While the platform area on the external gas path side of the bucket is being exposed to hot gas temperatures, the bottom of the platform is subjected to relatively low temperatures due to the air leaking from the forward rotor wheel space through a radial pin. This temperature difference between the bottom and top of the platform leads to a large thermal gradient and high stress field and therefore requires an optimal cooling scheme to reduce the thermal stresses in the platform area.

This invention relates to a unique methodology in designing the required bucket platform cooling hardware, including an impingement plate located within the hollow bucket shank, beneath the bucket platform. The impingement plate is spaced a substantially uniform distance from the surface (i.e., the target surface), and includes an optimized array of impingement cooling holes divided by a rib to thereby establish impingement zones on the pressure side of the bucket platform.

The cooling methodology consists of air being fed by wheelspace flow which is pumped up toward and through the plate, with the post-impingement flow being discharged via optimally located rows of film holes drilled through the platform wall, also on the pressure side of the bucket.

The invention includes systematically defining the most efficient combination of hole diameters, hole spacing and the optimal separation distance of the impingement plate from the cooled platform under-surface. The rib bifurcating the impingement zones is designed to diminish the impact of two-dimensional cross-flow degradation on the local heat transfer coefficients. Subdividing the target surface into three different impingement zones also aids in the following:

(a) Controlling the static pressure variation in the post-impingement region.

(b) Controlling the momentum flux between the jet flow and cross-stream flow; and

(c) Optimizing the required magnitude of the heat transfer coefficients based on the varying thermal stress distribution of the target surface.

In addition to the cooling configuration and optimized jet array in the impingement plate, the platform wall itself is optimized for a varying wall thickness configuration. In order to balance the stress distribution on the pressure side of the platform and airfoil-platform fillet area, the platform thickness is varied along the axial direction. A lower uniform thickness on the leading edge side of the platform, and a higher uniform thickness on the trailing edge of the platform has been proved to be the best configuration, based on experimental studies. The platform thickness along the tangential direction may or may not be varied.

Accordingly, in one aspect, the invention relates to a turbine bucket comprising an airfoil extending from a platform, having high and low pressure sides; a wheel mounting portion; a hollow shank portion located radially between the platform and the wheel mounting portion, the platform having an under surface; and an impingement cooling plate located in the hollow shank portion, spaced from the under surface, the impingement plate having a plurality of impingement cooling holes therein.

In another aspect, the invention relates to a gas turbine bucket comprising an airfoil extending from a platform, having high and low pressure sides; a wheel mounting portion; a hollow shank portion located radially between the platform and the wheel mounting portion, the platform having an under surface; means for enabling impingement cooling of the under surface, and means for discharging cooling air from the hollow shank portion.

In still another aspect, the invention relates to a method of cooling a turbine bucket platform located radially between an airfoil and a mounting portion, the platform forming a radially outer wall of a hollow shank portion comprising fixing an impingement cooling plate within the hollow shank portion, spaced from an under surface of the platform, the impingement cooling plate having a plurality of impingement cooling holes therein; providing discharge holes in the platform; and directing turbine wheelspace air flow through the impingement cooling holes and the discharge holes in the platform.

FIG. 1 is a partial elevation, partly in section, of a gas turbine bucket, illustrating an impingement plate in the hollow shank portion of the bucket;

FIG. 2 is a plan view of the bucket illustrated in FIG. 1, and showing generally, in phantom, the impingement plate within the shank portion of the bucket;

FIG. 3 is a plan view of the impingement plate in accordance with the invention; and

FIG. 4 is a partial side section of the bucket shown in FIG. 2 .

With reference initially to FIGS. 1 and 2, a turbine bucket 10 includes an airfoil 12 extending vertically upwardly from a horizontal, substantially planar platform 14 . The airfoil portion has a leading edge 15 and a trailing edge 17 . Below the platform 14 , there are two pair of so-called “angel wings” 16 , 18 extending in opposite directions from the leading and trailing sides 20 , 22 of the root or shank portion 24 of the bucket. The platform 14 is joined with and forms part of the shank portion 24 that also includes side walls or skirts 26 . Below the hollow shank portion, there is a dovetail 28 (only partially shown) by which the bucket is secured to a turbine wheel (in a preferred embodiment, the stage 1 or stage 2 wheels of a gas turbine).

The airfoil 12 has a high pressure side 30 and a low pressure side 32 , and thus, platform 14 also has a high pressure side 34 and a low pressure side 36 . The hollow shank portion 24 lies directly and radially beneath the platform, and within that hollow shank portion, an impingement plate 38 is fixed (by brazing or other appropriate means) to the interior of the shank portion along integral ledges or shoulders 40 , 42 (see FIG. 4) on the undersurface 44 of the platform that conform to the outer periphery of the plate. As illustrated in FIG. 3, the impingement plate is relatively close to the undersurface 44 of the platform 14 , and generally conforms thereto such that the distance between the impingement plate 38 and the undersurface 44 of the platform 14 remains substantially constant.

The impingement plate 38 is best seen in FIG. 3, illustrating a plan view thereof. The plate 38 is bifurcated generally by an upstanding rib 46 , the thickness of which conforms to the spacing between the platform undersurface and the plate. Such spacing may be between about 0.10″ and 0.30″, and preferably about 0.20″.

The plate 38 is formed with a first array or zone of impingement holes or jets 48 closest to the airfoil; a second array or zone of impingement holes or jets 50 on the other side of rib 46 , remote from the airfoil; and a third array or zone of impingement holes or jets 52 in a corner of the plate 38 , proximate the trailing edge 17 of the airfoil. As can be seen from FIG. 3, these three arrays of holes surround a blank area 54 of the plate that lies directly beneath the array of film cooling holes 56 formed in the platform 14 (shown in phantom in FIG. 3) to facilitate an understanding of the spatial relationship between the impingement holes in the plate 38 and the film holes in the platform 14 . It will be appreciated that all of the impingement holes are not shown in FIG. 3, nor are the few holes illustrated drawn to scale. Nevertheless, arrays of lines 58 , 60 and 62 represent centerlines of rows of holes in each of the respective arrays. Flow arrows 64 indicate the direction of flow of cooling air after passing through the impingement plate 38 , along the undersurface of the platform, toward the discharge location at the film cooling holes 56 in the platform 14 .

The holes in each array are spaced from each other in a given row in a “span-wise” direction, while the rows themselves are spaced in a “flow-stream” direction. Depending upon the particular application, the spacing in both directions may vary. In one example, spacing of rows in the flow-stream direction may vary between 0.16 and 0.43 inch. Spacing of holes in the span-wise direction may vary between 0.14 and 0.27 inch.

All of the impingement cooling holes 48 , 50 , 52 in the impingement plate are drilled perpendicular to the upper and lower surfaces of the plate, and may have diameters of about 0.020 inch. The film cooling holes 56 are drilled through the platform at an angle, to promote attachment to the platform surface, thus providing an additional cooling function.

By judicious selection of impingement hole diameters; spacing in both span-wise and flow-stream directions; as well as the optimal separation distance between the impingement plate 38 and the under surface 44 of the platform 14 , several benefits are obtained. For example, the total pressure drop across the impingement plate can be minimized, and high heat transfer coefficient distribution on the target surface (i.e., under surface 44 ) can be achieved by also controlling the momentum flux (by decreasing the impact of cross-flow degradation of the jet array configuration).

Moreover, the incorporation of rib 46 that bifurcates the impingement zones as defined by the respective arrays of holes 48 , 50 and 52 , diminishes the impact of two-dimensional cross-flow degradation on the local heat transfer coefficients. This also helps in diminishing deflection of the plate 40 due to the pressure ratio across the plate as well as the centrifugal loading due to the influence of the rotation field.

In addition to the cooling configuration and optimized jet array and impingement plate configuration, the wall of the platform 14 itself is optimized via a varying wall thickness configuration. In order to balance the stress distribution on the pressure side of the platform and airfoil-platform fillet area, the platform thickness is varied along the axial direction as best seen in FIG. 1. A lower uniform thickness on the leading edge side of the platform (e.g., 0.160 inch), a higher uniform thickness on the trailing edge of the platform (e.g., 0.380 inch) and in-between variation around the center of the platform has been proved to be the best configuration based on the experimental studies. This specific platform geometric configuration in conjunction with the described cooling arrangement is believed to provide the best LCF life.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.