Patent Application
20050152785
This invention relates to the manufacture of gas turbine blades or buckets and specifically, to an internal core arrangement utilized in the casting of turbine buckets, and to a bucket having cooling inlet passages formed by the core.
Single five-pass aft-flowing serpentine circuits have been proven to be an efficient and cost effective means of air cooling the shank and airfoil portions of a gas turbine bucket. This design represented a step forward in turbine cooling technology since air cooled stage 2 buckets have historically been cooled by stem-drilled radial holes. Since the source of the coolant air for the serpentine circuit is at the bottom or radially inner end of the dovetail mounting portion of the bucket, a passage is provided for feeding air through the shank portion of the bucket. In the prior arrangement, the cast inlet passage to the serpentine circuit is large and fills most of the shank in order to minimize the amount of solid metal in the shank. Weight minimization is important since extra weight increases the centrifugal loading on the rotor wheel. The problem with this prior design, however, is the lack of a continuous rib along the entire length of the bucket including the shank and airfoil portions, which is an important mechanical design criteria for bucket stiffness.
Another core arrangement, is disclosed in copending application Ser. No. 10/604,220, filed Jul. 1, 2003. This so-called “pant-leg” core is used in certain stem cooled buckets but like the core discussed above, it does not allow for a continuous center rib from the dovetail mounting portion to the bucket tip.
This invention relates to a new bucket shank internal core feature that has been developed for the shank portion of a turbine bucket with a single multi-pass serpentine cooling circuit in the airfoil portion of the bucket. Two separate core sections are provided in the shank area of the bucket. The core section of particular interest here is shaped to form a substantially inverted horseshoe-shape that is purged through an elliptical-shaped core tie passage to the primary inlet passage formed by the other adjacent core section. This core arrangement produces a bucket having advantages such as weight reduction as well as thermal and geometric symmetry in the shank, permitting the casting of a full length center rib from the shank portion to the tip of the airfoil portion.
More specifically, the shank portion of the bucket is formed utilizing a pair of internal core sections located on either side of a radial centerline through the shank and airfoil portions of the bucket. To one side of the centerline, a first inlet core section is arranged to produce the primary cooling supply passage to the serpentine cooling circuit. Specifically, the core section is shaped to provide two passages that merge at the inlet to the serpentine circuit. On the other side of the radial centerline, an inverted horseshoe-shaped core section is arranged to produce a cavity of generally similar shape to the primary inlet passage. A cast-in elliptical core tie feature is incorporated whereby the horseshoe-shaped cavity will be fluidly connected to the primary inlet passage by a relatively small passage. The elliptical core tie thus serves two purposes. One is to provide additional core stability during casting. The second purpose is to form a purge passage that will purge the cooling air in the horseshoe-shaped cavity. Specifically, in use, the cooling air enters the horseshoe-shaped cavity from the bottom of the dovetail portion of the bucket and is metered by the elliptical core tie passage into the primary inlet passage. Without the purge flow, the horseshoe cavity would be a dead-end cavity filled with hot stagnant air. This stagnant hot air would result in a thermal disparity in the shank, i.e., the forward half of the shank with the serpentine inlet would be cool and the aft half with the dead-end horseshoe cavity would be hot. Such a thermal mismatch would produce undesired thermally induced stresses in the shank.
Ball-braze chutes at the top of both core sections support the sections during casting. After casting, the chutes are plugged because, otherwise, the flow in the serpentine circuit would be disturbed if the coolant air were allowed to enter the serpentine circuit at these locations.
The core tie passage is elliptically shaped in cross-section in order to reduce its stress concentration factor, since it passes through the radially oriented center rib which is carrying a significant radial load. The shape of the elliptical core tie is engineered to balance the stress concentration factor in the effective flow area, and to set the amount of purge flow. The purge flow must be metered such that it has minimal impact on the flow within the inlet to the serpentine circuit.
Accordingly, in its broader aspects, the present invention relates to a turbine bucket comprising an airfoil portion, a shank portion and a dovetail mounting portion; an internal cooling circuit including inlet passages in the shank portion and the dovetail mounting portion connected to a cooling circuit in the airfoil portion, the inlet passages including a primary inlet passage on one side of a radial centerline of the bucket, and a secondary inlet cavity on an opposite side of the radial centerline; and a purge passage connecting the secondary inlet passage to the primary inlet passage.
In another aspect, the invention relates to a turbine bucket comprising an airfoil portion; a shank portion; a mounting portion; and an internal cooling circuit that includes a serpentine cooling passage in the airfoil portion and an inlet passage configuration in the shank and the mounting portion; the inlet passage configuration comprising a primary inlet passage adjacent a leading edge side of the bucket and a secondary inlet passage adjacent a trailing edge side of the bucket, and wherein a purge passage of elliptical cross-sectional shape connects the primary inlet passage and the secondary inlet passage to thereby purge cooling air from the secondary inlet passage.
The invention will now be described in detail in connection with the drawings identified below.
With reference to
As will be appreciated from
The cast-in elliptical core tie 66 provides additional core stability during casting. In addition, it creates an elliptical purge passage that allows a small amount of cooling air to purge the horseshoe-shaped cavity produced by the core section 58 as described further herein. The elliptical core tie 66 is elliptical in cross sectional shape to reduce its stress concentration factor since it passes through the center rib that carries a significant radial load. Preferably, the major axis is 0.2 inch (in the radial direction) and the minor axis is 0.070 inch (in the circumferential direction). These dimensions may change depending on factors such as air flow required through the passage created by the tie, stress concentration, and core stability during casting. The shape of the elliptical core tie is also engineered to balance the stress concentration factor and the effect of flow area (thus setting the amount of purge flow). The purge flow must be metered so it has minimal impact on the flow in the serpentine circuit.
So-called “ball braze chutes” 70 and 68 connect the respective core sections 48 and 58 to respective serpentine cooling circuit core portions 72, 74. These temporary core features serve to support the core sections 48 and 58 during casting. After casting, these “chutes” will be plugged by brazing steel balls 76, 78 within the passages formed by the chutes 68, 70 (
Turning now to
This arrangement also results in the formation of a pair of radial ribs 99, 100 located in the dovetail and shank portions of the bucket, adding desirable stiffness to this area of the bucket. With this arrangement, a continuous radially extending center rib 102 from dovetail to bucket tip is created between cooling circuit passages 104, 106. This center rib is important for overall bucket stiffness, like the center of an I-beam, and acts to carry a radial load to raise the bucket's natural frequencies.
It will further be recognized that the inlet section and horseshoe-shaped cavity creates a geometric symmetry with the serpentine inlet in the shank. This symmetry helps keep the center of mass of a bucket near the centerline of the bucket which reduces any moment imposed on the rim of the rotor wheel when spinning.
The coolant air enters the horseshoe cavity 90 from the bottom of the dovetail via passage 106 and is metered into the area of the primary inlet 80 by the elliptical passage 98. Without this purge flow, the horseshoe cavity 90 would be a dead-end cavity in which the stagnant air would become hot. This stagnant hot air would result in a thermal disparity in the shank, i.e., the forward half of the shank with the serpentine inlet would be cool and the aft half with the dead-end horseshoe cavity would be hot. This thermal mismatch would produce undesired thermally induced stresses in the shank.
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.
