Internal cooling circuit for gas turbine bucket

This is a continuation of Ser. No. 09/236,714 filed Jan. 25, 1999 now abandoned.

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 an internal cooling circuit for a stage two bucket in a gas turbine.

High gas path temperatures are required to achieve high output and high efficiency in gas turbine machines. Several rows (or stages) of rotating blades or buckets, made from various high temperature alloys, are used in the gas turbine to extract energy from the hot gas path. To maintain temperatures of the first and second stage buckets within the material design limits, internal cooling is required. For the high gas path temperatures expected in advanced gas turbine engines, cooling air is not attractive due to the high cycle efficiency penalties associated with using compressor discharge air. Steam is attractive as a viable alternative cooling medium due to its high heat capacity and its availability in a combined cycle unit which includes both steam and gas turbines. This invention addresses the design of an internal cooling circuit for a closed circuit, steam cooled, stage two bucket in a gas turbine engine.

The internal cooling circuit for a stage two bucket in accordance with this invention incorporates a closed loop serpentine passage in the airfoil portion of the bucket, with multiple 180° turns, and connected to inlet and outlet passages in the radially inner dovetail portion of the bucket. The cooling passages are used to direct the cooling medium (steam in the preferred embodiment), around the bucket, removing heat from the bucket walls. The serpentine path includes alternating radially inward and outward passages, extending from the root of the bucket to the tip, turning and then extending from the tip back to the root. Multiple turns may be employed in the serpentine path, depending on turbine size, temperature requirements, etc.

Turbulators are used to enhance heat transfer from the bucket walls to the cooling medium, and to direct flow into the otherwise hard-to-reach apex of the trailing edge passage. In addition, a turn guide vane is used in the radially outer or tip trailing edge passage to direct flow toward the tip trailing edge corner region. The passage aspect ratios (length to width cross-section dimensions of the various passages) are designed to minimize Buoyancy Numbers in outward flowing passages, thereby maximizing the heat transfer enhancements due to rotation. A discussion of suitable aspect ratios and Buoyancy Numbers can be found in commonly owned U.S. Pat. No. 5,536,143.

In one aspect, the invention thus provides in a gas turbine bucket having a shank portion and an airfoil portion having leading and trailing edges and pressure and suction sides, an internal cooling circuit, the internal cooling circuit having a serpentine configuration including plural radial outflow passages and plural radial inflow passages, and wherein a coolant inlet passage communicates with a first of the radial outflow passages along the trailing edge, the first of the radial outflow passages having a plurality of radially extending and radially spaced elongated rib segments extending between and connecting the pressure and suction sides in a middle region of the first radial outflow passage to prevent ballooning of the pressure and suction sides at the first radial outflow passage.

In another aspect, the invention provides a gas turbine bucket having a shank portion and an airfoil portion having leading and trailing edges and pressure and suction sides, an internal cooling circuit, the internal cooling circuit having a serpentine configuration including plural radial outflow passages and plural radial inflow passages, and wherein a coolant inlet passage communicates with a first of the radial outflow passages along the trailing edge, the internal cooling circuit including turbulator ribs in each of the plurality of radial outflow and radial inflow passages extending at an acute angle to a direction of coolant flow in all but a radial inflow passage along the leading edge.

Advantages of the closed circuit serpentine design, with turbulators and tip turn guide vanes, include improved heat transfer from the buckets to the steam by using a high capacity cooling medium, i.e., the steam, as well as improved overall turbine cycle efficiency over conventional air cooled buckets since steam is extracted from the top cycle of the steam turbine, used to cool the bucket and is then returned to the bottom cycle of the steam turbine in a closed loop. This results in improved overall turbine cycle efficiency over conventional arrangements where compressor discharge air is used for cooling, and then discharged into the hot gas path.

FIG. 1 is a plan view of a second stage bucket in accordance with the invention;

FIG. 2 is a section view taken along the line 2 — 2 in FIG. 1 , but with an airfoil tip cap in place;

FIG. 3 is a section taken along the line 3 — 3 of FIG. 2 ;

FIG. 4 is an enlarged detail illustrating turbulators on opposite side walls of a cooling passage in the stage 2 bucket in accordance with this invention;

FIG. 5 is a partial section of a turbulator profile in accordance with the invention; and

FIG. 6 is a partial side elevation of a lower portion of the bucket shown in FIG. 1 , illustrating the coolant inlet and outlets.

Referring to FIGS. 1 and 2 , a second stage bucket 10 in accordance with this invention includes an airfoil portion 12 attached to a platform portion 14 which seals the shank 16 of the bucket from the hot gases in the combustion flow path. The shank 16 is attached to a rotor disk by a conventional dovetail 18 . Angel wing seals 20 , 22 provide sealing of the wheel space cavities. With reference also to FIG. 6 , the dovetail 18 includes an extension 24 below the dovetail which serves to supply and remove cooling steam from the bucket via axially arranged passages 26 and 28 which communicate with axially oriented rotor passages (not shown). The airfoil portion 12 has leading and trailing edges 13 , 15 , respectively, and pressure and suction sides 17 , 19 , respectively.

With specific reference now to FIG. 2 , the internal cooling passages in the second stage bucket define a closed serpentine circuit having a total of six radially extending passages 30 , 32 , 34 , 36 , 38 and 40 , with alternating radially inward and radially outward passages extending from the primary radial supply passage 27 at root of the bucket to the tip, turning 180° and then extending from the tip and ultimately back to the root to the primary return passage 29 . Note that FIG. 2 also shows a tip cap 41 which seals the radially outer end of the airfoil portion 12 .

In the illustrated embodiment, there are three radially outer tip turns and two radially inner root turns, forming the six passages, three ( 30 , 34 and 38 ) in the radially outward direction and three ( 32 , 36 and 40 ) in the radially inward direction. The various passages are separated by five radially extending ribs or interior partitions 42 , 44 , 46 , 48 and 50 which form the tip and root turns. These ribs extend the full width of the airfoil portion, i.e., from the suction side of the airfoil to the pressure side. As shown, steam flows initially upwardly or radially outwardly through the trailing edge passage 30 first, and radially downwardly or inwardly through the leading edge passage 40 last. The steam is input at the trailing edge cooling passage 30 , via passages 26 , 27 , first, since the trailing edge of the bucket is typically the most difficult region to cool.

Turbulators 52 are used in passages 32 - 38 to enhance heat transfer from the bucket walls to the cooling medium. These turbulators extend outwardly from opposite walls of the passage as best seen in FIGS. 3 , 4 and 5 . The turbulators extend only into the cooling passage, not so far as to restrict coolant flow, but far enough to enhance heat transfer from the bucket walls to the cooling medium. In the preferred arrangement, the turbulators 52 are arranged at 45° angles to the direction of flow. Turbulators may be staggered in the radial direction (i.e., so that no turbulators are directly opposite each other) or they may be in lateral alignment (i.e., directly opposite each other) if desired.

In passage 40 , turbulators 54 are arranged at a 90° angle to the direction of cooling flow, providing superior heat transfer in the leading edge passage compared to staggered, overlapping turbulators 52 in the remaining passages. Turbulators 54 also “wrap around” the interior leading edge wall as best seen in FIG. 3 . Passage 30 , extending along the trailing edge of the bucket or blade has a large aspect ratio and requires segmented ribs 56 extending between opposite walls of the bucket in order to prevent ballooning of the walls of the bucket while still allowing free distribution of the steam from the forward part of the passage into the apex trailing edge of the passage.

In addition, a turn guide vane 58 is located at the radially outermost portion of the trailing edge passage 30 . This curved guide vane is located to provide a split of the cooling medium around the tip turn between passages 30 and 32 so as to direct needed coolant flow into the trailing edge tip corner region. The crescent shaped guide vane 58 has been found to provide the best flow split with minimum flow losses. Note that the guide vane extends completely between the opposite side walls of the bucket, thereby completely splitting the cooling flow on either side of the vane. This guide vane 58 is a cast-in feature (as are the turbulators 52 , 54 ) included in the ceramic core used to define the internal cooling passages of the bucket in the conventional investment casting process. Turbulator placement, size, height to width ratio, pitch, orientation and corner radii are all selected to provide for the most efficient heat transfer from the bucket walls to the cooling medium.

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.