Nozzle airfoil having movable nozzle ribs

This application is continuation of application Ser. No. 09/372,190, filed Aug. 11, 1999, now abandoned, the entire content of which is hereby incorporated by reference in this application.

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 nozzle airfoil design, and more particularly, to an improved airfoil to airfoil rib joint for reducing thermal stresses at that junction.

Gas turbines typically include a compressor section, a combustor and a turbine section. The compressor section draws in ambient air and compresses it. Fuel is added to the compressed air in the combustor and the air fuel mixture is ignited. The resultant hot fluid enters the turbine section where energy is extracted by turbine blades, which are mounted to a rotatable shaft. The rotating shaft drives the compressor in the compressor section and drives, e.g., a generator for generating electricity or is used for other functions. The efficiency of energy transfer from the hot fluid to the turbine blades is improved by controlling the angle of the path of the gas onto the turbine blades using non-rotating, airfoil shaped vanes or nozzles. These airfoils direct the flow of hot gas or fluid from a nearly parallel flow to a generally circumferential flow onto the blades. Since the hot fluid is at very high temperature when it comes into contact with the airfoil, the airfoil is necessarily subject to high temperatures for long periods of time. Thus, in conventional gas turbines, the airfoils are generally internally cooled, for example by directing a coolant, which is compressed air in some systems and/or nozzle stages and steam in others, through internal cooling cavities in the airfoil.

Inside the airfoil, ribs are conventionally provided to extend between the convex and concave sides of the airfoil to provide mechanical support between the concave and convex sides of the airfoil. The ribs are needed to maintain the integrity the nozzle and reduce ballooning stress of the cavities. The ribs concurrently define at least part of the coolant flow path(s) through the airfoil. Thus, during engine operation, the internal ribs will be at a temperature level close to that of the coolant flowing through the airfoil, while the peripheral airfoil metal will generally be at a much higher temperature level. The mismatched temperatures result in high thermal stresses at the junctures of the ribs and the airfoil sidewalls. This high stress level combined with the high operating temperature results in fast deterioration of the vane at that area and thus deteriorated component life.

The invention is embodied in a vane or nozzle airfoil structure in which one or more of the nozzle ribs are connected to the airfoil side walls in such a way that the ribs provide the requisite mechanical support between the concave side and convex side of the airfoil but are not locked in the radial direction of the vane or nozzle assembly, longitudinally of the vane or airfoil. This configuration minimizes the stress caused by the mismatch of the material temperature between the airfoil outer, side walls and the support ribs.

The mechanical support ribs may be bi-cast onto a preformed nozzle airfoil side wall structure or fastened to the airfoil by an interlocking slide connection and/or welding. This substantially independent formation and mechanical interconnection enables some play in the radial direction of the nozzle assembly, longitudinally of the airfoil. By attaching the nozzle ribs to the nozzle airfoil metal in such a way that allows play longitudinally of the airfoil, the temperature difference induced radial thermal stresses at the nozzle airfoil/rib joint area are reduced while maintaining proper mechanical support of the nozzle side walls.

These, as well as other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view of an airfoil showing the internal rib or baffle connection between the airfoil convex side and the airfoil concave side;

FIG. 2 is a schematic elevational view of a nozzle airfoil;

FIG. 3 is a perspective view showing the typical disposition of ribs in a nozzle airfoil;

FIG. 4 is a schematic view of detail IV in FIG. 1 showing a conventional airfoil to airfoil rib joint; and

FIG. 5 is a schematic view of detail IV in FIG. 1 showing an airfoil to airfoil rib joint provided in accordance with an exemplary embodiment of the invention.

A conventional nozzle airfoil 10 having a series of ribs or baffles 12 extending between convex 14 and concave 16 sides thereof is shown by way of example in FIGS. 1-3. Typically, cooling air or steam enters one end of the airfoil and passes in a radial direction, longitudinally of the airfoil, along a serpentine path through the passages 18 defined between the respective mechanical support ribs or baffles 12 . Conventionally, the airfoil walls 14 , 16 and ribs or baffles 12 are cast as a single vane section or unit. Thus, as shown for example in FIG. 4, the airfoil walls and the airfoil ribs 12 are conventionally integrally formed in one piece from a common material.

As noted above, during engine operation, the ribs 12 are at a temperature level close to that of the coolant flowing through passages 18 , whereas the airfoil walls 14 , 16 are at a much higher temperature level. This mismatch in temperatures causes high thermal stress in the area of the joints 20 between the ribs 12 and the airfoil walls 14 , 16 . The high stress level combined with the high operating temperature reduces nozzle life at that area.

In accordance with an exemplary embodiment of the invention stress is reduced at the joints 120 of the nozzle ribs or baffle walls 112 and the nozzle side wall 114 . More specifically, rather than integrally cast with the nozzle airfoil wall, the nozzle ribs 112 are connected to the nozzle airfoil wall 114 in such a way that the ribs 112 provide the requisite mechanical support between the convex 114 and concave (not shown) sides of the nozzle airfoil, and define at least a part of the coolant flow path(s) as in the conventional structure, but they are not locked in a radial direction of the nozzle section 114 (longitudinally of the respective airfoil). Such a connection, which is locked in a direction B transverse to the longitudinal extent of the airfoil, but which allows play longitudinally of the airfoil reduces the stresses caused by the temperature mismatch between the airfoil wall 114 and the ribs 112 .

An exemplary coupling of the airfoil rib to the airfoil outer wall is schematically shown in FIG. 5 . In the illustrated embodiment, a dovetail groove or receptacle 122 is defined on the interior of the airfoil outer (side) wall 114 . A complimentary dovetail feature 124 is defined on the longitudinal side edge 126 of the rib 112 for locking engagement with the dovetail receptacle 122 of the airfoil wall 114 , so as to substantially preclude relative movement of the rib 112 and the airfoil wall 114 in direction B, transverse to the longitudinal extent of the rib. However, relative movement between the rib 112 and the wall 114 is allowed, longitudinally of the joint 120 . Thus, the nozzle ribs 112 are connected to the nozzle airfoil section in such a way that the ribs provide mechanical support between the concave and convex sides of the nozzle airfoil 110 but they are not locked in a radial direction.

To couple the ribs 112 to the airfoil side wall 114 , the ribs 112 may be bi-cast on the nozzle airfoil wall 114 . In this case, the airfoil is cast or machined with a sliding connection feature such as a dovetail receptacle, as shown, at the designated locations for rib attachment. A lower melting point alloy is then used to form the ribs, by casting onto the dovetail features of the airfoil. Because the ribs 112 and airfoil wall 114 are not integrally formed, the requisite play for reducing thermal stress at the joint 120 is possible.

As an alternative, the dovetail feature(s) 112 may be cast with or into the airfoil. Suitably cast or machined ribs may be then be slid into or onto the respective dovetail features to engage the complimentary connector structures of the ribs and airfoil walls respectively. The ribs may be welded to the airfoil outer wall e.g. at spaced locations along joint 120 , if necessary or desirable to retain the ribs in position in the nozzle wall while still allowing for sufficient play to reduce thermal stresses.

While in the illustrated embodiment, the airfoil outer wall is illustrated as formed with a dovetail receptacle for receiving a suitable bulbous or dovetail portion on the rib, it is to be understood that the coupling configuration may be altered or reversed such that the dovetail or bulbous portion is defined on the airfoil outer wall and the receptacle therefor defined along the longitudinal side edges of the respective rib. Moreover, while in the illustrated embodiment the complementary coupling structures have been illustrated and described as dove-tail type configurations, it is to be understood that any of a variety of complementary coupling structures which allow for relative longitudinal sliding movement but limit or preclude play in a direction transverse thereto could be provided to interconnect the nozzle rib to the nozzle outer wall without departing from the invention disclosed herein.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, 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.