Patent Application
20140023497
The subject matter disclosed herein relates generally to gas turbine engines, and, more specifically, to a gas turbine engine rotor blade having improved tip cooling, and more specifically to a turbine blade having a cooled tip shroud.
A gas turbine engine includes one or more turbine blade rows, or stages, each row or stage having buckets or blades which project radially outwardly into the hot combustion gas path of the turbine, and disposed downstream of the combustor, which stages extract energy from the combustion gases generated by the combustor. Disposed radially outwardly of the rotor blade tips may be a stator shroud which is spaced from the blade tips to provide a relatively small clearance between the blade tips and shroud for reducing leakage of the combustion gases over the blade tips during operation. Each of the rotor blades includes conventionally known pressure and suction sides which are preferentially aerodynamically contoured for extracting as much energy as possible from the combustion gases flowing over the rotor blades. The pressure and suction sides extend to the blade tip and are disposed as close as possible to the stator shroud for maximizing the amount of energy extracted from the combustion gases. The clearance gap, however, between the blade tips and the stator shroud must nevertheless be adequate to minimize the occurrence of blade tip rubs during operation, which may damage the blade tips.
The efficiency of the turbine assembly is limited in part by “spillover:” the escape of hot combustion gases through the clearance gap between the turbine blade and the wall of the turbine assembly, which is commonly referred to as the turbine shroud. To reduce spillover, it is a common practice in the art to provide a tip shroud on the end of the airfoil opposite the end attached to the rotating shaft. The tip shroud includes a shelf and, optionally, one or more blade teeth that reduces spillover by decreasing the size of the clearance gap and interrupting the hot gas path around the end of the turbine blade.
Tip shrouds are subject to creep damage due to the combination of high temperature and centrifugally induced bending stresses. The creep is usually manifested by the formation of “dog ears” along unsupported edges of the shelf formed by the tip shroud. “Dog ears” as used herein, means the folding or degrading of the metal edges of the shelf formed by the tip shroud. Because it has been generally found that reinforcing the shelf simply transfers the stress from tip shroud to the root of the airfoil, the approach to reducing creep in this region of the turbine blade has been to “scallop” i.e., remove unsupported portions of the shelf. Scalloping, however, leads to increased hot gas path leakage past the turbine blade. If the tip shroud and shelf could be adequately cooled, the need to scallop the shelf could be substantially reduced. Consequently spillover would also be reduced and turbine efficiency could be improved.
One approach to cooling the tip shroud involves providing in the shroud tip an internal cooling cavity defining primary cooling flow passages through which cooling air flows and exits proximate the tip shroud edges (i.e. slashface). Inherent in such internal cooling cavities, however, are regions which are susceptible to flow recirculation or stagnation, which can cause localized hot spots in that region of the tip shroud.
These and other features of the present disclosure will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the drawings and the appended claims.
According to one embodiment of the disclosure, there is provided a turbine blade comprising a blade attachment portion; a radially extended turbine airfoil integral with the blade attachment portion, the turbine airfoil comprising one or more internal cooling passages; a tip shroud affixed to a top portion of the airfoil, the tip shroud comprising an internal cooling cavity in communication with the one or more internal cooling passages, the internal cooling cavity comprising at least one high velocity region and at least one low velocity region, the regions configured to pass cooling air therethrough; at least one exit port in communication with the internal cooling cavity configured to exhaust spent cooling air from the at least one high velocity region; and at least one purge hole in communication with the internal cooling cavity configured to exhaust spent cooling air from the at least one low velocity region.
According to another embodiment of the disclosure, there is provided a method of cooling a turbine blade shroud tip comprising providing a turbine blade comprising a radially extended turbine airfoil comprising one or more internal cooling passages; providing a tip shroud affixed to a top portion of the airfoil, the tip shroud comprising an internal cooling cavity in communication with the one or more internal cooling passages, the internal cooling cavity comprising at least one high velocity region and at least one low velocity region; passing cooling air through the one or more internal cooling passages and the internal cooling cavity; exhausting spent cooling air through at least one exit port in communication with the at least one high velocity region; and exhausting spent cooling air through at least one purge hole in communication with the at least one low velocity region.
According to another embodiment of the invention, there is provided a tip shroud configured for being disposed at the tip portion of an airfoil, comprising an internal cooling cavity, the internal cooling cavity comprising a plurality of high flow velocity regions and a plurality of low flow velocity regions, the regions configured to pass cooling air therethrough; at least one exit port in communication with each high flow velocity region configured to exhaust spent cooling air therefrom to a side edge of the tip shroud; and at least one purge hole in communication with each low flow velocity region configured to exhaust spent cooling air therefrom to a top or bottom surface of the tip shroud.
Referring now to
As illustrated in
The internal tip shroud cavity 22 may be of conventional design, for example, as illustrated and described in Brittingham, et al., U.S. Publication No. US2008/0170946A1. The internal tip shroud cavity 22 may extend across the tip shroud 12 substantially from front to back and side to side, within the plane of the tip shroud 12. As illustrated in
The internal tip shroud cavity 22 may be created in the tip shroud 12 by a ceramic core and formed during the investment casting process. This core may be held in place by one or more tabs extending out the edges 28 of the tip shroud 12. Spent cooling air may exit into the hot gas path from the interconnected primary cooling cavities through exit channels 30 that lead to exit holes 32 in the tip shroud edges or slash face 28.
As further illustrated in
It is believed that cooling air flows through the internal tip shroud cavity 22 at a higher velocity in high flow velocity regions 25, than through low flow velocity regions 34, in part because such high flow velocity regions 25 may exhaust spent cooling air along primary flow paths through the relatively large exit holes 32 in the shroud edges 28, as represented by the arrows A. Indeed, it is believed that the low flow velocity regions 34, because of the tortuous and/or constricted flow path and/or lack of proximal exit holes, experience cooling air flow recirculation and/or stagnation, which may lead to hot spots and premature tip shroud failure. The relatively low air flow through the low flow velocity regions 34 may further be the result of flow moving past what is sometimes referred to as a “bluff” body, for example, the supporting ribs previously described, which may separate the moving air and create a wake recirculation zone, similar to what occurs when air moves over a large blunt obstruction like a vehicle and then separates behind the rear face of the object.
According to the present disclosure, there may be provided one or more purge holes 36 positioned in the low flow velocity regions 34 of the tip shroud 12. The purge holes 36 may be circular, and may be drilled from the outer top surface 38 and/or interior or bottom surface 41 of the tip shroud 12. As illustrated in
The number and diameter of the purge holes 36 may depend on the design requirements and manufacturing process capability. For example, as illustrated in
The purge holes 36 may, as illustrated in
Incorporating purge holes 36 in the low flow velocity regions 34 of the tip shroud 12 may provide for exit of cooling air from the outer top surface 38 and/or bottom surface 41 of the tip shroud 12, also represented by solid line arrows B and dotted line arrows C, respectively, in
Furthermore, the incorporation of purge holes 36 may result in exiting film flow of spent cooling air, which may in turn, cause a reduction in external film temperature proximate the outer top surface 38 and/or bottom surface 41 of the tip shroud, which may also enhance part durability.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Where the definition of terms departs from the commonly used meaning of the term, applicant intends to utilize the definitions provided below, unless specifically indicated. 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 It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any, and all, combinations of one or more of the associated listed items. As used herein, the phrases “coupled to” and “coupled with” as used in the specification and the claims contemplates direct or indirect coupling.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person of ordinary skill in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The steps recited in the accompanying method claims need not be taken in the recited order, where other orders of conducting the steps to achieve the desired result would be readily apparent to those of ordinary skill in the art. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those of ordinary skill 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 languages of the claims.
