Cooling circuits for trailing edge cavities in airfoils

Details Cooling circuits for trailing edge cavities in airfoils Cooling circuits for trailing edge cavities in airfoils

1999-11-30

2001-02-06

Lopez, F. Daniel (Department: 3745)

Rotary kinetic fluid motors or pumps

With passage in blade, vane, shaft or rotary distributor...

C416S09700R, C416S09600A, C416S09600A, C416S092000

Reexamination Certificate

active

06183194

ABSTRACT:

TECHNICAL FIELD
This invention relates generally to turbine construction, and more specifically, to cooling arrangements for gas cooled airfoils with trapezoidal and/or triangular shaped cooling passages along the trailing edges thereof.
BACKGROUND
In gas turbine engines and the like, a turbine operated by burning gases drives a compressor which, in turn, furnishes air to one or more combustors. Such turbine engines operate at relatively high temperatures. The capacity of an engine of this kind is limited to a large extent by the ability of the material, from which the higher temperature components (such as turbine rotor blades, stator vanes or nozzles, etc.) are made, to withstand thermal stresses which can develop at such relatively high operating temperatures. The problem may be particularly severe in an industrial gas turbine engine because of the relatively large size of certain engine parts, such as the turbine blades and stator vanes. To enable higher operating temperatures and increased engine efficiency without risking blade failure, hollow, convectively-cooled turbine blades and stator vanes are frequently utilized. Such blades or vanes generally have interior passageways which provide flow passages to ensure efficient cooling whereby all the portions of the blades or vanes may be maintained at relatively uniform temperature. The traditional approach for cooling blades and vanes (referred to herein collectively as “airfoils”) is to extract high pressure cooling air from a source, for example, by extracting air from the intermediate and last stages of a turbine compressor. In modern turbine designs, it has been recognized that the temperature of the hot gas flowing past the turbine components could be higher than the melting temperature of the metal. It is, therefore, necessary to establish a cooling scheme to protect hot gas path components during operation. The invention focuses on gas cooled airfoils, and particularly those with trapezoidal or triangular cooling passages along trailing edges of such airfoils.
In general, compressed air is forced through small cavities close to the trailing edges of gas turbine airfoils for cooling. These trailing edge cavities assume trapezoidal (usually generally triangular) cross sectional areas with extremely low acute wedge angles, of less than 5°. Other cavities not necessarily at the trailing edge but located nearby in the airfoil can also assume similar geometrical attributes. In cooling passages having such geometrical attributes, poor cooling flow distribution results in excessive airfoil metal temperatures, resulting in premature loss of component life.
Examples of cooling circuits for gas turbine airfoils, including stator vanes, may be found in U.S. Pat. Nos. 5,125,798; 5,340,274; and 5,464,322.
DISCLOSURE OF THE INVENTION
It is the object of this invention to circumvent the above cooling problems by utilizing guide vanes placed radially in the trailing edge cavity of hollow airfoils to force flow in a more efficient way towards the apex or the convergent points of a triangular/trapezoidal cooling passage. As cooling flow proceeds toward these hard to cool areas, the cooling function is performed by convection.
Several cooling arrangements are described in this application. Each arrangement is designed for incorporation within an airfoil which has a triangular/trapezoidal trailing edge cooling passage with acute wedge angles of less than about 5°.
In accordance with a first exemplary embodiment, a series of small guide vanes are located in the radially outer portion of the trailing edge cooling passage or cavity of the airfoil and are arranged to force flow supplied from the top of the vane towards the apex of the triangular passage. A pair of larger guide vanes or flow splitters located substantially midway of the blade in the radial direction, cooperating to form discharge channels, force most of the cooling gas to return towards the leading wall of the vane cavity. A substantial portion of the cooling gas is then forced to flow back toward the trailing edge through another series of relatively small guide vanes located radially inwardly of the flow splitters. The cooling gas is then returned toward the leading wall of the cavity by another pair of flow splitters arranged similarly to the first pair of splitters. The cooling gas is then free to expand toward the trailing edge at the radial inner portion of the airfoil, before flowing out of the airfoil at the radially inner end thereof. All of the guide vanes and flow splitters in this first embodiment extend fully between the interior side walls of the airfoil.
It was found, however, that this design was not totally effective in forcing flow towards the trailing edge in that very large pressure drops were located in the discharge channels instead of being located along the guide vanes and towards the convergent portion of the airfoil cavity.
In a second disclosed embodiment, additional guide vanes are employed in the trailing edge cavity of the airfoil to force the flow against the convergent points of the trailing edge. Specifically, three sets of guide vanes are arranged in vertically spaced relationship within the trailing edge cavity to cause the cooling gas to follow a generally serpentine path from the radially outer end to the radially inner end of the airfoil. Each set of guide vanes includes vanes of increasing length in the flow direction, with some radial flow permitted around both the leading and trailing edges of each guide vane. Here again, all of the guide vanes extend fully between the side walls of the airfoil. However, in this case, most of the cooling gas escapes from the trailing edge after passing the first series of guide vanes and particularly after passing the final or longest guide vane of the first set. This is because the resistance offered by the converging airfoil walls was too difficult to overcome by the gas which found lower resistance flow paths away from the trailing edge. In addition, hot spots were found to exist behind at least the first set of guide vanes nearest the radially outer end of the airfoil.
In third and fourth preferred embodiments, the problems of the first two embodiments as described above are substantially circumvented. In the third embodiment, the guide vanes do not span the trailing edge cavity from wall to wall. Rather, ribs are provided on the opposed inner surfaces of the cavity, in generally matched pairs, inclined downwardly in the direction of flow towards the trailing edge. These ribs can be formed in horizontally aligned or horizontally offset pairs. In addition, the height of the guide vanes (in the horizontal direction, measured as the extent of the projection of the rib toward-the opposite side wall and transverse to the direction of flow) is selected to be greater than the boundary layer height of the flow passing radially downward, thus providing a means to trap the flow with lower momentum, and effectively forcing this trapped flow towards the apex of the trailing edge cavity.
The guide vanes in this third embodiment do not span the length of the entire cavity, thus allowing the trapped flow to spill over towards the apex of the passage. The cooling of the apex is therefore controlled by the height of the guide vanes and their relative orientation.
In the fourth embodiment, the trailing edge cavity is divided into two adjacent trapezoidal passages. Each passage has its own guide vane arrangement, substantially as described above in connection with the third embodiment. This arrangement is achieved by partitioning the trailing edge cavity by a single radially extending rib. Communication holes are located in the radial rib separating the two cavities to improve cross flow along the guide vanes in the trailing passage for improved flow distribution and cooling. With the guide vane arrangements described above for the third and fourth embodiments, hot spots behind the guide vanes are substantially eliminated.
It is also a feature of this invention to provide, optionally, a plurality of apertures at th

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