Nozzle band overhang cooling

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קירור בלי©ה של סבעת פיה

NOZZLE BAND OVERHANG COOLING

13DV-9452

NOZZLE BAND OVERHANG COOLING

BACKGROUND OF TOE INVENTION

1. Field of the Invention

The present invention relates generally to gas turbine engines, and more particularly, to a nozzle assembly for a high pressure turbine of a gas turbine engine.

2. Description of Related Art

Gas turbine engines typically include an outer casing ar nacelle which forms an inlet sized to provide a predetermined airflow to a core engine. The core engine generally includes a ocnpressor for pressurizing inlet air which is discharged to a conbustor where fuel is burned to provide high energy canbustion gases which drive a core engine turbine ar high pressure turbine. The high pressure turbine, in turn, drives the compressor.

The high pressure turbine cairnonly includes one or more rows or stages of a plurality of turbine blades which are circunferentially spaced apart about a turbine rotor, lhe high pressure turbine also includes a nozzle assembly for directing high pressure gases from the combustor onto the turbine blades at the proper angle and velocity to rotate or drive the turbine blades and rotors.

Generally, the nozzle assembly includes a plurality of nozzle vanes which extend radially between and are circunferentially spaced apart about an annular inner and outer nozzle band, lhe inner nozzle band provides a platform for the nozzle vane and includes a mounting flange extending radially inwardly for mounting the inner nozzle band to a nozzle support to react against the pressure loads imposed on the nozzle vanes. To provide a continuous flowpath to the platform of the nozzle vane, the inner nozzle band extends axially aft of the mounting flange to form a solid continuous lip or band overhang.

Typically, the nozzle vanes are cooled by ccnpressor discharge air which flows through a series of leading edge holes and gill holes or apertures on each side of the nozzle vanes. Air flowing from these holes forms a thin film of cool air over the outer surface of the nozzle vane. Internally, the nozzle vane is divided into two cavities and air flaring into the aft cavity is discharged through trailing edge slots. Moreover, the nozzle bands are cooled by impinging air on the radially inward or back side in combination with film cooling on the radially outward or gas path side.

One disadvantage of the above nozzle vane cooling scheme is that it is difficult to cool the band overhang which typically causes an area of distress. As a result, axial cracks may develop in the band overhang due to the existence of a high tenperature gradient between the hot aft end of the band overhang and the cool mounting flange, particularly during transient operation.

One proposed solution to cool the band overhang by back side air impingement involved an impingement manifold system. An example of this is disclosed in U.S. Patent No. 4,187,054, issued February 5, 1990, to inventors Landis, Jr. et al., which is assigned to the same assignee as the present invention. This manifold system allows cooling air to be introduced to the back side of the band overhang and past the mounting flange. The manifold system may be integral to the gas path side. As a result, a high tenperature * I gradient may exist between the hot gas path side and the cool back side which critically limits the life of the band overhang.

Previously, other attempts have been made to cool the band overhang, but have not proved successful or have produced poor results. One attempt was to internally 0001 the band overhang by serpentine cooling. An example of this is disclosed in U.S. Patent No. 4,353,679, issued October 12, 1982, to inventor Hauser, which is assigned to the same assignee of the present invention. Serpentine passages were cored into the nozzle band and band overhang. Air entered and flowed through the serpentine passages. Air fran the serpenti^np passages was vented as a film upstream of a nozzle throat, the minimum passage between an adjacent pair of vanes where the gas path static pressure is the highest. However, the serpentine passages proved to be an ineffective means of conducting heat fran the gas path side to the back side, resulting in high temperature gradients between the gas path side and the back side. As a result, axial cracks developed in the band overhang and cooling air leaked out through the serpentine passages rather than flowing through the canplete passage. This leakage reduced both the film flew and hole backflow margin which caused the gas path side to increase in temperature, further increasing the temperature gradient.

Another attempt to cool the band overhang utilized pin-fin cooling. The back side of the band overhang was a sheet metal cover brazed onto a plurality of pins extending radially inwardly fran the nozzle band. However, tiie sheet metal cover could not be effectively brazed onto all of the pins. As a result, heat conduction fran the gas path side to the back side was localized, resulting in high temperature gradients. Additionally, the braze material tended to fill some of the pins, resulting in hot spots.

SlMMMOf OF TOE INVENTION

It is, therefore, a primary object of the present invention to provide cooling for the band overhang on a nozzle assembly for a high pressure turbine of a gas turbine engine.

It is another object of the present invention to reduce axial cracks in the band overhang of a nozzle band for a nozzle assembly.

It is yet another object of the present invention to reduce the temperature gradient between the band overhang and mounting flange of a nozzle band.

It is a further object of the present invention to provide a simple structure for cooling the band overhang.

Briefly stated, the above objects are accomplished in the preferred embodiment of the present invention wherein a unique nozzle assembly of a high pressure turbine is included in a gas turbine engine. TOe nozzle assembly includes a plurality of circunferentially spaced vanes and a nozzle band. The nozzle band supports the vanes and includes a mounting flange extending radially from the nozzle band and adapted to be secured to the gas turbine engine. The nozzle band has an overhang portion axially downstream of the mounting flange. A means forms a pin fin bank within the overhang portion for allowing cooling fluid to flow therethrough to cool the overhang portion.

One advantage of the present invention is that cooling of the band overhang of a nozzle assembly for a high pressure turbine is accomplished. Another advantage of the present invention is that the pin fin bank yields higher life capability of the band overhang with less cooling air flew than a solid overhang. Yet another advantage of the present Invention is that the use of a thermal barrier coating reduces temperature gradients between the gas path side and the back side of the pin fin bank, particularly during transient operation. A further advantage of the present invention is that the pin fin bank provides a sinpler structure that assures a direct conductivity path from the gas path side to the back side of the band overhang.

Other objects, features and advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following description when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a gas turbine engine incorporating nozzle band overhang cooling according to the present invention.

FIG. 2 is an enlarged partial perspective view of the circled portion of FIG. 1.

FIG. 3 is an elevational view of the nozzle vanes of FIG. 2 incorporating nozzle band overhang cooling according to the present invention.

FIG. 4 is an enlarged partial fragmentary view of the circled portion of FIG. 3.

FIG. 5 is a partial plan fragmentary view of the nozzle vanes of FIGS. 2 through 4 incorporating nozzle band overhang cooling according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, wherein like numerals correspond to like elements throughout, attention is first directed to FIG. 1. In FIG. 1, there is shewn a conventional gas turbine engine 10, such as a turbofan gas turbine engine. The gas turbine engine 10 Includes an outer casing or nacelle 12, the upstream end of which forms an inlet, generally indicated at 14, sized to provide a predetermined airflow to the internal components of the engine 10. Disposed within the inlet 14 is a fan, generally indicated at 16. The fan 16 pressurizes the airflow from the inlet 14. Downstream of the fan 16 is a core engine, generally shown at 18. The core engine 18 includes an axial flew compressor, generally indicated at 20. Pressurized air from the fan 16 enters the core engine 18 through compressor inlet 22 and is further compressed by the compressor 20 and discharged to a canbustor, generally indicated at 24. In the canbustor 24, fuel is burned to provide high energy canbustion gases which drive a core engine turbine or high pressure turbine, generally indicated at 26. The high pressure turbine 26, in turn, drives the compressor 20 through a shaft 28 in the usual manner of a gas turbine engine. Hot combustion gases can then pass to and drive a fan turbine or lew pressure turbine, generally indicated at 30, which, in turn, drives the fan 16 through a shaft 32 in the usual manner of a gas turbine engine. A more detailed description of the gas turbine engine 10 is disclosed in either U.S. Patent NO. 3,879,941 — Sargisson or U.S. Patent NO. 4,080,785 — Koff et al., both of which are assigned to the same assignee as the present invention.

״The high pressure turbine 26 typically includes one or more rows or stages of a plurality of turbine blades 34 and 36 which are circumferentially spaced apart about turbine rotors 38 and 39, respectively. The high pressure turbine 26 also includes a nozzle assembly, generally indicated at 40, such as a first stage nozzle assenfcly for directing high pressure gases from the canbustor 26 onto the turbine blades 34 at a predetermined angle and velocity to rotate or drive the turbine blades 34 and turbine rotor 38. It should be appreciated that the turbine rotor 38 is connected to the shaft 28 to drive a rotor of the compressor 20. It should also be appreciated that a similar nozzle assembly 40 may be used for other stages of the high pressure turbine 26.

Referring to FIG. 2, the nozzle assembly 40 includes a nozzle support 42 which is an annular component. The nozzle support 42 forms an inner flowpath wall and is typically secured to a rear frame of the catpressor by means such as fasteners (not shewn). The nozzle assembly 40 also includes an outer ring seal 44 and an inner ring seal 46. The nozzle assembly 40 further includes a plurality of circumferentially spaced and generally radial turbine nozzle vanes, generally indicated at 48. The nozzle vanes 48 include radially extending vanes 49 carried by a segmented annular outer nozzle band 50 and similarly segmented inner nozzle band 52. It should be appreciated that the outer and inner ring seals 44 and 46 prevent leakage of the hot combustion gases around the outer and inner nozzle bands 50 and 52, respectively. A more detailed description of the nozzle assembly is disclosed in U.S. Patent No. 4,353,679 — Hauser, which is assigned to the sane assignee of the present invention and the disclosed material is incorporated herein by reference.

Referring to FIGS. 2 and 3, the inner nozzle band 52 provides an annular wall or platform 54 extending laterally of the nozzle vane 49. The inner nozzle band 52 also includes a mounting flange 56 protruding radially inwardly for mounting the inner nozzle band 52 to the nozzle support 42. The mounting flange 56 is connected to the nozzle support 42 by means such as a segmented retainer strip 58. The retainer strip 58 extends radially and is partially disposed in corresponding grooves of the mounting flange 56 and nozzle support 42. The inner nozzle band 52 further includes an aft axially extending lip or band overhang 60 to provide a continuous flowpath to the platform 54. It should be appreciated that the bolt-less retention of the

(' - - — .״ mounting flange 56 reduces the axial length of the band overhang 60. It should also be appreciated that the mounting flange 56 could be connected to the nozzle support 42 by means of a conventional bolted connection, but would result in an increased axial length for the band overhang 60.

Referring to FIG. 4, the inner nozzle band 52 includes a plurality of cooling holes or apertures 62 extending through the platform 54 at an acute angle. Air, represented by arrows 63, is exhausted from a back_side 64 of the inner nozzle band 52 through the cooling apertures 62 to a gas path side 66 and forms a thin film of cool air over the platform 54. The inner nozzle band 52 also includes a plurality of cooling air feed holes or apertures 68 extending at an angle through the mounting flange 56 fran the back side 64 to the band overhang 60.

Referring to FIGS. 4 and 5, the band overhang 60 includes a pin fin array or bank, generally indicated at 70, to enhance cooling and conduction to the back side 64. The pin fin bank 70 is cast or cored into the band overhang 60 and integral therewith. Ihe pin fin bank 70 canprises a plurality of axially and laterally spaced pins 72, 74 and 76 aligned in laterally extending rows. Ihe pins 72, 74 and 76 are typically round and spaced two diameters apart. The pins 72, 74 and 76 have a predetermined diameter such as 0.060 inches. B1e pins 72, 74 and 76 are axially spaced to form passages 78, 80, 82 and 84 extending laterally between the rows. It should be appreciated that passages extend axially between the pins 72, 74 and 76. The inlet passage 78 has a radial -height approximately 0.060 inches and the outlet passage 84 has a radial height approximately 0.040 inches. A plurality of discharge holes or apertures 86 extend fran the outlet passage 84 and are directed tangentially, The discharge apertures 86 are Electro-Discharge Machined (EDM) or laser drilled to reduce turbine performance losses.

Ihe band overhang 60 also includes a thermal barrier coating, generally indicated at 88, an the gas path side 66 to reduce heat flux and transient temperature gradients. Ihe thermal barrier coating 88 has a first or inner layer 90 of a high oxidation resistant rough material disposed on a recessed portion 91 of the band overhang 60. The first layer 90 is approximately 0.005 inches thick and is applied to the recessed portion 91 by conventional vacuum plasma spraying. The thermal barrier coating 88 also has a second or outer layer 92 of a ceramic material. Preferably, the ceramic material has a composition of eight percent (8%)Y₂O3 and ninety-tvo percent (92%)ZzO₂. Ihe second layer 92 is approximately 0.015 inches thick and is applied on top of or adjacent the first layer 90 by conventional air plasma spraying such that the outer surface of the second layer 92 is flush or continuous with the outer surface of the platform 54.

The band overhang 60 further includes an outer or gas path wall 93 radially spaced from an inner or back side wall 94. The back side wall 94 has a predetermined thickness such as 0.050 inches to reduce or minimize transient temperature gradients between the gas path side 66 and the back side 64. Preferably, the inner nozzle band 52 comprises a monocrystal alloy material oriented such that the monocrystal secondary axis (primary is radial) is circumferentially along the inner nozzle band 52. It should be appreciated that the secondary axis of the monocrystal alloy may be orientated to a suitable predetermined direction by conventional and known process.

In operation, cooling discharge air fran the compressor 20 flows through a passage to the back side 64 of the inner nozzle band 52 foxward of the mounting flange 56. The cooling air, represented by arrows 63, then flews through cooling apertures 62 and is exhausted as a thin film over the platform 54. Cooling air, represented by arrows 96, also flows fran the back side 64 forward of the mounting flange 56 through feed apertures 68 and into the inlet passage 78 of the pin fin bank 70. Air then flews through the other passages 80 and 82 to the outlet passage 84. Air is discharged from the outlet passage 84 through the discharge apertures 86, represented by arrows 96, and directed tangentially away to reduce turbine performance losses.

Accordingly, coolant air introduced through the feed apertures 68 provides convective cooling, resulting in high internal heat transfer coefficients. Cooling of the backside 64 of the band overhang 60 greatly enhances the thermal insulating effect of the thermal barrier coating 88. Combined, these result in acceptable temperature levels for the band overhang 60 with minimal cooling requirements.

Additionally, the pin fin bank 70 yields higher life capability with less cooling flow than a solid continuous band overhang. The use of the thermal barrier coating 88 is particularly effective during transient operation to reduce temperature gradients between the gas path side 66 and the back side 64 of the band overhang 60. The pin fin band 70 provides uniform thru-flow of air and also assures a direct conductivity path between the gas path side 66 and the back side 64 of the overhang 60, providing significantly more conduction than serpentine passages.

Further, cooling air that is discharged out the aft end of the band overhang 60 is not a serious performance penalty since this is the prime area of flow metering and the pressure drop is used to produce a significant velocity in the direction of the gas flew. This also helps to reduce the gas ingestion into the forward rotor cavity. Backflcw margin is not affected if axial cracks develop in the band overhang 60 since most of the flew metering is done at the trailing edge discharge apertures 86.

Also, orientation of the nonocrystal secondary axis circumferentially along the band overhang 60 greatly reduces thermal stress and increases life of the band overhang 60. Due to the bolt-less retention of the mounting flange 56, the axial length of the band overhang 60 is short as compared with the present lengths, thereby reducing thermal gradients. Further, the thickness of the back side wall 94 of the band overhang 60 was adjusted to a predetermined thickness to reduce transient taiperature gradients between the gas path side 66 and the back side 64. Moreover, the radial height of the pin bank passages 78, 80, 82 and 84 was adjusted to predetermined heights to reduce steady-state temperature gradients in the axial direction. As a result, the forward or inlet end of the passages is large where the upstream film is high and the aft or outlet end of the passages is reduced to increase the back side cooling where the film effectiveness is decaying.

The present invention has been described in an illustrative manner. It is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation.

Many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described.