# Patent

## Title
Mid-turbine frame buffer system

## Abstract

A mid-turbine frame buffer system for a gas turbine engine includes a mid-turbine frame that supports a shaft by a bearing. An air compartment and a bearing compartment are arranged radially inward of the mid-turbine frame. The bearing compartment is arranged within the air compartment and includes first and second contact seals arranged on either side of the bearing. The air compartment includes multiple air seals. A high pressure compressor is fluidly connected to the air compartment and is configured to provide high pressure air to the air compartment. A method of providing pressurized air to a buffer system includes sealing a bearing compartment with contact seals, surrounding the bearing compartment with an air compartment, and supplying high pressure air to the air compartment.

## Background
BACKGROUND
This disclosure relates to a mid-turbine frame buffer system for a gas turbine engine.
A mid-turbine frame is a structural case that is used to support the aft end of the high spool shaft of a turbofan engine. The mid-turbine frame is located between the high pressure turbine and low pressure turbine, a location where the core flowpath pressure and temperature are high. The static mid-turbine frame supports a rotating high speed shaft through a bearing enclosed in a buffered bearing compartment. Since the bearing must be cooled and lubricated with a constant oil flow, seals are used to contain the oil at static-to-rotating interfaces. The buffer air pressure outside of the bearing compartment must remain higher than the pressure inside the compartment so that air always leaks into the compartment, not out, so that the oil is contained within the bearing compartment.
SUMMARY
A mid-turbine frame buffer system for a gas turbine engine includes a mid-turbine frame that supports a shaft by a bearing. An air compartment and a bearing compartment are arranged radially inward of the mid-turbine frame. The bearing compartment is arranged within the air compartment and includes a seal assembly provided adjacent the bearing. The air compartment includes two or more air seals. An air source is fluidly connected to the air compartment and is configured to provide pressurized air to the air compartment.
In a further embodiment of the above, the bearing may be a roller bearing.
In a further embodiment of any of the above, the mid-turbine frame may be arranged axially between the high and low pressure turbines.
In a further embodiment of any of the above, the air source may be a high pressure compressor. Further, the high pressure compressor and the high pressure turbine may be supported on the shaft.
In a further embodiment of any of the above, a lubrication pump may be fluidly connected to the bearing compartment by a scavenge line.
In a further embodiment of any of the above, the seal assembly may include first and second contact seals. Further, each of the first and second contact seals may have first and second members in engagement with one another. At least one of the first and second members may be constructed from a carbon material.
In a further embodiment of any of the above, the air seals may include a labyrinth seal.
In a further embodiment of any of the above, the air seals may include a brush seal.
In a further embodiment of any of the above, the mid-turbine frame may include a member arranged in a core flow path. The member may provide a passage fluidly connecting mid-turbine frame outer and inner areas to one another for delivering the pressurized air to the air compartment.
A method of providing pressurized air to a buffer system includes sealing a bearing compartment with two or more seals, surrounding the bearing compartment with an air compartment, and supplying pressurized air to the air compartment.
In a further embodiment of the above, the two or more seals may be contact seals that may include a carbon material.
In a further embodiment of any of the above, the two or more seals may include contact seals and two or more air seals. Further, at least one of the two or more air seals may be at least one of a labyrinth seal and a brush seal.
In a further embodiment of any of the above, the pressurized air may include high pressure compressor air.
In a further embodiment of any of the above, the bearing compartment may be provided between low and high pressure turbines.
Another embodiment address a gas turbine engine that includes, among other possible things, a fan; a compressor section fluidly connected to the fan; a combustor fluidly connected to the compressor section; a turbine section fluidly connected to the combustor, the turbine section including a high pressure turbine coupled to the high pressure compressor via a shaft; a low pressure turbine; and a mid-turbine frame positioned between the high pressure turbine and the low pressure turbine, the mid-turbine frame supporting the shaft by a bearing; and a mid-turbine frame buffer system that includes an air compartment; and a bearing compartment arranged radially inward of the mid-turbine frame, the bearing compartment arranged within the air compartment, the bearing compartment including a seal assembly provided adjacent the bearing, the air compartment including two or more air seals. The compressor is fluidly connected to the air compartment and is configured to provide pressurized air to the air compartment.
In a further embodiment of the foregoing as turbine engine, the bearing may be a roller bearing.
In another further embodiment of any of the foregoing gas turbine engine embodiments, the engine may further include a lubrication pump fluidly connected to the bearing compartment by a scavenge line.
In another further embodiment of any of the foregoing gas turbine engine embodiments, the seal assembly may include first and second contact seals. Further, each of the first and second contact seals may have first and second members in engagement with one another. In addition, at least one of the first and second members may be constructed from a carbon material.
In another further embodiment of any of the foregoing gas turbine engine embodiments, the air seals may include at least one of a labyrinth seal and a brush seal.
In another further embodiment of any of the foregoing gas turbine engine embodiments, the gas turbine engine may be a high bypass geared aircraft engine having a bypass ratio of greater than about six (6).
In another further embodiment of any of the foregoing gas turbine engine embodiments, the gas turbine engine may include a low Fan Pressure Ratio of less than about 1.45.
In another further embodiment of any of the foregoing gas turbine engine embodiments, the low pressure turbine may have a pressure ratio that is greater than about 5.



BRIEF DESCRIPTION OF THE DRAWINGS
The disclosure can be further understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
FIG. 1 schematically illustrates a gas turbine engine.
FIG. 2 schematically depicts a mid-turbine frame buffer system for the gas turbine engine illustrated in FIG. 1.



DETAILED DESCRIPTION
FIG. 1 schematically illustrates a gas turbine engine 20. The gas turbine engine 20 is disclosed herein as a two-spool turbofan that generally incorporates a fan section 22, a compressor section 24, a combustor section 26 and a turbine section 28. Alternative engines might include an augmentor section (not shown) among other systems or features. The fan section 22 drives air along a bypass flowpath B while the compressor section 24 drives air along a core flowpath C for compression and communication into the combustor section 26 then expansion through the turbine section 28. Although depicted as a turbofan gas turbine engine in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use with turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures.
The engine 20 generally includes a low speed spool 30 and a high speed spool 32 mounted for rotation about an engine central longitudinal axis A relative to an engine static structure 36 via several bearing systems 38. It should be understood that various bearing systems 38 at various locations may alternatively or additionally be provided.
The low speed spool 30 generally includes an inner shaft 40 that interconnects a fan 42, a low pressure compressor 44 and a low pressure turbine 46. The inner shaft 40 is connected to the fan 42 through a geared architecture 48 to drive the fan 42 at a lower speed than the low speed spool 30. The high speed spool 32 includes an outer shaft 50 that interconnects a high pressure compressor 52 and high pressure turbine 54. A combustor 56 is arranged between the high pressure compressor 52 and the high pressure turbine 54. A mid-turbine frame 57 of the engine static structure 36 is arranged generally between the high pressure turbine 54 and the low pressure turbine 46. The mid-turbine frame 57 supports one or more bearing systems 38 in the turbine section 28. The inner shaft 40 and the outer shaft 50 are concentric and rotate via bearing systems 38 about the engine central longitudinal axis A which is collinear with their longitudinal axes.
The core airflow is compressed by the low pressure compressor 44 then the high pressure compressor 52, mixed and burned with fuel in the combustor 56, then expanded over the high pressure turbine 54 and low pressure turbine 46. The mid-turbine frame 57 includes airfoils 59 which are in the core airflow path. The turbines 46, 54 rotationally drive the respective low speed spool 30 and high speed spool 32 in response to the expansion.
The engine 20 in one example a high-bypass geared aircraft engine. In a further example, the engine 20 bypass ratio is greater than about six (6), with an example embodiment being greater than ten (10), the geared architecture 48 is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3 and the low pressure turbine 46 has a pressure ratio that is greater than about 5. In one disclosed embodiment, the engine 20 bypass ratio is greater than about ten (10:1), the fan diameter is significantly larger than that of the low pressure compressor 44, and the low pressure turbine 46 has a pressure ratio that is greater than about 5:1. Low pressure turbine 46 pressure ratio is pressure measured prior to inlet of low pressure turbine 46 as related to the pressure at the outlet of the low pressure turbine 46 prior to an exhaust nozzle. The geared architecture 48 may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.5:1. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present invention is applicable to other gas turbine engines including direct drive turbofans.
A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section 22 of the engine 20 is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet. The flight condition of 0.8 Mach and 35,000 ft, with the engine at its best fuel consumption—also known as bucket cruise Thrust Specific Fuel Consumption (“TSFC”). TSFC is the industry standard parameter of lbm of fuel being burned divided by lbf of thrust the engine produces at that minimum point. “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.45. “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tambient deg R)/518.7)^0.5]. The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second.
A buffer system 58 is schematically illustrated in FIG. 2. A bearing 60 is structurally supported by the mid-turbine frame 57 and supports the outer shaft 50 for rotation. The bearing may be, for example, a roller bearing a ball bearing or other type of bearing. The bearing 60 is arranged within a bearing compartment 62, which provides an enclosure for retaining lubrication for the bearing. This enclosure is provided, for example, by walls 64 that support first and second contact seals 66, 68 arranged on either side of the bearing 60. Each of the first and second contact seals 66, 68 includes first and second members 70, 72 that are in engagement with one another such that no gaps are provided between the first and second members 70, 72. At least one of the first and second members 70, 72 is constructed from a carbon material. The use of high pressure ratio face carbon seals reduces airflow into the oil compartment (as compared to conventional buffer systems), thereby eliminating the need for a conventional breather air tube.
A lubrication pump 88 is fluidly connected to the bearing compartment 62 by a scavenge line 90. Any air entering the bearing compartment 62 is exhausted through a vent 92. The lubrication pump 88 has sufficient capacity to evacuate any small amount of leakage past the first and second contact seals 66, 68 into the bearing compartment 62.
The bearing compartment 62 is enclosed within an air compartment 74. A pressurized air source 75 is fluidly coupled to the air compartment 74. “Pressurized” air is air that is provided by, e.g., the high pressure compressor 52. The mid-turbine frame 57 includes outer and inner areas 98, 100 arranged on opposing sides of the mid-turbine frame 57. A passage 102 fluidly interconnects the inner and outer areas 98, 100 to one another. In one example, the passage 102 is provided by the mid-turbine frame airfoil 59.
The air compartment 74 is provided, for example, by walls 82 that support first, second and third air seals 76, 78, 80. The first, second and third air seals 76, 78, 80 respectively cooperate with first, second and third surfaces 77, 79, 81. The first surface 77 is provided by a high pressure rotor 86. The second surface 79 is provided by the outer shaft 50. The third surface 81 is provided by a low pressure rotor 84. The air seals are provided, for example, by one or more labyrinth/knife-edge seals and/or brush seals. In the example illustrated in FIG. 2, the first seal 76 is a labyrinth seal, and the second and third air seals 78, 80 are brush seals. Air seals typically are not air tight in that they may provide small gaps. Air seals are typically designed to inhibit leakage by creating a tortuous path through which the air must pass. Any air leakage past the first, second and third air seals 76, 78, 80 vents to the low and high pressure turbines 46, 54, as depicted by the arrows in FIG. 2, which provide multiple air sinks.
In light of the foregoing design, the high pressure buffer system surrounding the bearing compartment 62 can vent directly into the turbo machinery primary core flowpath C at the mid turbine frame 57. Since the buffer air can vent directly into the core flowpath C, the number and diameter of the high pressure ratio air seals is reduced as compared to conventional turbine engines and, therefore, far less buffer airflow bypassing the turbo machinery is required. The energy in the buffer air entering the core flowpath C is captured in the downstream turbo machinery, thereby improving engine performance. In contrast, in conventional engines, the low buffer pressure systems therein must vent a larger mass flow of buffer air to a lower pressure sink in the core flowpath C downstream from turbo machinery, thereby losing the associated energy of this air and, correspondingly, negatively impacting engine efficiency.
A turbine engine case 94 supports another bearing 96, which rotationally supports the inner shaft 40 at a location aft of the bearing 60.
Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.

## Claims

1. A mid-turbine frame buffer system for a gas turbine engine comprising:
a mid-turbine frame supporting a shaft by a bearing;
an air compartment and a bearing compartment arranged radially inward of the mid-turbine frame, the bearing compartment arranged within the air compartment, the bearing compartment including a seal assembly provided adjacent the bearing and separating the bearing compartment front the air compartment, the air compartment including two or more air seals;
an air source fluidly connected to the air compartment and configured to provide pressurized air to the seal assembly in the air compartment to buffer the bearing compartment; and
wherein the mid-turbine frame includes a member arranged in a core flow path, the member providing a passage fluidly connecting mid-turbine frame outer and inner areas to one another for delivering the pressurized air to the air compartment.

2. The buffer system according to claim 1, wherein the bearing is a roller bearing.

3. The buffer system according to claim 1, comprising high and low pressure turbines, the mid-turbine frame arranged axially between the high and low pressure turbines.

4. The buffer system according to claim 3, wherein the air source is a high pressure compressor, and wherein the high pressure compressor and the high pressure turbine are supported on the shaft.

5. The buffer system according to claim 1, comprising a lubrication pump fluidly connected to the bearing compartment by a scavenge line.

6. The buffer system according to claim 1, wherein the seal assembly comprises first and second contact seals, wherein each of the first and second contact seals has first and second members in engagement with one another, and wherein at least one of the first and second members is constructed from a carbon material.

7. The buffer system according to claim 1, wherein the air seals includes a labyrinth seal.

8. The buffer system according to claim 1, wherein the air seals includes a brush seal.

9. A gas turbine engine comprising:
a fan;
a compressor section fluidly connected to the fan;
a combustor fluidly connected to the compressor section;
a turbine section fluidly connected to the combustor, the turbine section comprising:
a high pressure turbine coupled to the high pressure compressor via a shaft;
a low pressure turbine; and
a mid-turbine frame positioned between the high pressure turbine and the low pressure turbine, the mid-turbine frame supporting the shaft by a bearing; and

a mid-turbine frame buffer system comprising:
an air compartment; and
a bearing compartment arranged radially inward of the mid-turbine frame, the bearing compartment arranged within the air compartment, the bearing compartment including a seal assembly provided adjacent the bearing and separating the bearing compartment from the air compartment, the air compartment including two or more air seals;

wherein the compressor is fluidly connected to the air compartment and is configured to provide pressurized air to the seal assembly in the air compartment to buffer the bearing compartment; and
wherein the mid-turbine frame includes a member arranged in a core flow path, the member providing a passage fluidly connecting mid-turbine frame outer and inner areas to one another for delivering the pressurized air to the air compartment.

10. The gas turbine engine according to claim 9, wherein the bearing is a roller bearing.

11. The gas turbine engine according to claim 9, comprising a lubrication pump fluidly connected to the bearing compartment by a scavenge line.

12. The gas turbine engine according to claim 9, wherein the seal assembly comprises first and second contact seals, wherein each of the first and second contact seals has first and second members in engagement with one another, and wherein at least one of the first and second members is constructed from a carbon material.

13. The gas turbine engine according to claim 9, wherein the air seals includes at least one of a labyrinth seal and a brush seal.

14. The gas turbine engine according to claim 9, wherein the gas turbine engine is a high bypass geared aircraft engine having a bypass ratio of greater than about six (6).

15. The gas turbine engine according to claim 9, wherein the gas turbine engine includes a low Fan Pressure Ratio of less than about 1.45.

16. The gas turbine engine according to claim 9, wherein the low pressure turbine has a pressure ratio that is greater than about 5.