Low noise turbine for geared gas turbine engine

Abstract

A gas turbine engine according to an exemplary aspect of the present disclosure includes, among other things, a propulsor section, a geared architecture, a high spool and a low spool. The high spool includes a high pressure compressor and a high pressure turbine. The low spool includes a low pressure compressor and a low pressure turbine. At least one stage of the turbine section includes an array of rotatable blades and an array of vanes. A ratio of the number of vanes to the number blades is greater than or equal to 1.55. A mechanical tip rotational Mach number of the blades is greater than or equal to 0.5 at an approach speed.

Claims

1. A gas turbine engine comprising: a propulsor section including a propulsor having a propulsor rotor and a plurality of propulsor blades; a geared architecture; a high spool including a high pressure compressor, a high pressure turbine and an outer shaft that connects the high pressure compressor and the high pressure turbine; a low spool including a low pressure compressor, a low pressure turbine and an inner shaft that connects the propulsor and the low pressure compressor to the low pressure turbine, wherein the inner shaft drives the propulsor through the geared architecture, the low pressure turbine includes at least three turbine rotors, the high pressure turbine includes two stages, and the inner and outer shafts are concentric and rotate via bearing systems about an engine central longitudinal axis; and wherein at least one stage of the low pressure turbine includes an array of rotatable blades and an array of vanes, the array of vanes of the at least one stage are immediately upstream or downstream from the array of blades, a ratio of the number of vanes to the number of blades of the at least one stage is greater than or equal to 1.55, and a mechanical tip rotational Mach number of the array of blades is greater than or equal to 0.5 at an approach speed, the approach speed taken at an approach certification point as defined in Part 36 of the Federal Airworthiness Regulations.

2. The gas turbine engine as recited in claim 1, wherein the geared architecture is an epicyclic gear train.

3. The gas turbine engine as recited in claim 2, wherein the at least one stage comprises more than one stage of the low pressure turbine.

4. The gas turbine engine as recited in claim 2, wherein the gear train includes a gear reduction ratio of greater than 2.3.

5. The gas turbine engine as recited in claim 4, wherein the low pressure turbine drives both the low pressure compressor and an input of the gear train.

6. The gas turbine engine as recited in claim 5, wherein the gas turbine engine is rated to produce 15,000 pounds of thrust or more.

7. The gas turbine engine as recited in claim 6, further comprising: a pressure ratio of less than 1.50 across the propulsor blade alone at cruise at 0.8 Mach and 35,000 feet.

8. The gas turbine engine as recited in claim 7, wherein: the low pressure turbine includes no more than six turbine rotors; and the propulsor has less than twenty propulsor blades.

9. The gas turbine engine as recited in claim 8, wherein the at least one stage comprises more than one stage of the low pressure turbine.

10. The gas turbine engine as recited in claim 9, further comprising: a mid-turbine frame between the high pressure turbine and the low pressure turbine, the mid-turbine frame supporting bearing systems in a turbine section comprising the high pressure turbine and the low pressure turbine, and the mid-turbine frame including vanes in a core flow path.

11. The gas turbine engine as recited in claim 10, wherein: the low pressure compressor includes three stages; and the pressure ratio is less than 1.45 across the propulsor blade alone at cruise at 0.8 Mach and 35,000 feet.

12. The gas turbine engine as recited in claim 11, wherein the at least one stage comprises less than all of the stages of the low pressure turbine.

13. The gas turbine engine as recited in claim 11, wherein the at least one stage comprises all of the stages of the low pressure turbine.

14. The gas turbine engine as recited in claim 7, wherein: the propulsor has a low corrected tip speed of less than 1150 ft/second; and the propulsor has less than twenty-six propulsor blades.

15. The gas turbine engine as recited in claim 14, wherein the epicyclical gear train is a star gear system.

16. The gas turbine engine as recited in claim 15, wherein: the low pressure turbine includes no more than six turbine rotors; and the pressure ratio is less than 1.45 across the propulsor blade alone at cruise at 0.8 Mach and 35,000 feet.

17. The gas turbine engine as recited in claim 16, wherein the at least one stage comprises more than one stage of the low pressure turbine.

18. The gas turbine engine as recited in claim 17, wherein: the propulsor has less than twenty propulsor blades; the low pressure compressor includes three stages; and the low pressure turbine includes an inlet, an outlet and a pressure ratio of greater than 5, the pressure ratio being pressure measured prior to the inlet as related to pressure at the outlet prior to an exhaust nozzle.

19. The gas turbine engine as recited in claim 14, wherein the epicyclical gear train is a planetary gear system.

20. The gas turbine engine as recited in claim 19, wherein: the low pressure turbine includes no more than six turbine rotors; and the pressure ratio is less than 1.45 across the propulsor blade alone at cruise at 0.8 Mach and 35,000 feet.

21. The gas turbine engine as recited in claim 20, wherein the at least one stage comprises more than one stage of the low pressure turbine.

22. The gas turbine engine as recited in claim 21, wherein: the propulsor has less than twenty propulsor blades; the low pressure compressor includes three stages; and the low pressure turbine includes an inlet, an outlet and a pressure ratio of greater than 5, the pressure ratio being pressure measured prior to the inlet as related to pressure at the outlet prior to an exhaust nozzle.

23. The gas turbine engine as recited in claim 6, wherein the geared architecture is positioned intermediate the low pressure turbine and low pressure compressor such that the propulsor rotor and the low pressure compressor are rotatable at a common speed.

24. The gas turbine engine as recited in claim 23, wherein: the low pressure turbine includes no more than six turbine rotors; and the low pressure compressor includes three stages.

25. The gas turbine engine as recited in claim 24, wherein the at least one stage comprises more than one stage of the low pressure turbine.

26. The gas turbine engine as recited in claim 25, wherein the epicyclical gear train is a star gear system.

27. The gas turbine engine as recited in claim 25, wherein the epicyclical gear train is a planetary gear system.

28. The gas turbine engine as recited in claim 27, further comprising: a pressure ratio of less than 1.45 across the propulsor blade alone at cruise at 0.8 Mach and 35,000 feet; a mid-turbine frame between the high pressure turbine and the low pressure turbine, the mid-turbine frame supporting bearing systems in a turbine section comprising the high pressure turbine and the low pressure turbine, and the mid-turbine frame including vanes in a core flow path and that function as inlet guide vanes for the low pressure turbine; and wherein the propulsor has less than twenty propulsor blades.

29. The gas turbine engine as recited in claim 28, wherein the at least one stage comprises less than all of the stages of the low pressure turbine.

30. The gas turbine engine as recited in claim 28, wherein the at least one stage comprises all of the stages of the low pressure turbine.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an example gas turbine engine.

(2) FIG. 2 shows another embodiment.

(3) FIG. 3 shows yet another embodiment.

DETAILED DESCRIPTION

(4) FIG. 1 schematically illustrates an example gas turbine engine 20 that includes a fan section 22, a compressor section 24, a combustor section 26, and a turbine section 28. Alternative engines might include an augmenter section (not shown) among other systems or features. The fan section 22 drives air along a bypass flow path B while the compressor section 24 draws air in along a core flow path C where air is compressed and communicated to a combustor section 26. In the combustor section 26, air is mixed with fuel and ignited to generate a high pressure exhaust gas stream that expands through the turbine section 28 where energy is extracted and utilized to drive the fan section 22 and the compressor section 24.

(5) Although the disclosed non-limiting embodiment depicts a turbofan gas turbine engine, 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; for example a turbine engine including a three-spool architecture in which three spools concentrically rotate about a common axis and where a low spool enables a low pressure turbine to drive a fan via a gearbox, an intermediate spool that enables an intermediate pressure turbine to drive a first compressor of the compressor section, and a high spool that enables a high pressure turbine to drive a high pressure compressor of the compressor section.

(6) The example 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.

(7) The low speed spool 30 generally includes an inner shaft 40 that connects a fan 42 and a low pressure (or first) compressor section 44 to a low pressure (or first) turbine section 46. The inner shaft 40 drives the fan 42 through a speed change device, such as 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 (or second) compressor section 52 and a high pressure (or second) turbine section 54. The inner shaft 40 and the outer shaft 50 are concentric and rotate via the bearing systems 38 about the engine central longitudinal axis A.

(8) A combustor 56 is arranged between the high pressure compressor 52 and the high pressure turbine 54. In one example, the high pressure turbine 54 includes at least two stages to provide a double stage high pressure turbine 54. In another example, the high pressure turbine 54 includes only a single stage. As used herein, a “high pressure” compressor or turbine experiences a higher pressure than a corresponding “low pressure” compressor or turbine.

(9) The example low pressure turbine 46 has a pressure ratio that is greater than about five (5). The pressure ratio of the example low pressure turbine 46 is measured prior to an inlet of the low pressure turbine 46 as related to the pressure measured at the outlet of the low pressure turbine 46 prior to an exhaust nozzle.

(10) A mid-turbine frame 58 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 58 further supports bearing systems 38 in the turbine section 28 as well as setting airflow entering the low pressure turbine 46.

(11) The core airflow C is compressed by the low pressure compressor 44 then by the high pressure compressor 52 mixed with fuel and ignited in the combustor 56 to produce high speed exhaust gases that are then expanded through the high pressure turbine 54 and low pressure turbine 46. The mid-turbine frame 58 includes vanes 60, which are in the core airflow path and function as an inlet guide vane for the low pressure turbine 46. Utilizing the vane 60 of the mid-turbine frame 58 as the inlet guide vane for low pressure turbine 46 decreases the length of the low pressure turbine 46 without increasing the axial length of the mid-turbine frame 58. Reducing or eliminating the number of vanes in the low pressure turbine 46 shortens the axial length of the turbine section 28. Thus, the compactness of the gas turbine engine 20 is increased and a higher power density may be achieved.

(12) The disclosed gas turbine engine 20 in one example is a high-bypass geared aircraft engine. In a further example, the gas turbine engine 20 includes a bypass ratio greater than about six (6), with an example embodiment being greater than about ten (10). The example geared architecture 48 is an epicyclical gear train, such as a planetary gear system, star gear system or other known gear system, with a gear reduction ratio of greater than about 2.3.

(13) In one disclosed embodiment, the gas turbine engine 20 includes a bypass ratio greater than about ten (10:1) and the fan diameter is significantly larger than an outer diameter of the low pressure compressor 44. It should be understood, however, that the above parameters are only exemplary of one embodiment of a gas turbine engine including a geared architecture and that the present disclosure is applicable to other gas turbine engines.

(14) 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’)”—is the industry standard parameter of pound-mass (lbm) of fuel per hour being burned divided by pound-force (lbf) of thrust the engine produces at that minimum point.

(15) “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 1.50. In another non-limiting embodiment the low fan pressure ratio is less than about 1.45.

(16) “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram ° R)/(518.7° R)].sup.0.5. The “Low corrected fan tip speed,” as disclosed herein according to one non-limiting embodiment, is less than about 1150 ft/second.

(17) The example gas turbine engine includes the fan 42 that comprises in one non-limiting embodiment less than about twenty-six (26) fan blades. In another non-limiting embodiment, the fan section 22 includes less than about twenty (20) fan blades. Moreover, in one disclosed embodiment the low pressure turbine 46 includes no more than about six (6) turbine rotors schematically indicated at 34. In another non-limiting example embodiment the low pressure turbine 46 includes about three (3) turbine rotors. The example low pressure turbine 46 provides the driving power to rotate the fan section 22 and therefore the relationship between the number of turbine rotors 34 in the low pressure turbine 46 and the number of blades in the fan section 22 disclose an example gas turbine engine 20 with increased power transfer efficiency.

(18) The use of the gear reduction between the low speed spool 30 and the fan 42 allows an increase of speed to the low pressure turbine 46. In the past, the speed of the low pressure turbine 46 and the low pressure compressor 44 has been somewhat limited in that the fan speed cannot be unduly large. The maximum fan speed is at its outer tip, and in larger engines, the fan diameter is much larger than it may be in smaller power engines. However, the use of the gear reduction has freed the designer from limitation on the speeds of the low pressure turbine 46 and the low pressure compressor 44 speeds caused by a desire to not have unduly high fan speeds.

(19) In geared gas turbine engines, such as the engine 20, a careful design between the number of vanes and blades in the low pressure turbine 46, and the mechanical tip rotational Mach number of the low pressure turbine 46 can be selected to reduce turbine noise through the use of the mechanism referred to as “cutoff.” This “cutoff” mechanism occurs when the vane-to-blade ratio is selected such that the fundamental blade passage tone is prevented from propagating to the far-field. This mechanism has been used previously in non-geared engines, which have low pressure turbines that operate at low tip Mach numbers, typically no greater than 0.5. However, “cutoff” has not been used in geared engines, such as those described herein, which have low pressure turbines that operate at high tip Mach numbers, typically greater than 0.5. On geared engines with such turbines, the “cutoff” mechanism requires a larger vane-to-blade ratio than it would on non-geared engines.

(20) The mechanical tip rotational Mach number, M.sub.tip, is generally defined as:

(21) $M_{\mathrm{tip}}=\frac{\pi }{720c}\mathrm{DN}$
wherein N is a rotor rotational speed in revolutions per minute, c is the local speed of sound in feet per second and D is the local tip diameter in inches.

(22) The mechanical tip rotational Mach number for any blade row may be calculated in this manner.

(23) Although described with reference to the two-spool engine 20, the relationship between the number of vanes and blades in the low pressure turbine 46, and the mechanical tip rotational Mach number of the low pressure turbine 46 may be applicable to three-spool direct drive engines or three-spool engines having a gear reduction as well.

(24) In the example engine 20, a ratio of the number of vanes to blades in a stage of the low pressure turbine is greater than or equal to R.sub.A. In this example, a mechanical tip rotational Mach number of the blade of the low pressure turbine is greater than or equal to M.sub.A at approach speed. In the example engine 20, R.sub.A is about 1.55 and M.sub.A is about 0.5. This novel design will result in reduced low pressure turbine noise because at least one stage of the low pressure turbine is “cutoff” at its rotor blade passing frequency.

(25) The stage including the vanes and blades greater than or equal to R.sub.A, can be any stage of the low pressure turbine 46.

(26) The stage may also be a stage of the high pressure turbine 54, or, if present, an intermediate pressure turbine. In a high or intermediate pressure turbine example, R.sub.A may be greater than or equal to 1.55.

(27) It is envisioned that all of the stages in the low pressure turbine 46 (or high pressure turbine 54 or, if present, an intermediate pressure turbine) would include a ratio of vanes to blades that is greater than or equal to R.sub.A. However, this disclosure may also extend to turbines wherein only one of the stages has a ratio of vanes to blades that is greater than or equal to R.sub.A. This disclosure also extends to turbines wherein more than one, but less than all, of the stages has a ratio of vanes to blades that is greater than or equal to R.sub.A.

(28) The mechanical tip rotational Mach number is measured at engine operating conditions corresponding to one or more of the noise certification points defined in Part 36 of the Federal Airworthiness Regulations. More particularly, the rotational speed may be taken as an approach certification point as defined in Part 36 of the Federal Airworthiness Regulations. For purposes of this application and its claims, the term “approach speed” equates to this certification point.

(29) The disclosed examples are most applicable to jet engines rated to produce 15,000 pounds (66,723 N) of thrust or more.

(30) FIG. 2 shows an embodiment 200, wherein there is a fan drive turbine 208 driving a shaft 206 to in turn drive a fan rotor 202. A gear reduction 204 may be positioned between the fan drive turbine 208 and the fan rotor 202. This gear reduction 204 may be structured and operate like the gear reduction disclosed above. A compressor rotor 210 is driven by an intermediate pressure turbine 212, and a second stage compressor rotor 214 is driven by a turbine rotor 216. A combustion section 218 is positioned intermediate the compressor rotor 214 and the turbine section 216.

(31) FIG. 3 shows yet another embodiment 300 wherein a fan rotor 302 and a first stage compressor 304 rotate at a common speed. The gear reduction 306 (which may be structured as disclosed above) is intermediate the compressor rotor 304 and a shaft 308 which is driven by a low pressure turbine section.

(32) Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.