US3918832A

US3918832A - Stator construction for an axial flow compressor

Info

Publication number: US3918832A
Application number: US492574A
Authority: US
Inventors: Richmond G Shuttleworth; Donald L Williams
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1974-07-29
Filing date: 1974-07-29
Publication date: 1975-11-11
Anticipated expiration: 1992-11-11
Also published as: FR2280812A1; FR2280812B1; BR7504846A; DE2532554A1; DE2532554C2; CH589229A5; SE7508371L; GB1511019A; JPS5138112A; NO149010B; IT1040063B; NO149010C; NO752644L; CA1027870A; JPS6050999B2; SE419260B

Abstract

A multistage compressor stator for an axial flow gas turbine engine is disclosed. A plurality of vane assemblies is mounted in a circumferential track of the case inner wall. Each vane assembly includes one or more stator vanes extending radially inward from the case inner wall into the axial flow path of the working medium, and an arcuate retainer which slidably engages the base portion of each vane. During operation of the engine the vanes are subject to vibration which is dissipatively transferred to both the working medium and the compressor case.

Description

[ 1 Nov. 11, 1975 STATOR CONSTRUCTION FOR AN AXIAL FLOW COMPRESSOR Inventors: Richmond G. Shuttleworth, Vernon;

Donald L. Williams, Manchester, both of Conn.

United Technologies Corporation, Hartford, Conn.

Filed: July 29, 1974 Appl. No.: 492,574

Assignee:

US. Cl. 415/217; 415/219 R Int. Cl. F01D 25/24; F04D 19/02 Field of Search 415/216, 217, 218, 219 R,

References Cited UNITED STATES PATENTS 8/1961 Bean et a1 415/218 2/1967 Bobo 415/216 6/1967 Bobo 415/218 11/1973 Canova et al.v 415/216 FOREIGN PATENTS OR APPLICATIONS 918.522 2/1963 United Kingdom 415/217 1.021.495 3/1966 United Kingdom 415/218 1.252.179 12/1960 France 415/217 Primary E.\'aminer-Henry F. Raduazo Attorney, Agent, or FirmRobert C. Walker [5 7 ABSTRACT A multistage compressor stator for an axial flow gas turbine engine is disclosed. A plurality of vane assemblies is mounted in a circumferential track of the case inner wall. Each vane assembly includes one or more stator vanes extending radially inward from the case inner wall into the axial flow path of the working medium, and an arcuate retainer which slidably engages the base portion of each vane. During operation of the engine the vanes are subject to vibration which is dissipatively transferred to both the working medium and the compressor case.

9 Claims, 6 Drawing Figures US. Patent Nov. 11, 1975 Sheet 1 of3 3,918,832

Shet 2 of 3 US. Patent Nov. 11, 1975 US. Patent Nov. 11, 1975 Sheet 3 013 3,918,832

WWW

STA'IOR CONSTRUCTION FoRAN' AXIAL FLOW COMPRESSOR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relatesv to gas turbine engines and more specificallyto compressor stators for gas turbine engines. s M

2. Description of the Prior Art Gas turbine engines are designed and constructed to provide adequate structural integrity of the individual components while maintaining acceptable levels of aer odynamic performance. In the compressor section a stator encases the rotor assembly. The tips of the compressor blades, which extend radially outward from the rotor into the flow path of the working medium, coopcrate with a mating surface on the compressor stator which surrounds the tips of the blades to form a gas seal between therotor and stator. The tips of the compressor vanes, which extend radially inward from the compressor case into the flow path of the working medium, cooperate with a corresponding surface on the rotor to form a gas seal between the rotor and the stator. The aerodynamic performance of the compressor is highly dependent on the clearances between the rotor and the stator at the .blade and vane tips. Even a slight reductionin tip clearance can improve the aerodynamic performance of the compressor significantly.

The larger the flow path diameter of the gas turbine, the more difficult it becomes to maintain acceptable tip clearances. The. sealing surfaces of the rotor and the arc circumscribed by the rotating blade tips are held concentric with the axis of the rotor within very limited tolerances tomaintain rotor balance. The compressor stator does not structurally require this precision balancingand concentricity tolerances of the stator are, therefore, generally relaxed to reduce manufacturing cost. The extent to which these tolerances are relaxed, directly affects the concentricity of the blade and vane tips with their corresponding sealing surfaces.

In an ideal condition all sealing surfaces of the rotor are concentric with their corresponding vane tips, and all sealing surfaces of the stator are concentric with their corresponding blade tips. This construction permits a minimum clearance between the rotating and stationary parts. As eccentricity between the stationary and rotatingparts is introduced the clearances must be increased to prevent destructive contact between the rotor and the stator during operation of the engine.

Additional clearance between the cooperating sealing surfaces of the rotor and stator is also provided to allow for distortion of the compressor case under transient thermal conditions. A compressor case havinga nonuniform mass distributed about its circumference, such as an axially split case, experiences distorted thermal growth due to the concentration of mass in the flange areas..As the case is exposed to changing thermal environments the areas of the case having low mass concentration expand more rapidly than those areas of the case having high mass concentration due to the time differential required for these areas-to reach steady state thermal conditions.

Most axial flow gas turbines have a double compressor case comprising-an outer case which provides structural support to the engine bearings and an inner case which radially defines the flow path of the working medium and which supports the compressor vanes extending from the inner wall into the flow path. The inner case comprises a series of annular rings bolted together. Alternate rings support rows of stator vanes and are joined by an intermediate ring which is the sealing surface opposing the corresponding row of rotor blade tips. The joining features of each ring are formed with respect to the axis of each individual ring within limits of a concentricity tolerance. As adjacent rings are joined together concentricity tolerances can accumulate so that, potentially, the misalignmentbetween the first and the last stages of the stator assembly can be excessive. Consequently design clearance between the rotor and the stator assemblies is increased to accommodate for this potential misalignment.

Other gas turbines, particularly those used in industrial applications, have a single compressor case which is split axially and joined by longitudinal flanges on opposing halves of the compressor case. The mass distribution about the circumference of the compressor case is nonuniform and causes eccentric distortion of the compressor case during transient thermal conditions.

. Components supported by the areas having a high mass concentration assume in a radially inward position with respect to those components supported by an area having a low mass concentration as the case temperature is increasing and assume a radially outward position as the case temperature is decreasing. Sufficient clearance between the blade and vane tips and the corresponding sealing surfaces is provided to prevent destructive contact between the rotor and the stator in .the areas where the compressor case has greater mass.

A single compressor case with uniform mass distribution has the potential for high aerodynamic performance. Although adequate mechanical support of the vanes from such a case is difficult. The vanes are necessarily cantilevered from the case and, therefore, tend to vibrate during operation of the engine. Vanes which are rigidly mounted to the compressor case have a limited lifetime since they tend to crack or fracture due to vibratory stresses in the area where the airfoil is joined to a supporting structure. In addition, a single-wall case requires vane loading slots into which the vanes are inserted and repositioned circumferentially about the compressor case. The loading slot region frequently experiences excessive stress concentration in the areas where small radii interrupt uniform stress patterns.

In US. Pat. No. 2,857,093, Warnkin discloses a plurality of stator vanes which are assembled on an arcuate segment and, subsequently, mounted in a circumferential track of an axially split compressor case. Each compressor vane has a wedge shaped root which is in; serted through an aperture in the arcuate segment where it is held firmly in position.

In US. Pat. No. 2,928,586 Hart describes a stator for a multistage axial flow compressor having a cylindrical casing wherein spacer rings are assembled between rows of blades and are held in place by bolts penetrating the case to engage each ring. Two axially adjacent spacer rings form a tee shaped retainer which engages the correspondingly shaped base of each compressor vane. In Hart the potential misalignment due to concentricity buildup is increased over a single wall case with integrally mounted vanes.

Efforts are continuing toward improvements in aerodynamic performance while maintaining the structural integrity of compressor stators including efforts to nondestructively dissipate vibrational energy from the vanes.

SUMMARY OF THE INVENTION A primary object of the present invention is to improve the structural integrity of the compressor case and compressor vanes of a gas turbine engine. A further object of the present invention is to improve the aerodynamic performance of the compressor.

According to the present invention a plurality of compressor vanes having a retaining slot in the base portion of each vane is slidably mounted on an arcuate retainer having a correspondingly shaped cross section, the vanes extending from the arcuate retainer in a radially inward direction toward the center of curvature of the arc; a plurality of arcuate retainers and the vanes mounted thereupon comprise a set of vanes for a single compressor stage and are bolted into a circumferential retaining track of uniform cross section which has been machined into the inner wall of the compressor case, one or more bolts penetrating the case from the outer wall engage each arcuate retainer.

A primary feature of the present invention is the dissipative transfer of vibrational energy from the vanes to the working medium and to the compressor case. An additional feature is the one piece compressor case having circumferential tracks of uniform cross section for retaining detachable vane assemblies.

Principal advantages of the present invention are the ability of the stator assembly to dissipate vibrational energy without cracking the stator vanes and the ability of the compressor case to maintain uniform blade and vane tip clearances around the circumference of the rotor. An additional advantage of the present invention is the confinement of high stress concentrations to the vanes and to the arcuate retainers which are easily replaceable and low cost parts.

The foregoing and other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of the preferred embodiments thereof as illustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a simplified elevation view showing an axial flow gas turbine engine;

FIG. 2 is a simplified cross-sectional view of a portion of the compressor of the gas turbine engine shown in FIG. 1;

FIG. 3 is a section view of the compressor taken in the direction 3 as shown in FIG. 2;

FIG. 4 is a section view taken along the line 4-4 as shown in FIG. 3;

FIG. 5 is a section view of the compressor vane attachment under gas pressure loads; and

FIG. 6 is a section view of the compressor vane attachment under a condition of vibrational excitation in which the vibratory loads are opposite to and exceed the gas pressure loads.

DESCRIPTION OF THE PREFERRED EMBODIMENT The gas turbine 10 shown in FIG. 1 is an axial flow engine having a multistage compressor 12 joined to a multistage turbine 14 by a combustor 16. Air is compressed in the compressor, is mixed with fuel and burned in the combustion section to produce hot gases which are expanded through a series of nozzles within the turbine section. The more air that an engine can 4 compress and use the greater is the power or thrust that can be produced within the engine.

A portion of the multistage compressor is shown in cross section in FIG. 2. A rotor assembly 18 comprises a plurality of compressor wheels 20 which are separated axially by spacers 22. Each compressor wheel includes a disk 24 and a plurality of blades attached thereto as represented by the single blade 26 on each wheel. Each blade has a platform 28 at the base of an airfoil section 30. An axial gap between the blade platforms of adjacent wheels is spanned by an inner air seal 32. The rotor assembly is radially enclosed by a compressor stator 34 comprising a plurality of vane stages 36 each mounted within a circumferential track 38 in a compressor case 40.

As is shown in FIG. 3 each vane stage comprises a plurality of vane assemblies 42 which include one or more vanes 44, an arcuate retainer 46 and a pair of end plates 48. Each vane assembly is held within the circumferential track by one or more bolts 50 which penetrate the case to engage the arcuate retainer.

Each compressor vane has an airfoil section 52, a base 54 including retaining slot 56 and a tip 58 as shown in FIG. 4. An axial clearance 60 is provided between the vane base and the compressor case and a radial clearance 62 is provided between the vane base and the arcuate retainer. As is shown in FIG. 5 each vane has a case bearing surface 64 which is opposed by a vane bearing surface 66 of the case and a retainer bearing surface 68 which is opposed by a vane bearing surface 70 of the retainer.

During assembly of the compressor one or more vanes 44 are slidably mounted on each arcuate retainer 46, the tee shaped arcuate retainer of the preferred embodiment engaging the correspondingly shaped slot in the base of each vane. An end plate 48 is affixed to each end of the arcuate retainer to trap the vanes on the retainer. A plurality of vane assemblies is bolted into each circumferential track to form each compressor vane stage 36, each assembled vane extending radia'lly inward across the flow path of the working medium. The end plates perform the additional function of preventing circumferential movement of the vanes about the track during operation of the engine.

The number of vanes mounted within each vane assembly is varied according to the size and weight of the individual components. Including a large number of vanes in each vane assembly lessens the number of steps required to assemble a complete vane stage in the compressor case. Including a smaller number of vanes in each vane assembly reduces the weight of the assembly and makes it more easily mountable within the compressor case. In one embodiment a single vane stage comprises five vane assemblies which weigh approximately thirty pounds each and include fourteen vanes. As many as ten vane assemblies are commonly used.

The number of vanes on the vane assembly to be last assembled is limited by the cord length of its arcuate retainer. The cord length of the last arcuate retainer must be smaller than the distance between the tips 58 of the vanes through which the vane assembly passes as it is positioned into the circumferential track from a radially inward direction. In the embodiment just described the fifth vane assembly is split and includes one vane assembly having thirteen vanes and one assembly having a single vane as shown in FIG. 3. The vane assemblies are bolted into the circumferential track by vane assembly comprising a'single vane is mounted within a circumferential track in the same manner as vane assemblies having a plurality of vanes.

One of the significant aspects ofa compressor constructed in accordance with the present invention is the damping of vibratory energy in the compressor vanes during engine operation. The axial clearances 60 between the base of each vane and the compressor case, and a radial clearance 62 between the base of each vane and the corresponding arcuate retainer permit limited movement of the vanes after the arcuate retainer of each vane assembly is secured to the compressor case. In a typical embodiment both the radial and axial clearances are one thousandth to thirteen thousandths of an inch. During operation'of the compressor the vanes are pressure loaded and assume a canted position as shown in FIG. 5. The vanes tilt toward the front or low pressure end of the compressor until the retainer bearing surface of the vane 68 comes to rest against the vane bearing surface of the retainer 70 and simultaneously the case bearing surface of the vane 64 comes to rest against the vane bearing surface of the case 66. Within the normal operating ranges of the engine, inherent vibrational loads cyclically exceed the static pressure loads on the vane causing the vane to tilt from its forward position to a rearward position as shown in FIG. 6; Rearward movement of the vane is op-- posed by the pressure loading forces which cushion the airfoil surface to dissipate vibrational energy from the vane. Additionally, friction damping occurs between the side bearing surfaces of adjacent vanes and between the bearing surfaces on each vane which are in contact with the retainer or the case.

In contrast to the present invention, vibrational energy is most commonly removed from the compressor vanes of gas turbine engines through a rigidly fixed attachment joining the vane and to the case. With this construction the life of the vane is shortened because vibratory stresses concentrate at the juncture between the vane airfoil and the vane base. The cumulative vibratory stresses at this juncture ultimately crack or fracture the vanes. Vanes attached in accordance with the present invention are not rigidly affixed to the case and do not experience excessive vibratory stresses.

The compressor case has essentially u-shaped tracks machined into the circumferential inner wall. The absence of vane loading slots allows case stresses to uniformly distribute around the circumference of the case thereby maximizing the case life. Stress concentrations within the stator do occur at the internal structural corners of the retainer and the vane base. However, the retainers and the vanes are easily replaced at minimal cost when structural cracks appear.

Movement of the vanes relative to the case and the arcuate retainer can cause wear along the surfaces of contact. To prevent excessive wear the bearing surfaces are treated with a hardfacing material. The construction shown in the preferred embodiment has a simple geometry which permits the application of hardfacing material to the bearing surfaces.

Significant aerodynamic improvements result from a compressor constructed in accordance with the present invention. In one embodiment an increase in the blade and vane tip clearance of ten thousandths throughout the length of the compressor decreases the compressor efficiency by one percent. A common design goal is the 6 control of the tip clearances to within one percent of the 'spanwidth of the airfoil sections which is for one typical embodiment a thirty eight thousandths nominal clearance between each vane or blade tip and its corresponding sealing surface at a diameter of approximately fifty inches.

Control of the tip clearance requires control of the distortion of the compressor case and control of the concentricity of mating surfaces with respect to a common axis. Control of the compressor case distortion is achieved by use of a nonsplit compressor having a uniform cross-sectional area about its circumference at any axial position. The use of a nonsplit compressor case allows a uniform cross section by eliminating the mass concentration of the flanges joining a split compressor case. The areas of a split case which have high mass, such as the flange areas, exhibit a retarded rate of thermal response. A nonuniform thermal response distorts the sealing surfaces of the case at the blade tips from a circular configuration, and alters the radially inward position of effected compressor vanes. In a split case the tip clearances must be adjusted to compensate for the range of thermal response rather than a single thermal response as in the preferred embodiment.

A second principal problem in holding minimum tip clearances is the buildup of concentricity tolerances between opposing compressor parts. Most gas turbines in use today utilize stator constructions of the double case type wherein the inner case supports the compressor vanes and the outer wall provides structural support to the engine bearings. The inner case supporting the vanes comprises a plurality of cylindrically shaped vane supports placed in axially adjacent positions and bolted together. Each cylindrical support is machined to concentricity tolerances in relation to its own axis. As the support cylinders are bolted together the potential concentricity misalignment increases from the first support to the last support. In the compressor case of the present invention each circumferential track is machined in relation to the same axis which is the axis of the compressor case. There is, therefore, a single uniform concentricity tolerance at all axial stages of the compressor. Although, the compressor rotor is subject to the same type of tolerance buildup experienced with the double compressor case, the magnitude of the tolerance buildup is not as great because rotor concentrici; ties are already accurately controlled to maintain rotor balance.

In addition, elimination of the inner case of the compressor reduces the stator cost significantly and in the embodiment described by about one third. The single case construction has a reduced number of pieces and, therefore, has reduced assembly complexity. The single case construction is lighter than the double case construction and can be mass balanced for improved blade tip clearance.

In mass balancing, the mass of the compressor case at any axial position can be increased to match the predicted thermal response of the rotor at that axial position. Although the thermal response of a compressor having a double case construction may be similarly controlled, the geometry is more complex and accurate distortion prediction is more difficult.

Although the invention has been described with respect to a preferred embodiment having a pair of end plates attached one to each end of each arcuate retainer, a single end plate attached to one end of each arcuate retainer is equally effective in preventing the 7 circumferential rotation of the vanes around the track in the inner wall of the case.

Although the invention has been shown and described with respect to a preferred embodiment thereof, it should be understood to those skilled in the art that the foregoing and other changes and omissions in the form and detail thereof can be made therein without departing from the spirit and the scope of the invention.

We claim:

1. In a gas turbine engine, a compressor stator including a plurality of vane assemblies detachably mounted within a circumferential track in the inner wall of the case of the compressor wherein each vane assembly comprises:

an arcuate retainer which is attached to the case and which is coextensive with the case over a segment of the case circumference;

at least one vane having a base including a retaining slot which is slidably engaged by the arcuate retainer, the vane extending radially inward from the retainer toward the center of curvature of the retainer; and

a first end plate attached to one end of the arcuate retainer for preventing the circumferential rotation of the vanes around the track in the inner wall.

2. The invention according to claim 1 further including a second end plate attached to the end of the arcuate retainer opposite said first end plate for trapping the vanes on the arcuate retainer.

3. A stator construction for an axial flow compressor having means for damping vane vibration including a compressor case having, in the case inner wall, a circumferential track containing a plurality of vane assemblies each of which comprises an arcuate retainer slidably engaged to a slot in the base of the stator vanes which are attached to and extend radially inward from the arcuate retainer toward the center of curvature of the retainer, and a first end plate attached to one end of the retainer to prevent circumferential movement of the vanes about the compressor case, each of said vanes having an axial and radial clearance between the vane base and the compressor case which permits movement of the vane under the influence of vibrational excitation to dissipatively transfer vibrational energy from the blade to the working medium by pressure load forces on the airfoil and to the compressor case by friction forces between the vanes, case and retainer.

4. The invention according to claim 3 wherein the axial and radial clearances between the vane base and the compressor case are in the range of one to thirteen thousandths of an inch.

5. The invention according to claim 4 wherein the slot in the base of each vane has a tee shaped cross section.

6. The invention according to claim 5 wherein the compressor case has a substantially uniform cross-sectional area at any axial position along the length of the case.

7. The invention according to claim 6 wherein the number of vane assemblies comprising a single compressor stage is between five and ten.

8. The invention according to claim 7 wherein at least one assembly includes no more than one vane 9. The invention according to claim 8 further including a second end plate attached to the end of the arcuate retainer opposite said first end plate for trapping the vanes on the arcuate retainer.

Claims

1. In a gas turbine engine, a compressor stator including a plurality of vane assemblies detachably mounted within a circumferential track in the inner wall of the case of the compressor wherein each vane assembly comprises: an arcuate retainer which is attached to the case and which is coextensive with the case over a segment of the case circumference; at least one vane having a base including a retaining slot which is slidably engaged by the arcuate retainer, the vane extending radially inward from the retainer toward the center of curvature of the retainer; and a first end plate attached to one end of the arcuate retainer for preventing the circumferential rotation of the vanes around the track in the inner wall.

8. The invention according to claim 7 wherein at least one assembly includes no more than one vane.

9. The invention according to claim 8 further including a second end plate attached to the end of the arcuate retainer opposite said first end plate for trapping the vanes on the arcuate retainer.