EP0194957B1

EP0194957B1 - Compressor blade tip seal

Info

Publication number: EP0194957B1
Application number: EP86630032A
Authority: EP
Inventors: Franz Harter
Original assignee: United Technologies Corp
Current assignee: RTX Corp
Priority date: 1985-03-11
Filing date: 1986-03-06
Publication date: 1990-01-31
Anticipated expiration: 2006-03-06
Also published as: DE194957T1; EP0194957A3; EP0194957A2; DE3668661D1; JPS61207802A

Description

This invention relates to axial flow fans/com- pressors of gas turbine engines and particularly to the relationship of the tips of the blades to the adjacent shroud or rub strip.
A fan or compressor according to the precharacterizing portion of claim 1 is disclosed in US-A 4 239 452 and in US-A 4 238 170. In US-A 4 238 170, for example, the tips of the compressor blades extend adjacent the surrounding shroud or rub strip that is trenched or recessed to the dimension complementary to the outer station and tip of the blade. In some instances, say at the low pressure stages where soft abradable materials such as a synthetic rubber can be utilized, the blades which move radially outward during engine acceleration, machine the groove. Obviously, this technique assures a close fit of the mating parts and helps in avoiding leakage around the tips of the blade. When the tips penetrate into the trench, as operating speed increases, a pumping action of air against the vertical side wall of the trench adjacent the blade trailing edges is provided so as to prevent working medium from migrating from the high pressure side of the blades to the low pressure side thereof. The pumping action accordingly opposes leakage but at high operating speed it may become too large creating turbulence in the fan/compressor flow path, adversely affecting performance. To avoid unreasonable turbulence and performance penalties at maximum operating speed it was usual to limit the maximum penetration of the blade tips into the trench by having the blade not extending into the trench at low operating speeds. This, however, permits increased blade tip leakage at low operating speeds.
Reference is also made to CH-A 414 681 which discloses in Fig. 1 an ambodiment similar to the US patents referred to. In Fig. 2 of CH-A 414 681 the trench receiving the blade tips is angularly disposed with respect to the outer flow path surface but the blade tip is parallel with respect thereto. A stator vane is also shown having a tip extending into a trench at the inner flow path wall. The tip of the stator vane and the outwardly oriented surface of the trench are angularly disposed with respect to the inner flow path wall. The vane is adapted to extend deeper into the trench at its leading edge. There is no leakage opposing pumping action against a downstream vertical side wall of the trench.
In FR-A 1 348 186, in Fig. 6, a ducted propeller is disclosed wherein the inwardly facing surface of the trench converges or diverges with respect to the outer flow path surface, but the blade is parallel with respect thereto, thus providing the disadvantage referred to above.
In GB-A 882 015 the blade tip and the inwardly oriented surface of the trench are angularly disposed with respect to the outer flow path surface. The maximum penetration is at the blade leading edge. There is no or substantially no leakage opposing pumping action against a vertical side wall of the trench adjacent the blade trailing edge.
Further, reference is made to GB-A 2 034 435, Fig. 4, wherein a turbine blade tip is shown received in an annular trench in the outer flow path wall. Cooling fluid under pressure is discharged into a cavity formed between the turbine blade tip and the trench to oppose blade tip leakage. The inwardly oriented surface of the trench and a portion of the turbine blade tip are angularly disposed with respect to the outer flow path wall.
The object of the invention is to provide a fan or compressor of the type disclosed avoiding or reducing leakage around the blade tip without adversely affecting performance.
In accordance with the invention this is achieved by the features recited in the characterizing portion of claim 1.
By the provision of these features the blade tip does not penetrate along its full width into the trench when the operating speed causing penetration is obtained. The tip aft portion first penetrates into the trench. This allows deeper penetration without causing too substantial turbulence and accordingly penetration is acceptable at low operation speeds to reduce or avoid leakage in the low speed operating range.
The angular contour is designed to effectuate a closure in the gap between the inner wall of the trench and the tip of the blad upon displacement of the compressor and/or fan blade arising out of the growth of the materials resulting from stable hand temperature operating conditions.
Other advantageous features of the fan/compressor are recited in the dependent claims.
The fan/compressor will now be described in greater detail with reference to the drawings, wherein:

Fig. 1 ist a partial view in section of a compressor section of a gas turbine engine schematically showing the slanted trench of the casing wall or rub strip of this invention,
Fig. 2 is an enlarged view of a nonslanted trench adjacent the tip station of a compressor blade of the prior art design,
Fig. 3 is an enlarged view of one of the blades and the attendant slanted trench in the engine casing, and
Fig. 4 is a partial view of the tip stations and trench illustrating another embodiment of this invention.

The invention in its preferred embodiment is illustrated for use in the lower temperature stations of a gas turbine engine and particularly in the compressor section where a soft material circumscribes the engine's inner diameter of the engine case and is abradable so as to be susceptible of being machined by the operation of the rotating blades. Thus, as disclosed in the US-A 4 238 170, supra, the blades at zero rotational speeds are spaced from the inner diameter of the rub strip and when accelerated to its highest operating speed, cut into the rub strip to define the trench. It is, however, to be understood and as will be obvious to one skilled in this art, the trench shape can be machined out prior to engine operation. What is considered the improvement by the teachings of this invention is the particular contour of the tips of the blades and its cooperating trench.
A portion of a compression section 10 of an axial flow compressor of a gas turbine engine is illustrated in Fig. 1. A flow path 16 for working medium gases extends axially through the compression section. An outer wall 18 having an inwardly facing surface to 20 and an inner wall 22 having an outwardly facing surface 24 form the flow path. A plurality of axially spaced rows of rotor blades as represented by the single blades 26 extend outwardly from the rotor 12 across the flow path into proximity with the outer wall. Each blade has an unshrouded tip 28 and is contoured to an airfoil cross section. Accordingly, each blade has a pressure side and a suction side and, as illustrated, has an upstream edge 30 and a downstream edge 32. Extending over the tips of each row of rotor blades is a stator seal land 34. Each land has a circumferentially extending groove or trench 36 formed therein to a depth D at an inwardly facing surface 37 thereof paralleling the blade tip 28.
A plurality of rows of stator vanes represented by the single vanes 38 are cantilevered inwardly from the stator 14 across the flow path 16 into proximity with the inner wall 22. Each vane, which in this illustration has an unshrouded tip 40, is contoured to an airfoil section. Accordingly, each vane has a pressure side and a suction side and, as illustrated, has an upstream edge 42 and a downstream edge 44. Extending over the tips of each row of stator vanes is a rotor seal land 46. Each land has a circumferentially extending groove 48 formed therein.
In the nonoperating condition the blade tips 28 are spaced from the inwardly facing surface 37. The gap between tips and surface enables assembly of the components. In response to centrifugally and thermally generated forces as the machine is accelerated to high operating speeds the rotor tips grow radially outward machining the groove 36 in the abradable material of the stator seal land 34. The point of closes proximity of the blades to the bottom of the groove is referred to as the «pinch point» and normally occurs during a transient engine operating to a maximum speed or power condition. As the engine reaches thermal stability at a given operating speed the outer wall including the land, moves both axially and radially relative to the blade tips to a position at which the blade tips 28 and inner surface 37 define a gap D.
A problem with the heretofore design as illustrated in Fig. 2 which is a prior art design is that the blade 50 penetration along its full width into the trench 54 toward the inwardly facing surface 52 as operating speed/increases causes a substantial pumping of air against the trench vertical wall 53 adjacent the blade trailing edge which creates turbulence.
The turbulence as shown by arrow A, essentially becomes a blockage in the flow path of the gas engine's working medium and adversely affects performance. The maximum depth of blade tip penetration must be controlled to avoid unreasonable turbulence losses at the maximum operating speed. At low speed operating the blade 50 will not penetrate into the trench 54 and leakage can readily occur between the flow path outer wall and the blade tip.
Ideally, it is desirable to match the pressure gradient across the tip which tends to leak air from the high pressure side to the low pressure side by the pressure created by the tip pumping action. In the heretofore shown embodiment the full width of the blade works on the air and has the tendency of over pressurizing this air and hence, creates the undesirable turbulence.
The tip of the blade and the inwardly facing surface 37 of the trench 36 are contoured to be angularly disposed relative to the outer gas path wall 18. This is best seen in Fig. 3. As the trench 36 is machined as described above, the trench 36 is formed to define the contour of the inwardly facing surface 37. The depth of the trench 36 as measured from the inwardly facing surface 20 of the outer wall 18 increases toward the vertical side wall 39 of the tench 36 adjacent the blade trailing edge 32. Looking at the cross section of the trench it is apparent that the axial extension of surface 37 relative to the flow path defined by surface 20 forms angle alpha a. The tip 28 of the blade 26 slants from a given diameter at the leading edge 30 to a higher diameter at the trailing edge 32. By virtue of this contour, two important features are realized:
1. The full width of the blade 50 pumped against the vertical trench wall 53 in the situation of the heretofore design as soon as any portion of the blade tip penetrated into the trench 54, Fig. 2. Thus the blade penetration is minimal prior to creating undesirable turbulence. Only the aft or trailing portion of the blade tip pumps against the trench vertical side wall 39 in Figure 3 when the speed is attained to cause the blade tip 28 to penetrate into the trench 36. Thus the blade tip can penetrate deeper into the trench prior to creating the limiting condition of turbulence. At lower operating speed conditions the revised tip design will permit penetration whereas the heretofore design did not permit penetration.
2. By slanting the trench in the proper direction, the gap will be reduced by the relative axial motion between the blade tip and trench outer wall as these engine parts achieve thermal stability at any given engine speed condition. Thus knowing the axial growth direction of the case, say in the direction of the arrow B relative to the blade's axial motion, it is apparent the gap D tends to become smaller.
Fig. 4 exemplifies another configuration on how the tip can be contoured to combat the leakage problem alluded to in the above. As noted the tip of blade 70 is contoured in a sawtooth fashion providing a plurality of parallel channels 72. In each channel the inner surface 74 is angularly disposed to the gas path wall providing similar benefits as was described above.
The preferred embodiment described in connection with Fig. 3 has proven to be particularly efficacious resulting in perhaps a 0.1 or 0.2% improvement in specific fuel consumption as evidenced on the PW2037 engine manufactured by Pratt & Whitney Aircraft of United Technologies Corporation, the assignee of this patent application.

Claims

1. Axial flow fan/compressor for a gas turbine engine, having high and low speed operating conditions and having a stator (14) and a rotor (12) rotatably supported in the stator (14), a working medium flow path (16) being defined between an inwardly facing surface (20) of an outer wall (18) of said stator (14) and an outwardly facing surface (24) of said rotor (12), said rotor (12) supporting a plurality of radially extending blades (26), each having a tip (28), a leading edge (30) and a trailing edge (32), a circumferentially extending trench (36) being formed in the inwardly facing surface (20) of the stator outer wall (18), said blade tips (28) being adapted to penetrate into said trench (36) and said trench having an inwardly oriented surface and a vertical side wall (39) adjacent the trailing edges (32) of the blades (26), the inwardly facing surface of the trench (36) paralleling the contour of the blade tips (28), characterized in that the blade tips (28) and the inwardly facing surface (37) of the trench (36) are angularly disposed relative to the inwardly facing surface of the stator outer wall (18) and are slanted relative to the fan/ compressor centerline, the depth of the trench (36) as measured from said inwardly facing surface (20) of the stator outer wall (20) increasing toward said vertical side wall (39) of the trench (36) whereby a trailing edge portion of each of the blade tips (28) is adapted to extend into the trench (36) when said fan/compressor (10) is in the low speed operating condition.

2. Fan/compressor according to claim 1, characterized in that the tip (28) of each blade (26) slants from a given diameter at the leading edge (30) to the higher diameter at the trailing edge (32).

3. Fan/compressor according to claim 2, characterized in that said stator (14) has a particular direction of axial growth and the direction of said slant is in said direction of axial growth to minimize the gap between the tips (28) of said blades (26) and the inwardly facing surface (37) of said trench (36) upon axial growth of said stator (14).

4. Fan/compressor according to claim 1, characterized by an abradable material (34) lining said outer wall (18) adjacent the tips (28) of said blades (26) and said trench (36) being machined into said abradable material (34) by accelerating said fan/ compressor to said high speed operating condition whereby said blades (26) expand radially.