The invention relates to a compressor blade for an axial compressor, comprising a blade root for attaching the compressor blade to a carrier and a cantilevered airfoil extending from said blade root in a span direction from said blade root at 0% height to a free ending airfoil tip at 100% height, the airfoil comprising a suction side and a pressure side extend ^¬ ing in chord direction from an upstream-sided leading edge to a downstream-sided trailing edge, wherein the airfoil com ^¬ prises a tip-sided airfoil area extending from approximately 70 % height of the airfoil to the airfoil tip, wherein for each profile of the airfoil a stagger angle is determinable. Further the invention relates to an axial compressor with at least one ring of said compressor blades.

Modern turbo engines and especially their bladings achieved already very high aerodynamical efficiency. This is also val ^¬ id for axial compressors. With the tendency to continuously increased profile loadings an increasing part of overall losses originates in the areas closed to the endwalls of the flow path, especially due to tip leakage flows. A reduction of these losses would enable an improvement of the efficiency of the axial compressor.

Up to now conventional flow path geometries of compressors do not consider directly the problem of tip leakage flows over the rotor blades. Especially airfoils with a rather short span height and relative large radial tip gaps have potential for improvements.

To address this problem rotor blades with a corrugated, but constant radial tip clearance design between their airfoil tips and the opposing casing endwall are known. Especially their constant tip wall gap reduces the resulting tip losses.

However, there is still a need for further improvement and higher efficiency in axial compressors. Therefore, the object of the invention is the provision of a compressor blade and an axial compressor with increased efficiency.

Concludingly the object of the invention is achieved by the independent claim. The dependent claims describe advantageous developments and modifications of the invention.

The invention directs to cantilevered compressor blades suf- fering under tip losses. To reduce the tip losses of compres ^¬ sor blades the invention proposes to distribute the work con ^¬ version or the turning of the airfoil along the span direction in that way, that tip losses are reduced. Compressor ro ^¬ tor blades shall perform more work conversion in the tip- sided airfoil area than in the remaining airfoil areas. For compressor stator vanes the turning is increased towards the tips. To achieve both shifts in the radial distribution of the flow, the stagger angles of the airfoil are selected along the airfoil span such that compared to a conventional blading the flow turning is shifted partially from the tip- sided airfoil area into an intermediate airfoil area and from intermediate airfoil area to a root-sided airfoil area. In other words, the work conversion rate in the tip-sided air ^¬ foil is decreased. This leads to the result, that the mass flow of air to be compressed is pushed away from that radial free end of the airfoil towards the endwall being a part of the blade root. Due to less main mass flow of air in the tip- sided airfoil area, resulting tip losses and endwall block ^¬ ages are reduced. The local re-staggering of the tip-sided profiles of compres ^¬ sor rotor blades next to the stationary endwalls and/or of the tip-sided profiles of compressor stator vanes next to the rotating endwalls leads to a change in the degree of reaction in the airfoil areas close to said endwalls and therefore to a change of the local relative Mach numbers. This contributes also to the desired gain of efficiency. However, but not only the tip losses are reduced. Also, the surge-behavior is im- proved and the tendency for pumping phenomena is reduced. In total increases this the operating range of the airfoil. Fur ^¬ ther beneficial effects are a decreased inclination to flow separation and a more homogeneous pressure distribution along the span direction of the airfoil.

To achieve said benefits the invention proposes for a com ^¬ pressor blade as mentioned into the introduction, that the profile comprising the largest stagger angle of all profiles of the tip-sided airfoil area is located between 78% height and 86% height and that starting from the profile with the largest stagger angle of the tip-sided airfoil area the stag ^¬ ger angles of the remaining profiles of the tip-sided airfoil area decrease monotonously towards both 70 % height and the blade tip.

The invention is based on the knowledge that with the inven ^¬ tion the spanwise distribution of work conversion or turning is distributed in that way that more work conversion or turn ^¬ ing is established in the free ending tip region and less at the root-span of the airfoil. This reduces significantly losses within the row of compressor blades. Further endwall losses can be reduced by shifting the secondary flows. These features are very beneficial for compressor rotor blades as well as for compressor guide vanes. Hence the com ^¬ pressor blade could be embodied as a rotor blade or a stator vane while the aerodynamical effects are different, e.g. flow can be redistributed to one endwall only and/or flutter in ^¬ clination .

The airfoil comprises further a root-sided airfoil area ex ^¬ tending between the blade root and a first virtual separative profile located between 20% and 40% height, especially at 30% height, and an intermediate airfoil area extending between the first virtual separative profile and a second virtual separative profile located at said approximately 70% height. According to a first preferred embodiment of the invention the stagger angles in the root-sided airfoil area are approx ^¬ imately constant. In that regard a stagger angle can be un ^¬ derstood as approximately constant, if its value does not change more than +/-2°. This increases the stiffness of the airfoil and enables a better monolithic attachment of the airfoil to the blade root for increasing lifetime of the com ^¬ pressor blade.

According to a second preferred embodiment the profile with the smallest stagger angle of all profiles of the root-sided airfoil area is located between 17% height and 25% height and starting from said profile with the smallest stagger angle of the root-sided airfoil area

the stagger angles of the remaining profiles of the root- sided airfoil area increase monotonously towards both the first virtual separative profile and the blade root. Instead of increased stability and lifetime provides this an in ^¬ creased aerodynamical efficiency. In another preferred embodiment the stagger angles of the root-sided airfoil area increase continuously from the first virtual separative profile towards the root-sided profile for at least 1,5°, but not more than 7°. Such a decrease of the stagger angle from the blade root towards the first virtual separative profile enables a snooze transition of the pres ^¬ sure side surface and a suction side surface for redistrib ^¬ uting the local turning of the airfoil, which has been shown in detailed analysis and in comprehensive developments.

Advantageously and for the same reason, in the tip-sided air ^¬ foil area beginning with the second virtual separative pro ^¬ file the stagger angles continuously increase towards to the profile section of the airfoil tip. Again, detailed investi ^¬ gations have shown that a preferred re-distribution of the local work conversion or turning of the airfoil can be achieved, if the airfoil comprises in span direction an over ^¬ all height and wherein the root-sided airfoil area and/or the tip-sided airfoil area comprises an extend parallel to the span, which is not larger than 30% of the overall height.

To provide a reliable compressor rotor blade with highest ef ^¬ ficiency in the intermediate area in a further preferred em- bodiment the stagger angle of the profiles of the intermedi ^¬ ate airfoil area changes monotonously from first virtual separative profile to the second virtual separative profile in a range between 4° and 8°.

Embodiments of the invention are now described, but in the way of example only, with reference to the accompanying draw ^¬ ings, of which: Figure 1 shows a longitudinal section through the axial compressor of a gas turbine,

Figure 2 shows a compressor rotor blade according to the invention as a first exemplary embodiment,

Figure 3 shows schematically a graph for the stagger an ^¬ gle of a preferred compressor rotor blade,

Figure 4 shows a compressor guide vane according to the invention as a second exemplary embodiment and

Figure 5 shows schematically a graph for the stagger an ^¬ gle of a preferred compressor guide vane.

In all drawings similar or identical elements are provided with the same reference numbers. The illustration in the drawings is only in schematic form.

Figure 1 shows in a longitudinal section an axial compressor

10 according to the invention. Within an annular flow path 13 of the compressor for compressing air two compressor stages

11 are shown exemplarily. Each compressor stage 11 comprises an upstream positioned compressor rotor blade 12 and a downstream positioned compressor stator vane 14. To address simultaneously both the compressor rotor blade 12 and the com ^¬ pressor guide vane 14 herein the term compressor blade 42 is used. The compressor rotor blades 12 are attached to the ro ^¬ tor 16, in example to a compressor rotor disk, in a conventional way. Also, in a conventional way the compressor stator vanes 14 are attached accordingly to compressor casing 18. Due to the schematic structure of the drawings the details of the attachment and especially the compressor blade roots are not shown. Both the compressor rotor blades 12 and the compressor stator vanes 14 are embodied with cantilevered air ^¬ foils 20, 22 integrally attached to their roots, so that the airfoil 20 of the compressor rotor blade comprises a free ending tip 24, which faces the stationary compressor casing 18 under establishing a tip gap 26. In an analogous way the airfoils 22 of the compressor stator vanes 14 comprise a tip 28 which faces the rotating rotor hub endwall 30 establishing a tip gap 32. During operation the rotor 16 rotates about its machine axis 17.

Figure 2 shows in a perspective view both a conventionally rotor blade 13 and a rotor blade 12 according to a first ex ^¬ emplary embodiment of the invention. The conventional com- pressor rotor blade 13 is drawn in dashed line, while the compressor rotor blade 12 according to a first exemplary embodiment of the invention is drawn in an uninterrupted line.

Each compressor blade 42 comprises a blade root 19, from which the airfoil 20, 22 extend in span direction to the re ^¬ spective blade tip 24, 28. The span axis is aligned with the radial direction R (figure 1) according to the machine axis 17. The airfoil 20, 22 comprises a leading edge 25 and a trailing edge 27 between which a suction side 29 and a pres- sure side 31 extends in chord direction. The airfoil height is determined at the leading edge 25 from its inner end at the blade root 19 with 0% height to its outer end at the blade tip 24, 28 with 100% height. For each height of the airfoils 20, 22 a profile can be de ^¬ termined. The profile represents for a specific height of the airfoil the two-dimensional outer airfoil shape defined by a cross section through said airfoil at said height, the cross section being substantially parallel to the machine axis. For each profile a stagger angle can be determined in a conven ^¬ tional way between a chord line of the respective profile and the compressor axial direction. The chord line is the line between the points where the front and the rear of the pro- file would touch the surface, when a profile were laid convex side up on a flat surface.

Hence, larger stagger angles can be understood as those that turns a profile into a more closed position and smaller stag- ger angles can be understood as those that turns a profile into a more open position. Stagger angles are usually in the range between 90° and the 180°.

The airfoil 20 is virtually separated in a number of airfoil areas, which follows successively one after one. According to the first exemplary embodiment the compressor rotor blade 12 comprises three airfoil sections: a root-sided airfoil area 20a, an intermediate airfoil area 20b and a tip-sided airfoil area 20c. The neighboring airfoil areas 20a, 20b, 20c are separated accordingly by a first virtual separative profile

21 and a second virtual separative profile 23 from one anoth ^¬ er. According to the first exemplary embodiment the first virtual separative profile 21 is located at 30% height and the second virtual separative profile 23 is located at 70% height.

As especially can be seen in comparison to the conventionally compressor rotor blade 13 the tip-sided airfoil area 20c of the compressor rotor blade 12 according to the first embodi- ment of the invention comprises a profile 50 with the largest stagger angle of the tip-sided airfoil area 20c at about 82% height. With increasing distance to said height of 82% the stagger angles of the remaining profiles of the tip-sided airfoil area 20a decreases monotonously towards both the blade tip 24 and the second virtual separative profile 23.

Only within the intermediate airfoil area 20b the stagger an- gles of its profiles change monotonously very slightly from the first virtual separative profile 21 to the second virtual separative profile 23. Especially for compressor stator blades the change of the stagger angle within their interme ^¬ diate airfoil areas 20b, 22b is not larger than 3° from one end of said area to other end of said area and for compressor rotor blades the change is not larger than 12°.

With regard to the root-sided airfoil area 20a of said first embodiment the stagger angles of the root-sided airfoil area 20a increase continuously about 4° from the first virtual separative profile 21 towards the innermost root-sided pro ^¬ file at 0% height. In other words and be seen in comparison to the conventional compressor rotor blade 13, the profiles of the root-sided airfoil area 20a have stagger angles that are turned close in comparison to the stagger angle of the intermediate airfoil area 20b. This means that the leading edge 25 of the profiles are turned towards the suction side 29 while the trailing edges 27 are turned to the pressure side 31, also compared with the profiles of the intermediate airfoil area 20b, especially for compressor stator blades.

Figure 3 shows only schematically a graph 44 for the stagger angles for all profiles of the airfoil 20 of the compressor rotor blade 12 over to the span direction from 0% height to 100% height at the free ending airfoil tip, related to said first exemplary embodiment in full line. Within the root- sided airfoil area 20a the stagger angles decrease monoto ^¬ nously towards 0% height such that the smallest stagger angle is located approximately at 0% height. In the range between approximately 0% height and approximately 10% height the stagger angles remain approximately constant. In the interme ^¬ diate area 20b the stagger angles increase monotonously from the first virtual separation profile 21 towards the second virtual separation profile 23 with an approximately constant gradient. According to the invention within the tip-sided airfoil area 20c the profile comprising the largest stagger angle of all profiles of the tip-sided airfoil area 20c is located between 78% height and 86% height and that starting from the profile with the largest stagger angle of the tip- sided airfoil area 20c the stagger angles of the remaining profiles of the tip-sided airfoil area 20c, 22c towards both the second virtual 70% height separative profile (23) and the blade tip decrease monotonously.

A dashed line 46 in figure 3 represents a variant of the graph for the stagger angles to the first preferred embodi ^¬ ment. Said variant differs from the first embodiment only in the root-sided airfoil area 20a. According to said variant in the root-sided airfoil area 20a the profile with the smallest stagger angle is located at 18% height. With increasing dis ^¬ tance to said height of 18% the stagger angles of the remain ^¬ ing profiles of the root-sided airfoil area 20a increases mo ^¬ notonously towards both the profile next to the blade root 19 at 0% height and the first virtual separative profile 21.

This is even more beneficial for counteracting aerodynamical losses and increases the stability margin of the respective airfoil in the compressor setup. In the same way as figure 2 shows figure 4 both a convention ^¬ al compressor stator vane 15 and a compressor stator vane 14 according to the invention. The conventional compressor stator vane 15 is drawn in dashed line, while the compressor stator vane 14 according to a second exemplary embodiment of the invention is drawn in an uninterrupted line.

Each compressor stator vane 14 comprises a blade root 19, from which the airfoil 22 extend in span direction to the respective blade tip 28. The span axis is aligned with the ra ^¬ dial direction R (figure 1) according to the machine axis 17. The airfoil 22 comprises a leading edge 25 and a trailing edge 27 between which a suction side 29 and a pressure side 31 extends in chord direction. The airfoil height is deter ^¬ mined again at the leading edge 25 again from blade root 19 at 0% height to the blade tip 24 at 100% height. It is empha ^¬ sized that the blade root 19 of each compressor rotor blade 12 of the axial compressor 10 is located on the inner diame- ter of the annular flow path 13 with respect to the machine axis 17, whereas the blade root 19 of each compressor stator vane 14 is located on the outer diameter of the annular flow path 13. Also this airfoil 22 of the compressor stator vane 14 is sep ^¬ arated virtually in a number of airfoil areas, which follows successively one after one. According to the second exemplary embodiment the compressor stator vane 14 comprises again three airfoil sections: a root-sided airfoil area 22a, an in- termediate airfoil area 22b and a tip-sided airfoil area 22c. The respective airfoil areas 22a, 22b, 22c are separated by a first virtual separative profile 21 and a second virtual separative profile 23 from one another. As can be seen also in comparison to the conventional com ^¬ pressor stator vane 15, the profiles of the root-sided air ^¬ foil area 22a have stagger angles that are turned close in comparison to the stagger angle of the intermediate airfoil area 22b. In the tip-sided airfoil area 22c the profiles com- prise stagger angles that are larger than the stagger angle of the intermediate airfoil area 22b.

Figure 5 shows only schematically a graph 48 for the stagger angles for all profiles of the airfoil 22 of a compressor stator vane 14 over to the span direction from 0% height to 100% height at the free ending airfoil tip. According to fig ^¬ ure 5 and similarly to compressor rotor blades 12 also compressor stator vanes 14 comprise a tip-sided airfoil area 22c in which the profile with the largest stagger angle of all profiles of the tip-sided airfoil area 22c is located at about 82% height. With increasing distance to said height the stagger angles of the remaining profiles of the tip-sided airfoil area decreases monotonously towards both the blade tip and the second virtual separative profile 23. The differ ^¬ ence between said largest stagger angle and the stagger angle of the second virtual separative profile 23 is larger than 3°, but not more than 12°. Within the intermediate area 22b the profiles comprise a stagger angle with only a moderate increase in size with de ^¬ creasing distance to the first virtual separating profile 21. Further towards the blade root 19 the size of the stagger an ^¬ gles the increases further, but with a larger gradient.

Only within the intermediate airfoil area 22b the stagger an ^¬ gles of the profiles are rather uniform. Preferably the dif ^¬ ference between the largest and smallest stagger angle of the profiles of the intermediate airfoil area 22b is not greater than 3°. As displayed by the graph of figure 5, if the stag ^¬ ger angles are not constant in intermediate airfoil area 22b, their size increase only moderate for those profiles with de ^¬ creasing distance to the first virtual separating profile 21. In the root-sided airfoil area 22a the stagger angle increas- es monotonously significantly i.e. about more than 3°, but not more than 12° from the first virtual separative profile 21 towards the profile at 0% height. Of course, the individual features of the all exemplary em ^¬ bodiments can be combined arbitrarily.