The present invention relates generally to hydroelectric turbine installations with at least one blade containing a hollow passage. Blades with a hollow passage are used in hydroelectric installations with means for enhancing the level of dissolved gas in water passing through the turbine. In this case the hollow passage is passed by gas. Blades with a hollow passage are also used for reducing axial thrust in radial flow turbines of the Francis type (compare e.g., WO 2018/137821 A1 ). In this case the hollow passage is passed by water. The present invention relates also to a runner comprising a blade made by the claimed method.

Methods of making a turbine blade with a hollow passage for a hydroelectric turbine installation are known from prior art. For example, US 5,896,657 A discloses a method of making a such a turbine blade. Initially, a blank is cut from mill plate to the overall blade profile. Then the blank is cut into a leading blade portion or member 48 and a trailing blade portion or insert 50 (see Fig.5). Insert 50 preferably includes a region of inner edge 36 of blade 32. However, an alternative embodiment is illustrated in FIGS. 9 and 10 in which an insert 50' is spaced from an inner edge 36', and a shallow groove 52 end an overlying cover plate 54 extend from inner edge 36' to a rear edge 56' of a member 48'. Cover plate 54 is welded within an upper area of shallow groove 52 and is generally flush with a suction side 42' of the blade. A rearwardly opening slot or groove 58 is machined along a length of a rear or trailing edge 56 of member 48, leaving an upper shoulder 60 and a lower shoulder 62. Lower shoulder 62 is machined to include a beveled surface 64 extending from pressure side 40 (i.e. , the lower surface as illustrated in FIGS. 3 and 4) to a rearwardly projecting lip 66. A frontwardly opening slot or groove 68 is machined along a front or leading edge 70 of insert 50, leaving an upper shoulder 72 and a lower shoulder 74. Upper shoulder 72 is machined to include a beveled surface 76 extending from suction side 42 (i.e., the upper surface as illustrated in FIGS. 3 and 4) to a frontwardly projecting lip 78. A plurality of discharge passages 80 are formed in insert 50 (e.g., by drilling) extending from groove 68 to trailing edge 46. Finally, member 48 and insert 50 are connected by welding. As shown in FIGS. 3 and 4, rearwardly opening groove 58 and frontwardly opening groove 68 thus cooperate to form an integral aeration conduit or passage 47 through which an oxygen containing gas, such as air, can be transported.

It is the intention of the present invention to disclose a method of making a turbine blade with a hollow passage, which is simpler and less costly compared to the method known from prior art.

The task is solved by a method according to the independent claim. Advantageous embodiments of the invention are disclosed in the dependent claims.

The invention will hereinafter be described in conjunction with the appended drawings:

Fig.1 Francis type runner and blade

Fig.2 Kaplan type runner and blade

Fig.3 Blade according to the present invention comprising a passage

Fig.4 Parts of a blade according to the present invention

Fig.5 Pressure side surface of a blade according to the present invention Fig.6 Suction side surface of a blade according to the present invention

Figure 1 shows on the left side a Francis type runner in schematic rendering. The runner is designated by 1 and comprises a hub designated by 2 and a band designated by 3. The hub 1 is also called crown. There are also Francis type runners without a band. Such runners are called ‘shroudless’ (compare e.g., WO 2016/011537 A1 ). The runner 1 comprises a multitude of blades. Such a blade is shown separately on the right side of figure 1 and is designated by 4. The blade comprises four edges: A first edge, which is called inner edge and is designated by 5, a second edge, which is called outer edge and is designated by 6, a third edge, which is called leading edge and is designated by 7 and a fourth edge, which is called trailing edge and is designated by 8. The inner edge 5 is connected to the hub 2 and the outer edge 6 is connected to the band 3, in case there is a band 3. Figure 2 shows the same elements as figure 1 but for a Kaplan type runner. A Kaplan type runner comprised a multitude of pivotable blades 4. Kaplan type runners comprise no such thing as a band and the outer edges 6 of the blades are shaped in a way to form a narrow gap between the runner ring and the blade 4. The runner ring is designated as 11 . Since the gap should be homogenous for each pivot position of the blade 4 the inner surface of the runner ring 11 and the outer edges 6 of the blade 4 are shaped spherical. Since Kaplan type blades are sometimes rounded in the region between the outer edge 6 and the adjacent edges it is not immediately clear where the outer edge 6 ends. This can be clarified using the mentioned gap: The outer edge ends at the points, where the blade starts to deviate from forming the homogenous gap. This is indicated by the dashed lines on the right part of figure 2.

Another type of turbine comprising blades according to the present invention is called Propeller type turbine. As the Kaplan type turbine, the Propeller type turbine is an axial flow turbine. The main difference between Kaplan and Propeller type turbine is that the blades of the latter cannot be pivoted. Therefore, the inner surface of the runner ring and the outer edges 6 can be shaped simpler, that means cylindric.

Independent of the turbine type each blade comprises a first and a second surface. In figure 2 the first surface is designated by 9 and the second surface is designated by 10. In figure 2 and the following the first surface is the pressure side surface and the second surface is the suction side surface. For the method according to the present invention, it makes no difference whether the first surface is the pressures side surface or the suction side surface, and the same holds consequently for the second surface. Therefore, in the claims the phrases ‘pressure side surface’ and ‘suction side surface’ won’t be used. One of them is the first surface and the other consequently is the second surface.

Blades of the mentioned turbine types can comprise a passage. A passage is a hollow channel inside the blade. Such a passage can be used in all three turbine types for enhancing the level of dissolved gas in water passing through the turbine. In case of a Francis type turbine such a passage can also be used for reducing the axial thrust. Blades comprising a passage manufactured by the method according to the present invention are described with the help of figures 3 to 6. This is carried out exemplarily for the case of a blade of a Francis turbine.

Generally, passages in blades extend between two apertures, wherein the two apertures are located at different edges of the blade. Figure 3 shows on the left and right side two different embodiments of passages respectively. In the embodiment shown on the left side of figure 3 a first aperture is located at the inner edge and a second aperture is located at the trailing edge of the blade. In the embodiment shown on the right side of figure 3 a first aperture is located at the inner edge and a second aperture is located at the outer edge of the blade. At the lower part of figure 3 the cross sections along the lines A-A and B-B are shown. In both cases the blade consists of two parts which are welded together by two welding seams, wherein one welding seam is located on one surface of the blade and the other welding seam is located on the other surface of the blade. The welding seams are indicated by the solid triangles.

Figure 4 shows the same embodiments as figure 3, wherein in figure 4 the two parts of the blade are shown separately, that means before they are welded together. In the lower left part of figure 4 a first part of the blade is designated by 4.1 and a second part of the blade is designated by 4.2.

Figure 5 focuses on the pressure side surface of the blade according to the left-side embodiment of figure 3 and 4. The pressure side surface consists of three disjoint areas. A first area is designated by 9.1 , a second area is designated by 9.2 and a third area is designated by 9.3. The three areas are defined by projecting the outline of the passage on the pressure side surface. The right side of figure 5 shows the corresponding cross section according to figure 3. The second area 9.2 is located above the passage and the other areas 9.1 and 9.3 are located besides the passage. Similar areas exist on the suction side surface, which will be described in the next paragraph. Figure 6 focuses on the suction side surface of the blade according to the right-side embodiment of figure 3 and 4. The suction side surface consists of three disjoint areas. A first area is designated by 10.1 , a second area is designated by 10.2 and a third area is designated by 10.3. The three areas are defined by projecting the outline of the passage on the suction side surface. The right side of figure 6 shows the corresponding cross section according to figure 3. The second area 10.2 is located above the passage and the other areas 10.1 and 10.3 are located besides the passage.

It is clear from figures 5 and 6, that corresponding areas on the two surfaces are named and numbered in the same way. That means that the first area 9.1 of the pressure side surface is located opposite to the first area 10.1 of the suction side surface, and so on.

For the directions of the mentioned projections exist several possibilities. The projection can for example be perpendicular to the corresponding surfaces or perpendicular to the centerline of the blade profile. In figures 5 and 6 the projection lines are indicated by the dashed lines. However, deviations from the perpendicular directions up to 30° does make almost no difference. This is due to the fact, that the blades are rather slim.

The definitions based on figures 3 to 6 can be used to formulate the method of the present invention and to generally define the two parts 4.1 and 4.2 composing the blade. The first part 4.1 comprises the first area 9.1 of the pressure side surface and the first 10.1 and the second area 10.2 of the suction side surface, whereas the second part 4.2 comprises the second 9.2 and the third area 9.3 of the pressure side surface and the third area 10.3 of the suction side surface.

The method of making a turbine blade with a passage according to the present invention comprises the following steps:

- Providing a first part 4.1 and a second part 4.2;

- Machining the first part 4.1 and the second part 4.2 in order to form the profile of the first and the second surface of the blade 4; - Composing the blade 4 by welding the first part 4.1 to the second part 4.2, wherein the welding comprises a first weld seam located on the first surface and a second weld seam located on the second surface of the blade 4;

- Machining of the weld seams.

The steps can be performed in the above stated order. It is also possible that the step ‘machining the first part 4.1 and the second part 4.2 ... ’ is performed after the step ‘composing the blade 4 by welding ... ’.

The first part 4.1 and the second part 4.2 are provided by casting. The machining of the parts 4.1 and 4.2 is done in the regions which form a part of the first and second surface of the blade 4. That are the areas 9.1 , 9.2, 9.3, 10.1 , 10.2 and 10.3. The parts 4.1 and 4.2 can comprise bevels to accommodate the weld seams. If the welding is done using a narrow gap welding method no bevels are needed. The machining of the weld seams is performed to get smooth first and second surfaces.

In a variation of the described method the first part 4.1 and the second part 4.2 are first connected to the runner hub 2 and afterwards welded together.

The method according to the present invention is less complex than the methods known from prior art, since the two parts composing the blade can be provided by casting. In the case that one of the apertures of the passage is located at the rather thin trailing edge of the blade, the welding seams are running almost perpendicular to the trailing edge near this edge, which causes less deformation of the outlet blade angles due to the welding compared to welds running parallel to the trailing edge as are needed according to the method known from prior art. Table of reference signs

1 Runner

2 Hub I Crown

3 Band

4 Blade

4.1 First part of a blade

4.2 Second part of a blade

5 Inner edge

6 Outer edge

7 Leading edge

8 Trailing edge

9 Pressure side surface

9.1 First area of the pressure side surface

9.2 Second area of the pressure side surface

9.3 Third area of the pressure side surface

10.1 First area of the suction side surface

10.2 Second area of the suction side surface

10.3 Third area of the suction side surface

10 Suction side surface

11 Runner ring