DESCRIPTION

TECHNICAL FIELD AND PRIOR ART

The invention relates to the field of the hydropower industry.

More specifically, the invention relates to Francis turbines, pump- turbines or other designs that have a leakage flow. The internal structure of a known turbine is illustrated in figure 1.

It comprises blades 2 (of which only one is represented on figure 1), which can rotate around an axis XX'. Each blade has a leading edge 8 and a trailing edge 10. Water flows from an upper reservoir (or high pressure region) 1, across the blades and exits into a discharge region (or low pressure region) 3 after the runner and then into a draft tube (not shown of figure 1). The blades are mounted on a runner crown 12 under a head cover 14.

Hydroturbines can have a leakage flow (represented by arrows I and II on figure 1 below the head cover 14) through labyrinth seals 13, 15 (which are between rotating and stationary parts), which prevent excessive flow from going around the turbine through gaps between rotating and stationary elements, such as the runner and the head cover, or the runner and the bottom ring. Said leakage flow then flows through any space between the seal 13 and the slots or holes through which the leakage flow exits; for example it flows through a cavity 16 (see figure 1) and:

- either exits to the main flow through balancing holes 18 (arrow I) in the crown 12, just below the intersection of the runner blade trailing edge 10 and the crown 12;

- or exits through the runner crown tip (arrow II).

This leakage flow impacts the mechanical stresses, the axial thrust, and the performance of the runner.

First, in all turbines with leakage flow, performance is impacted due to the quantity of leakage flow: volumetric losses are incurred when the volume of leakage flow increases, reducing the volume of water working to generate power ; in other words, more leakage flow means less water generating movement and less power output.

Second, hydrodynamic losses are also incurred because the leakage flow is returned to the main flow in such a way that it is misaligned with the main flow and disturbs the hydrodynamic flow through the turbine.

Furthermore the mechanical structure of the turbine can be affected by a high pressure in the leakage flow (in particular any space between the seal 13 and the slots or holes through which the leakage flow exits) between the runner crown 12 and the head cover 14, which causes increased forces on mechanical components such as the head cover 14 and thrust bearings. This can be due both to centrifugation of water and to the static pressure of the water in the leakage flow channel.

It is therefore a technical problem to find a new device and a new method to reduce the effects of high pressures of said leakage flow on the mechanical structure of the turbine and the disturbances of the hydrodynamic flow through the turbine.

It is also a technical problem to find a new device and a new method to guide the flow of leakage water, to improve the efficiency of the turbine.

SUMMARY OF THE INVENTION

The invention first concerns a hydraulic turbine comprising:

- blades fixed to a runner crown and to be actuated in rotation around an axis of rotation, each blade being comprised between a leading edge and a trailing edge,

- a stationary head cover and at least one passage for water, for example at least one chamber or channel, located between said runner crown and said head cover or within the head cover.

Said runner may further comprise:

- means forming at least one water passage, for example at least one hole, between said passage for water (for example: a chamber) and a chamber in the runner tip or in the lowest part of the runner crown; - an upper portion and a lower portion of the said runner crown, said upper portion having a larger diameter than said lower portion so as to define a channel between them, said channel leading to a discharge region below the runner.

The runner tip, also called the runner cone, forms or comprises the lowest part of the runner crown, below the attachment points of the blade trailing edge.

Said channel can be circular in shape and have a circular symmetry around the axis of rotation.

A turbine according to the present invention may comprise two portions. The first (upper) portion can resemble a standard runner and crown and extends to just below the location where the runner blades are joined to the crown. The second (lower) portion can be part of the runner crown and can be shaped with a discontinuity in diameter where the lower portion has a smaller diameter than the first (upper) portion; it may leave a channel between the upper and the lower portions, the leakage water flowing through or along this channel and returning to the main flow in the discharge region.

The leakage flow can be directed by the orientation of the channel between the upper and lower crown portions and is thus well oriented with the main flow; preferably it has the same flow direction as the main flow.

The invention reduces the axial thrust on the bearings and the mechanical forces on other elements above the runner crown. Further, the invention improves the efficiency of the turbine by improving the hydrodynamics in the runner water passages.

In a preferred embodiment, support elements join said upper portion and said lower portion of said runner crown. They hold the lower crown portion in place relative to the upper crown portion. These support elements also have a directional effect that provides rotation to the leakage flow to match the main flow of water. Preferably, they are shaped such that the leakage flow is best aligned with the main flow.

Preferably said support elements guide water flowing through said channel so that water exiting said channel has the same direction as water which has flowed through the blades. The crown tip, or the entire crown tip, comprising the lower crown portion, the support elements and part of the upper crown portion can be joined to the rest of the upper crown portion by welding, bolting or fastening by any other method.

Said at least one hole between said chamber and said runner tip is preferably cylindrical, and has an axis substantially parallel to a surface of said upper portion. But said at least one hole can also have other shapes and/or orientations relative to the surface of said upper portion.

In a preferred embodiment, there can be two positions along the rotation axis, any plane perpendicular to said axis and comprised between said two positions crossing the upper and/or the lower portions of the runner.

A hydraulic turbine according to the invention may comprise a central pipe extending from a hole in the runner shaft or from air holes located in the head cover to the tip of the crown.

The invention also concerns a method of operating a hydraulic turbine according to the invention.

In particular such method may comprise:

- flowing water through said blades, to rotate said blades around said axis of rotation;

- while flowing leakage water through said chamber, between said runner crown and said head cover, then through said at least one hole, said chamber in the runner tip and said channel, said leakage water exiting said channel and flowing into said discharge region below the runner.

Said leakage water exits said channel and flows into said discharge region with approximately the same direction as the main flow of water.

BRIEF DESCRIPTION OF THE DRAWINGS

- Figure 1 shows an internal structure of a known turbine;

- Figures 2A and 2B show an internal structure of a turbine according to the invention and a detailed view of the evacuation channel;

- Figure 3 shows a top view of a turbine according to the invention; - Figures 4A and 4B are perspective partial views of runners according to particular embodiments of the invention ;

- Figure 5 shows the variation of pressure with radius and with centrifugation.

DETAILLED DESCRIPTION OF SPECIFIC EMBODIMENTS

An example of an internal structure of a turbine according to the invention is illustrated on figure 2A, where the same reference numbers as on figure 1 designate the same technical elements or features.

The runner crown 12, which is attached to shaft 4 and can rotate around axis XX', has a lateral runner flange 20 in which an exit hole (or conduit or channel) 22 establishes a communication between chamber 16 (between runner crown 12 and head cover 14 or within head cover 14) and a chamber 28 inside the runner tip, which is located below the attachment points of the blade trailing edge 10 (the runner tip is also called the runner cone and forms or comprises the lowest part (along axis XX') of the runner crown); alternatively chamber 16 can be within head cover 14 and be opened, for example through a hole like exit hole 20, to chamber 28. Two examples with different chambers 16 are illustrated on figures 4A and 4B.

More generally, means forming at least one passage, for example an exit hole 22 or channel of conduit, establish a hydraulic communication between:

- any passage or space, for example like chamber 16 or a channel, located between runner crown 12 and head cover 14 or located within head cover 14,

- and said chamber 28.

Arrows III indicate the direction of the leakage flow that flows through chamber 16 and below the head cover 14, then through hole 22 and the chamber 28.

The runner crown has an upper portion 12i to which the runner flange 20 is connected and a lower portion 12 ₂ . which comprises the runner tip. The lower end 12i _e of the upper portion 12i is located at a lower level (with reference to the vertical axis XX') than the upper end 12 _2e of the lower portion 12 ₂ . so that the parts of said upper and lower portions face each other; a distance d is maintained between the lower part of the upper portion 12i and the upper part of the lower portion 122, so as to form a gap or a channel 24 between said upper portion 12i and said lower portion 12 ₂ . Said channel is circular in shape and has a circular symmetry around the axis XX'.

A detailed illustration of this channel 24 and of the relative position of the upper and lower portions 12i and 12 ₂ can be seen on figure 2B.

The lower portion 12 ₂ , which has an upper end 12 _2e, has a side 12 _2i turned toward the axis XX' of rotation of the runner and a side 12 ₂₂ opposite to said axis XX' and turned to the blades 2.

The upper portion 12i , which has an lower end 12i _e , has a side 12n turned toward the axis XX' of rotation of the runner and partly to lower portion 12 ₂ and a side 12i ₂ opposite to said axis XX' and facing the blades 2.

The channel 24 defined between the upper portion 12i and the lower portion 12 ₂ extends between side 12 ₂₂ of the lower portion 12 ₂ and side 12n of the upper portion 122.

The surfaces of the different sides 12 _2i, 12 _22, 12n, 12i ₂ are substantially parallel to each other: channel 24 thus guides water in a direction substantially identical to the direction of water which exits from the blades to the discharge region 3 (which is also a low pressure region).

In any plane AA' (figure 2B) perpendicular to the rotation axis XX' of the runner and located between the lower end 12i _e of the upper portion and the upper end 12 _2e of the lower portion, the distance di between axis XX' and the side 12 ₂₂ of the lower portion 12 ₂ is smaller than the distance d ₂ between axis XX' and side 12n of the upper portion 12i.

The hole 22 which connects said chamber 16 and said runner tip is preferably cylindrical with a cylindrical axis which can be substantially parallel to the surface 12i2 of the upper portion 12i of the runner crown; a cylindrical hole offers the advantage of easier machining.

The upper portion 12i and the lower portion 12 ₂ of the runner crown are maintained at distance d (width of channel 24) from each other by support structures 26 (or ribs or stiffeners) used both to mechanically join the upper and lower portions and to direct the leakage flow so that it has a direction at least partially aligned with the main flow of water; since the ribs are in rotation like the runner crown, they also have a rotational effect on the leakage flow; they can be hydro-dynamically shaped and oriented to the flow.

Only one hole 22 and one rib 26 are represented on figures 2A and 2B. But the device, including said channel 24, has a circular symmetry around axis XX' and a plurality of such holes, preferably regularly spaced from each other, and a plurality of ribs are located around axis XX': figure 3 is a top view of part of the runner and of the upper 12i and of the lower 122 portions of the runner crown; several holes 22i-22 _d are also represented on this figure (all having the technical function of hole 22 on figure 2A), as well as several support structures 26i-26 _n (which, on the top view, are located under the lower portion 122).

In this example, the support structures 26 are essentially straight metal sections but can have any other direction and/or orientation and/or curvature to orient the flow of water exiting channel 24 in a direction as close as possible to that of the main water flow at a specific operating point (which flows through the blades 2 and then to the low pressure region 3).

Leakage water flows through chamber 16, then exits this chamber through hole 22 and enters chamber 28 and leaves chamber 28 through channel 24. Water is forced to leave the chamber due to the pressure difference between the inlet upstream section (in chamber 16) and the discharge region 3 below the runner. In cavity 28, water is also subject to centrifugal force and thus forced against wall 12n, from which it can exit through channel 24.

As illustrated on figures 2A and 2B, there can be a junction (obtained by welding, bolting or fastening by any other method) between:

- the crown tip, or the entire crown tip, comprising the lower crown portion 122, the support element(s) 26 and part of the upper crown portion 12i;

- and the rest of the upper crown portion 12i. In other words, an assembly is made comprising the crown tip (including the entire lower portion), the support structure (s) 26 and part of the upper crown. This assembly can then be fastened to the upper crown portion.

A perspective partial view of a runner according to a particular embodiment of the invention is shown on figure 4A. Chamber 16 is located between labyrinth seal 13 and an intermediate labyrinth seal 17 (which is a seal at an intermediate radius between the stationary head cover and the rotating upper crown).

Another perspective partial view of a runner according to another particular embodiment of the invention is shown on figure 4B. Chamber 16 is located above the upper crown, including passages in the head cover, between the intermediate labyrinth seal 17 and the central shaft.

In both examples of figures 4A and 4B reference 22 designates a conduit or a channel establishing communication between said chamber 16 and chamber 28.

Unlike the prior art structures (in which the leakage flow exits the crown at an angle to the main flow (see figure 1, arrow I), disrupting the hydrodynamics of the runner and increasing losses) the leakage flow in a runner according to the invention exits the runner crown and is already quite well aligned with the main flow, thus reducing losses. In this invention, the exit of passages 24 are at a radius larger than the radius of the runner tip and similar to the radius of the prior art structures (see Figure 1, arrow II). This feature reduces the pressure in chambers 28 and 16, reducing mechanical stresses and axial thrust on the runner.

In addition, the chamber 28 inside the runner tip has a low pressure very close to the pressure in the discharge region 3 below the runner. Below the runner, the pressure is close to constant. If the holes through which the leakage flow exits to the discharge region 3 are at a small radius (close to axis XX'), the pressure at this location will be the pressure below the runner. As the radius increases, centrifugation will increase the pressure. If the holes are at a larger radius, the pressure will be the pressure below the runner at this larger radius. Centrifugation still increases the pressure, but the pressure will be lower than if the leakage flow exits the runner at a smaller radius. This is illustrated on figure 5, which shows the dependence of the pressure on the radius, both for a discharge flow released at runner tip (curve A) and for a discharge flow released from the channel 24 according to the invention (curve B). The pressure below the runner is essentially constant, independent of radius. The pressure from the starting point (small radius ri for runner tip, intermediate radius r2 for the channel 24) is the same. Centrifugation causes an increase in pressure. Since the starting pressure at the larger radius r2 is the same as the starting pressure at the smaller radius ri, the pressure above the runner with the smaller radius leakage flow exit (curve A) is higher than the pressure above the runner with the larger radius leakage flow exit (curve B).

In other words, centrifugation causes the pressure to increase with radius, but since the radius of the exit of channel 24 is larger than the radius of the runner tip, the pressure in chamber 16 between the head cover 14 and the runner crown 12 is reduced with respect to the configuration of the prior art in which the leakage flow is released through the runner tip; this, in turn, reduces the load on mechanical components, increasing lifetime of the turbine and other components and allowing the use of smaller, less expensive components. In the case of turbine rehabilitations, the lower pressures can result in reuse of an existing component rather than replacement. An additional central pipe 30 can be added extending from the hole in the runner shaft to the crown tip such that air can be injected into the flow as required without impact on the leakage flow.