WO2008060158A2

WO2008060158A2 - Hydraulic reaction turbine and method for reducing pressure fluctuations

Info

Publication number: WO2008060158A2
Application number: PCT/NO2007/000078
Authority: WO
Inventors: Anders Wedmark
Original assignee: Andritz Technology And Asset Management Gmbh
Priority date: 2006-11-16
Filing date: 2007-02-27
Publication date: 2008-05-22
Also published as: CA2669946C; CA2669946A1; NO325509B1; NO20065276L; WO2008060158A3

Abstract

Hydraulic reaction turbine comprising, a runner, a draft tube (1) downstream of the runner, and - injection means (4,10) for introducing water into the draft tube in order to reduce pressure fluctuations therein, with one or more openings (10) in the draft tube wall (1a) adapted to substantially evenly distribute injected water in the axial and/or circumferential direction of the draft tube (1).

Description

HYDRAULIC REACTION TURBINE AND METHOD FOR REDUCING PRESSURE FLUCTUATIONS

In the draft tube downstream of the runner in Francis turbines and other hydraulic reaction turbines with fixed runner blades there have been problems in connection with vibrations and pulsations when operated at higher and/or at lower loads than the load for highest efficiency. Various methods are known for the purpose of reducing these pressure fluctuations; these being related to an undesired rotational component of the water in the draft tube when the unfavourable load conditions occur.

Mechanical means are known for reducing or eliminating the above problems. One example of a mechanical solution is found in US patent publication 2004/0037698, describing fixed stay vanes after the runner.

Other prior art solutions resort to air or gas injection for the same purpose, as for example described in JP-02238177 and US-2004/0265117.

Moreover, injection of water into the draft tube in order to reduce pressure fluctuations therein has been described in WO-2006/043824.

Mention may further be made of JP-61178563, where also introduction of water through the draft tube wall is shown. However, this is for suppressing exfoliation in a turbine having a low head (typically a Kaplan-type of turbine) in which there is not any problem with pressure pulsations of the kind contemplated here. Such problems occur regularly at certain load conditions in reaction turbines having a runner with fixed blades, whereas Kaplan-type turbines have adjustable pitch blades.

By injecting water, the unfavourable flow vortex formation that may occur, will be impeded or destroyed, thus making it possible to operate the turbines over a broader range of loads . More specifically the injection of water (under high pressure) will serve to eliminate the rotational or vortex component mentioned above, occurring in the water leaving the turbine runner. Thus, the downstream flow will be more close to axial, and the undesired pressure fluctuations will no longer represent a serious problem. It may, however, be considered to be a certain drawback that injection of water under pressure will require some energy, which may reduce the total efficiency of such turbines .

The present invention aims at improving the water injection apparatus and method in reaction turbines as mentioned above. Thus, the invention in a general aspect is directed to a hydraulic reaction turbine comprising a runner, a draft tube downstream of the runner, and injection means for introducing water into the draft tube in order to reduce pressure fluctuations therein. The novel and specific features according to this invention, in general relate to said injection means comprising one or more openings in the draft tube wall adapted to substantially evenly distribute injected water in the axial and/or circumferential direction of the draft tube. As to the distribution around the circumference of the draft tube, this may be all around the circumference or partially so, preferably covering at least one half of the circumference. A main point is to provide for an essentially continuous injection of water over an area or separate areas of the draft tube wall, in contrast to the concentrated jets of water from typical nozzles according to prior art. Thus, with this invention a substantial portion or volume of the main water flow having an undesired rotational component, will be favourably influenced by the injected water. Other specific features according to the invention and advantages obtained, will be understood from the following description. The invention will now be explained more in detail with reference to embodiments thereof, as illustrated in the accompanying drawings, where: Fig. 1 is a schematic overview illustration in axial cross-section, showing essential parts of a reaction turbine, including the runner and the draft tube as well as a water injection apparatus,

Fig. 2 is a simplified cross-section of the draft tube in Fig. 1 as indicated at I-I, with a slot-like water injection opening,

Fig. 2A shows the shape of the water injection opening as seen according to arrows A-A in Fig. 2, Fig. 3 in a similar cross-sectional view as Fig. 2, shows a second embodiment with a number of water injection openings distributed around the periphery of the draft tube, and

Fig. 3A shows a simplified vertical cross-section at one of the water injection openings in Fig. 3.

In Fig. 1 there is illustrated a conventional Francis turbine with a runner 100 and a draft tube 1, with an axis of rotation 100a, being also the central axis of draft tube 1 immediately downstream of the runner 100. In the wall of draft tube 1 there is schematically indicated an injection opening 10 for water to be introduced so as to reduce pressure fluctuations as discussed above. In the case of Fig. 1 the opening 10 has an extension deviating somewhat from the axial direction 100a of the draft tube 1, this being a preferred embodiment under some circumstances. The angle of inclination may be closer or larger in relation to axis 100a, than illustrated in Fig. 1, depending on, inter alia, the magnitude of rotational components to be expected in the main water flow from the runner 100. Fig. 2 being a cross-sectional view as indicated at I-I in Fig. 1, illustrates more in detail an embodiment where the injection apparatus comprises a water distribution chamber 4 external to the draft tube wall Ia and in communication with a slot or opening 10 in the wall. The slot-like injection opening 10 is here shown as extending parallel to the draft tube axis and like the one in Fig. 1 has a substantially elongated shape. Thus, according to the invention it is preferred that the longitudinal or axial dimension of opening 10 is a multiple of the lateral dimension thereof. More specifically, this dimensional ratio may correspond to much as one or two orders of magnitude, whereby the opening may take the form of a quite narrow, long slit in the draft tube wall Ia. In the embodiment of Fig. 3 a comparatively large number of slot-like openings 20 are provided and evenly distributed along the circumference of draft tube wall Ia. It will be understood that although complete coverage of the whole circumference as illustrated in Fig. 3, is favourable, there may be modifications where some portions of the circumference do not have such injection openings. This may be in order to ensure that the water has the proper angle relative to the axis of the draft tube when it enters the draft tube. It is to be noted that for obtaining the continuous or even distribution of injected water, as explained above, it is advantageous to have a total extension of openings 20 around the circumference of draft tube 1, being more than a small fraction of the draft tube diameter immediately downstream of the runner 100 (see Fig. 1). The same preferred relationship applies with respect to the axial length or extension of opening 10 in Figs. 1 and 2/2A. Water is supplied under pressure through a conduit 2 in fig. 3 and distributed around the draft tube by means of an external, closed channel 3. Thus, water injection will take place as indicated with two arrows 25, through all openings 20. The cross-sectional detail of Fig. 3A further shows how the water injection is provided for in this embodiment. From an annular chamber 5 formed by channel structure 3, each opening 20 at draft tube wall Ia has a diverging flow cross-sectional area towards the interior of draft tube 1. The same applies to opening 10 in Fig. 2. This diverging shape contributes to the even distribution discussed above, since the diverging or conical shape will have a favourable effect of spreading the injected water into a large volume of the main, vertical water flow from the runner 100 (Fig. 1) . Moreover, this spreading effect of the injected water will be obtained without any significant loss of energy.

In a modification of the embodiment in Fig. 3 the row^' of openings 20 may extend along an inclined or helical line instead of all openings being located in the same radial plane, thus providing for an extension as mentioned above, also in the axial direction.

Whereas some practical embodiments according to the invention have been described and schematically illustrated in the drawings, various modifications may be possible, such as in the number and shape of injection openings or the direction of water injection, depending on the requirements in individual hydraulic reaction turbines where pressure fluctuations in the draft tube may be a problem.

Claims

1. Hydraulic reaction turbine comprising a runner (100) , - a draft tube (1) downstream of the runner, and injection means (10,20) for introducing water into the draft tube in order to reduce pressure fluctuations therein, characterised in that said injection means comprise one or more openings (10,20) in the draft tube wall (Ia) adapted to substantially evenly distribute injected water in the axial and/or circumferential direction of the draft tube (1).

2. Turbine according to claim 1, wherein said one or more openings has/have together a total extension in the axial and/or circumferential direction of the draft tube, being larger than a small fraction of the draft tube diameter immediately downstream of the runner.

3. Turbine according to claim 1 or 2, wherein said extension is inclined in relation to the draft tube axis .

4. Turbine according to claim 1, 2 or 3, wherein at least one opening (10) has an elongate shape with a longitudinal dimension being a multiple of the lateral dimension the opening, the longitudinal dimension being preferably one or two orders of magnitude larger than said lateral dimension.

5. Turbine according to claim 1, 2, 3 or 4, wherein said plurality of openings (20) are distributed around a substantial part of the circumference of the draft tube (1), preferable around at least half the circumference.

6. Turbine according to any one of claims 1-5, wherein said opening (s) (10,20) has/have on outwardly diverging flow cross-sectional area.

7. Method for reducing pressure fluctuations in the draft tube downstream of the runner in hydraulic reaction turbines, by injecting water through the draft tube wall at a level spaced from the runner, characterized in that water is injected so as to be substantially evenly distributed in the axial and/or circumferential direction of the draft tube.