An apparatus for eliminating vortex and recovering energy in the draft tube of a Francis turbine

Abstract

The invention relates to a vortex elimination and energy recovery device for a Francis turbine tailwater pipe, wherein the front end of the volute is provided with a water inlet, the interior of the volute is provided with a cabin, a fixed shaft is provided in the cabin, the fixed shaft is respectively connected to a fixed guide vane and a movable guide vane, and the cabin is connected to a turbine blade; the rear end of the volute is connected to the tailwater pipe, the end of the tailwater pipe is provided with a water outlet, a straight cone section of the tailwater pipe is provided with a plurality of grooves, a compressed air barrel is provided in the middle section of the tailwater pipe, the compressed air barrel is a hollow cylindrical structure, a sealing piston is provided in the compressed air barrel, the sealing piston is connected to a spring provided in the compressed air barrel, one end of the spring is fixed to the sealing piston, and the other end of the spring is fixed to the bottom surface of the compressed air barrel; a first guide plate is connected to the pipe wall of the middle section of the tailwater pipe, the first guide plate is connected to the compressed air barrel, a second guide plate is fixed to the inner wall of the straight cone section of the tailwater pipe, an air guide hole is provided on the first guide plate, and the air guide hole is connected to the compressed air barrel; and a guide cover is connected to the end of the tailwater pipe.

Description

Technical Field

[0001] The invention relates to a device for eliminating vortex and recovering energy from a draft tube of a Francis turbine, and is applied in the technical field of turbine equipment. Background Art

[0002] As is well known, the draft tube is a critical energy recovery component in a turbine, and its performance directly determines the overall efficiency of the turbine unit. Typically, the flow within the draft tube is highly complex, with varying pressure gradients in different directions, accompanied by "horseshoe vortices" and "cutting vortices" in the wake and flow path. The flow exhibits strong vorticity, representing a highly complex three-dimensional, unsteady viscous fluid motion that is difficult to simulate and test using technical equipment. However, when the turbine is operating at low load, the draft tube typically experiences significant pressure fluctuations, which not only reduces turbine efficiency but also affects the stability of the unit's operation. To address this vortex in the water flow, researchers have studied the turbulent boundary layer on the grooved surface, specifically the drag-reducing properties and mechanisms of the grooved surface, thereby improving flow stability. Grooved drag reduction occurs when the secondary vortices induced by the grooves interact with the streamwise vortices, retaining the low-speed fluid within the grooves and reducing the efficiency of momentum exchange between the high- and low-speed fluids, thereby reducing drag. The grooved surface can suppress the sudden onset of coherent structures, shortening the timescales of their ejection and sweep, thereby achieving drag reduction. The grooved surface not only increases the laminar boundary layer area but also increases the Reynolds number at which laminar flow transitions to turbulent flow by approximately four times, thereby reducing the drag of the flat-plate boundary layer. Therefore, how to leverage the properties of the grooves to reduce the efficiency of momentum exchange between high- and low-speed fluids, implement energy recovery in the draft tube, and improve the power generation efficiency of the original turbine are all pressing issues.

[0003] Patent publication number CN109441691B discloses a Francis turbine with a draft tube and straightening plates. It specifically discloses the use of spaced long and short blades within the runner to improve the flow field within the runner. It also cleverly utilizes tapered blades evenly distributed along the circumferential inner wall of the draft tube to separate the circumferential velocity of the runner outflow and prevent the formation of vortices. The tapered blades also serve to guide and straighten the flow, effectively addressing the secondary effects of the draft tube flow caused by improving the runner flow field characteristics and significantly improving turbine performance. However, this invention does not address the aforementioned technical issues. Summary of the Invention

[0004] In order to solve the above technical problems, the present invention provides a vortex elimination and energy recovery device for the tail tube of a mixed flow turbine, which forms a reciprocating motion when the water pressure pulsation is large, drives the air generator to generate electricity, thereby achieving the effect of recovering electrical energy and improving the power generation efficiency of the original turbine.

[0005] The technical solutions of the present invention are as follows:

[0006] A device for eliminating vortexes and recovering energy in a draft tube of a Francis turbine comprises a volute, wherein a water inlet is formed at the front end of the volute, a nacelle is disposed within the volute, a fixed shaft is disposed within the nacelle, fixed guide vanes and movable guide vanes are respectively connected to the fixed shaft, and turbine blades are connected to the nacelle; a draft tube is connected to the rear end of the volute, a water outlet is formed at the end of the draft tube, a plurality of grooves are provided in the straight tapered section of the draft tube, a pressurized air cylinder is disposed in the middle section of the draft tube, the pressurized air cylinder is a hollow cylindrical structure, a sealing piston is disposed in the pressurized air cylinder, the sealing piston is connected to a spring disposed in the pressurized air cylinder, one end of the spring is fixed to the sealing piston, and the other end of the spring is fixed to the bottom surface of the pressurized air cylinder; a first guide plate is connected to the wall of the middle section of the draft tube, the first guide plate is connected to the pressurized air cylinder, a second guide plate is fixed to the inner wall of the straight tapered section of the draft tube, an air guide hole is formed in the first guide plate, and the air guide hole is connected to the pressurized air cylinder; and a deflector is connected to the end of the draft tube.

[0007] The turbine blades are axial flow blades.

[0008] The grooves are rectangular parallelepiped groove structures and are evenly distributed on the inner wall surface of the straight cone section of the tailwater pipe.

[0009] The relationship between the width a of the groove and the depth h of the groove is a=(1-2)*h, and the length of the groove is the same as the length of the straight cone section of the tailwater pipe.

[0010] The circumferential coverage ratio of the grooves is 50% of the entire circumference, and the angle between two adjacent grooves is 7.2°.

[0011] The first guide plate and the second guide plate are each three pieces. The first guide plate is evenly distributed on the middle section of the tailwater pipe wall, and the second guide plate is evenly distributed on the inner wall of the straight cone section of the tailwater pipe. The cross-section of the second guide plate is a trapezoidal structure with a circular arc side.

[0012] Fins are provided on the upper portion of the second guide plate.

[0013] The air guide hole is arranged in the middle of the clamping plate at the root of the first guide plate, and the air guide hole is used for air exchange between the compressed air cylinder and the air generator.

[0014] The deflector is a trumpet-shaped structure, and is fixed on the tailwater pipe through a first deflector plate.

[0015] The present invention has the following beneficial effects:

[0016] 1. The present invention provides a plurality of grooves in the straight tapered section of the draft tube. The secondary vortices induced by the grooves interact with the streamwise vortices, thereby reducing the efficiency of momentum exchange between high-speed and low-speed fluids, thereby reducing resistance. As a result, the grooves have a more significant inhibitory effect on the vortex band phenomenon in the draft tube during the operation of the Francis turbine.

[0017] 2. The present invention installs a compressed air power generation device in the turbine draft tube. When the water pressure pulsates greatly, it generates reciprocating motion, driving the air generator to generate electricity, thereby achieving the purpose of recovering electrical energy. By installing an axial flow energy recovery device with a deflector at the tail end of the draft tube, it can achieve the purpose of recovering the draft tube energy. At the same time, a second deflector with a trapezoidal cross-section is provided on the inner wall of the straight cone rear section of the draft tube, and a fin is provided on the inner side of the upper part of the second deflector. The small vortex belt can further influence the main vortex belt and stabilize the water flow. In addition, the device is equipped with a first deflector, which can not only fix the compressed air power generation device but also improve the flow state of the draft tube.

[0018] 3. The present invention has a simple structure, is easy to install, economical to maintain, and can be made of a wide range of materials. It can improve the stability and efficiency of hydropower units with different water heads and different working conditions.

[0019] 4. The present invention utilizes the high-amplitude pressure pulsation of the turbine tailwater pipe under low-load operation to drive the sealing piston to reciprocate in the compressed air barrel, pushing the gas into the air generator to generate electricity; it fully utilizes the energy lost in the tailwater pipe, improves the power generation efficiency of the turbine, and also plays a rectifying role while collecting the energy in the tailwater pipe.

[0020] 5. The present invention can improve the stability of the turbine tailwater pipe, enhance the operational stability of the entire hydroelectric generator set, extend the life of the unit, reduce the number of equipment repairs, reduce the maintenance costs of the hydropower system, and save costs; at the same time, the present invention improves the utilization rate of water, which has important engineering practical significance in today's increasingly scarce water resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the overall structure of the present invention;

[0022] Figure 2 Schematic diagram of the cross-sectional structure of the air guide cover in the present invention;

[0023] Figure 3 Schematic diagram of the structure of the sealing piston and spring in the present invention;

[0024] Figure 4 Schematic diagram of the structure of the first guide plate and the air guide hole in the present invention;

[0025] Figure 5 This is a pressure pulsation change curve diagram of the middle section of the tailwater pipe in the present invention.

[0026] The reference numerals in the figures are as follows:

[0027] 1. Volute; 2. Fixed guide vane; 3. Movable guide vane; 4. Turbine blade; 5. Groove; 6. Draft tube; 7. First guide plate; 8. Guide cover; 9. Engine room; 10. Fixed shaft; 11. Air guide hole; 12. Compressed air cylinder; 13. Sealing piston; 14. Water inlet; 15. Spring; 16. Water outlet; 17. Second guide plate. DETAILED DESCRIPTION

[0028] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] See also Figures 1 to 5 The device for eliminating vortex and recovering energy for the tailwater pipe of a Francis turbine comprises a volute 1, a water inlet 14 is provided at the front end of the volute 1, a cabin 9 is provided inside the volute 1, a fixed shaft 10 is provided in the cabin 9, a fixed guide vane 2 and a movable guide vane 3 are connected to the fixed shaft 10, and a turbine blade 4 is connected to the cabin 9; the rear end of the volute 1 is connected to the tailwater pipe 6, a water outlet 16 is provided at the end of the tailwater pipe 6, a plurality of grooves 5 are provided in the straight cone section of the tailwater pipe 6, a compressed air cylinder 12 is provided in the middle section of the tailwater pipe 6, and the compressed air cylinder 12 is a hollow cylindrical structure for storing air, and a sealing piston 13 is provided in the compressed air cylinder 12, and the sealing piston 1 3 is connected to a spring 15 provided in the air compressor 12. One end of the spring 15 is fixed to the sealing piston 13, and the other end of the spring 15 is fixed to the bottom surface of the air compressor 12. The sealing piston 13 moves back and forth in the air compressor 12 under the action of the spring 15. The stiffness of the spring 15 is proportional to the average value of the pressure pulsation of the draft tube 6. A first guide plate 7 is connected to the middle pipe wall of the draft tube 6, and the first guide plate 7 is connected to the air compressor 12. A second guide plate 17 is fixed to the inner wall of the straight cone section of the draft tube 6. The first guide plate 7 is formed with an air guide hole 11, and the air guide hole 11 is connected to the air compressor 12. The end of the draft tube 6 is connected to the deflector 8.

[0030] The turbine blades 4 are axial flow blades.

[0031] The grooves 5 are rectangular parallelepiped groove structures and are evenly distributed on the inner wall surface of the straight cone section of the tailwater pipe 6 .

[0032] The relationship between the width a of the groove 5 and its depth h is a = (1-2) * h. The length of the groove 5 is the same as the length of the straight tapered section of the draft tube 6. In the present invention, the width of the groove 5 is preferably set to 128 mm and the depth is set to 100 mm. Under low flow conditions, the draft tube 6 generates an eccentric vortex. The secondary vortex in the groove 5 interacts with the eccentric vortex, keeping the low-speed fluid within the groove 5, thereby achieving the effect of vortex elimination.

[0033] The circumferential coverage ratio of the grooves 5 is 50% of the entire circumference, and the angle between two adjacent grooves 5 is 7.2°.

[0034] The first guide plate 7 and the second guide plate 17 are each three pieces. The first guide plate 7 is evenly distributed on the middle section of the tailwater pipe 6, and plays the role of fixing the compressed air barrel 12 while rectifying the flow; the second guide plate 17 is evenly distributed on the inner wall of the rear end of the straight cone section of the tailwater pipe 6. The cross-section of the second guide plate 17 is a trapezoidal structure with an arc side. It is evenly distributed on the inner wall of the tailwater pipe 6 around the axis of the turbine, and the outer side is fixed inside the tailwater pipe 6. The thickness of the outer side is the angle swept by the inner side along the axis of the tailwater pipe 6, and the range covered is the thickness. The second guide plate 17 can improve the pressure pulsation of the water flow in the elbow section, and also improve the energy recovery coefficient of the tailwater pipe 6, thereby improving the operation stability on the basis of improving the operation efficiency of the turbine.

[0035] The vortexes within the tailwater tube 6 are primarily formed by the interaction between the main flow area driven by the runner and the dead water area within the tailwater tube 6, with the backflow of water in the dead water area forming vortices. Under low flow conditions, the number of vortices within the tailwater tube 6 increases dramatically, filling the entire tailwater tube 6. The vortexes caused by the backflow are more prominent in the middle and rear parts of the tailwater tube 6. Under low flow conditions, due to the reduced flow rate, the force exerted by the runner on the water flow increases, making the flow pattern within the tailwater tube 6 more turbulent.

[0036] The second guide plate 17 is provided with fins on its upper portion. After the fins are added, in addition to the existing vortex in the main flow area, an additional small vortex appears in the flow field, thereby affecting the main vortex and stabilizing the flow pattern in the tailwater pipe 6.

[0037] The air guide hole 11 is set in the middle of the clamping plate at the root of the first guide plate 7. The air guide hole 11 is used to exchange air between the compressed air cylinder 12 and the air generator. When the pressure is high, the gas is pressed into the air generator, and when the pressure is low, negative pressure is formed to suck in the air.

[0038] The deflector 8 is a trumpet-shaped structure, and is fixed to the tailwater pipe 6 via a first deflector plate 7. The deflector 8 collects water flow and improves power generation efficiency.

[0039] The present invention utilizes the large pressure pulsation changes at the tailwater pipe 6 of a low-load turbine. When the pressure is high, the sealing piston 13 is pushed to move, and the air in the air compressor 12 is pressed into the air generator through the air guide hole 11, so that it generates electricity; when the pressure in the tailwater pipe 6 is low, the spring 15 pushes the sealing piston 13 to move outward, so that negative pressure is formed in the air compressor 12, and the air is drawn back into the air compressor 12.

[0040] Furthermore, a fault monitoring device is installed in the draft tube 6, based on a decision tree algorithm using wavelet packet characteristic entropy. First, a pressure sensor in the draft tube 6 collects a measured pressure pulsation signal. This signal is then decomposed using wavelet packet decomposition to continuously decompose the high- and low-frequency components of the pulsation signal. This wavelet packet decomposition sequence is then generated, using information entropy as a criterion to define a partitioning measure. This constructs a characteristic entropy feature vector, which is then normalized. Finally, through learning and generating a decision tree, using the feature vector as input and the fault category vector as output, real-time monitoring of vortex belt faults in the Francis turbine draft tube 6 is achieved.

[0041] Furthermore, during the identification process of the tailwater pipe 6 fault monitoring device, vortex bands will cause abnormal pulsation, resulting in uneven energy distribution in the tailwater pipe 6. The wavelet packet characteristic entropy increases with the appearance of vortex bands in the tailwater pipe 6, and the two are positively correlated. The greater the characteristic entropy, the greater the fault occurrence rate. By learning and generating a decision tree, fault classification is performed on existing data, and fault pattern recognition of the decision tree can be realized, that is, different fault categories are output by different feature vector inputs, so as to judge whether vortex bands are generated in the tailwater pipe 6 and the severity of the vortex bands.

[0042] Working principle of the present invention:

[0043] By utilizing the larger pressure pulsation changes at the tailwater pipe 6 of the low-load turbine, when the pressure is high, the sealing piston 13 is pushed to move, and the air in the compressed air tank 12 is pressed into the air generator through the air guide hole 11, so that it generates electricity; when the pressure in the tailwater pipe 6 is low, the spring 15 pushes the sealing piston 13 to move outward, so that negative pressure is formed in the compressed air tank 12, and the air is drawn back into the compressed air tank 12.

[0044] The calculation formula of spring 15 pressure is:

[0045]

[0046] k is the spring stiffness, N / mm; F is the spring working load, N; f is the deformation of the spring under the working load (mm).

[0047] The value of F is the pressure acting on the sealing piston 13:

[0048] F=PS

[0049] P is the pressure pulsation intensity in the middle of the tailwater pipe 6 (Pa); S is the contact area of the sealing piston 13 (m 2 ).

[0050] After simulation research, it is recommended that the f value in actual use is half of the depth of the compressed air cylinder 12. This can realize the periodic reciprocating motion of the sealing piston 13.

[0051] By comparing the vortex bands before and after the installation of the grooves 5, simulation studies have shown that the vortex bands in the draft tube 6 are effectively eliminated after the grooves 5 are installed. Before the installation of the grooves 5, a single spiral vortex band with a strong spiral strength existed in the straight cone section of the draft tube 6, with a distinct eccentric vortex band. The vortex band became increasingly eccentric towards the lower end of the draft tube 6, and its shape was extremely unstable. After the installation of the grooves 5, the vortex bands were essentially eliminated, with only a very small amount remaining at the inlet of the draft tube 6. This demonstrates that the installation of the grooves 5 in the straight cone section of the draft tube 6 has a significant effect on improving the vortex bands in the draft tube 6.

[0052] Table 1 shows the monitoring and calculation of the turbine efficiency under low flow conditions. It can be seen from the table that the installation of groove 5 in the straight cone section has little effect on the unit efficiency, with the difference before and after being 0.62%. It can be considered that the installation of groove 5 has almost no effect on the turbine efficiency and will not affect the normal operation of the turbine.

[0053] Table 1 Efficiency of turbine before and after installation of groove 5 under low flow conditions

[0054]

[0055] Table 2 shows the maximum pressure pulsation amplitudes corresponding to various measuring points in the tailwater pipe 6 under low-flow conditions for the traditional tailwater pipe 6 and the scheme with the groove 5. The results show that after the installation of the groove 5, the pressure pulsation amplitude of the tailwater pipe 6 is significantly reduced, with the maximum pressure pulsation reduction ratio reaching 71.59%, indicating that the installation of the groove 5 has significantly improved the reduction of the overall pressure pulsation amplitude of the tailwater pipe 6.

[0056] Table 2 Pressure pulsation amplitude of the draft tube vortex belt under low flow conditions

[0057]

[0058] The tailwater pipe 6 adopts a fault diagnosis method based on a decision tree algorithm using wavelet packet characteristic entropy. The working steps are as follows:

[0059] (1) Using the pressure sensor in the tailwater pipe 6 to detect and obtain the measured pressure pulsation signal x(t);

[0060] (2) The measured pressure pulsation signal x(t) is decomposed by wavelet packets, that is, the signal is continuously decomposed into high-frequency and low-frequency signals h(t) and l(t). The decomposition formula is as follows:

[0061]

[0062] If the signal in this embodiment is decomposed into i layers, the wavelet packet decomposition sequence D is obtained. (i，j) ={j=0~2 i -1),

[0063] Then we get the partition measure N is the length of the original signal, D f(i，j) (k) is D (i，j) The kth value of the Fourier transform sequence of the sequence.

[0064] Then, the wavelet packet characteristic entropy is obtained by the basic information entropy theory, and processed to finally form the signal wavelet packet characteristic entropy vector M:

[0065]

[0066]

[0067] (3) Decision tree learning and generation, that is, the feature entropy extracted from the measured data under various environments is trained in the early stage, and then the feature vector is programmed according to the definition of the output of the existing fault category to generate a decision tree. The feature vector can be used as the input vector of the decision tree to perform vortex pattern recognition and complete fault monitoring.

[0068] In this embodiment, the characteristic entropy vector M is normalized to avoid increased difficulty in analysis and calculation due to high entropy.

[0069] In this embodiment, the selection of the optimal feature requires that the purity of the nodes in each branch be as high as possible during feature selection. The purity of the subsets after division is evaluated using a criterion based on information entropy.

[0070] In this embodiment, when a vortex appears in the draft tube 6, the pulsation in the tube increases and disrupts the original energy distribution characteristics in the draft tube 6. The larger the vortex, the greater the wavelet packet characteristic entropy, and the two are positively correlated. The possibility of a fault also increases, and the wavelet packet characteristic entropy changes accordingly.

[0071] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention's description and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A device for eliminating vortex and recovering energy in the draft tube of a Francis turbine, characterized by: The invention comprises a volute (1), wherein a water inlet (14) is provided at the front end of the volute (1), a cabin (9) is provided inside the volute (1), a fixed shaft (10) is provided inside the cabin (9), a fixed guide vane (2) and a movable guide vane (3) are respectively connected to the fixed shaft (10), and a turbine blade (4) is connected to the cabin (9); the rear end of the volute (1) is connected to a tailwater pipe (6), a water outlet (16) is provided at the end of the tailwater pipe (6), a straight cone section of the tailwater pipe (6) is provided with a plurality of grooves (5), a compressed air barrel (12) is provided in the middle section of the tailwater pipe (6), the compressed air barrel (12) is a hollow cylindrical structure, a sealing piston (13) is provided in the compressed air barrel (12), the sealing piston (13) is connected to a spring (15) provided in the compressed air barrel (12), one end of the spring (15) is fixed to the sealing piston (13), and the other end of the spring (15) is connected to the compressed air barrel (12). The bottom surface of the barrel (12) is fixed; a first guide plate (7) is connected to the middle pipe wall of the tailwater pipe (6), the first guide plate (7) is connected to the compressed air barrel (12), a second guide plate (17) is fixed to the inner wall of the straight cone section of the tailwater pipe (6), an air guide hole (11) is opened on the first guide plate (7), and the air guide hole (11) is connected to the compressed air barrel (12); the end of the tailwater pipe (6) is connected to the guide cover (8 ); the groove (5) is a rectangular parallelepiped groove structure, and the grooves (5) are evenly distributed on the inner wall surface of the straight cone section of the tailwater pipe (6); the relationship between the width a of the groove (5) and the depth h of the groove (5) is a=(1-2)h, and the length of the groove (5) is the same as the length of the straight cone section of the tailwater pipe (6); the circumferential ratio covered by the groove (5) is 50% of the entire circumference, and the angle between two adjacent grooves (5) is 7.2°.

2. The device for eliminating vortex and recovering energy in the draft tube of a Francis turbine according to claim 1, characterized in that: The turbine blades (4) are axial flow blades.

3. The device for eliminating vortex and recovering energy in the draft tube of a Francis turbine according to claim 1, characterized in that: The first guide plate (7) and the second guide plate (17) are each composed of three pieces. The first guide plate (7) is evenly distributed on the middle pipe wall of the tailwater pipe (6), and the second guide plate (17) is evenly distributed on the inner wall of the straight cone section of the tailwater pipe (6). The cross section of the second guide plate (17) is a trapezoidal structure with a circular arc side.

4. The device for eliminating vortex and recovering energy in the draft tube of a Francis turbine according to claim 3, characterized in that: Fins are provided on the upper portion of the second guide plate (17).

5. The device for eliminating vortex and recovering energy in the draft tube of a Francis turbine according to claim 1, characterized in that: The air guide hole (11) is arranged in the middle of the clamping plate at the root of the first guide plate (7), and the air guide hole (11) is used for air exchange between the compressed air cylinder (12) and the air generator.

6. The device for eliminating vortex and recovering energy in the draft tube of a Francis turbine according to claim 1, characterized in that: The deflector (8) is a trumpet-shaped structure, and the deflector (8) is fixed on the tailwater pipe (6) via a first deflector plate (7).

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