Joint structure of tail guide combination section of hydropower station tail water system and modeling method

Abstract

The invention discloses a hydropower station tail water system tail guide combination section connecting structure and a modeling method, and the section shape of the tail guide combination section connecting structure is a circle, a first city gate opening type and a second city gate opening type in sequence from a tail water tunnel to a diversion tunnel; the circular section is connected with the first city gate opening type section through a first gradual transition section; the first door opening type section and the second door opening type section are connected through a second transition section; and the projection of the second transition section on the bottom surface is arc-shaped. According to the invention, a circular-gate opening type-gate opening type arrangement form is adopted, smooth transition is realized through the gradual transition section, sudden change of water flow caused by a square section in the prior art is avoided, the turbulence intensity can be effectively reduced, the pressure fluctuation is reduced, and the water flow efficiency is optimized. According to the modeling method, a model is accurately constructed through parameterization design, then grid division is carried out, structural design is optimized through numerical simulation analysis, and reliable guidance is provided for engineering practice.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of water conservancy and hydropower engineering, and particularly relates to a tail guide joint segment connection structure of a tail water system of a hydropower station and a modeling method. BACKGROUND

[0002] In the construction of a hydropower station, the arrangement form of a diversion tunnel converted into a tail water tunnel can realize compact arrangement of an underground hub and has practicability and economy, such as Xiluodu Hydropower Station and Wudongde Hydropower Station. When the downstream water level of a tail guide combined hydropower station is low or close to the top of the diversion tunnel, a clear-full flow alternation phenomenon will appear in the tail water system, resulting in pressure fluctuation, water flow turbulence and other problems, and the hydraulic conditions are complex, which seriously affects the stable operation of the hydropower station. Different tail guide combined types will have different effects on the transition process characteristics, and the determination of a reasonable tail guide combined type has important engineering significance.

[0003] In most hydropower station constructions, the joint between the tail water tunnel and the diversion tunnel is smoothly transitioned, and the end of the diversion tunnel does not have a protruding blocking end. However, for the tail water system with a protruding blocking end at the end of the diversion tunnel, due to the blind end cavity caused by the blocking end, it is easy to cause turbulence of water flow at the joint between the tail water tunnel and the diversion tunnel, and large pressure fluctuation, which affects the efficiency of water flow and the stability of the structure. At the same time, it may exacerbate the clear-full flow alternation phenomenon in the unsteady flow transition process, increase the risk of closed air mass and water flow interruption, and bring hidden dangers to the safe and stable operation of the hydropower station.

[0004] Therefore, for the tail water system with a protruding blocking end at the end of the diversion tunnel, the connection structure of the tail guide joint segment needs to be optimized and designed, and a tail guide joint segment connection structure and a corresponding modeling method are needed, which can optimize the transition process, reduce water flow disturbance, and improve the stability of the structure. SUMMARY

[0005] The application aims to provide a tail guide joint segment connection structure of a tail water system of a hydropower station and a modeling method, which solves the problems of complex water flow pattern, large pressure fluctuation, and easy clear-full flow alternation in the prior art, and improves the operation safety and efficiency of the tail water system.

[0006] To solve the above technical problems, the application realizes the following technical scheme: A tail guide joint segment connection structure of a tail water system of a hydropower station, the tail water system of the hydropower station comprising a tail water tunnel and a diversion tunnel, and an included angle existing between the center lines of the tail water tunnel and the diversion tunnel; wherein the end face of the tail water tunnel is circular; and the end face of the diversion tunnel is overall in the shape of a city gate, the upper part is arched, and the lower part is rectangular.

[0007] The cross-sectional shape of the tail guide joint section connection structure from the tail water tunnel to the diversion tunnel is circular cross-section, first city gate tunnel type cross-section and second city gate tunnel type cross-section in turn; the circular cross-section and the first city gate tunnel type cross-section are connected through a first gradual transition section to realize smooth transition from the circular cross-section to the city gate tunnel type cross-section; the first city gate tunnel type cross-section and the second city gate tunnel type cross-section are connected through a second transition section; wherein the projection of the second transition section on the bottom surface is arc-shaped.

[0008] Further optimization, the side length of the lower rectangle of the first city gate tunnel type cross-section is smaller than the diameter corresponding to the circular cross-section.

[0009] The corresponding sizes of the first city gate tunnel type cross-section and the second city gate tunnel type cross-section are the same, that is, the second transition section is a continuous extension of the first city gate cross-section; or, the sizes of the first city gate tunnel type cross-section and the second city gate tunnel type cross-section are different, that is, the second transition section is a gradual transition section.

[0010] Further optimization, the top of the first gradual transition section and / or the second transition section is provided with a vent hole, and the arrangement position of the vent hole is determined through three-dimensional CFD simulation to ensure that there is sufficient ventilation space above the free flow section during the transition process.

[0011] The modeling method of the tail guide joint section connection structure of the tail water system of the above hydropower station, comprising the following steps: Step S1: according to the design drawing of the tail water system of the hydropower station, determining the modeling range as the outlet of the unit tail water pipe to the outlet of the diversion tunnel; establishing a horizontal reference line and a vertical reference line for positioning the spatial coordinates of each cross-section; setting a modeling angle reference line to control the transition angle of the circular, first city gate tunnel type and second city gate tunnel type cross-sections, and ensure the consistency of the axis.

[0012] Step S2: determine the parameters of the circular cross-section, the first city gate tunnel type and the second city gate tunnel type cross-section, and then perform guide line arrangement and lofting operation.

[0013] Using parametric modeling software, the shapes of the circular cross-section, the first city gate tunnel type cross-section and the second city gate tunnel type cross-section are established respectively, and then the circular cross-section and the first city gate tunnel type cross-section are connected through the top arch-shaped guide line and the side guide line, and the lofting modeling is performed, so as to generate the first gradual transition section from the circular cross-section to the first city gate tunnel type cross-section, realize the smooth transition between them, and also generate the second transition section through lofting modeling.

[0014] Step S3: combine the first gradual transition section, the second transition section and the diversion tunnel model into an integral structure through stretching and Boolean sum operation; Step S4: according to the actual engineering requirement, the fillet modeling processing is carried out at the tail guide joint to optimize the smoothness of water flow connection.

[0015] Step S5: meshing the built model, adopting hexahedron and polyhedron combined mesh form, and locally encrypting key areas such as gradual transition section and tail guide joint to improve calculation precision and ensure accuracy of subsequent simulation analysis.

[0016] Step S6: simulating water flow state and pressure fluctuation under different working conditions by adopting VOF multiphase flow model and Reliable k-ε turbulence model to simulate and analyze and optimize the design of the connecting structure.

[0017] Further optimization, the parameters to be determined in the step S2 include the diameter of the circular section, the lower end rectangular size and the vault radius of the first gate hole type section, the lower end rectangular size and the vault radius of the second gate hole type section, the distance between the circular section and the first gate hole type, and the included angle and the radius of the corresponding arc between the first gate hole type section and the second gate hole type section.

[0018] Further optimization, the midpoint of the upper arch of the first gate hole type section is denoted as point A, the two ends of the arch and the upper end two vertices of the lower rectangle are respectively coincided and denoted as points B and D, the midpoint of the lower rectangle bottom edge is denoted as point C, and the four angle points of the circular section are divided into four points A', B', C' and D' in clockwise direction from the upper vertex, points B and B' are located on the same side, points D and D' are located on the other side, and the lower vertex C' of the circular section and the midpoint C of the lower rectangle bottom edge of the first gate hole type section are located on the same horizontal plane.

[0019] The four guide lines of the first gradual transition section are straight line segments AA', BB', DD' and CC'.

[0020] The guide lines of the second transition section include the arc line segment formed by connecting the corresponding midpoints of the upper arches of the first gate hole type section and the second gate hole type section, and the four arc line segments formed by connecting the four corresponding vertices of the lower rectangle.

[0021] Further optimization, the simulation working conditions in the step S6 include the unit load shedding working condition under the upstream design flood level and the unit load increasing working condition under the downstream design flood level, and the design of the connecting structure is optimized according to the simulation results to obtain better hydraulic performance.

[0022] Compared with the prior art, the present application has the following beneficial effects: 1. Improve the water flow characteristics: the present application is aimed at the tail water system with a convex blocking end at the end of the diversion tunnel, adopts the arrangement form of circular-gate hole type-gate hole type, realizes smooth transition through the gradual transition section, avoids water flow mutation, can effectively reduce the turbulence intensity, reduce the pressure fluctuation, and optimize the water flow efficiency.

[0023] 2. Improve structural stability: Smooth transition reduces the impact of water flow on the structure, reduces the risk of damage to the structure due to water flow disturbance, and improves the stability and safety of the tail guide joint section structure.

[0024] 3. Reduce the risk of clear full flow alternation: reasonable structure design combined with the setting of air holes can effectively control the clear full flow alternation phenomenon, avoid the generation of closed air mass and water flow interruption, and ensure the safe and stable operation of the hydropower station.

[0025] 4. Accurate and efficient modeling: the modeling method adopted accurately constructs the model through parameterized design, then performs mesh division, and then optimizes the structure design through numerical simulation analysis, providing reliable guidance for engineering practice. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is the tail guide structure in Example 1; Figure 2 is the overall model diagram of the tail water system of Cixia Gorge; Figure 3 is the tail guide structure in Comparative Example 1; Figure 4 is a schematic diagram of 9 monitoring points provided at the tail guide joint part and the top of the diversion tunnel in Example 1; Figure 5 is the water-gas two-phase distribution of the tail water system of CT2 type 1 under the working condition; Figure 6 is Figure 4 is an enlarged view of the tail guide joint section; Figure 7 is the water-gas two-phase distribution of the tail water system of CT2 type 2 under the working condition; Figure 8 is Figure 6 is an enlarged view of the tail guide joint section; Figure 9 is the pressure fluctuation diagram of monitoring point up5 at the top of the tail guide joint under the working condition CT2; Figure 10 is the pressure fluctuation diagram of monitoring point up6 at the top of the tail guide joint under the working condition CT2; Figure 11 is the pressure fluctuation diagram of monitoring point down7 at the bottom of the diversion tunnel under the working condition CT2; Figure 12 is the pressure fluctuation diagram of monitoring point down8 at the bottom of the diversion tunnel under the working condition CT2; Figure 13 is the water-gas two-phase distribution during the transition process of the tail guide joint type 1 under the working condition CT2; Figure 14 is the water-gas two-phase distribution during the transition process of the tail guide joint type 2 under the working condition CT2; Figure 15The velocity vector diagram in the transition process of the tail guide arrangement type 1 for the working condition CT2; Figure 16 The velocity vector diagram in the transition process of the tail guide arrangement type 2 for the working condition CT2; Figure 17 The boundary flow change process of the working condition CT8; Figure 18 The water-gas two-phase volume fraction distribution of the tail water system type 1 for the working condition CT8; Figure 19 The tail guide combination section is enlarged; Figure 17 The tail guide combination section is enlarged; Figure 20 The water-gas two-phase volume fraction distribution of the tail water system type 2 for the working condition CT8; Figure 21 The tail guide combination section is enlarged; Figure 19 The tail guide combination section is enlarged; Figure 22 The water-gas two-phase distribution in the transition process of the tail guide combination type 1 for the working condition CT8; Figure 23 The water-gas two-phase distribution in the transition process of the tail guide combination type 2 for the working condition CT8; Figure 24 The velocity vector diagram in the transition process of the tail guide combination type 1 for the working condition CT8; Figure 25 The velocity vector diagram in the transition process of the 1#-4# air hole for the working condition CT8. DETAILED DESCRIPTION

[0027] The specific embodiments of the present application are described below with reference to the accompanying drawings, so that those skilled in the art can understand the present application. Obviously, the present application is not limited in the scope of the specific embodiments. For those skilled in the art, as long as various changes are within the spirit and scope of the present application defined and determined by the appended claims, these changes are non-creative labor, and all the inventions utilizing the inventive concept of the present application are within the scope of protection.

[0028] Taking the 3# tail water system of Cihaxia Hydropower Station on the Yellow River as an example, the following is described.

[0029] Embodiment 1: The tail guide combination section connection structure of the tail water system of the hydropower station, from the tail water tunnel to the diversion tunnel, the cross-sectional shape of the tail guide combination section connection structure is circular cross-section, first city gate type cross-section and second city gate type cross-section in turn; the circular cross-section and the first city gate type cross-section are connected through the first gradual transition section to realize the smooth transition of the circular cross-section to the city gate type cross-section; the first city gate type cross-section and the second city gate type cross-section are connected through the second transition section; wherein, the projection of the second transition section on the bottom surface is arc-shaped. The air hole is arranged at the top of the first gradual transition section and / or the second transition section.

[0030] In the present embodiment, the edge length of the lower rectangle of the first gate-shaped section is smaller than the diameter corresponding to the circular section; the corresponding dimensions of the first gate-shaped section and the second gate-shaped section are the same, i.e., the second transition section is a continuous extension of the first gate-shaped section.

[0031] In other embodiments, the dimensions of the first gate-shaped section and the second gate-shaped section can be different, i.e., the second transition section is a gradual transition section.

[0032] In the present embodiment, the modeling method of the tail guide junction structure of the tailrace system of the hydropower station is as follows: 1. Structure size determination: the diameter of the circular section 11 is 14.5 m, which matches the diameter of the tailrace tunnel. The dimensions of the first gate-shaped section 12 and the second gate-shaped section 13 are the same, including the lower rectangle and the upper vault, wherein the size of the lower rectangle is 13.5 m x 13.5 m, and the radius R of the vault is 7.79 m. The tailrace system is provided with 4 air vents.

[0033] 2. Modeling process: as shown in Figure 1 , a parametric model is established using SolidWorks, the center coordinates of the circular section are defined, and the vault elevation of the gate-shaped section is 2769.0 m. A guide line is added, and the first gradual transition section 2 is generated by the lofting function, as shown in Figure 1 (a). Ensure that the area change rate of the section is ≤1.5% / m. Similarly, the second transition section 3 is generated between the first gate-shaped section 12 and the second gate-shaped section 13, as shown in Figure 1 (b).

[0034] Then, the second transition section 3 is subjected to Boolean sum with the plugging end model 4 protruding from the end of the diversion tunnel, as shown in Figure 1 (c); and the diversion tunnel is stretched forward to form a supplemental length section 5, as shown in Figure 1 (d); finally, the first gradual transition section, the second transition section, and the diversion tunnel are combined into an overall structure by the above operations, so that the tailrace tunnel 1 and the diversion tunnel 6 section are combined together, and the overall structure is as shown in Figure 1 (e). Finally, according to the actual engineering requirements, the tail guide junction is subjected to round corner modeling processing to optimize the smoothness of the water flow connection.

[0035] The Fluent meshing is used to divide the constructed tailrace system model into unstructured grids, the grid size of the transition section is ≤1.5 m, the total number of grid elements is 2.25 million, and the number of grid nodes is about 4.73 million.

[0036] Figure 2 is a diagram of the overall model of the tailrace system of Cixia Gorge, wherein Figure 2The modules in the dotted boxes in (a)-(d) are the downstream surge chamber, vent, tail guide joint connection structure and the local model of the downstream tailwater. Figure 2 The dotted box (c) is Figure 1 The structure shown in (e).

[0037] Comparative Example 1: The tailwater system of a hydropower station features a tailwater junction structure with circular, square, and gate-shaped cross-sections, from the tailwater tunnel to the diversion tunnel. The circular and square sections are connected by a first gradual transition section, while the square and gate-shaped sections are connected by a second gradual transition section. Ventilation holes are provided at the tops of the first and second gradual transition sections.

[0038] In this comparative example, the circular section has a diameter of 14.5m, the square section measures 14.5m x 14.5m, and the first gradual transition section is 10m long. The gate-shaped section measures 13.5m x 13.5m, with a dome radius of 7.79m and a second gradual transition section of 18m. The top arch curve uses a quadratic parabola to provide a smooth transition to the gate-shaped section. The tailwater system is equipped with four air vents.

[0039] The difference from Example 1 is that the transition section after the circular section is different. In Comparative Example 1, the square section is connected to the gate-type diversion tunnel, while in Example 1, the gate-type section is connected to the gate-type diversion tunnel, without the middle square section. The other parts are the same in structure. Figure 3 shown.

[0040] CFD simulation verification was performed on the corresponding structures in Example 1 and Comparative Example 1: A total of 9 monitoring points, up1 to up9, are set at the tail conductor joint part and the top of the diversion tunnel of the hydropower station constructed in Example 1 and Comparative Example 1, respectively. Figure 4 As shown, nine common monitoring points (not labeled in the figure) are set at the cave bottom, corresponding to the cave top monitoring points, from down1 to down9. This means that the corresponding models in Example 1 and Comparative Example 1 each have a total of 18 monitoring points, and the corresponding positions of the monitoring points in Example 1 and Comparative Example 1 are identical, for monitoring and analyzing tailwater system pressure fluctuations. Corresponding monitoring sections, plane1 to plane6, are set at points up4 to up9 to monitor the flow rate of the water flow section.

[0041] The tail conductor connection section of the hydropower station constructed in Example 1 and Comparative Example 1 was subjected to CFD simulation verification under the two operating conditions of CT2 and CT8, respectively. The specific parameters of the two operating conditions are shown in Table 1.

[0042] For the convenience of description, the circular-square-arch type cross section in Comparative Example 1 is referred to as Type 1, and the circular-arch type cross section-arch type cross section in Example 1 is referred to as Type 2.

[0043] Table 1: Checking working conditions of large fluctuation transition process Calculation condition Upstream water level (m) Plant water level (m) Load change Water level combination and load change description CT2 2991 2768.69 2→0 Upstream design flood level, all units in the same hydraulic unit run at the maximum output power under the corresponding water head, while shedding load, and the guide vane is closed urgently CT8 2990 2767.45 0→2 Downstream design flood level, two units in the same hydraulic unit, from no load to load increase to the maximum power under the corresponding water head I. For CT2 working condition: Considering that the flow condition of the tail guide joint section is complex when the downstream is the checking flood level, the working condition CT2 is selected for the hydraulic transition process analysis to analyze and demonstrate the advantages and disadvantages of the hydraulic characteristics of the two arrangement types. The upstream of the working condition CT2 is the design flood level, the downstream is the checking flood level, all the units in the same hydraulic unit are operated at the maximum output power under the corresponding water head, and the load is shed at the same time, and the guide vane is closed urgently. According to the one-dimensional calculation method, the flow rate at the outlet of the unit tail water pipe is 373.61 m 3 / s, the tail water level is 2768.69 m, the bottom plate elevation is 2751 m, and the water depth is 17.59 m.

[0044] The inlet boundary of the tail water system is set as a mass flow inlet, the initial mass flow size is the steady flow working condition, the inlet flow rate change process of the transition process working condition is obtained by the one-dimensional calculation method, and the UDF programming is used for setting. The downstream water level is set as the initial water level of the calculation domain, and the steady flow calculation is first performed, and then the transition process calculation is performed by taking the initial condition.

[0045] The outlet of the downstream tail water hole is opened to the Openchannel algorithm model, which is set as a pressure outlet under the open channel boundary condition, and the water body is below the 2768.69 m elevation, and the gas area is above the elevation.

[0046] The top of the downstream surge tank, the tail water gate well, and the top of the downstream pool are directly in contact with the atmosphere, in order to meet the smooth air exhaust and air intake in the system when the water level fluctuates, the top surface of each working well is set as a pressure outlet boundary condition, and the pressure outlet value is 0 Pa. The gas-liquid two-phase flow VOF algorithm is adopted, the turbulence model adopts the Reliable k-ε model, and the body of influence gravity is selected for solving. The solid wall boundary is set as a non-slip wall surface, and the standard wall surface function method is used for processing the near wall surface. The boundary condition settings of the two tail guide arrangement types are the same.

[0047] 1) Steady flow working condition calculation and analysis: After numerical simulation calculation, the inlet and outlet flux conservation is monitored to obtain the stable water flow state in the water conveyance tunnel. The water-gas two-phase volume fraction distribution of the tail water system of Type 1 under the working condition CT2 is shown in Figure 5 and Figure 6 The water-gas two-phase volume fraction distribution of the tail water system of Type 2 under the working condition CT2 is shown in Figure 7 and Figure 8As shown, red represents water, blue represents gas, and the rainbow segment in the middle is the water-gas transition area.

[0048] The steady flow calculation results show that, due to the existence of the contraction gradient section, the clear full flow of type 1 mainly occurs in the diversion tunnel, the 1# to 3# vents are submerged in water, and a small air mass exists in the 4# vent. The clear full flow of type 2 occurs at the junction of the tailwater guide and the guide, the 1# to 3# vents are submerged in water, and some air mass exists near the 4# vent. At the same time, some gas remains in the blind-end cavity at the end of the diversion tunnel. The clear full flow phenomenon mainly occurs near the tailwater outlet gate well.

[0049] 2) Transition process calculation and analysis: 2.1) Comparison of pressure fluctuations at the tail guide junction: Under working condition CT2, monitoring points are set at the top and bottom of the tail guide joint section to monitor the pressure fluctuation of the corresponding section during the transition process. The pressure fluctuation at the top of the tail guide joint section is as follows: Figure 9 and Figure 10 As shown, Figure 9 This is the pressure fluctuation diagram corresponding to the monitoring point up5 at the top of the diversion tunnel. Figure 9 The pressure fluctuation diagram corresponding to the monitoring point up6 at the top of the diversion tunnel is shown in Figure 2. The pressure fluctuation at the bottom of the diversion tunnel is shown in Figure 2. Figure 11 and Figure 12 As shown, Figure 11 This is the pressure fluctuation diagram corresponding to the monitoring point down7 at the bottom of the diversion tunnel. Figure 12 This is the pressure fluctuation diagram corresponding to the monitoring point down8 at the bottom of the diversion tunnel.

[0050] According to the analysis of the calculation results, the overall pressure fluctuation trends in the tail guide joint section and diversion tunnel are the same for the two tail guide joint types. The positive pressure fluctuation of type 2 is more severe than that of type 1. At point up5, the minimum pressure of type 1 reaches -2.87m, while the minimum pressure of type 2 is -1.63m.

[0051] 2.2) Flow pattern analysis at the tail guide joint: The flow pattern of the transition process of type 1 is as follows Figure 13 As shown, Figure 13 (a)-(e) in the figure represent the water-gas two-phase distribution during the transition process of the tail guide combined with type 1 at different times from 0 to 300 seconds. During the load rejection transition process, the pressure reduction wave propagates downstream, and the water-gas interface moves downstream. As the water level in the surge tank rises and falls, the interface fluctuates back and forth. Since the circle gradually changes to a square, there is a contraction section in the vertical section connecting the city gate. After a long strip of air bag is generated at about 200 seconds, the gas is gradually discharged through the 3# vent and the 4# vent. The flow state of the transition process of type 2 under working condition CT2 is as follows: Figure 14 As shown, Figure 14(a)-(e) in the figure represent the water-air two-phase distribution in the transition process of the tail guide junction of type 2 at different time points from 0 to 300 s.

[0052] 2.3), the velocity vector distribution of the tail guide junction: The velocity vector diagram of the tail guide junction of type 1 in the transition process of working condition CT2 is shown in Figure 15 . Figure 15 (a)-(e) in the figure represent the velocity vector diagram of type 1 in the transition process at different time points from 0 to 300 s. The density of streamlines is affected by the density of the grid, and the grid near the air vent is locally encrypted, so the streamlines are relatively dense.

[0053] The velocity vector diagram of the tail guide junction of type 2 in the transition process of working condition CT2 is shown in Figure 16 .

[0054] II. For CT8 working condition: The working condition CT8 is the downstream design flood level, and the two units of the same hydraulic unit are loaded from no load to maximum power under the corresponding water head. According to the one-dimensional calculation method, the flow rates of the tail water pipe outlets 1 and 2 are both 22.14 m3 / s, the tail water level is 2767.45 m, the bottom plate elevation is 2751 m, and the water depth is 16.45 m. The inlet boundary of the tail water system is set as a mass flow inlet, the constant flow working condition is the initial mass flow, and the inlet flow variation history boundary in the transition process is obtained by the one-dimensional calculation method, and the flow variation history curve is shown in Figure 17 . The initial water level of the calculation domain is set according to the downstream water level setting, and the constant flow calculation is first carried out, and then the transition process calculation is carried out with this as the initial condition.

[0055] The downstream tail water outlet opens the Openchannel algorithm model, which is set as a pressure outlet under the boundary condition of open channel, and the water body below 2759.13 m elevation and the gas area above this elevation.

[0056] The downstream surge tank, 1-4# air vents, tail gate well top and downstream pool top are directly in contact with the atmosphere, in order to meet the smooth air exhaust and air intake in the system when the water level fluctuates, the top surface of each working well is set as a pressure outlet boundary condition, and the pressure outlet value is 0 Pa. The VOF algorithm of gas-liquid two-phase flow is adopted, the turbulence model adopts the Reliable k-ε model, and the body of influence gravity is selected for solution.

[0057] 1), constant flow working condition calculation and analysis: Based on 3D numerical simulation calculation, the inlet and outlet flux conservation is monitored to obtain the stable water flow state in the water conveyance tunnel. The water-air two-phase volume fraction distribution of type 1 tail water system under working condition CT8 is shown in Figure 18 and Figure 19 .

[0058] The water-gas two-phase volume fraction distribution of the type 2 tail water system under the working condition CT8 is shown in Figure 20 and Figure 21 , wherein the red color represents the water body, the blue color represents the gas, and the middle rainbow section is the water-gas transition area.

[0059] Through the constant flow calculation result analysis, it is known that the clear full flow of the type 1 mainly occurs in the diversion tunnel, and the air vents 1#-3# are submerged by water; the clear full flow of the type 2 occurs at the joint, the air vents 1#-2# are submerged by water, and the air vent 3# is at the water-gas interface.

[0060] 2), calculation and analysis of the transition process: 2.1), comparison of pressure fluctuations at the tail guide joint: Under the working condition CT8, monitoring points are arranged at the bottom and top of the tail guide joint to monitor the pressure fluctuations in the transition process. According to the calculation result, the overall pressure fluctuation trends of the two positions under the two types of tail guide joints are the same, the lowest pressure of the type 1 is-2.01 m, and the lowest pressure of the type 2 is-1.53 m.

[0061] 2.2), flow state analysis of the tail guide joint: The water-gas variation flow state of the type 1 under the working condition CT8 is shown in Figure 22 , wherein (a)-(f) in Figure 22 represent the water-gas two-phase distribution of the type 1 in the transition process at different times from 0-350 s. The transition process flow state of the type 2 under the working condition CT8 is shown in Figure 23 , wherein (a)-(f) in Figure 23 represent the water-gas two-phase distribution of the type 2 in the transition process at different times from 0-350 s.

[0062] According to the 3D calculation result analysis, when the unit increases the load, the pressure rise wave propagates downstream, the tail water tunnel is blocked by water, the water-gas interface rises upward, and moves towards the downstream outlet, the clear full flow phenomenon appears, and with the rise and fall of the surge chamber water level, it presents a periodic attenuation, and gradually returns to the initial state after 350 s. Under the two arrangement forms, the transition process flow state is similar, at 270 s, there is a residual gas at the sealing end of the diversion tunnel.

[0063] 2.3), velocity vector distribution of the tail guide joint: The velocity vector diagrams of the type 1 and the type 2 in the transition process under the working condition CT8 are shown in Figure 24 and Figure 25 . Figure 24 (a)-(e) in Figure 25(a)-(e) in the figure represent the velocity vector diagrams in the transition process of type 2 at different time points from 0 to 350 s.

[0064] The density of the streamline is affected by the density of the grid, and the grid near the air hole is locally densified, so the streamline is relatively dense. The analysis of the calculation results shows that at the typical time point 0 s, the initial maximum speed of type 1 is 6.49 m / s, and the initial maximum speed of type 2 is 1 m / s. Compared with other time points, the maximum speed of type 1 is greater than that of type 2, because type 1 has a contraction section at the top when the square section gradually changes to the city gate type, and the water flow increases in speed through this place, compared with type 2 which connects by the diversion tunnel section, the transition section is smoother and the speed is smaller.

[0065] Based on two different tail guides combined with transition types, CFD three-dimensional numerical simulation research is carried out. According to the model characteristics of the transition type, CT2 and CT8 working conditions are selected for comparative analysis under two kinds of arrangement forms, and the numerical simulation analysis obtains the pressure wave fluctuation along the line of the tailwater system tunnel, the water level fluctuation of the surge chamber and the water flow pattern evolution law in the transition process under two kinds of working conditions.

[0066] Through the above research, it is shown that under two kinds of working conditions, negative pressure will be generated, the minimum pressure of type 1 under CT2 working condition is-2.87 m, the minimum pressure of type 2 is-1.63 m, the minimum pressure of type 1 under CT8 working condition is-2.01 m, and the minimum pressure of type 2 is-1.53 m, the minimum pressure of type 2 is higher than that of type 1, and the negative pressure condition is improved. In the transition process, the water flow speed of type 1 under two kinds of calculation working conditions is greater than that of type 2, which is related to the existence of part of the contraction section when the square section of the transition section is transitioned to the city gate type section, and the hydraulic condition is relieved; at the same time, under two kinds of arrangement schemes, it is possible that some air masses are left in the blind end of the diversion tunnel in the transition process, but the air masses will be finally discharged through the air hole, and the blind end part will produce multiple eddies in the transition process, and the turbulent effect is obvious. Based on the above analysis, the scheme of type 2 is better than that of type 1.

[0067] The above only describes the preferred embodiments of the present application and does not limit the present application, any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application. The contents not described in detail in the specification of the present application are the existing technology known by the professional technical personnel.

Claims

1. A tailwater system tailwater joint section connection structure of a hydropower station, wherein the tailwater system of the hydropower station includes a tailwater tunnel and a diversion tunnel, and an angle exists between the center lines of the tailwater tunnel and the diversion tunnel; wherein: The tailwater tunnel has a circular end face; the diversion tunnel has a gate-shaped end face overall, with an arched upper portion and a rectangular lower portion. The characteristic is that, from the tailwater tunnel to the diversion tunnel, the cross-sectional shapes of the tailwater-guide joint connection structure are, in order, a circular cross-section, a first gate-shaped cross-section, and a second gate-shaped cross-section. The circular section is connected to the first city gate arch section by a first gradual transition section to achieve a smooth transition from the circular section to the city gate arch section; the first city gate arch section and the second city gate arch section are connected by a second transition section; wherein the projection of the second transition section on the bottom surface is arc-shaped.

2. The tailwater system tailwater joint section connection structure of a hydropower station according to claim 1, characterized in that: The side length of the lower rectangle of the first city gate arch section is smaller than the corresponding diameter of the circular section; The corresponding dimensions of the first city gate arch section and the second city gate arch section are the same, that is, the second transition section is a continuous extension of the first city gate arch section; or, the dimensions of the first city gate arch section and the second city gate arch section are different, that is, the second transition section is a gradual transition section.

3. The tailwater system tailwater joint section connection structure of a hydropower station according to claim 1, characterized in that: Ventilation holes are provided on the top of the first gradual transition section and / or the second transition section, and the arrangement positions of the ventilating holes are determined by three-dimensional CFD simulation.

4. A modeling method for the tailwater system tailwater joint section connection structure of a hydropower station according to any one of claims 1 to 3, characterized in that: The following steps are involved: Step S1: Based on the hydropower station tailwater system design drawing, determine the modeling range from the unit tailwater pipe outlet to the diversion tunnel outlet; establish horizontal and vertical reference lines for locating the spatial coordinates of each section; set the modeling angle reference line to control the transition angle of the circular, first gate tunnel type, and second gate tunnel type sections to ensure axis consistency; Step S2: Determine the parameters of the circular section, the first gate arch type, and the second gate arch type sections, and then perform guide line layout and stakeout operations; Step S3: combining the first gradual transition section, the second transition section, and the diversion tunnel model into an integral structure through stretching and Boolean sum operations; Step S4: Fillet modeling is performed on the tail guide joint to optimize the smoothness of water flow connection; Step S5: Meshing the constructed model, using a mesh form combining hexahedrons and polyhedrons, and locally encrypting key areas; Step S6: Using the VOF multiphase flow model and the Reliable k-ε turbulence model, simulate the flow pattern and pressure fluctuation under different working conditions, and analyze and optimize the connection structure.

5. The modeling method according to claim 4, characterized in that: In step S2, the parameters that need to be determined include the diameter of the circular section, the lower rectangular size and arch radius of the first city gate arch section, the lower rectangular size and arch radius of the second city gate arch section, the distance between the circular section and the first city gate arch section, and the angle and radius of the corresponding arc between the first city gate arch section and the second city gate arch section.

6. The modeling method according to claim 5, characterized in that: The midpoint of the upper arch of the first gate arch section is designated as point A. The two ends of the arch coincide with the two upper vertices of the lower rectangle, designated as points B and D, respectively. The midpoint of the base of the lower rectangle is designated as point C. The four angle-dividing points on the circular section are designated as points A', B', C', and D' in clockwise order, starting from the upper vertex. Points B and B' are located on the same side, while points D and D' are located on the other side. The lower vertex C' of the circular section is located on the same horizontal plane as the midpoint C of the base of the lower rectangle of the first gate arch section. The four guide lines of the first gradual transition segment are straight line segment AA', straight line segment BB', straight line segment DD' and straight line segment CC'; The guide line of the second transition section includes an arc segment formed by connecting the corresponding midpoints of the upper arches of the first city gate arch section and the second city gate arch section, and four arc segments formed by connecting the four corresponding vertices of the lower rectangle.

7. The modeling method according to claim 6, characterized in that: The simulated operating conditions in step S6 include the unit load shedding operating condition under the upstream design flood level, and the unit load increasing operating condition under the downstream design flood level.

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