A modeling method for the connection structure of the tailrace guide section of a hydropower station

Abstract

This invention discloses a connection structure and modeling method for the tailrace guide section of a hydropower station. From the tailrace tunnel to the diversion tunnel, the cross-sectional shapes of the connection structure are successively circular, first archway-shaped, and second archway-shaped. The circular cross-section and the first archway-shaped cross-section are connected by a first gradual transition section; the first archway-shaped cross-section and the second archway-shaped cross-section are connected by a second transition section. The projection of the second transition section on the bottom surface is arc-shaped. This invention adopts a circular-archway-archway-archway arrangement, achieving a smooth transition through gradual transition sections, avoiding abrupt changes in water flow caused by square cross-sections in existing technologies. This effectively reduces turbulence intensity, pressure fluctuations, and optimizes water flow efficiency. The modeling method uses parametric design to accurately construct the model, then performs mesh generation, and finally optimizes the structural design through numerical simulation analysis, providing reliable guidance for engineering practice.

Description

Technical Field

[0001] This invention belongs to the field of water conservancy and hydropower engineering technology, specifically relating to a modeling method for the connection structure of the tailrace guide section of a hydropower station tailrace system. Background Technology

[0002] In hydropower station construction, the arrangement of diversion tunnels into tailrace tunnels allows for a compact underground hub layout, combining practicality and economy, as seen in the Xiluodu and Wudongde hydropower stations. However, in hydropower stations with a tailrace-drainage combination, when the downstream water level is low or near the top of the diversion tunnel, alternating periods of open and full flow occur in the tailrace system, leading to pressure fluctuations, turbulent flow, and other problems. This results in complex hydraulic conditions that severely impact the stable operation of the power station. Different tailrace-drainage combination types have varying effects on the characteristics of the transition process, and determining a suitable tailrace-drainage combination type is of significant engineering importance.

[0003] In most hydropower station construction, the transition between the tailrace tunnel and the diversion tunnel is smooth, with no protruding end sealing the diversion tunnel. However, for tailrace systems with protruding end sealing the diversion tunnel, the blind cavity at the end can easily lead to turbulence and significant pressure fluctuations at the junction of the tailrace tunnel and the diversion tunnel, affecting flow efficiency and structural stability. Furthermore, it may exacerbate the alternation of open and closed flow during the unsteady flow transition, increasing the risk of forming enclosed air masses and flow interruptions, posing a threat to the safe and stable operation of the hydropower station.

[0004] Therefore, for tailwater systems with protruding blocking ends at the end of the diversion tunnel, it is necessary to optimize the design of the connection structure of the tail guide section. A tail guide section connection structure and corresponding modeling method are needed that can optimize the transition process, reduce water flow disturbance, and improve structural stability. Summary of the Invention

[0005] The purpose of this invention is to provide a modeling method for the connection structure of the tailrace guide section of a hydropower station, which solves the problems of complex flow patterns, large pressure fluctuations, and easy alternation of open and closed flow in the tailrace guide section in the prior art, thereby improving the operational safety and efficiency of the tailrace system.

[0006] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution:

[0007] A tailrace-guide connection structure for a hydropower station tailrace system includes a tailrace tunnel and a guide tunnel, with an included angle between the centerlines of the tailrace tunnel and the guide tunnel; wherein the end face of the tailrace tunnel is circular; and the end face of the guide tunnel is generally arched, with an arched upper part and a rectangular lower part.

[0008] From the tailrace tunnel to the diversion tunnel, the cross-sectional shapes of the connecting structure of the tailrace-diversion section are successively a circular cross-section, a first archway-shaped cross-section, and a second archway-shaped cross-section; the circular cross-section and the first archway-shaped cross-section are connected by a first gradual transition section to achieve a smooth transition from the circular cross-section to the archway-shaped cross-section; the first archway-shaped cross-section and the second archway-shaped cross-section are connected by a second transition section; wherein, the projection of the second transition section on the bottom surface is arc-shaped.

[0009] Further optimization involves making the side length of the lower rectangle of the first city gate-shaped cross-section smaller than the diameter of the corresponding circular cross-section.

[0010] The corresponding dimensions of the first and second city gate-shaped cross sections are the same, meaning the second transition section is a continuous extension of the first city gate-shaped cross section; or, the dimensions of the first and second city gate-shaped cross sections are different, meaning the second transition section is a gradual transition section.

[0011] Further optimization involves providing ventilation holes at the top of the first gradual transition section and / or the second transition section. The arrangement of the ventilation holes is determined by three-dimensional CFD simulation to ensure sufficient ventilation space above the surface of the open channel during the transition process.

[0012] The modeling method for the connection structure of the tailrace guide section of the aforementioned hydropower station includes the following steps:

[0013] Step S1: Based on the tailrace system design drawings of the hydropower station, determine the modeling range as from the tailrace pipe outlet of the unit to the diversion tunnel outlet; establish horizontal and vertical baselines to locate the spatial coordinates of each section; set modeling angle reference lines to control the transition angles of the circular, first archway, and second archway sections to ensure axis consistency.

[0014] Step S2: Determine the parameters of the circular cross-section, the first city gate type, and the second city gate type cross-section, and then carry out the guide line layout and layout operation.

[0015] Using parametric modeling software, the shapes of the circular section, the first city gate-shaped section, and the second city gate-shaped section are established respectively. Then, the circular section and the first city gate-shaped section are connected by the top arched guide line and the side guide line. Lofting modeling is performed to generate the first gradual transition section from the circular section to the first city gate-shaped section, realizing a smooth transition between the two. Similarly, the second transition section is generated by lofting modeling.

[0016] Step S3: Combine the first gradient transition section, the second transition section, and the guide tunnel model into a whole structure through stretching and Boolean summation operations;

[0017] Step S4: Based on actual engineering requirements, perform rounded corner modeling at the tail guide joint to optimize the smoothness of water flow connection.

[0018] Step S5: Mesh the established model using a combination of hexahedral and polyhedral meshes. At the same time, refine the mesh locally in key areas such as the gradual transition section and the tail section to improve the calculation accuracy and ensure the accuracy of subsequent simulation analysis.

[0019] Step S6: Using the VOF multiphase flow model and the Reliablek-ε turbulence model, simulate and analyze the flow regime and pressure fluctuations of water under different working conditions, and optimize the design of the connection structure.

[0020] Further optimization involves determining the following parameters in step S2: the diameter of the circular cross-section, the lower rectangular dimensions and arch radius of the first city gate-shaped cross-section, the lower rectangular dimensions and arch radius of the second city gate-shaped cross-section, the distance between the circular cross-section and the first city gate-shaped cross-section, and the included angle and radius of the corresponding arcs between the first and second city gate-shaped cross-sections.

[0021] Further optimization involves designating the midpoint of the upper arch of the first city gate-shaped cross-section as point A; the two ends of the arch and the two upper vertices of the lower rectangle as points B and D, respectively; the midpoint of the bottom edge of the lower rectangle as point C; and the four angle bisectors on the circular cross-section, starting from the upper vertex and proceeding clockwise, as points A', B', C', and D'. Points B and B' are located on the same side, and points D and D' are located on the other side, with the lower vertex C' of the circular cross-section and the midpoint C of the bottom edge of the lower rectangle of the first city gate-shaped cross-section on the same horizontal plane.

[0022] The four guide lines of the first gradual transition segment are line segment AA', line segment BB', line segment DD', and line segment CC'.

[0023] The guide lines of the second transition section include the arc-shaped line segments formed by connecting the corresponding midpoints of the upper arches of the first and second city gate-shaped cross sections, and the four arc-shaped line segments formed by connecting the four corresponding vertices of the lower rectangle.

[0024] Further optimization involves simulating operating conditions in step S6, including load shedding at the upstream design flood level and load increase at the downstream design flood level. Based on the simulation results, the design of the connection structure is optimized to obtain better hydraulic performance.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] 1. Improved water flow characteristics: This invention addresses tailwater systems with protruding blocking ends at the diversion tunnels by adopting a circular-gate-gate-gate arrangement. Through gradual transition sections, a smooth transition is achieved, avoiding abrupt changes in water flow. This effectively reduces turbulence intensity, minimizes pressure fluctuations, and optimizes water flow efficiency.

[0027] 2. Improved structural stability: The smooth transition reduces the impact of water flow on the structure, lowers the risk of damage caused by water flow disturbance, and improves the stability and safety of the tail guide joint section structure.

[0028] 3. Reduce the risk of alternating open and full flow: A reasonable structural design combined with the setting of ventilation holes can effectively control the phenomenon of alternating open and full flow, avoid the generation of closed air masses and water flow interruption, and ensure the safe and stable operation of the hydropower station.

[0029] 4. Precise and efficient modeling: The modeling method used accurately constructs the model through parametric design, then performs mesh generation, and finally optimizes the structural design through numerical simulation analysis, providing reliable guidance for engineering practice. Attached Figure Description

[0030] Figure 1 This is a diagram of the tail guide structure in Example 1;

[0031] Figure 2 A schematic diagram of the overall tailrace system of the Tszha Gorge;

[0032] Figure 3 This is a diagram of the tail guide structure in Comparative Example 1;

[0033] Figure 4 This is a schematic diagram of the nine monitoring points set at the tail guide junction and the top of the diversion tunnel in Example 1;

[0034] Figure 5 For the CT2 type tailrace system, the water-air two-phase distribution is shown.

[0035] Figure 6 for Figure 4 Enlarged view of the mid-tail guide joint segment;

[0036] Figure 7 For the CT2 type 2 tailrace system, the water-air two-phase distribution is shown.

[0037] Figure 8 for Figure 6 Enlarged view of the mid-tail guide joint segment;

[0038] Figure 9 This is a pressure fluctuation diagram at monitoring point up5 at the top of the CT2 tail guide junction under operating conditions.

[0039] Figure 10 This is a pressure fluctuation diagram at monitoring point up6 at the top of the CT2 tail guide junction under operating conditions.

[0040] Figure 11 This is a pressure fluctuation diagram at monitoring point down7 at the bottom of the CT2 diversion tunnel under operating conditions.

[0041] Figure 12This is a pressure fluctuation diagram at monitoring point down8 at the bottom of the CT2 diversion tunnel under operating conditions.

[0042] Figure 13 The water-gas two-phase distribution during the transition process of CT2 tail guide combined with type 1 under working condition;

[0043] Figure 14 The water-gas two-phase distribution during the transition process of CT2 tail guide combined with type 2 under working condition;

[0044] Figure 15 Vector diagram of velocity during the transition process of CT2 tail guide arrangement type 1 under working conditions;

[0045] Figure 16 A vector diagram showing the velocity during the transition of the CT2 tail guide arrangement type 2 under working conditions;

[0046] Figure 17 The process of change in the inlet boundary flow of CT8 under operating conditions;

[0047] Figure 18 The volume fraction distribution of water and air phases in the tailrace system of CT8 type under operating conditions;

[0048] Figure 19 for Figure 17 Enlarged view of the mid-tail guide joint segment;

[0049] Figure 20 The volume fraction distribution of water and air phases in the CT8 type 2 tailrace system under operating conditions;

[0050] Figure 21 for Figure 19 Enlarged view of the mid-tail guide joint segment;

[0051] Figure 22 The water-gas two-phase distribution during the transition process at the type 1 tail guide junction of CT8 under operating conditions;

[0052] Figure 23 The water-gas two-phase distribution during the transition process at the type 2 tail guide junction of CT8 under operating conditions;

[0053] Figure 24 A velocity vector diagram of the transition process for CT8 tail-guided coupling type 1 under working conditions;

[0054] Figure 25 This is a vector diagram showing the velocity transition process of vents 1# to 4# under CT8 operating conditions. Detailed Implementation

[0055] The specific embodiments of the present invention will now be described with reference to the accompanying drawings to enable those skilled in the art to understand the invention. Obviously, the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, any modifications within the spirit and scope of the invention as defined and determined by the appended claims are considered non-creative and all inventions utilizing the inventive concept of this method are protected.

[0056] Taking the No. 3 tailrace system of the Yellow River Cihaxia Hydropower Station as an example, the following explanation will be given.

[0057] Example 1: The tailrace system of a hydropower station has a tailrace guide section connection structure. From the tailrace tunnel to the diversion tunnel, the cross-sectional shapes of the tailrace guide section connection structure are successively a circular cross-section, a first archway-shaped cross-section, and a second archway-shaped cross-section. The circular cross-section and the first archway-shaped cross-section are connected by a first gradual transition section to achieve a smooth transition from the circular cross-section to the archway-shaped cross-section. The first archway-shaped cross-section and the second archway-shaped cross-section are connected by a second transition section. The projection of the second transition section on the bottom surface is arc-shaped. Ventilation holes are provided at the top of the first gradual transition section and / or the second transition section.

[0058] In this embodiment, the side length of the lower rectangle of the first city gate-shaped cross-section is smaller than the diameter of the corresponding circular cross-section; the corresponding dimensions of the first city gate-shaped cross-section and the second city gate-shaped cross-section are the same, that is, the second transition section is a continuous extension of the first city gate-shaped cross-section.

[0059] In other embodiments, the dimensions of the first gate-shaped section and the second gate-shaped section may be different, that is, the second transition section is a gradual transition section.

[0060] In this embodiment, the modeling method for the connection structure of the tailrace guide section of the hydropower station is implemented in the following steps:

[0061] 1. Structural Dimensions: The diameter of the circular cross-section 11 is 14.5m, matching the diameter of the tailrace tunnel. The first archway-type cross-section 12 and the second archway-type cross-section 13 have the same dimensions, including a lower rectangle and an upper arch. The lower rectangle measures 13.5m × 13.5m, and the radius R of the arch is 7.79m. The tailrace system has a total of 4 vents.

[0062] 2. Modeling process: such as Figure 1 As shown, a parametric model was built using SolidWorks, defining the coordinates of the center of the circular cross-section, and the elevation of the arched section's crown was 2769.0m. Guide lines were added, and the first gradual transition segment 2 was generated using the lofting function, as shown... Figure 1As shown in (a), ensure that the rate of change of cross-sectional area is ≤1.5% / m. Similarly, a second transition section 3 is generated between the first archway-shaped cross-section 12 and the second archway-shaped cross-section 13, as shown in (a). Figure 1 As shown in (b) of the diagram.

[0063] Then, the second transition section 3 and the protruding sealing end model 4 at the end of the diversion tunnel are summed using Boolean summation, such as... Figure 1 As shown in (c); and the additional length segment 5 formed by forward stretching to create a guide tunnel, as shown in... Figure 1 As shown in (d) in the diagram; ultimately, through the above operations, the first gradual transition section, the second transition section, and the diversion tunnel are combined into an integral structure, so that tailrace tunnel 1 and diversion tunnel 6 are combined together, and the overall structure is as follows. Figure 1 As shown in (e) in the diagram. Finally, based on actual engineering requirements, the tail guide joint is modeled with rounded corners to optimize the smoothness of the water flow connection.

[0064] Fluentmeshing was used to generate an unstructured mesh for the constructed tailrace system model. The mesh size of the transition section was ≤1.5m, the total number of mesh cells was 2.25 million, and the number of mesh nodes was approximately 4.73 million.

[0065] Figure 2 This is a model diagram of the overall tailrace system of the Tszha Gorge, in which... Figure 2 The modules within the dashed boxes in (a)-(d) represent, in order, the downstream surge tank, vent, tailpipe connection structure, and a local model of the downstream tailwater. Figure 2 The dashed box (c) is Figure 1 The structure shown in (e) is shown in the diagram.

[0066] Comparative Example 1: The tailrace system of a hydropower station has a tailrace guide section connection structure. From the tailrace tunnel to the diversion tunnel, the cross-sectional shapes of the tailrace guide section connection structure are successively circular, square, and archway-shaped. The circular and square cross-sections are connected by a first gradual transition section, and the square cross-section and the archway-shaped cross-section are connected by a second gradual transition section. Ventilation holes are provided at the top of the first and second gradual transition sections.

[0067] In this comparative example, the circular cross-section has a diameter of 14.5m, the square cross-section has side dimensions of 14.5m × 14.5m, and the first gradual transition section is 10m long. The arch-shaped cross-section has dimensions of 13.5m × 13.5m, an arch radius R of 7.79m, and a second gradual transition section length of 18m. The arched curve at the top adopts a quadratic parabola to achieve a transition buffer to the arch-shaped cross-section. The tailrace system has a total of 4 vents.

[0068] The difference from Example 1 lies in the transition section after the circular cross-section. In Comparative Example 1, a square cross-section connects to a gate-shaped diversion tunnel; in Example 1, a gate-shaped cross-section connects to a gate-shaped diversion tunnel, without the intermediate square cross-section segment. The other structural parts are the same, as detailed below. Figure 3 As shown.

[0069] CFD simulations were performed to verify the structures corresponding to Example 1 and Comparative Example 1, respectively:

[0070] Nine monitoring points, up1 to up9, were set at the tailrace connection and the top of the diversion tunnel of the hydropower stations constructed in Example 1 and Comparative Example 1, respectively. Figure 4 As shown, nine common monitoring points (down1 to down9) are set at the bottom of the tunnel corresponding to the monitoring point at the top of the tunnel (not marked in the figure). This means that both Example 1 and Comparative Example 1 have a total of 18 monitoring points, and the corresponding positions of each monitoring point are the same in both examples. These points are used to monitor and analyze pressure fluctuations in the tailrace system. Furthermore, corresponding monitoring sections (plane1 to plane6) are set at points up4 to up9 to monitor the flow rate across the cross-section.

[0071] CFD simulations were performed on the tail section of the hydropower station constructed in Example 1 and Comparative Example 1 under two working conditions, CT2 and CT8 respectively. The specific parameters for the two working conditions are shown in Table 1.

[0072] For ease of description, the circular-square-gate-archway cross section in Comparative Example 1 will be referred to as Type 1, and the circular-gate-archway cross section in Example 1 will be referred to as Type 2.

[0073] Table 1 Verification of Large Fluctuation Transition Process

[0074]

[0075] I. For CT2 operating conditions:

[0076] Considering the complex flow conditions at the tailrace section when the downstream is at the check flood level, operating condition CT2 is selected for hydraulic transient process analysis to analyze and demonstrate the advantages and disadvantages of the hydraulic characteristics under the two arrangement types. In operating condition CT2, the upstream is at the design flood level, and the downstream is at the check flood level. All units in the same hydraulic unit operate at their maximum output power at the corresponding head, while simultaneously shedding load and closing the guide vanes in an emergency. Based on the one-dimensional calculation method, the flow rate at the tailrace outlet is obtained as 373.61 m³ / s. 3 / s, tailwater level 2768.69m, bottom elevation 2751m, water depth 17.59m.

[0077] The inlet boundary of the tailrace system is set as the mass flow rate inlet. The steady flow condition is based on the initial mass flow rate. The inlet flow rate variation history for the transient process condition is obtained using a one-dimensional calculation method and given by UDF programming. The downstream water level is set as the initial water level of the computational domain. First, steady flow calculations are performed, and then this is used as the initial condition for transient process calculations.

[0078] The downstream tailrace tunnel outlet is set to a pressure outlet under open channel boundary conditions using the Openchannel algorithm model. The area below the 2768.69m elevation is water, and the area above this elevation is gas.

[0079] The tops of the downstream surge tank, tailrace gate well, and downstream pool are in direct contact with the atmosphere. To ensure smooth air venting and intake during water level fluctuations, the top surfaces of each working well are set as pressure outlet boundary conditions, with a pressure outlet value of 0 Pa. The gas-liquid two-phase flow VOF algorithm is used, the Reliable k-ε turbulence model is employed, and gravity calculation is selected based on body of influence. The solid wall boundary is set as a no-slip wall, and the near-wall surface is treated using the standard wall function method. The boundary conditions are identical for both tailrace arrangement types.

[0080] 1) Calculation and analysis of steady flow conditions:

[0081] Numerical simulation calculations were performed to monitor the conservation of inlet and outlet fluxes, thus obtaining a stable flow state within the water conveyance tunnel. The volume fraction distribution of the water and air phases in the Type 1 tailrace system under operating condition CT2 is shown below. Figure 5 and Figure 6 As shown, the volume fraction distribution of water and air phases in the Type 2 tailrace system under operating condition CT2 is as follows: Figure 7 and Figure 8 As shown, red represents water, blue represents gas, and the rainbow segment in the middle is the transition zone between water and gas.

[0082] The steady flow calculation results show that, in Type 1, due to the presence of a contraction transition section, the open full flow mainly occurs inside the diversion tunnel, with vents 1# to 3# submerged in water and small air masses present at vent 4#. In Type 2, the open full flow occurs at the tail guide junction, with vents 1# to 3# submerged in water, and some air masses present near vent 4#. At the same time, some gas remains in the blind end cavity at the end of the diversion tunnel. The open full flow phenomenon mainly occurs near the tailwater outlet gate well.

[0083] 2) Transient process calculation and analysis:

[0084] 2.1) Comparison of pressure fluctuations at the tail guide joint:

[0085] Under CT2 operating conditions, monitoring points are set at the top and bottom of the tail guide joint section to monitor the pressure fluctuations at the corresponding cross-sections 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, where Figure 9 This is a pressure fluctuation diagram corresponding to monitoring point up5 at the top of the diversion tunnel. Figure 9 This is a pressure fluctuation diagram corresponding to monitoring point up6 at the top of the diversion tunnel. The pressure fluctuation at the bottom of the diversion tunnel is shown below. Figure 11 and Figure 12 As shown, where Figure 11 This is a pressure fluctuation diagram corresponding to monitoring point down7 at the bottom of the diversion tunnel. Figure 12 This is a pressure fluctuation diagram corresponding to monitoring point down8 at the bottom of the diversion tunnel.

[0086] Analysis of the calculation results shows that under the two tail guide connection types, the overall pressure fluctuation trend in the tail guide connection section and the diversion tunnel is the same. The positive pressure fluctuation of type 2 is more intense than that of type 1. At point up5, the lowest pressure of type 1 reaches -2.87m, while the lowest pressure of type 2 is -1.63m.

[0087] 2.2) Flow regime analysis at the tail guide junction:

[0088] Type 1 Transient process flow pattern as follows Figure 13 As shown, Figure 13 In the diagram, (a)-(e) represent the water-gas two-phase distribution during the transition process of Type 1 with tail-guided connection at different times from 0-300s. During the load shedding transition, the pressure drop 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. Due to the shrinkage section in the vertical section connecting the gate after the circle gradually changes to a square, a long strip-shaped air bladder is generated at around 200s, and the gas gradually exits through vents #3 and #4. The flow pattern of Type 2 during the transition process under operating condition CT2 is as follows: Figure 14 As shown, Figure 14 In the figure, (a)-(e) represent the water-gas two-phase distribution during the transition of tail-leader combination type 2 at different times from 0 to 300s.

[0089] 2.3) Velocity vector distribution at the tail guide junction:

[0090] The velocity vector diagram of the tail guide joint section during the transition of CT2 under operating condition 1 is as follows: Figure 15 As shown, Figure 15 In the diagram, (a)-(e) represent the velocity vector diagrams during the transition of Type 1 at different times from 0 to 300s. The density of streamlines is affected by the mesh density; the mesh near the vent is locally refined, resulting in denser streamlines.

[0091] Type 2 velocity vector diagram of the tail guide joint section during the CT2 transition process, as shown below. Figure 16 As shown.

[0092] II. For CT8 operating conditions:

[0093] The calculation condition CT8 is based on the downstream design flood level. Two generating units within the same hydraulic unit are increased from no-load to maximum power operation at the corresponding head. According to the one-dimensional calculation method, the flow rates at outlets 1 and 2 of the tailrace pipe are both 22.14 m³ / s, the tailrace level is 2767.45 m, the bottom elevation is 2751 m, and the water depth is 16.45 m. The inlet boundary of the tailrace system is set as the mass flow rate inlet. The steady flow condition is based on the initial mass flow rate. The transition process condition's inlet flow rate change history boundary is obtained using the one-dimensional calculation method. The flow rate change history curve is shown below. Figure 17 As shown. The initial water level of the computational domain is set based on the downstream water level. First, a steady flow calculation is performed, and then this is used as the initial condition for the transient process calculation.

[0094] The downstream tailwater outlet is set to a pressure outlet under open channel boundary conditions using the Openchannel algorithm model. The area below the 2759.13m elevation is water, and the area above this elevation is gas.

[0095] The downstream surge tank, vents 1-4, the top of the tailrace gate well, and the top of the downstream pool are in direct contact with the atmosphere. To ensure smooth venting and intake of air within the system during water level fluctuations, the top surface of each working well is set as a pressure outlet boundary condition with a pressure outlet value of 0 Pa. The gas-liquid two-phase flow VOF algorithm is used, the Reliable k-ε turbulence model is adopted, and the body of influence gravity solution is selected.

[0096] 1) Calculation and analysis of steady flow conditions:

[0097] Based on 3D numerical simulation calculations, the inlet and outlet fluxes are monitored and conserved to obtain the stable water flow state within the water conveyance tunnel. The volume fraction distribution of water and air phases in the Type 1 tailrace system under operating condition CT8 is shown below. Figure 18 and Figure 19 As shown.

[0098] The volume fraction distribution of water and air phases in the Type 2 tailrace system under CT8 operating conditions is as follows: Figure 20 and Figure 21 As shown, red represents water, blue represents gas, and the rainbow segment in the middle is the transition zone between water and gas.

[0099] Analysis of the steady flow calculation results shows that, due to the presence of a contraction transition section, the open full flow of Type 1 mainly occurs inside the guide tunnel, and the vents 1#~3# are submerged in water; the open full flow of Type 2 occurs at the junction, the vents 1#~2# are submerged in water, and the vent 3# is located at the water-air interface.

[0100] 2) Transient process calculation and analysis:

[0101] 2.1) Comparison of pressure fluctuations in the tail guide joint section:

[0102] Under CT8 operating conditions, monitoring points were set at the bottom and top of the tail guide joint section to monitor pressure fluctuations during the transition process. The calculation results show that the overall pressure fluctuation trend at the two tail guide joint types is the same. The lowest pressure of type 1 is -2.01m and the lowest pressure of type 2 is -1.53m.

[0103] 2.2) Flow regime analysis of the tail section:

[0104] Under operating condition CT8, the flow regime of water vapor change in type 1 is as follows: Figure 22 As shown, Figure 22 In the diagram, (a)-(f) represent the water-gas two-phase distribution during the transition process of Type 1 at different times from 0 to 350 s. The flow regime of Type 2 during the transition process under operating condition CT8 is shown below. Figure 23 As shown, Figure 23 In the diagram, (a)-(f) represent the water-gas two-phase distribution during the transition of Type 2 at different times from 0 to 350s.

[0105] Analysis of the 3D calculation results shows that when the unit increases its load, the pressure rise wave propagates downstream, the downstream tailrace tunnel is backed up, the water-air interface rises upward and moves towards the downstream outlet, resulting in an alternating flow phenomenon of open and full flow. This flow exhibits periodic decay as the surge tank water level rises and falls, gradually returning to its initial state after 350 seconds. Under both layout configurations, the flow pattern during the transition process is similar; at 270 seconds, some residual gas is observed at the sealed end of the diversion tunnel.

[0106] 2.3) Velocity vector distribution of the tail guide joint section:

[0107] Velocity vector diagrams of Type 1 and Type 2 during the CT8 transition process, as shown below. Figure 24 and Figure 25 As shown. Figure 24 In the diagram, (a)-(e) represent the velocity vector diagrams during the transition of Type 1 at different times from 0 to 350s. Figure 25 In the diagram, (a)-(e) represent the velocity vector diagrams during the transition of Type 2 at different times from 0 to 350s.

[0108] The density of streamlines is affected by the mesh density. The mesh near the vents is locally denser, resulting in denser streamlines. Analysis of the calculation results shows that at a typical time 0s, the initial maximum velocity of Type 1 is 6.49 m / s, and the initial maximum velocity of Type 2 is 1 m / s. Comparing other times, the maximum velocity of Type 1 is greater than that of Type 2. This is because Type 1 has a contracting section at the top where the square cross-section gradually transitions to a gate-like shape, increasing the velocity of the water flow at this point. Compared to Type 2, where the transition is achieved through a guide tunnel, the transition section is smoother and the velocity is lower.

[0109] CFD three-dimensional numerical simulation studies were conducted based on two different tailrace connection types. According to the model characteristics of the connection type, working conditions CT2 and CT8 were selected, and comparative analyses were carried out under the two arrangement forms. The numerical simulation analysis obtained the pressure wave fluctuation, surge tank water level fluctuation, and flow regime evolution law of the tailrace system along the tunnel under the two working conditions.

[0110] The above research shows that negative pressure is generated under both operating conditions. Under CT2 condition, the minimum pressure for Type 1 is -2.87 m, and for Type 2 it is -1.63 m. Under CT8 condition, the minimum pressure for Type 1 is -2.01 m, and for Type 2 it is -1.53 ​​m. The minimum pressure of Type 2 is higher than that of Type 1, indicating an improvement in the negative pressure situation. Under both calculation conditions, the water flow velocity during the transition process is greater in Type 1 than in Type 2. This is related to the partial contraction of the cross-section when transitioning from the square section to the arch-shaped section, which alleviates the hydraulic conditions. Simultaneously, during the transition, some air masses may remain at the blind end of the guide tunnel under both layout schemes. However, these air masses will eventually be discharged through the vents. Multiple vortices will be generated at the blind end during the transition, resulting in significant turbulence. Based on the above analysis, the Type 2 scheme is superior to the Type 1 scheme.

[0111] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention. Contents not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A modeling method for a tail guide joint segment connection structure of a tail water system of a hydropower station, characterized in that, The tailrace system of a hydropower station comprises a tailrace tunnel and a diversion tunnel, and an included angle exists between the center lines of the tailrace tunnel and the diversion tunnel; wherein, the end face of the tailrace tunnel is circular; the end face of the diversion tunnel is overall in the shape of a city gate, the upper part is in the shape of an arch, and the lower part is in the shape of a rectangle; from the tailrace tunnel to the diversion tunnel, the cross-sectional shapes of the tail-conjunction segment connection structure are in turn circular, first city gate shape and second city gate shape; The circular cross section and the first city gate shape cross section are connected through a first gradual transition section to realize smooth transition from the circular cross section to the city gate shape cross section; the first city gate shape cross section and the second city gate shape cross section are connected through a second transition section; wherein, the projection of the second transition section on the bottom face is in the shape of an arc; The side length of the lower part rectangle of the first city gate shape cross section is smaller than the diameter corresponding to the circular cross section; The corresponding sizes of the first city gate shape cross section and the second city gate shape 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 shape cross section and the second city gate shape cross section are different, that is, the second transition section is a gradual transition section; 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; The modeling method comprises the following steps: Step S1: according to the design drawing of the tailrace system of the hydropower station, the modeling range is determined to be from the outlet of the unit tailrace pipe to the outlet of the diversion tunnel; a horizontal reference line and a vertical reference line are established for positioning the spatial coordinates of each cross section; a modeling angle reference line is set to control the transition angle of the circular, first city gate shape and second city gate shape cross sections, and to ensure the consistency of the axis; Step S2: the parameters of the circular cross section, the first city gate shape and the second city gate shape cross sections are determined, and then the guide line arrangement and lofting operation are performed; Step S3: the first gradual transition section, the second transition section and the diversion tunnel model are combined into an overall structure through stretching and Boolean sum operation; Step S4: the tail-conjunction is subjected to round corner modeling processing to optimize the smoothness of the water flow connection; Step S5: the built model is subjected to mesh division, and a hexahedron and polyhedron combined mesh form is adopted, and the key areas are subjected to local encryption; Step S6: the VOF multiphase flow model and the Reliable k-ε turbulence model are adopted to simulate the water flow pattern and pressure fluctuation under different working conditions, and the connection structure is analyzed and optimized.

2. The modeling method of claim 1, wherein, In the step S2, the parameters to be determined include the diameter of the circular cross section, the lower end rectangle size and arch top radius of the first city gate shape cross section, the lower end rectangle size and arch top radius of the second city gate shape cross section, the distance between the circular cross section and the first city gate shape, and the included angle and radius of the corresponding arc between the first city gate shape cross section and the second city gate shape cross section.

3. The modeling method of claim 2, wherein, The midpoint of the upper arch of the first gate-shaped section is denoted as point A; the two ends of the arch and the upper end of the lower rectangle are respectively denoted as points B and D; the midpoint of the bottom edge of the lower rectangle is denoted as point C; the four angle points of the circular section are denoted as points A', B', C' and D' in clockwise order 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 bottom edge of the lower rectangle of the first gate-shaped section are located on the same horizontal plane; The four guide lines of the first gradual transition section are straight line segments AA', BB', DD' and CC' respectively; The guide lines of the second transition section include the arc-shaped line segments formed by connecting the corresponding midpoints of the upper arches of the first gate-shaped section and the second gate-shaped section, and the four arc-shaped line segments formed by connecting the four corresponding vertices of the lower rectangle.

4. The modeling method of claim 3, wherein, The simulation working condition in the step S6 includes a unit load shedding working condition under an upstream design flood level and a unit load increasing working condition under a downstream design flood level.

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