A method for predicting Karman vortex resonance in mixed-flow turbines based on the TBR model

Abstract

This invention relates to a method for predicting Karman vortex resonance in mixed-flow turbines based on a TBR model, comprising: constructing a three-dimensional geometric model of the turbine's flow components; determining the turbine's operating conditions where Karman vortices occur; performing fluid simulation using a TBR model based on the turbine's operating conditions and the three-dimensional geometric model to obtain the Karman vortex frequencies at the fixed guide vanes, movable guide vanes, and runner; performing component modal analysis on the fixed guide vanes, movable guide vanes, and runner using the finite element method to obtain the natural frequencies of the components; and comparing the Karman vortex frequencies at the fixed guide vanes, movable guide vanes, and runner with the natural frequencies to determine whether resonance may occur. This invention is highly efficient, accurate, and practical in the early design stage of turbines, and has significant application value for shortening the turbine design cycle.

Description

Technical Field

[0001] This invention relates to the field of water turbine technology, and in particular to a method for predicting Karman vortex resonance in mixed-flow water turbines based on the TBR model. Background Technology

[0002] In recent years, the hydropower market has flourished, with a large number of power plants and generating units put into operation. However, during the commissioning phase or after a period of operation, some units, especially large and medium-sized mixed-flow units, have experienced severe vibration, noise, and even cracking problems. Test analysis suggests that this may be related to resonance caused by Karman vortices at the blade outlet.

[0003] Karman vortices are widely present in various fluid machinery. In mixed-flow turbines, Karman vortices mainly appear at the outlet edges of three types of blades with an airfoil structure: fixed guide vanes, movable guide vanes, and runner blades. The mechanism by which Karman vortices induce unit resonance is as follows: When the turbine is operating under certain conditions, the water flow bypasses the blade surface. When a specific Reynolds number is reached, boundary layer separation occurs at the blade trailing edge, detaching and forming vortices at the outlet edge of the airfoil. (e.g.) Figure 1 Asymmetric vortices with opposite rotation directions formed on either side of the airfoil's outlet edge are called Karman vortices. Decoupled Karman vortices exert a periodic alternating force on the blades, causing them to vibrate. When the frequency of the Karman vortices falls within the blade's natural frequency range or a harmonic, Karman vortex resonance occurs. Karman vortex resonance can cause vibration and noise in the unit, and even component cracking and damage, seriously threatening the safe and stable operation of the power plant.

[0004] Currently, in the design phase of hydraulic turbines, major manufacturers generally use the theoretical calculation formula for the Karman vortex frequency. To determine the Karman vortex frequency in the water turbine. Sh This is the Strauhall number, which is generally chosen from 0.22 to 0.25 based on experience; w The velocity at the water outlet of the blade; d The Karman vortex frequency is a characteristic dimension of the blade, typically the diameter at the flow separation point. Because the selection of relevant parameters relies heavily on empirical methods, the calculated Karman vortex frequency often deviates significantly from the actual value. Furthermore, Karman vortex resonance is generally only detected after it occurs, requiring subsequent blade redesign. This theoretical formula-based calculation of the Karman vortex frequency is no longer sufficient to meet the current design requirements for eliminating the influence of Karman vortices in turbine design.

[0005] Numerical simulation for calculating Karman vortex resonance in turbine components offers lower uncertainty and higher accuracy compared to theoretical formulas. However, some studies indicate that conventional turbulence models, except for Large Eddy Simulation (LES), generally cannot successfully calculate the Karman vortex phenomenon at the turbine airfoil tail. LES typically requires extremely fine computational meshes, limiting the time step to very small values. Furthermore, to avoid resonance during design, the number of guide vanes and runner blades in mixed-flow turbines are coprime, causing periodic boundary conditions to fail during calculation. Therefore, accurate calculation of the Karman vortex phenomenon in turbines necessitates transient calculations across the entire flow path, consuming enormous computational resources and significantly extending the turbine design cycle. Summary of the Invention

[0006] The purpose of this invention is to provide a method for predicting Karman vortex resonance in mixed-flow turbines based on the TBR model, thereby addressing the problems existing in the prior art. The TBR model is a method provided by ANSYS CFX for rotating machinery that can calculate transient effects using one or several flow channels. The advantages of this method are that it can significantly reduce the number of meshes, thus reducing computational resource consumption. Combined with the LES turbulence model, it can accurately simulate the Karman vortex phenomenon in turbines, greatly improving the accuracy and efficiency of Karman vortex frequency prediction. This allows for meeting the design requirements of turbines being unaffected by Karman vortices during the design phase and shortening the turbine design cycle, which has positive implications for improving turbine design capabilities and ensuring the safe and stable operation of hydropower stations.

[0007] To achieve the above objectives, the present invention provides the following solution:

[0008] A method for predicting Karman vortex resonance in mixed-flow turbines based on the TBR model includes:

[0009] Construct a three-dimensional geometric model of the flow passage components of the water turbine;

[0010] Determine the operating conditions of the turbine exhibiting Karman vortices;

[0011] Based on the turbine operating conditions where Karman vortices occur and the aforementioned three-dimensional geometric model, a TBR model is used for fluid simulation to obtain the Karman vortex frequencies at the fixed guide vane, the movable guide vane, and the runner.

[0012] The finite element method was used to perform modal analysis on the fixed guide vane, the movable guide vane, and the impeller to obtain the natural frequencies of the components.

[0013] By comparing the Karman vortex frequencies and natural frequencies at the fixed guide vane, the movable guide vane, and the runner, it can be determined whether resonance may occur.

[0014] Optionally, the method of constructing a three-dimensional geometric model of the turbine's flow passage components includes:

[0015] Design the turbine structure according to the preset power station requirements and draw a single-line model of the flow-through components; the flow-through components include: volute, fixed guide vanes, movable guide vanes, runner, and draft tube;

[0016] Based on the preliminary design model single-line diagram, a three-dimensional geometric model of the flow-through component is established.

[0017] Optionally, fluid simulation using a TBR model includes:

[0018] The volute, fixed guide vane, movable guide vane, impeller, and tailrace pipe are each divided into a hexahedral structured mesh model;

[0019] Based on the hexahedral structured mesh model, steady-state numerical simulation of the entire flow channel was performed;

[0020] The hexahedral structured mesh model is processed based on the TBR-FT method; the TBR-FT method includes: based on the hexahedral structured mesh model, dividing the blade cascade channels used in the TBR model, and performing numerical simulation on the blade cascade channels by adopting the FT pitch variation model and dividing them into dual channels;

[0021] Based on the steady-state numerical simulation results, the TBR-FT method is used for transient numerical calculations; the results of the full-channel steady-state calculation are used as the initial conditions for the transient calculations using the TBR-FT method.

[0022] Based on transient numerical calculations, it is determined whether the Karman vortex phenomenon exists, and the Karman vortex frequencies at the fixed guide vane, the movable guide vane, and the runner are obtained.

[0023] Optionally, the volute, fixed guide vane, movable guide vane, impeller, and draft tube are respectively divided into hexahedral structured mesh models, including:

[0024] The grids for the spiral casing and tailrace pipe are divided into overall sections respectively;

[0025] The fixed guide vane is divided into a full-circumference grid.

[0026] The flow channels of the movable guide vane and the impeller are meshed separately, and then the mesh of the entire ring is obtained by rotation copying.

[0027] Optionally, based on the steady-state numerical simulation results, transient numerical calculations using the TBR model include:

[0028] Based on the steady-state calculation settings, modify the analysis type, changing it from Steady to TBR;

[0029] Further settings for the TBR model include the following:

[0030] Selection of pitch variation model: Select the FT pitch variation model and pre-compile the relevant sampling surface and periodic surface specifications;

[0031] Setting the time step: Specify the preset time step;

[0032] Based on the steady-state calculation settings, the turbulence model was modified to the LESSmagorinsky model.

[0033] Modify the dynamic-static interface settings based on the steady-state calculation settings; change the dynamic-static interface to TransientRotor Stator;

[0034] Set up monitoring variables: Set up a series of monitoring points after the fixed guide vane, after the movable guide vane and before the runner inlet, and at the junction of the runner blades with the upper crown and lower ring to dynamically monitor the changes in integral variables and pressure and velocity values.

[0035] Optionally, determining the existence of the Karman vortex phenomenon based on transient numerical calculations includes:

[0036] Based on the monitored changes in pressure and velocity values, pressure and velocity cloud maps are constructed.

[0037] The presence of the Karman vortex phenomenon is initially determined from the cloud map. If the Karman vortex phenomenon exists, the pressure pulsation at the set monitoring points is extracted, and a fast Fourier transform is performed to obtain the dominant frequency of the pressure pulsation at each monitoring point. This dominant frequency of the pressure pulsation is the dominant frequency of the Karman vortex under the corresponding calculation condition. If there is no Karman vortex phenomenon on the cloud map, it indicates that the turbine will not be affected by the Karman vortex under the corresponding calculation condition.

[0038] Optionally, comparing the Karman vortex frequency with the natural frequency at the fixed guide vane, the moving guide vane, and the runner includes:

[0039] If the Karman vortex frequency is within the natural frequency range of the component or a certain preset harmonic of the natural frequency, it indicates that the turbine has a preset probability of Karman vortex resonance. Therefore, the airfoil design of the blades needs to be modified, the three-dimensional geometric model of the turbine flow components needs to be reconstructed, and the Karman vortex frequencies at the fixed guide vane, the movable guide vane and the runner need to be recalculated.

[0040] If the Karman vortex frequency is not within the natural frequency range of the component or a certain preset harmonic, it indicates that the turbine will not experience Karman vortex resonance under the corresponding operating conditions.

[0041] The beneficial effects of this invention are as follows:

[0042] This invention first constructs a three-dimensional geometric model of the turbine's flow components; secondly, it determines the turbine's operating conditions where Karman vortices occur; then, based on the turbine's operating conditions and the aforementioned three-dimensional geometric model, it uses a TBR model to perform fluid simulation, obtaining the Karman vortex frequencies at the fixed guide vanes, movable guide vanes, and runner; next, it uses the finite element method to perform modal analysis on the fixed guide vanes, movable guide vanes, and runner respectively, obtaining the natural frequencies of the components; finally, it compares the relationship between the Karman vortex frequencies at the fixed guide vanes, movable guide vanes, and runner and the natural frequencies to determine whether resonance may occur. The advancement of this invention lies in:

[0043] Accuracy: This method abandons the traditional design approach that heavily relies on theoretical formulas based on Karman vortex frequencies to predict whether a turbine will experience Karman vortex resonance. Instead, it employs numerical simulation methods for more reasonable and accurate prediction of Karman vortex resonance.

[0044] High efficiency: The proposed three-dimensional numerical simulation method for water turbines based on the TBR model greatly reduces the number of computational grids and the consumption of computational resources, thus greatly improving the efficiency of the numerical simulation method in predicting Karman vortex resonance.

[0045] In summary, this invention helps to accurately and efficiently predict Karman vortex resonance during the turbine design phase, thereby avoiding the adverse effects of Karman vortices on turbine operation. It helps to shorten the construction cycle of hydropower stations and extend the service life of turbines, which is of great significance to the safe and stable operation of hydropower stations. Attached Figure Description

[0046] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0047] Figure 1 A schematic diagram of the Karman vortex around the airfoil;

[0048] Figure 2 This is a flowchart illustrating the implementation of an embodiment of the present invention;

[0049] Figure 3 This is a schematic diagram illustrating the principle of the FT pitch variation model.

[0050] Figure 4 This is a full three-dimensional model of the mixed-flow turbine used in the embodiments of the present invention;

[0051] Figure 5 The dual-channel three-dimensional geometry used in the embodiments of the present invention;

[0052] Figure 6This is a schematic diagram of a dual-channel mesh according to an embodiment of the present invention;

[0053] Figure 7 This is a schematic diagram of the FT dual-channel method used in an embodiment of the present invention. Detailed Implementation

[0054] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0055] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0056] This embodiment proposes a method for predicting Karman vortex resonance in mixed-flow turbines based on the TBR model, including:

[0057] Construct a three-dimensional geometric model of the flow passage components of the water turbine;

[0058] Determine the operating conditions of the turbine exhibiting Karman vortices;

[0059] Based on the turbine operating conditions and three-dimensional geometric model in which Karman vortices occur, the TBR model is used for fluid simulation to obtain the Karman vortex frequencies at the fixed guide vane, movable guide vane and runner with the "airfoil" structure.

[0060] The finite element method was used to perform modal analysis on the fixed guide vane, the movable guide vane, and the impeller to obtain the natural frequencies of the components.

[0061] By comparing the Karman vortex frequencies and natural frequencies at the fixed guide vane, the movable guide vane, and the runner, it can be determined whether resonance may occur.

[0062] Specifically, in this embodiment, the turbine components are initially designed based on the actual hydrological conditions of the power station, and a single-line diagram of the model is drawn. Then, a three-dimensional geometric model is established based on the designed turbine single-line diagram. After selecting the operating conditions where Karman vortices may occur, numerical simulations in the fluid domain and structural modal analyses in the solid domain are performed respectively. The numerical simulation in the fluid domain adopts a method based on the TBR model to reduce the number of computational grids, thereby reducing the computational resource consumption of the numerical simulation and quickly and accurately obtaining the Karman vortex frequency. In the solid domain, the traditional method of analyzing the structural modes of components is used to obtain the natural frequencies of each component. By comparing and analyzing the Karman vortex frequencies obtained from the fluid calculations and the natural frequencies obtained from the solid calculations, it is determined whether the Karman vortex resonance phenomenon can occur. This invention has high efficiency, accuracy, and practicality in the early design stage of turbines, and has important application value for shortening the turbine design cycle.

[0063] Furthermore, constructing the three-dimensional geometric model of the turbine's flow-through components includes:

[0064] Design the turbine structure according to the preset power station requirements and draw a single-line model of the flow-through components; the flow-through components include: volute, fixed guide vanes, movable guide vanes, runner, and draft tube;

[0065] Based on the preliminary design model single-line diagram, a three-dimensional geometric model of the flow-through component is established.

[0066] Furthermore, fluid simulations using the TBR model include:

[0067] The volute, fixed guide vane, movable guide vane, impeller, and tailrace pipe are each divided into a hexahedral structured mesh model;

[0068] Steady-state numerical simulation of the entire flow channel is performed based on a hexahedral structured mesh model.

[0069] Based on the hexahedral structured mesh model, the blade cascade channels used in the TBR model are divided. For the blade cascade channels, the FT pitch variation model is used and a dual-channel model is divided for numerical simulation; this method is referred to as the TBR-FT method.

[0070] Based on the steady-state numerical simulation results, the TBR-FT method is used for transient numerical calculations; the results of the full-channel steady-state calculation are used as the initial conditions for the transient calculations using the TBR-FT method.

[0071] Based on transient numerical calculations, it is determined whether the Karman vortex phenomenon exists, and the Karman vortex frequencies at the fixed guide vane, the movable guide vane, and the runner are obtained.

[0072] Karman vortices arise from flow separation at the boundary of a solid as fluid flows around it. In hydraulic turbines, fluid flow around airfoil-designed blades leads to flow separation at the blade trailing points, resulting in Karman vortices. Fixed guide vanes, movable guide vanes, and runner blades are airfoil-designed flow components in hydraulic turbines, and Karman vortices may occur after fluid flows around them. Therefore, research on Karman vortices in hydraulic turbines primarily focuses on these three components. The volute and draft tube, being flow-through components, are extremely unlikely to experience Karman vortices.

[0073] Furthermore, the volute, fixed guide vanes, movable guide vanes, impeller, and tailrace pipe are respectively divided into hexahedral structured mesh models, including:

[0074] The grids for the spiral casing and tailrace pipe are divided into overall sections respectively;

[0075] The fixed guide vane is divided into a full-circumference grid.

[0076] The flow channels of the movable guide vane and the impeller are meshed separately, and then the mesh of the entire ring is obtained by rotation copying.

[0077] Furthermore, based on the steady-state numerical simulation results, transient numerical calculations using the TBR model include:

[0078] Based on the steady-state calculation settings, modify the analysis type, changing it from Steady to TBR;

[0079] Further settings for the TBR model include the following:

[0080] Selection of pitch variation model: Select the FT pitch variation model and pre-compile the relevant sampling surface and periodic surface specifications;

[0081] Setting the time step: Specify the preset time step;

[0082] Based on the steady-state calculation settings, the turbulence model was modified to the LESSmagorinsky model.

[0083] Modify the dynamic-static interface settings based on the steady-state calculation settings; change the dynamic-static interface to TransientRotor Stator;

[0084] Set up monitoring variables: Set up a series of monitoring points after the fixed guide vane, after the movable guide vane and before the runner inlet, and at the junction of the runner blades with the upper crown and lower ring to dynamically monitor the changes in integral variables and pressure and velocity values.

[0085] Furthermore, based on transient numerical calculations, determining the existence of the Karman vortex phenomenon includes:

[0086] Based on the monitored changes in pressure and velocity values, pressure and velocity cloud maps are constructed.

[0087] The presence of the Karman vortex phenomenon is initially determined from the cloud map. If the Karman vortex phenomenon exists, the pressure pulsation at the set monitoring points is extracted, and a fast Fourier transform is performed to obtain the dominant frequency of the pressure pulsation at each monitoring point. This dominant frequency of the pressure pulsation is the dominant frequency of the Karman vortex under the corresponding calculation condition. If there is no Karman vortex phenomenon on the cloud map, it indicates that the turbine will not be affected by the Karman vortex under the corresponding calculation condition.

[0088] Furthermore, the relationship between the Karman vortex frequency and the natural frequency at the fixed guide vane, the moving guide vane, and the runner includes:

[0089] If the Karman vortex frequency is within the natural frequency range of the component or a certain preset harmonic of the natural frequency, it indicates that the turbine has a preset probability of Karman vortex resonance. Therefore, the airfoil design of the blades needs to be modified, the three-dimensional geometric model of the turbine flow components needs to be reconstructed, and the Karman vortex frequencies at the fixed guide vane, the movable guide vane and the runner need to be recalculated.

[0090] If the Karman vortex frequency is not within the natural frequency range of the component or a certain preset harmonic, it indicates that the turbine will not experience Karman vortex resonance under the corresponding operating conditions.

[0091] like Figure 1 The diagram illustrates the phenomenon where water flows around a blade airfoil and separates from the airfoil near the water outlet edge, generating a Karman vortex. The appearance of the Karman vortex applies an alternating force to the blade tail, causing forced vibration of the blade.

[0092] Specifically, in this embodiment, such as Figure 2 This paper presents the implementation steps for predicting Karman vortex resonance in a mixed-flow turbine based on a TBR model. Specifically, it includes:

[0093] Step 1: Design the turbine structure according to the power station requirements and draw a single-line model diagram.

[0094] First, based on the hydrological conditions of the power station, parameters such as head and flow rate, the relevant structures of the turbine are initially designed, and single-line model diagrams of the five major flow components, including the spiral casing, fixed guide vanes, movable guide vanes, runner, and tailrace, are drawn.

[0095] Step 2: Establish a three-dimensional geometric model of the flow-through components based on the single-line diagram of the preliminary design model.

[0096] Based on the single-line diagram, a 3D geometric model of the flow-through component is created in 3D modeling software, and the flow-through component is assembled to further determine the accuracy of the preliminary design and to provide relevant geometric boundaries for subsequent numerical simulations.

[0097] Step 3: Select the operating conditions that may cause Karman vortices in the turbine blades.

[0098] Karman vortices typically occur under high flow rate conditions. Numerical simulations can be performed using the rated operating conditions. If Karman vortices do not occur under the rated operating conditions, other operating conditions can be selected within ±10% of the rated output.

[0099] Step 4: Three-dimensional fluid numerical simulation of the water turbine.

[0100] In this step, the TBR model will be used to simplify the structure of the fixed guide vane, moving guide vane, and impeller, thereby improving the efficiency of the numerical simulation. Performing numerical simulations of the fluid domain and setting relevant monitoring points allows for the acquisition of the Karman vortex frequency.

[0101] Step 4 is the most important part of this invention, and therefore will be described in more detail.

[0102] Step 4.1: Based on Step 2, perform mesh generation. Create high-precision hexahedral structured meshes for the volute, fixed guide vanes, movable guide vanes, impeller, and draft tube. Since the volute and draft tube are not blade cascade types, their meshes need to be generated as a whole. Due to the special nature of the fixed guide vanes, only the entire ring of the flow path can be meshed for the fixed guide vanes. The movable guide vanes and impeller have a certain periodicity, so only one flow path needs to be meshed, and then a rotation-copying method can be used to obtain the full-ring mesh.

[0103] Step 4.2: Perform steady-state numerical simulation on the entire flow channel. The purpose of steady-state numerical simulation is to provide initial conditions for transient simulation. Moreover, steady-state numerical simulation consumes less computational resources than transient simulation. Therefore, conventional steady-state numerical simulation methods for hydraulic turbines can be used. The relevant settings for steady-state simulation will not be elaborated here.

[0104] Step 4.3: Based on Step 4.1, mesh the blade cascade channels used in the TBR model. That is, it is not necessary to obtain the mesh of the entire ring of the active guide vane and runner; only the mesh of one or two channels needs to be rotated and copied for later calculations. Generally, the number of blades in the active guide vane and runner is prime. To accurately calculate the Karman vortex phenomenon, full-ring modeling and calculation are required. By using the TBR model, the flow field in one or several channels of the active guide vane and runner can be solved, greatly reducing computational memory and time while still obtaining relatively accurate results. The TBR model provides three pitch variation models to solve the problem of unequal pitch ratios between the active guide vane and runner during transient calculations: PT (Profile Transformation), TT (Time Transformation), and FT (Fourier Transformation). The PT and TT methods are suitable for small to medium pitch ratios, but the TT method is only applicable to compressible flows and therefore not suitable for this invention. The FT method can be used for both incompressible and compressible simulations, and it is applicable to large, medium, and small pitch ratios. Furthermore, the dual-channel method converges faster than the single-channel method. Therefore, this invention employs the FT pitch variation model and divides the flow into two channels for numerical simulation, hereinafter referred to as the TBR-FT method.

[0105] The TBR-FT method uses a sampling method to reconstruct the solved data and use historical data as a sampling point for the next calculation. Then, the sampling point is expanded into a Fourier series to obtain the functional relationship of the physical quantity of interest at a certain position, as shown in equation (1).

[0106] (1)

[0107] in For physical quantities on the sampling surface, t To calculate time, A m These are the Fourier coefficients. m For the number of samples, ω ω is the rotor angular velocity.

[0108] To convert the data from the sampling surface to the periodic surface, further, such as Figure 3 As shown, the TBR-FT method applies the phase-shift periodic boundary condition. Its basic principle is that the rotor and stator are periodic to each other in the circumferential direction and at different times. The flow variables are sampled on its intermediate surface 2 according to equation (1) to calculate the Fourier coefficients. Then, the method uses the phase lag based on the circumferential angle difference and the rotor angular velocity to process the flow variables on the sampled surface, thereby obtaining the flow variable information on the periodic surfaces 1 and 3, as shown in equations (2) and (3).

[0109] (2)

[0110] (3)

[0111] In the formula: , These are the flow variables on two periodic surfaces, Δ t For time step, P R and P S These are the circumferential angles of the rotor and stator within a flow channel, respectively. R Let be the radius.

[0112] In a specific application of this invention: the mixed-flow turbine model used in this embodiment includes 14 fixed guide vanes, 28 movable guide vanes, and a runner composed of 15 long blades and 15 segmental blades. The three-dimensional model of the entire flow channel is as follows: Figure 4 As shown in the figure. This embodiment divides the movable guide vanes and runner into a dual-channel design. Since the runner of this turbine model is of the long and short blade type, each flow channel of the runner must contain one long blade and one short blade. Therefore, the combination of four movable guide vanes and two pairs of long and short blades is used as the dual-channel calculation model, as shown in the figure. Figure 5 As shown; specifically, the single-channel movable guide vane mesh generated in step 4.1 is rotated and copied 4 times, and the single-channel impeller mesh is rotated and copied 2 times. The central angle of the movable guide vane dual-channel is 51.43° (4×360° / 28 blades), and the central angle of the impeller dual-channel is 48° (2×360° / 15 sets of blades). The pitch ratio (ratio of central angles) between the movable guide vane and the impeller dual-channel is 1.07. The FT model does not have strict restrictions on the pitch ratio, but it is recommended that the pitch ratio be as close to 1 as possible. Figure 6 This is a schematic diagram of a hexahedral structured mesh for a dual-channel system.

[0113] Step 4.4: Based on Step 4.2, modify the steady-state calculation settings and use the TBR model for transient numerical calculations. Specifically:

[0114] Step 4.4.1: Modify the analysis type based on the steady-state calculation settings. Change the analysis type from Steady to Transient Blade Row (TBR). This allows you to use one or more flow channels defined in Step 4.3 to reduce the overall mesh size and transiently simulate the transient flow behavior in the turbine.

[0115] Step 4.4.2: Further configure the TBR model. This mainly includes the following steps:

[0116] Step 4.4.2.1: Selection of Pitch Variation Model: Select the Fourier Transform (FT) pitch variation model, according to the following... Figure 7 The schematic diagram shown completes the specification of the relevant sampling surface and periodic surface.

[0117] Step 4.4.2.2: Setting the time step: Similar to traditional transient calculations, the TBR model is a transient calculation model and requires specifying the time step. This invention suggests that the time step can be selected as the time taken for each rotation of the wheel by 1~3°.

[0118] Step 4.4.3: Modify the turbulence model based on the steady-state calculation settings. The turbulence model for steady-state calculations is modified to the LES Smagorinsky model. This model uses direct numerical simulation to solve for vortices larger than the grid scale, while vortices smaller than the grid scale are solved through subgrid simulation. Using this turbulence model, the Karman vortex phenomenon within the turbine can be calculated more accurately.

[0119] Step 4.4.4: Modify the dynamic-static interface settings based on the steady-state calculation settings. Since the steady-state calculation ignores the dynamic characteristics of the runner rotation, the dynamic-static interface generally uses a frozen rotor model. However, the Karman vortex phenomenon is a transient flow phenomenon in a turbine, requiring the dynamic-static interface to be modified to a Transient Rotor Stator. There are two dynamic-static interfaces in a turbine: the interface between the runner inlet and the movable guide vane outlet, and the interface between the runner outlet and the draft tube inlet.

[0120] Step 4.4.5: Set monitoring variables. In areas susceptible to Karman vortices, such as after the fixed guide vane, after the movable guide vane and before the runner inlet, and at the junction of the runner blades with the upper crown and lower ring, a series of monitoring points are set to dynamically monitor changes in integral variables and values ​​such as pressure and velocity.

[0121] Step 4.5: Use the results of the full-channel steady-state calculation as the initial conditions for the transient calculation using the TBR-FT method.

[0122] Step 4.6: Post-processing. Create pressure and velocity contour maps on the relevant surfaces to preliminarily determine the presence of Karman vortex phenomena. If Karman vortex phenomena are present, extract the pressure pulsations at the monitoring points mentioned above and perform a Fast Fourier Transform (FFT) to obtain the dominant frequency of the pressure pulsations at each monitoring point. This dominant frequency is the Karman vortex frequency under this calculation condition. If no Karman vortex phenomena are found on the contour map, it indicates that the turbine will not be affected by Karman vortices under this calculation condition.

[0123] Step 5: Modal analysis of component structure.

[0124] The finite element method was used to perform structural modal analysis on the fixed guide vane, the movable guide vane, and the runner to obtain their respective natural frequencies, mode shapes, and modal parameters. During the analysis, it was possible to analyze the structural modes of only one or a few blades without having to analyze the entire component.

[0125] Step 6: Compare the calculated Karman vortex frequency with the component's natural frequency. If the Karman vortex frequency is within the component's natural frequency range or a harmonic of that natural frequency, it indicates a high probability of Karman vortex resonance in the turbine. Therefore, the blade airfoil design needs to be modified, repeating steps 2-5. If the Karman vortex frequency is not within the natural frequency range or a harmonic of that natural frequency, it indicates that the turbine will not experience Karman vortex resonance under this operating condition, and the design results can be used for power plant operation.

[0126] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for predicting Karman vortex resonance in a mixed-flow turbine based on a TBR model, characterized in that, include: Construct a three-dimensional geometric model of the turbine's flow-through components; including: Design the turbine structure according to the preset power station requirements and draw a single-line model of the flow-through components; the flow-through components include: volute, fixed guide vanes, movable guide vanes, runner, and draft tube; Based on the preliminary design model single-line diagram, a three-dimensional geometric model of the flow-through component is established; Determine the operating conditions of the turbine exhibiting Karman vortices; Based on the turbine operating conditions where Karman vortices occur and the aforementioned three-dimensional geometric model, a TBR model is used for fluid simulation to obtain the Karman vortex frequencies at the fixed guide vane, the movable guide vane, and the runner. Fluid simulation using the TBR model includes: The volute, fixed guide vane, movable guide vane, impeller, and tailrace pipe are each divided into a hexahedral structured mesh model; Based on the hexahedral structured mesh model, steady-state numerical simulation of the entire flow channel was performed; The hexahedral structured mesh model is processed based on the TBR-FT method; the TBR-FT method includes: based on the hexahedral structured mesh model, dividing the blade cascade channels used in the TBR model, and performing numerical simulation on the blade cascade channels by adopting the FT pitch variation model and dividing them into dual channels; Based on the steady-state numerical simulation results, the Transient Flow Ratio-FT (TBR-FT) method is used for transient numerical calculations. Specifically, the results of the full-channel steady-state calculations are used as the initial conditions for the transient calculations using the TBR-FT method, including: Based on the steady-state calculation settings, modify the analysis type, changing it from Steady to TBR; Further settings for the TBR model include the following: Selection of pitch variation model: Select the FT pitch variation model and pre-compile the relevant sampling surface and periodic surface specifications; Setting the time step: Specify the preset time step; Based on the steady-state calculation settings, the turbulence model was modified to the LES Smagorinsky model. Modify the dynamic-static interface settings based on the steady-state calculation settings; change the dynamic-static interface to Transient RotorStator; Set up monitoring variables: Set up a series of monitoring points after the fixed guide vane, after the movable guide vane and before the runner inlet, and at the junction of the runner blades with the upper crown and lower ring to dynamically monitor the changes in integral variables and pressure and velocity values; Based on transient numerical calculations, it is determined whether the Karman vortex phenomenon exists, and the Karman vortex frequencies at the fixed guide vane, the movable guide vane, and the runner are obtained; The finite element method was used to perform modal analysis on the fixed guide vane, the movable guide vane, and the impeller to obtain the natural frequencies of the components. By comparing the Karman vortex frequency with the natural frequency at the fixed guide vane, the movable guide vane, and the runner, it can be determined whether resonance occurs. The relationship between the Karman vortex frequency and the natural frequency at the fixed guide vane, the moving guide vane, and the runner includes: If the Karman vortex frequency is within the natural frequency range of the component or a certain preset harmonic of the natural frequency, it indicates that the turbine has a preset probability of Karman vortex resonance. Therefore, the airfoil design of the blades needs to be modified, the three-dimensional geometric model of the turbine flow components needs to be reconstructed, and the Karman vortex frequencies at the fixed guide vane, the movable guide vane and the runner need to be recalculated. If the Karman vortex frequency is not within the natural frequency range of the component or a certain preset harmonic, it indicates that the turbine will not experience Karman vortex resonance under the corresponding operating conditions.

2. The method for predicting Karman vortex resonance in a mixed-flow turbine based on a TBR model according to claim 1, characterized in that, The volute, fixed guide vanes, movable guide vanes, impeller, and draft tube are each divided into a hexahedral structured mesh model, including: The grids for the spiral casing and tailrace pipe are divided into overall sections respectively; The fixed guide vane is divided into a full-circumference grid. The flow channels of the movable guide vane and the impeller are meshed separately, and then the mesh of the entire ring is obtained by rotation copying.

3. The method for predicting Karman vortex resonance in a mixed-flow turbine based on the TBR model according to claim 1, characterized in that, Determining the existence of the Karman vortex phenomenon based on transient numerical calculations includes: Based on the monitored changes in pressure and velocity values, pressure and velocity cloud maps are constructed. The presence of the Karman vortex phenomenon is initially determined from the cloud map. If the Karman vortex phenomenon exists, the pressure pulsation at the set monitoring points is extracted, and a fast Fourier transform is performed to obtain the dominant frequency of the pressure pulsation at each monitoring point. This dominant frequency of the pressure pulsation is the dominant frequency of the Karman vortex under the corresponding calculation condition. If there is no Karman vortex phenomenon on the cloud map, it indicates that the turbine will not be affected by the Karman vortex under the corresponding calculation condition.

Images