Three-dimensional calculation system and method for blade dynamic stress of steam turbine under ultra-low load

Abstract

The present application relates to a steam turbine ultra-low load blade dynamic stress three-dimensional calculation system and a calculation method thereof, and belongs to the technical field of blade dynamic stress prediction. The problem solved is the calculation of blade dynamic stress under an ultra-low load working condition. The system comprises a thermal boundary calculation module, a blade three-dimensional flow field calculation module, a blade finite element calculation module and a blade dynamic stress calculation module. The thermal boundary calculation module is used to obtain the thermal data of blade inlet and outlet flow, total enthalpy and pressure, and provide the thermal boundary for the blade three-dimensional flow field calculation. The blade three-dimensional flow field calculation module is used to obtain the calculation results of blade steady-state and transient flow field, and provide the input data for the blade structure field static stress and dynamic stress calculation. The blade finite element calculation module is used to obtain the calculation results of blade static stress and resonance frequency, and provide the input data for the blade dynamic stress calculation. The blade dynamic stress calculation module is used to obtain the calculation results of blade dynamic stress, and provide the data for the blade dynamic stress examination module.

Description

Technical Field

[0001] This invention relates to a dynamic stress calculation system and method for turbine blades under ultra-low load, belonging to the field of blade dynamic stress prediction technology. Background Technology

[0002] The global energy landscape is shifting from reliance on traditional fossil fuels to increasing and promoting clean and efficient energy sources. New energy sources, such as wind and solar power, are beneficial for carbon reduction, but they are susceptible to natural environmental factors and exhibit significant fluctuations, intermittency, and instability. Domestic and international experience has demonstrated that while new energy sources are growing rapidly, it is essential to strengthen and match them with flexible, stable, and dispatchable power sources. China's energy resource endowment dictates that safe and reliable coal-fired power plants will play a crucial role for a considerable period. Adhering to the principle of "ensuring a safety net for energy supply," overcoming the key technical challenges of flexible operation of main power plant equipment under all operating conditions is imperative.

[0003] Extensive technological development and engineering practice by relevant enterprises both domestically and internationally have demonstrated that the key technologies for flexible operation of modern large steam turbines under all operating conditions should focus on the low-pressure flow path, particularly the long blades of the last stage. The level of development and research in this area is a crucial indicator of comprehensive technological capability. During long-term low-load, variable-condition operation of the unit, the long blades in the low-pressure module will inevitably experience eddy current-induced vibration responses, leading to fluctuations in blade dynamic stress and difficulty in identifying high-value regions. Furthermore, blade dynamic stress testing is costly and requires stringent conditions, making it difficult to implement and hindering the assessment and evaluation of the safety of long turbine blades operating under ultra-low load conditions. Therefore, it is necessary to propose a method for calculating blade dynamic stress suitable for ultra-low load operation of steam turbines, providing a technical means for evaluating blade dynamic stress.

[0004] Currently, the industry's three-dimensional simulation calculation and evaluation methods for blade dynamic stress mainly focus on blade harmonic vibration, with the excitation factor related to the harmonic number. For blade vibration characteristics under ultra-low load and turbulent flow fields, which are typically random vibrations caused by eddy current effects and unrelated to harmonic vibration, the excitation force should be extracted and analyzed based on the flow field characteristics for calculation. However, there is currently a lack of effective methods for calculating blade dynamic stress specifically for ultra-low load conditions.

[0005] Based on the above problems, it is urgent to propose a three-dimensional calculation system and calculation method for the dynamic stress of turbine blades under ultra-low load in order to solve the above technical problems. Summary of the Invention

[0006] This invention provides a three-dimensional calculation method for the dynamic stress of turbine blades under ultra-low load conditions. This invention addresses the problem of calculating blade dynamic stress under ultra-low load conditions. A brief overview of the invention is provided below to offer a basic understanding of certain aspects of it. It should be understood that this overview is not an exhaustive summary of the invention. It is not intended to identify key or essential parts of the invention, nor is it intended to limit the scope of the invention.

[0007] The technical solution of the present invention:

[0008] A three-dimensional calculation system for dynamic stress of turbine blades under ultra-low load includes a thermal boundary calculation module, a three-dimensional flow field calculation module for blades, a finite element calculation module for blades, and a dynamic stress calculation module for blades.

[0009] Thermodynamic boundary calculation module: used to obtain thermodynamic data of blade inlet and outlet flow rate, total enthalpy, and pressure, providing thermodynamic boundaries for blade three-dimensional flow field calculation;

[0010] Blade 3D Flow Field Calculation Module: Used to obtain steady-state and transient flow field calculation results of the blade, and provide input data for the calculation of static and dynamic stress of the blade structure field;

[0011] The blade 3D flow field calculation module includes: a blade flow field calculation domain modeling module, a steady-state flow field calculation module, and a transient flow field calculation module;

[0012] Blade finite element calculation module: used to obtain the calculation results of blade static stress and resonance frequency, and to provide input data for blade dynamic stress calculation;

[0013] The blade finite element calculation module includes: a blade structural field calculation domain modeling module, a blade static stress calculation module, and a blade resonance frequency calculation module;

[0014] Blade dynamic stress calculation module: Used to obtain the dynamic stress calculation results of the blade, providing data for the blade dynamic stress assessment module.

[0015] Preferably, it also includes a blade dynamic stress assessment module: used to evaluate and assess the dynamic stress safety of the blade and guide the optimal design of the blade.

[0016] A three-dimensional calculation method for dynamic stress in turbine blades under ultra-low load includes the following steps:

[0017] Step 1: Thermal boundary calculation; Using the thermal boundary calculation module, for the target blade, the thermal calculation program is used to obtain the blade thermal boundary under different turbine load conditions, and the blade inlet flow rate, total enthalpy, outlet pressure and temperature values ​​are extracted as the input boundary for the blade three-dimensional flow field calculation;

[0018] Step 2: Blade 3D flow field calculation; Using the blade 3D flow field calculation module, firstly, the blade flow field calculation domain modeling module is used to model the blade flow field calculation domain, then the steady-state flow field calculation module is used to perform steady-state calculation of the blade flow field, and finally the transient flow field calculation module is used to perform transient calculation of the blade flow field.

[0019] Step 3: Blade finite element calculation; Using the blade finite element calculation module, firstly, the blade structural field calculation domain modeling module is used to model the blade structural field calculation domain, then the blade static stress calculation module is used to calculate the blade static stress, and finally the blade resonance frequency calculation module is used to calculate the blade resonance frequency.

[0020] Step 4: Calculate the dynamic stress of the blade using the blade dynamic stress calculation module.

[0021] Preferred: The blade thermal boundary includes typical ultra-low load operating points of 5%, 10%, 15%, 20%, 25%, and 30%.

[0022] Preferred method: In step two, the blade flow field computational domain modeling is performed: a three-dimensional flow field computational domain model of the blade is established using CFD software, the blade is meshed, and the number of mesh nodes meets the mesh independence requirement. The computational domain is set with the stationary blade inlet as the computational domain inlet, and the inlet flow rate and total enthalpy are given. The computational domain is set with the moving blade outlet as the computational domain outlet, and the outlet pressure and temperature are given. The stationary blade computational domain is stationary, and the moving blade computational domain is rotating. The rotation speed is the turbine operating speed. The cyclic symmetric method is used for simulation, and steam is used as the working fluid in the computational domain.

[0023] Steady-state calculation of the blade flow field adopts steady-state CFD calculation mode to obtain steady-state results of the blade flow field at the blade thermal boundary, understand the distribution characteristics of flow field separation and de-flow, and use the steady-state CFD calculation result file as the input file for transient flow field calculation, and use the blade surface pressure data calculated by steady-state CFD as the input file for structural field finite element static stress calculation.

[0024] The transient flow field of the blade is calculated using a transient CFD calculation mode. During the transient calculation, the steam pressure on the blade surface is calculated using statistical parameters to obtain the transient results of the blade flow field at the blade thermal boundary. The mean square error of the steam pressure at each node on the blade surface is extracted, and the excitation force input file for the dynamic stress calculation module is compiled. The excitation force input file is in CSV format and contains four columns of data. The first three columns are the coordinates of the nodes in the x, y, and z directions, and the fourth column is the mean square error of the steam pressure.

[0025] Preferred method: In step three, the blade structure field computational domain is modeled: using finite element method (FEM) software, a blade structure field model is established. The blade structure field model includes the blade and the wheel groove, with the blade root connected to the wheel groove. The blade structure field model is meshed, and the computational domain includes all structural features of the blade, including the end band, the tie rod in the middle of the blade, the blade body, and the blade root at the end of the blade. Frictional contact is set between adjacent end bands and tie rods. A frictional foundation is set between the blade root and the wheel groove, and the cyclic symmetry method is used for simulation. The wheel groove is a fixed constraint, and the rotational speed of the moving blade and the wheel groove is the turbine operating speed to simulate the centrifugal force.

[0026] Blade static stress calculation: Based on the blade structural field model, the blade surface pressure results of the steady-state calculation of the blade flow field in the steady-state flow field calculation module are used as the input file for static stress calculation. The steam force is simulated to calculate the blade static stress and obtain the blade static stress distribution characteristics. The static stress calculation results are used as the input data for the blade dynamic stress assessment module.

[0027] Blade resonance frequency calculation: Based on the blade structural field model, the blade resonance frequency is calculated to obtain the first two blade resonance frequencies at the turbine operating speed. The resonance frequency results are used as input to the blade dynamic stress calculation module.

[0028] Preferred: Blade structure field computational domain modeling: When meshing the blade structure field model, refine the mesh at the rounded edges of the moving blade, the rounded edges of the moving blade inlet and outlet, and the rounded edges of the wheel groove.

[0029] Preferred method: In step four, based on the blade structural field model, the root mean square error of the blade surface pressure calculated by the transient flow field calculation module is used as the excitation force input file, and the resonance frequency corresponding to the resonance mode closest to the operating speed calculated by the blade resonance frequency calculation module is used as the input file. The blade dynamic stress is calculated to obtain the blade dynamic stress distribution characteristics, and the dynamic stress calculation results are used as the input data of the blade dynamic stress assessment module.

[0030] Preferably, it also includes step five: blade dynamic stress assessment, using a blade dynamic stress assessment module.

[0031] Preferred method: In step five, a dynamic stress assessment table for the blades is prepared;

[0032] The second column of the assessment table, “Leaf Height,” refers to the height of the blade, in mm. The third column of the assessment table, “Operating Condition,” refers to the calculated ultra-low load operating point. The “Assessment Position” given in the assessment table is divided into two categories: blade assessment and leaf root assessment. The blade assessment is divided into assessment position A and assessment position B, and the leaf root assessment is divided into assessment position C and assessment position D.

[0033] The allowable dynamic stress value given in the assessment table refers to the maximum value at which the dynamic stress of the blade meets the design safety requirements;

[0034] The process for obtaining the allowable dynamic stress value is as follows: based on the GOODMAN test curve of the blade material, using static stress data as input, querying the vibration resistance of the material, and using the safety factor specified by the manufacturer as the criterion, calculating the allowable dynamic stress value;

[0035] The allowable value of dynamic stress is calculated using the following formula: Allowable value of dynamic stress = Vibration resistance strength / Safety factor;

[0036] The safety criterion for blade dynamic stress assessment is: the dynamic stress at the assessment location on the blade body and blade root should be less than or equal to the allowable dynamic stress value. If the assessment fails, the blade design needs to be optimized by increasing blade damping, blade rigidity, or upgrading the blade material grade to ensure that the blade dynamic stress assessment is qualified.

[0037] The present invention has the following beneficial effects:

[0038] This invention is applicable to the vibration characteristics of blades under ultra-low load and turbulent flow fields, which are usually random vibrations caused by eddy current effects. Starting from the flow field characteristics, the excitation force is extracted and analyzed, thereby realizing the calculation of blade dynamic stress.

[0039] This invention extracts the pulsating pressure of the flow field as the excitation force for calculating the dynamic stress of the blade. Compared with the current two-way fluid-structure interaction calculation method, it can significantly reduce the dependence on computer resources and reduce the calculation time, making it suitable for engineering applications.

[0040] The effectiveness of the algorithm has been verified by blade dynamic stress test. The calculated value of the present invention deviates from the experimental value by about 13.6%, which meets the requirements of engineering application. Attached Figure Description

[0041] Figure 1 This is a framework diagram of a three-dimensional calculation method for dynamic stress of turbine blades under ultra-low load.

[0042] Figure 2 This is a schematic diagram of the computational domain modeling of the blade flow field;

[0043] Figure 3 The results are the steady-state flow field calculations for the blades under low load conditions (taking 15% as an example).

[0044] Figure 4 This is a schematic diagram of the blade structure.

[0045] In the diagram: 1-Stationary blade, 2-Moving blade, 3-Stationary blade inlet, 4-Moving blade outlet, 5-Wheel groove, 6-Blade root, 7-Stretcher, 8-Enclosure. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of this invention clearer, the invention is described below with reference to specific embodiments shown in the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0047] The connections mentioned in this invention are divided into fixed connections and detachable connections. Fixed connections, also known as non-detachable connections, include but are not limited to conventional fixed connection methods such as folded connections, riveted connections, adhesive connections, and welded connections. Detachable connections include but are not limited to conventional disassembly methods such as threaded connections, snap-fit ​​connections, pin connections, and hinged connections. When a specific connection method is not explicitly defined, it is assumed that at least one existing connection method can always be found to achieve the function, and those skilled in the art can choose according to their needs. For example, a welded connection can be chosen for fixed connections, and a hinged connection can be chosen for detachable connections.

[0048] Specific implementation method one: Combining Figures 1-4 This embodiment describes a three-dimensional calculation system for the dynamic stress of turbine blades under ultra-low load, which includes a thermal boundary calculation module, a blade three-dimensional flow field calculation module, a blade finite element calculation module, and a blade dynamic stress calculation module.

[0049] Thermodynamic boundary calculation module: used to obtain thermodynamic data of blade inlet and outlet flow rate, total enthalpy, and pressure, providing thermodynamic boundaries for blade three-dimensional flow field calculation;

[0050] Blade 3D Flow Field Calculation Module: Used to obtain steady-state and transient flow field calculation results of the blade, and provide input data for the calculation of static and dynamic stress of the blade structure field;

[0051] The blade 3D flow field calculation module includes: blade flow field calculation domain modeling module 2.1, steady-state flow field calculation module 2.2, and transient flow field calculation module 2.3;

[0052] Blade finite element calculation module: used to obtain the calculation results of blade static stress and resonance frequency, and to provide input data for blade dynamic stress calculation;

[0053] The blade finite element calculation module includes: blade structural field calculation domain modeling module 3.1, blade static stress calculation module 3.2, and blade resonance frequency calculation module 3.3;

[0054] Blade dynamic stress calculation module: Used to obtain the dynamic stress calculation results of the blade, providing data for the blade dynamic stress assessment module;

[0055] It also includes a blade dynamic stress assessment module: used to evaluate and assess the dynamic stress safety of blades and guide blade optimization design.

[0056] Specific Implementation Method Two: Combining Figures 1-4 This embodiment describes a three-dimensional calculation method for dynamic stress of turbine blades under ultra-low load, which includes a thermal boundary calculation module, a blade three-dimensional flow field calculation module, a blade finite element calculation module, and a blade dynamic stress calculation module.

[0057] Thermodynamic boundary calculation module: used to obtain thermodynamic data of blade inlet and outlet flow rate, total enthalpy, and pressure, providing thermodynamic boundaries for blade three-dimensional flow field calculation;

[0058] Blade 3D Flow Field Calculation Module: Used to obtain steady-state and transient flow field calculation results of the blade, and provide input data for the calculation of static and dynamic stress of the blade structure field;

[0059] The blade 3D flow field calculation module includes: blade flow field calculation domain modeling module 2.1, steady-state flow field calculation module 2.2, and transient flow field calculation module 2.3;

[0060] Blade finite element calculation module: used to obtain the calculation results of blade static stress and resonance frequency, and to provide input data for blade dynamic stress calculation;

[0061] The blade finite element calculation module includes: blade structural field calculation domain modeling module 3.1, blade static stress calculation module 3.2, and blade resonance frequency calculation module 3.3;

[0062] Blade dynamic stress calculation module: Used to obtain the dynamic stress calculation results of the blade, providing data for the blade dynamic stress assessment module;

[0063] It also includes a blade dynamic stress assessment module: used to evaluate and assess the dynamic stress safety of blades and guide blade optimization design;

[0064] Includes the following steps:

[0065] Step 1: Combining Figure 1 Thermal boundary calculation: Using the thermal boundary calculation module, for the target blade, the thermal calculation program obtains the blade thermal boundary under different turbine load conditions, and extracts the blade inlet flow rate, total enthalpy, outlet pressure and temperature values ​​as the input boundary for the blade three-dimensional flow field calculation;

[0066] The blade thermal boundary includes typical ultra-low load operating points such as 5%, 10%, 15%, 20%, 25%, and 30%.

[0067] Step 2: Blade 3D Flow Field Calculation; Using the blade 3D flow field calculation module, firstly, the blade flow field calculation domain modeling module 2.1 is used to model the blade flow field calculation domain, then the steady-state flow field calculation module 2.2 is used to perform steady-state calculation of the blade flow field, and finally the transient flow field calculation module 2.3 is used to perform transient calculation of the blade flow field.

[0068] Combination Figure 2 In step two, the blade flow field computational domain modeling is performed: a three-dimensional flow field computational domain model of the blade is established using CFD software. The blade consists of a stationary blade 1 and a moving blade 2, which are connected. The stationary blade side has a stationary blade inlet 3, and the moving blade side has a moving blade outlet 4. The blade is meshed, and the number of mesh nodes meets the mesh independence requirement. The computational domain is set with the stationary blade inlet as the computational domain inlet, with the inlet flow rate and total enthalpy value given, and the moving blade outlet as the computational domain outlet, with the outlet pressure and temperature given. The stationary blade computational domain is stationary, and the moving blade computational domain is rotating, with the rotation speed being the turbine operating speed (3000 r / min). The cyclic symmetry method is used for simulation, and the cyclic symmetry boundary can be automatically generated by the CFD software. The working fluid in the computational domain is steam, and the steam property data is sourced from the NIST library and can be called by the CFD software.

[0069] Combination Figure 3 Steady-state calculation of the blade flow field is performed using a steady-state CFD calculation mode to obtain the steady-state results of the blade flow field at the blade thermal boundary, to understand the distribution characteristics of flow field separation and de-flow, and to use the steady-state CFD calculation result file as the input file for transient flow field calculation, and to use the blade surface pressure data from the steady-state CFD calculation as the input file for finite element static stress calculation of the structural field.

[0070] The transient flow field of the blade is calculated using a transient CFD calculation mode. During the transient calculation, statistical parameters of the steam pressure on the blade surface are calculated, including the average pressure, root mean square error, maximum value, and minimum value within the calculation period. The transient results of the blade flow field at the thermal boundary of the blade are obtained. The root mean square error of the steam pressure at each node on the blade surface is extracted. The excitation force input file for the dynamic stress calculation module is compiled. The excitation force input file is in CSV format and contains four columns of data. The first three columns are the coordinates of the nodes in the x, y, and z directions, and the fourth column is the root mean square error of the steam pressure.

[0071] Combination Figure 4 Step 3: Blade finite element calculation; Using the blade finite element calculation module, firstly, the blade structure field calculation domain modeling module 3.1 is used to model the blade structure field calculation domain, then the blade static stress calculation module 3.2 is used to calculate the blade static stress, and finally the blade resonance frequency calculation module 3.3 is used to calculate the blade resonance frequency.

[0072] In step three, the blade structure field computational domain is modeled: using finite element method (FEM) software, a blade structure field model is established. The blade structure field model includes the blade and wheel groove 5, with the blade root 6 connected to the wheel groove 5. The blade structure field model is meshed, and the computational domain includes all structural features of the blade, including the shroud 8 at the blade tip, the tie rod 7 in the middle of the blade, the blade body, and the blade root 6 at the blade tip. Frictional contact is set between adjacent shrouds 8 and tie rods 7, with a friction coefficient of 0.2. A frictional base is set between the blade root 6 and the wheel groove 5, with a friction coefficient of 0.25. The cyclic symmetry method is used for simulation. The cyclic symmetry boundary is usually the periodic surface of the shroud and wheel groove. The wheel groove is a fixed constraint. The rotational speed of the moving blade and wheel groove is the turbine operating speed (3000 r / min) to simulate centrifugal force. The materials of the moving blade and wheel groove are derived from material test data, mainly including elastic modulus, Poisson's ratio, and density.

[0073] Blade structure field computational domain modeling: When meshing the blade structure field model, the mesh is refined at the rounded edges of the moving blade, the rounded edges of the moving blade inlet and outlet steam, and the rounded edges of the wheel groove to ensure computational accuracy;

[0074] Blade static stress calculation: Based on the blade structural field model, the blade surface pressure results of the steady-state calculation of the blade flow field in the steady-state flow field calculation module 2.2 are used as the input file for static stress calculation. The steam force is simulated to calculate the blade static stress and obtain the blade static stress distribution characteristics. The static stress calculation results are used as the input data for the blade dynamic stress assessment module.

[0075] Blade resonance frequency calculation: Based on the blade structural field model, the blade resonance frequency is calculated to obtain the first two blade resonance frequencies at the turbine operating speed (3000 r / min). The resonance frequency results are used as the input to the blade dynamic stress calculation module.

[0076] Step 4: Blade dynamic stress calculation, using the blade dynamic stress calculation module;

[0077] In step four, based on the blade structure field model, the root mean square error of the blade surface pressure calculated by the transient flow field calculation module 2.3 is used as the excitation force input file, and the resonance frequency corresponding to the resonance mode closest to the working speed (3000 r / min) calculated by the blade resonance frequency calculation module 3.3 is used as the input file to calculate the blade dynamic stress, obtain the blade dynamic stress distribution characteristics, and use the dynamic stress calculation results as the input data of the blade dynamic stress assessment module.

[0078] It also includes step five: blade dynamic stress assessment (monitoring), using the blade dynamic stress assessment module;

[0079] In step five, a dynamic stress assessment table for the blades is compiled, as shown in Table 1:

[0080] Table 1 Blade Dynamic Stress Assessment Table

[0081]

[0082] The second column of the assessment table, “Leaf Height,” refers to the height of the blade, in mm. The third column of the assessment table, “Operating Condition,” refers to the calculated ultra-low load operating point. The “Assessment Position” given in the assessment table is divided into two categories: blade assessment and leaf root assessment. The blade assessment is divided into assessment position A and assessment position B, and the leaf root assessment is divided into assessment position C and assessment position D.

[0083] The assessment location A refers to the peak position of the blade static stress; the peak static stress is extracted from the blade static stress calculation module 3.2; the dynamic stress value includes the first-order resonance assessment point and the second-order resonance assessment point; the resonance assessment point refers to the resonance mode closest to the operating speed (3000 r / min) calculated by the blade resonance frequency calculation module 3.3; the dynamic stress values ​​of the first-order resonance assessment point and the second-order resonance assessment point are extracted from the blade dynamic stress calculation;

[0084] The assessment location B refers to the peak position of the blade dynamic stress; the peak position of the dynamic stress includes the first-order resonance assessment point and the second-order resonance assessment point; the resonance assessment point refers to the resonance mode closest to the operating speed (3000 r / min) calculated by the blade resonance frequency calculation module 3.3; the peak dynamic stress of the first-order resonance assessment point and the second-order resonance assessment point is extracted from the blade dynamic stress calculation; the first-order and second-order static stress values ​​are extracted from the blade static stress calculation module 3.2.

[0085] The assessment location C refers to the peak position of the blade root static stress; the peak static stress is extracted from the blade static stress calculation in the blade static stress calculation module 3.2; the dynamic stress value includes the first-order resonance assessment point and the second-order resonance assessment point; the resonance assessment point refers to the resonance mode closest to the working speed (3000 r / min) calculated by the blade resonance frequency calculation module in the blade resonance frequency calculation module 3.3; the dynamic stress values ​​of the first-order resonance assessment point and the second-order resonance assessment point are extracted from the blade dynamic stress calculation module;

[0086] The assessment position D refers to the peak position of dynamic stress at the blade root; the peak position of dynamic stress includes the first-order resonance assessment point and the second-order resonance assessment point; the resonance assessment point refers to the resonance mode closest to the operating speed (3000 r / min) calculated by the blade resonance frequency calculation module 3.3; the peak dynamic stress of the first-order and second-order resonance assessment points is extracted from the blade dynamic stress calculation module; the static stress values ​​of the first and second orders are extracted from the blade static stress calculation module 3.2.

[0087] The allowable dynamic stress value given in the assessment table refers to the maximum value at which the dynamic stress of the blade meets the design safety requirements;

[0088] The process for obtaining the allowable dynamic stress value is as follows: based on the GOODMAN test curve of the blade material, using static stress data as input, querying the vibration resistance of the material, and using the safety factor specified by the manufacturer as the criterion, calculating the allowable dynamic stress value;

[0089] The allowable value of dynamic stress is calculated using the following formula: Allowable value of dynamic stress = Vibration resistance strength / Safety factor;

[0090] The safety criterion for blade dynamic stress assessment is: the dynamic stress at the assessment location of the blade body and blade root is less than or equal to the allowable value of dynamic stress. When the assessment fails, the blade needs to be optimized by increasing blade damping, blade rigidity, or upgrading the blade material grade to ensure that the blade dynamic stress assessment is qualified.

[0091] This invention is applicable to the calculation of blade dynamic stress under ultra-low load operating conditions of steam turbines, providing a technical means for blade dynamic stress prediction and assessment.

[0092] It should be noted that in the above embodiments, as long as the technical solutions are not contradictory, they can be permuted and combined. Those skilled in the art can exhaust all possibilities based on the mathematical knowledge of permutation and combination. Therefore, the present invention will not describe the technical solutions after permutation and combination one by one, but it should be understood that the technical solutions after permutation and combination have been disclosed by the present invention.

[0093] This embodiment is merely an exemplary description of this patent and does not limit its scope of protection. Those skilled in the art can make partial changes to it, as long as they do not exceed the spirit and essence of this patent, they are all within the scope of protection of this patent.

Claims

1. A three-dimensional calculation method for dynamic stress of turbine blades under ultra-low load, characterized in that: The process includes the following steps: Step 1: Thermal boundary calculation; Using the thermal boundary calculation module, for the target blade, the thermal calculation program is used to obtain the blade thermal boundary under different turbine load conditions, and the blade inlet flow rate, total enthalpy, outlet pressure and temperature values ​​are extracted as the input boundary for the blade three-dimensional flow field calculation; Step 2: Blade 3D flow field calculation; Using the blade 3D flow field calculation module, firstly, the blade flow field calculation domain modeling module (2.1) is used to model the blade flow field calculation domain, then the steady-state flow field calculation module (2.2) is used to perform steady-state calculation of the blade flow field, and finally the transient flow field calculation module (2.3) is used to perform transient calculation of the blade flow field. Step 3: Finite element calculation of the blade; Using the blade finite element calculation module, firstly, the blade structure field calculation domain modeling module (3.1) is used to model the blade structure field calculation domain, then the blade static stress calculation module (3.2) is used to calculate the blade static stress, and finally the blade resonance frequency calculation module (3.3) is used to calculate the blade resonance frequency. Step 4: Blade dynamic stress calculation, using the blade dynamic stress calculation module; In step two, the blade flow field computational domain is modeled as follows: A three-dimensional flow field computational domain model of the blade is established using CFD software. The blade is meshed, with the number of mesh nodes meeting the mesh independence requirement. The computational domain uses the stationary blade inlet as the inlet, with given inlet flow rate and total enthalpy, and the moving blade outlet as the outlet, with given outlet pressure and temperature. The stationary blade computational domain is stationary, while the moving blade computational domain is rotating at the turbine operating speed. A cyclic symmetric method is used for simulation, and steam is used as the working fluid. For steady-state calculation of the blade flow field, a steady-state CFD calculation mode is used to obtain the steady-state results of the blade flow field at the blade's thermal boundary. This reveals the distribution characteristics of flow field separation and decoupling. The steady-state CFD calculation result file is used as the input file for transient flow field calculation, and the blade surface pressure data from the steady-state CFD calculation is used as the input file for the finite element static stress calculation of the structural field. The transient flow field of the blade is calculated using a transient CFD calculation mode. During the transient calculation, the steam pressure on the blade surface is calculated using statistical parameters to obtain the transient results of the blade flow field at the blade thermal boundary. The mean square error of the steam pressure at each node on the blade surface is extracted, and the excitation force input file for the dynamic stress calculation module is compiled. The excitation force input file is in CSV format and contains four columns of data. The first three columns are the coordinates of the nodes in the x, y, and z directions, and the fourth column is the mean square error of the steam pressure.

2. The three-dimensional calculation method for dynamic stress of turbine blades under ultra-low load according to claim 1, characterized in that: The blade thermal boundary includes typical ultra-low load operating points of 5%, 10%, 15%, 20%, 25%, and 30%.

3. The three-dimensional calculation method for dynamic stress of turbine blades under ultra-low load according to claim 1 or 2, characterized in that: In step three, the blade structure field calculation domain is modeled: using finite element calculation software, a blade structure field model is established. The blade structure field model includes the blade and wheel groove (5). The blade root (6) is connected to the wheel groove (5). The blade structure field model is meshed. The calculation domain includes all structural features of the blade, including the end band (8), the middle tie rod (7), the blade body, and the end root (6). Frictional contact is set between adjacent end bands (8) and tie rods (7). Frictional foundation is set between the blade root (6) and the wheel groove (5). The cyclic symmetry method is used for simulation. The wheel groove is a fixed constraint. The rotation speed of the moving blade and the wheel groove is the turbine operating speed to simulate the centrifugal force. Blade static stress calculation: Based on the blade structural field model, the blade surface pressure results of the steady-state calculation of the blade flow field in the steady-state flow field calculation module (2.2) are used as the input file for static stress calculation. The steam force is simulated to calculate the blade static stress and obtain the blade static stress distribution characteristics. The static stress calculation results are used as the input data for the blade dynamic stress assessment module. Blade resonance frequency calculation: Based on the blade structural field model, the blade resonance frequency is calculated to obtain the first two blade resonance frequencies at the turbine operating speed. The resonance frequency results are used as input to the blade dynamic stress calculation module.

4. The three-dimensional calculation method for dynamic stress of turbine blades under ultra-low load according to claim 3, characterized in that: Blade structure field computational domain modeling: When meshing the blade structure field model, refine the mesh at the rounded edges of the moving blade, the rounded edges of the moving blade inlet and outlet, and the rounded edges of the wheel groove.

5. The three-dimensional calculation method for dynamic stress of turbine blades under ultra-low load according to claim 1 or 2, characterized in that: In step four, based on the blade structure field model, the root mean square error of the blade surface pressure calculated by the transient flow field calculation module (2.3) is used as the excitation force input file, and the resonance frequency corresponding to the resonance mode closest to the operating speed calculated by the blade resonance frequency calculation module (3.3) is used as the input file to calculate the blade dynamic stress, obtain the blade dynamic stress distribution characteristics, and use the dynamic stress calculation results as the input data of the blade dynamic stress assessment module.

6. The three-dimensional calculation method for dynamic stress of turbine blades under ultra-low load according to claim 1 or 2, characterized in that: The process also includes step five: blade dynamic stress assessment, using a blade dynamic stress assessment module; and in step five, a blade dynamic stress assessment table is compiled; the second column "blade height" of the assessment table refers to the blade height in mm; the third column "operating condition" of the assessment table refers to the calculated ultra-low load operating point; the "assessment position" given in the assessment table is divided into two categories: blade body assessment and blade root assessment. The blade body assessment is divided into assessment position A and assessment position B, and the blade root assessment is divided into assessment position C and assessment position D; the allowable dynamic stress value given in the assessment table refers to the maximum value at which the blade dynamic stress meets the design safety requirements. The process for obtaining the allowable dynamic stress value is as follows: based on the GOODMAN test curve of the blade material, using static stress data as input, the vibration resistance strength of the material is queried, and the allowable dynamic stress value is calculated using the safety factor specified by the manufacturer. The allowable dynamic stress value is calculated using the following formula: Allowable dynamic stress value = Vibration resistance strength / Safety factor. The safety criteria for blade dynamic stress assessment are: the dynamic stress at the assessment location of the blade body and blade root is less than or equal to the allowable value of dynamic stress. When the assessment fails, it is necessary to increase the blade damping, blade rigidity or improve the blade material to guide the blade optimization design. The blade is optimized in a graded manner to ensure that the blade dynamic stress assessment is qualified.

7. A three-dimensional calculation system for dynamic stress of turbine blades under ultra-low load, used in the method described in any one of claims 1-6, characterized in that: It includes a thermal boundary calculation module, a blade three-dimensional flow field calculation module, a blade finite element calculation module, and a blade dynamic stress calculation module; Thermodynamic boundary calculation module: used to obtain thermodynamic data of blade inlet and outlet flow rate, total enthalpy, and pressure, providing thermodynamic boundaries for blade three-dimensional flow field calculation; Blade 3D Flow Field Calculation Module: Used to obtain steady-state and transient flow field calculation results of the blade, and provide input data for the calculation of static and dynamic stress of the blade structure field; The blade 3D flow field calculation module includes: blade flow field calculation domain modeling module (2.1), steady-state flow field calculation module (2.2), and transient flow field calculation module (2.3); Blade finite element calculation module: used to obtain the calculation results of blade static stress and resonance frequency, and provide input data for blade dynamic stress calculation; The blade finite element calculation module includes: blade structural field calculation domain modeling module (3.1), blade static stress calculation module (3.2), and blade resonance frequency calculation module (3.3); Blade dynamic stress calculation module: used to obtain blade dynamic stress calculation results, providing data for blade dynamic stress assessment module.

8. The three-dimensional calculation system for dynamic stress of turbine blades under ultra-low load according to claim 7, characterized in that: It also includes a blade dynamic stress assessment module: used to evaluate and assess the dynamic stress safety of blades.

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