Fatigue calculation method for runner chamber of tubular turbine

Abstract

The invention relates to the technical field of water turbine fatigue analysis, and discloses a method for fatigue calculation of a runner chamber of a through-flow water turbine, which comprises the following steps of: 1, inputting known flow at an inlet position in simulation software, and giving a known pressure value at an outlet position; 2, coupling a fluid calculation module and a finite element calculation module in simulation software by adopting a coupling module, and simultaneously performing transient calculation to obtain a dynamic stress calculation result of the weak point of the runner chamber; 3, determining the stress amplitude before correction according to the dynamic stress result obtained in the step 2, and correcting the stress amplitude through a calculation formula; and 4, predicting the service life to obtain the allowable maximum cycle index under the stress amplitude. The fatigue life of the turbine runner chamber is accurately calculated, the calculation period is shortened, and the execution cost is reduced.

Description

Technical Field

[0001] This invention relates to the field of turbine fatigue analysis technology, specifically to a method for fatigue calculation of the runner chamber of a cross-flow turbine. Background Technology

[0002] The runner is one of the most important components of a axial-flow turbine. During actual operation, due to its periodic rotation, the runner's interior must withstand alternating stresses caused by the centrifugal force generated by rotation and water pressure. This can easily lead to fatigue damage in the runner chamber in high-stress areas, affecting the runner's service life and, in severe cases, even jeopardizing the operational safety of the hydropower station.

[0003] Currently, many manufacturers at home and abroad still use traditional methods to perform fatigue analysis on the turbine runner: (1) They often multiply the static analysis results by a correlation coefficient to simply calculate the alternating stress on the turbine runner. Then, they combine the linear Miners theory in fatigue life calculation to estimate the running life of the turbine runner. (2) They directly use relevant empirical formulas to estimate the running life of the turbine runner. Obviously, these two fatigue life analysis methods rely too much on the selection of empirical coefficients, which is not friendly to companies without relevant technical accumulation. Moreover, the acquisition of empirical coefficients often requires a large number of repeated experiments, which will inevitably lead to extremely high experimental costs and extremely long experimental cycles. At the same time, due to the different selection experience of different staff, the accuracy of the fatigue life prediction of the turbine runner is greatly reduced when facing the same fatigue calculation problem. Summary of the Invention

[0004] To address the technical problem of insufficient accuracy in predicting the fatigue life of turbine runners, this invention provides a method for fatigue calculation of runners in axial-flow turbines, characterized by the following steps:

[0005] Step 1: Input the known flow rate at the inlet position and the known pressure value at the outlet position in the simulation software;

[0006] Step 2: In the simulation software, the fluid calculation module and the finite element calculation module are coupled using a coupling module, and transient calculations are performed simultaneously to obtain the dynamic stress calculation results of the weak points in the turbine chamber;

[0007] Step 3: Determine the stress amplitude before correction based on the dynamic stress results obtained in Step 2, and correct the stress amplitude using the calculation formula:

[0008]

[0009] σ a2 Corrected stress amplitude; σ b The tensile ultimate strength of the material; σ m1σ represents the average stress magnitude corresponding to the SN curve material test; m2 σ is the average stress value corresponding to the cyclic stress of the load spectrum; a1 ε represents the stress amplitude before correction; ε is the stress correction factor for the geometric dimensions; β is the correction factor for the smoothness of the material surface finish.

[0010] Step 4, Life Prediction: The corrected stress amplitude σ a2 Substitute into σ a =1000.6N -0.1603 In this case, the maximum number of cycles allowed under that stress amplitude is obtained.

[0011] Preferably, in step 1, in the simulation software, the dynamic-static interface is set as the INTERFACE boundary condition, and the wall is given a no-slip wall boundary condition.

[0012] Preferably, in step 1, the SST k-Omega model is selected as the turbulence model, and the SIMPLEC algorithm is selected as the coupling of pressure and velocity.

[0013] Preferably, in step 1, the time taken for the rotor to rotate 3° is defined as a time step, and the total calculation time is the time taken for the rotor to rotate 10 times.

[0014] Preferably, in step 2, the pressure transmitted from the simulation software is received in the finite element analysis software, and displacement constraints are applied to the turbine chamber, constraining its axial and circumferential displacements. When inputting boundary conditions in the software, the corresponding rotational speed of the turbine chamber is given, and the gravity of the turbine chamber itself is considered. The time step and total calculation time used in the finite element calculation are the same as those used in the simulation software calculation, so as to obtain the dynamic stress calculation results of the weak points of the turbine chamber.

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

[0016] 1. This invention improves the aforementioned method for analyzing the fatigue life of turbine runners. It employs fluid-structure interaction calculations to obtain the alternating stresses at weak points in the runner, and then combines rainflow counting and linear Miners theory to evaluate the fatigue life of the runner. This method accurately considers the stress conditions within the runner and eliminates the influence of insufficient user experience on fatigue life. It has a short calculation cycle and low execution cost. Therefore, it simultaneously addresses the issues of fatigue life prediction accuracy and computational efficiency. This method has already been applied to some engineering problems. Attached Figure Description

[0017] Figure 1 The results of dynamic stress calculation at the weak point of the turbine chamber in this invention;

[0018] Figure 2The image shows the water fatigue curve of the 04Cr13Ni5Mo material of this invention. Detailed Implementation

[0019] The following detailed description illustrates the specific implementation method:

[0020] Those skilled in the art will understand that:

[0021] Computational Fluid Dynamics (CFD) is an interdisciplinary field and technology that uses numerical methods to simulate fluid flow, heat transfer, mass transfer, chemical reactions, and related physical phenomena on a computer. It integrates fluid mechanics, mathematical algorithms, and computer science, using "digital experiments" to replace or supplement traditional physical experiments, and is one of the core tools in modern engineering design and scientific research.

[0022] In CFD simulation software (such as ANSYS Fluent, Siemens STAR-CCM+, etc.), the INTERFACE boundary condition is a special virtual boundary used to connect two or more non-conformal mesh regions (i.e., regions where mesh nodes are discontinuous), enabling the transfer of physical quantities (such as pressure, velocity, temperature, etc.). Its core function is to simulate the interaction between different computational domains, and it is particularly suitable for handling scenarios where moving and stationary components meet.

[0023] In ANSYS software, CFD (Computational Fluid Dynamics) specifically refers to the core solution process that uses numerical methods to simulate physical processes such as fluid flow, heat transfer, and mass transfer. When transient coupled calculations are used, CFD calculations and finite element (FEA) calculations work together, dynamically exchanging data and jointly simulating the interaction between the fluid and the structure.

[0024] In the Goodman curve method, the AB line usually refers to the Goodman line itself, which is the core component of the graph and is used to predict the fatigue strength of a material in the presence of mean stress (non-zero mean stress).

[0025] Stress amplitude is a core concept in fatigue analysis. It specifically refers to half of the stress fluctuation amplitude in alternating stress (cyclic stress), representing the dynamic stress intensity that a material experiences under cyclic loading.

[0026] Example 1

[0027] A method for fatigue calculation of the runner chamber of a axial-flow turbine includes the following steps:

[0028] Step 1: In the CFD simulation software, input the known flow rate at the inlet position and the known pressure value at the outlet position. Set the dynamic-static interface to INTERFACE boundary condition, and set the wall to no-slip wall boundary condition. Select the SST k-Omega model for turbulence, and the SIMPLEC algorithm for pressure-velocity coupling. Define the time taken for the impeller to rotate 3° as a time step, and take the total calculation time as the time taken for the impeller to rotate 10 times (this value is variable and depends on the convergence of the calculation).

[0029] Step 2: In ANSYS software, the CFD fluid calculation module and the finite element calculation module are coupled using a coupled module, and transient calculations are performed simultaneously. The finite element analysis software receives the pressure transmitted from the CFD and applies displacement constraints to the runner chamber, restricting its axial and circumferential displacements. When inputting boundary conditions in the software, the corresponding rotational speed of the runner chamber and its own gravity are given. The time step and total calculation time used in the finite element calculation are the same as those in the CFD calculation. The dynamic stress calculation results at the weak points of the runner chamber are finally obtained.

[0030] Step 3, Stress Amplitude Correction:

[0031] The Goodman curve method was used for fatigue analysis of the turbine runner. The Goodman curve is a straight line assuming the fatigue limit is a point A of symmetrical cyclic stress and the yield limit B of static load. Points on the AB line represent the critical points where fatigue failure will occur, and it is assumed that the safety factor of the points on the line is equal to 1.

[0032] The Goodman mean stress correction method is currently the most widely used mean stress correction method, and its calculation formula is as follows:

[0033]

[0034] In the formula: σ a2 Corrected stress amplitude; σ b The tensile ultimate strength of the material; σ m1 The SN curve represents the average stress magnitude corresponding to the material test, typically σ. m1 =0MPa; σ m2 σ is the average stress value corresponding to the cyclic stress of the load spectrum; a1 ε is the stress amplitude before correction; ε is the stress correction factor for geometric dimensions, with a value of 0.67; β is the correction factor for the smoothness of the material surface, with a value of 1 when polishing is performed.

[0035] Depend on Figure 1 The stress amplitudes before and after correction at the weak point of the turbine chamber can be obtained by formula (1), as shown in Table 1.

[0036] Table 1. Calculation results of stress amplitude at weak points in the runner chamber before and after correction.

[0037]

[0038] 2. Lifespan prediction

[0039] According to formula (1), the tensile strength of the impeller chamber material needs to be obtained. The impeller chamber material is 04Cr13Ni5Mo, and its tensile strength is σ. b =780MPa, fatigue curve in water is shown below Figure 2 As shown in the figure. This project adopts σ from this figure. a =1000.6N -0.1603 (where σ is in the formula) a The stress amplitude (N) is used as the SN curve to evaluate the fatigue of the turbine chamber.

[0040] The corrected stress amplitude σ a2 Substitute into σ a =1000.6N -0.1603 In this case, the maximum number of cycles allowed under this stress amplitude is N = 2.38 × 10⁻⁶. 13 Second-rate.

[0041] If the unit operates for 365 days a year, the number of cycles the turbine chamber undergoes is: 365 × 24h × 60min × 93.8r / min × 4 (number of blades) = 197,205,120 (cycles).

[0042] For the stress amplitude caused by cyclic loading, a linear accumulation method is used to perform fatigue analysis on the turbine runner chamber. This project applies linear Miner's theory to predict the fatigue life of the turbine runner chamber as follows:

[0043] The service life of the turbine chamber is: 2.38×10¹³ / 197205120=1.2×10⁵ years.

[0044] The above descriptions are merely embodiments of the present invention. Commonly known structures and characteristics are not described in detail here. Those skilled in the art are aware of all common technical knowledge in the field prior to the application date or priority date, are aware of all existing technologies in that field, and have the ability to apply conventional experimental methods prior to that date. Those skilled in the art can, under the guidance of this application, improve and implement this solution in combination with their own capabilities. Some typical known structures or methods should not be obstacles for those skilled in the art to implement this application. It should be noted that those skilled in the art can make several modifications and improvements without departing from the structure of the present invention. These should also be considered within the scope of protection of the present invention, and will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for fatigue calculation of the runner chamber of a axial-flow turbine, characterized in that, Includes the following steps: Step 1: Input the known flow rate at the inlet position and the known pressure value at the outlet position in the simulation software; Step 2: In the simulation software, the fluid calculation module and the finite element calculation module are coupled using a coupling module, and transient calculations are performed simultaneously to obtain the dynamic stress calculation results of the weak points in the turbine chamber; Step 3: Determine the stress amplitude before correction based on the dynamic stress results obtained in Step 2, and correct the stress amplitude using the calculation formula: σ a2 Corrected stress amplitude; σ b The tensile ultimate strength of the material; σ m1 σ represents the average stress magnitude corresponding to the SN curve material test; m2 σ is the average stress value corresponding to the cyclic stress of the load spectrum; a1 The stress amplitude before correction; ε is the stress correction factor for the geometric dimensions; β is the correction factor for the smoothness of the material surface finish. Step 4, Life Prediction: The corrected stress amplitude σ a2 Substitute into σ a =1000.6N -0.1603 In this case, the maximum number of cycles allowed under that stress amplitude is obtained.

2. The method for fatigue calculation of the runner chamber of a axial-flow turbine according to claim 1, characterized in that: In step 1, in the simulation software, the dynamic-static interface is set as the INTERFACE boundary condition, and the wall is given a no-slip wall boundary condition.

3. The method for fatigue calculation of the runner chamber of a axial-flow turbine according to claim 2, characterized in that: In step 1, the SST k-Omega model is selected as the turbulence model, and the SIMPLEC algorithm is used for the coupling of pressure and velocity.

4. The method for fatigue calculation of the runner chamber of a axial-flow turbine according to claim 3, characterized in that: In step 1, the time taken for the wheel to rotate 3° is defined as a time step, and the total calculation time is the time taken for the wheel to rotate 10 times.

5. The method for fatigue calculation of the runner chamber of a axial-flow turbine according to claim 4, characterized in that: In step 2, the pressure transmitted from the simulation software is received in the finite element analysis software, and displacement constraints are applied to the turbine chamber, constraining its axial and circumferential displacements. When inputting boundary conditions in the software, the corresponding rotational speed of the turbine chamber is given, and the gravity of the turbine chamber itself is considered. The time step and total calculation time used in the finite element calculation are the same as those used in the simulation software calculation, and the dynamic stress calculation results of the weak points of the turbine chamber are obtained.

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