High-precision numerical prediction method for impulse turbine no-load characteristic curve

Abstract

The invention discloses a high-precision numerical prediction method for a no-load characteristic curve of an impulse turbine, which comprises the following steps of: on the basis of computational fluid dynamics, combining with a one-way fluid-solid coupling technology that load information is transmitted from a fluid region to a solid region, and solving dynamic response characteristics of a runner solid while performing flow field simulation, so as to obtain a high-precision numerical prediction result; according to the method, flow sudden change in a flow field in the non-load operation process of the impulse turbine and fatigue damage characteristics of a flow passage component can be analyzed in detail, the non-load operation characteristics of the turbine under different opening degree conditions are solved through high-precision numerical simulation, and prediction of a non-load operation characteristic curve is completed. According to the method, stable operation of the impulse turbine under the extreme working condition can be predicted.

Description

Technical Field

[0001] The present invention belongs to the technical field of prediction and evaluation of safe operation of hydraulic machinery under extreme working conditions, and in particular relates to a high-precision numerical prediction method for a no-load characteristic curve of an impulse turbine. Background Art

[0002] Hydropower, as a clean and renewable energy source recognized in the world today, has the ability to quickly adjust the load on the power grid. The low-carbon transformation of my country's energy structure can alleviate the security challenges brought about by the grid-connected power supply of large-scale unstable and intermittent energy.

[0003] The impulse turbine has a simple structure and rapid working condition conversion. Through different nozzle combinations, it can maintain efficient operation within a wide load range. Its installation elevation is not restricted by cavitation conditions, and it has a unique inherent advantage in developing high-head hydropower resources in southwest my country. Relying on the construction of some high-head and large-capacity power stations in Tibet, my country has preliminarily completed the hydraulic design of 500MW impulse turbines. Compared with the reaction turbine, the operating speed of the impulse turbine is extremely high. It is an extremely important task to determine the prediction of the no-load characteristic curve during the design stage. The hydraulic unit loses all loads due to system failure, and the speed control system fails at the same time. The inlet water flow cannot be cut off in time, and the runner speed will rise sharply until it reaches the maximum value. The unit output continues to decrease until it reaches the no-load operation state.

[0004] During no-load operation, the centrifugal force of rotating parts increases rapidly, which will induce a strong hydraulic excitation effect, endangering the safe and stable operation of the unit and even causing damage to the flow-through parts. Unit 2 of Tanghe Hydropower Station in my country once entered a no-load operation state due to an electrical fault. The rapid increase in speed caused the turbine runner to swing at a large value, eventually causing the steel belt of the flying pendulum motor in the governor to break. In order to ensure the safe and reliable operation of the impulse turbine, it is urgent to propose a prediction method for the unstable hydraulic excitation phenomenon of the impulse turbine in the no-load operation state and the stability of the unit operation. In-depth analysis of the unit's external characteristic parameters and internal flow evolution characteristics during the no-load operation of the turbine reveals the induction mechanism of the unstable flow structure and the energy dissipation characteristics, which is of great significance to ensure the safe and efficient operation of the hydraulic unit.

[0005] At present, the method for determining the runner speed of the impulse turbine during no-load operation mainly relies on empirical formulas and scaled model tests. Since there are still many unknown factors between the model test and the actual machine operation, the prediction results often have large errors and need to be corrected by empirical coefficients. Obviously, this prediction method is highly random and has a small scope of application. In the future design process of large-capacity and giant impulse turbines, it is obviously not scientific enough to use this method to predict the no-load operation characteristics, and the credibility is low. Summary of the invention

[0006] In order to solve the above technical problems, the present invention proposes a high-precision numerical prediction method for the no-load characteristic curve of the impulse turbine, which can predict the stable operation of the unit under extreme working conditions of the impulse turbine.

[0007] To achieve the above object, the present invention provides a high-precision numerical prediction method for a no-load characteristic curve of an impulse turbine, comprising:

[0008] S1. Construct a full three-dimensional numerical calculation model of the impulse turbine;

[0009] S2. Formulate a grid division strategy according to the different characteristics of each calculation area, perform grid division on the fluid area of ​​the full three-dimensional numerical calculation model of the impulse turbine, and obtain a grid division result;

[0010] S3, solving the flow distribution characteristics in the flow channel according to the grid division result;

[0011] S4, loading the fluid area load information onto the solid area surface of the flow-through component through the fluid-solid interface;

[0012] S5. Solve the dynamic response characteristics of the solid region of the runner during no-load operation of the turbine using the one-way fluid-solid coupling technology, and use the rain flow method to predict the fatigue life and operating stability of the runner components;

[0013] S6. Adjust the opening of the injection mechanism and repeat the numerical simulation process of S1-S5 to solve the no-load operation characteristics of the turbine under different opening conditions and complete the high-precision numerical prediction of the no-load characteristic curve.

[0014] Optionally, the fluid region of the full three-dimensional numerical calculation model of the impulse turbine includes: a water distribution ring pipe, an injection mechanism, a runner disc and bucket blades in the form of a cantilever beam.

[0015] Optionally, the solid components in the fluid region of the full three-dimensional numerical calculation model of the impulse turbine are the runner disk in the runner region and the bucket blades in the form of cantilever beams.

[0016] Optionally, meshing the fluid region of the full three-dimensional numerical calculation model of the impulse turbine includes: using polyhedral meshing for the fluid calculation domain of the water distribution ring pipe, the injection mechanism and the fluid part of the runner; and using unstructured tetrahedral meshing for the solid region of the runner.

[0017] Optionally, the meshing results include:

[0018] The SST k-ωDES and VOF turbulence models are adopted, the annular pipe inlet is set as a full-pressure inlet, and the injection mechanism outlet is an atmospheric pressure outlet. Under the conditions of optimal opening and optimal speed, the grid independence of the computational grid is verified to obtain a verification result; based on the verification result, the grid division result is obtained.

[0019] Optionally, grid independence verification of the computational grid includes:

[0020] The mesh division results of three different densities are constructed respectively, and the approximate relative error of the numerical simulation is e a , relative error e ext And the grid convergence index GCI, calculated as follows:

[0021]

[0022] Among them, φ is the key variable selected for calculation, r is the grid refinement factor, p is the apparent order calculated by the fixed-point iteration method, subscripts 1, 2 and 3 are the calculation variables of the grid division results of three different densities, and superscripts 21 and 32 are the error values ​​of the fine grid relative to the medium grid and the medium grid relative to the coarse grid, respectively.

[0023] Optionally, the dynamic response characteristics include the impeller speed, impeller torque, forces in all directions of the impeller, pressure distribution on the front and back sides of the bucket blades, and dynamic response characteristics of the impeller solid components.

[0024] Optionally, calculate the rotor speed as:

[0025]

[0026] Wherein, J is the moment of inertia of the runner, ω is the angular velocity of rotation, t is the time, and M is the dynamic torque of the runner impacted by the jet and the resistance torque caused by mechanical friction and airflow resistance.

[0027] Technical effect of the invention: The invention discloses a high-precision numerical prediction method for the no-load characteristic curve of the impulse turbine. In the numerical simulation, the jet impact force distribution and the torque characteristics of the impeller on the bucket surface are monitored, and the impeller speed is calculated in real time using the rotor dynamics equation, and the no-load operation process of the impulse turbine is successfully simulated. The stress-strain characteristics of the impeller are analyzed by the one-way fluid-solid coupling method, and the fatigue damage characteristics of the flow-through components are quantitatively evaluated. Based on the computational fluid dynamics method, the invention fully simulates the growth process of the impeller speed of the impulse turbine by relying on the rotor dynamics torque balance equation, and solves the dynamic response characteristics of the solid area of ​​the impeller in combination with the one-way fluid-solid coupling technology, and realizes the transient numerical simulation of the no-load operation process of the impulse turbine with high accuracy. The numerical calculation structure can clarify the internal flow field structure of the flow channel during the no-load operation of the impulse turbine, the force distribution on the bucket surface, and the stress-strain characteristics of the flow-through components, so as to fully predict the fatigue damage of the impeller and the stable operation of the unit. The connection between the speed growth law of the impulse turbine during no-load operation and the flow state evolution in the flow channel, the force distribution of the bucket blades and the dynamic response of the flow components is established. The present invention provides a scientific and reliable research method to improve the prediction accuracy of the no-load operation process of the impulse turbine and explore the unstable flow characteristics induced by the speed growth. The method can predict the stable operation of the unit under extreme working conditions of the impulse turbine and provide a reference for the optimal design of the impulse turbine. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0029] Figure 1 A schematic flow chart of a high-precision numerical prediction method for a no-load characteristic curve of an impulse turbine according to an embodiment of the present invention;

[0030] Figure 2 Schematic diagram of a method for real-time updating of rotating wheel speed and displacement according to an embodiment of the present invention;

[0031] Figure 3 It is a schematic diagram of the evolution characteristics of the wheel torque during the no-load operation process of an embodiment of the present invention;

[0032] Figure 4 Schematic diagram of the evolution characteristics of the rotor speed during no-load operation of an embodiment of the present invention;

[0033] Figure 5Schematic diagram of the flow distribution characteristics on the bucket blade surface under different working conditions of an embodiment of the present invention, where (a) is OP01 (Nopt), (b) is OP02 (1.21Nopt), (c) is OP03 (t=1.38Nopt), (d) is OP04 (1.55Nopt), and (e) is OP05 (Nmax=1.65Nopt). DETAILED DESCRIPTION

[0034] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0035] It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0036] like Figure 1 As shown, this embodiment provides a high-precision numerical prediction method for a no-load characteristic curve of an impulse turbine, comprising:

[0037] S1. Construct a full three-dimensional numerical calculation model of the impulse turbine;

[0038] S2. Formulate a grid division strategy according to the different characteristics of each calculation area, perform grid division on the fluid area of ​​the full three-dimensional numerical calculation model of the impulse turbine, and obtain a grid division result;

[0039] S3, solving the flow distribution characteristics in the flow channel according to the grid division result;

[0040] S4, loading the fluid area load information onto the solid area surface of the flow-through component through the fluid-solid interface;

[0041] S5. Solve the dynamic response characteristics of the solid region of the runner during no-load operation of the turbine using the one-way fluid-solid coupling technology, and use the rain flow method to predict the fatigue life and operating stability of the runner components;

[0042] S6. Adjust the opening of the injection mechanism and repeat the numerical simulation process of S1-S5 to solve the no-load operation characteristics of the turbine under different opening conditions and complete the high-precision numerical prediction of the no-load characteristic curve.

[0043] Furthermore, the fluid area load information is loaded onto the solid area surface of the flow-through component through the fluid-solid interface, and the dynamic response characteristics of the solid area of ​​the runner during the no-load operation of the turbine are solved by using the one-way fluid-solid coupling technology. The fatigue life and operating stability of the runner components are predicted and evaluated using the "rain flow method".

[0044] The increase of the impulsive turbine runner speed is calculated using the rotor dynamics torque balance equation. During the simulation, the force on the bucket surface and the runner torque characteristics are monitored in real time. The runner torque is used as the judgment standard. If the torque value is greater than zero, the simulation continues and the speed value and runner position are updated; if the runner torque is equal to zero, it is considered that the runner reaches a no-load operation state and the simulation process is completed.

[0045] Adjust the opening of the injection mechanism, repeat the numerical simulation process, solve the no-load operation characteristics of the turbine under different opening conditions, and complete the high-precision numerical prediction of the no-load characteristic curve.

[0046] Specifically, a full three-dimensional computational domain numerical calculation model of the impulse turbine is first established. In this embodiment, the fluid region of the full three-dimensional calculation model includes: a water distribution ring pipe, an injection mechanism, a runner disc, and bucket blades in the form of a cantilever beam; the solid components are the runner disc and bucket blades in the form of a cantilever beam in the runner region.

[0047] Then, according to the characteristics of complex and changeable flow structure, frequent flow state conversion and complex structural components in the flow channel of the impulse turbine, polyhedral meshing is used to divide the fluid calculation domain including the annular pipe, injection mechanism and impeller fluid part, and unstructured tetrahedral meshing is used to divide the impeller solid area.

[0048] Furthermore, the grid independence verification is carried out under the conditions of optimal opening and optimal speed. The SST k-ωDES and VOF turbulence models are used to verify the grid independence. The annular pipe inlet is set as the full-pressure inlet, and the outlet of the injection mechanism is the atmospheric pressure outlet.

[0049] Furthermore, the grid independence verification method uses the grid convergence index (GCI) recommended by ASME (American Society of Mechanical Engineers) to estimate the grid discretization error. The number of grids used in the final calculation is determined by the relative error value obtained and the GCI data, taking into account the computer performance; if the grid independence verification fails, return to the previous step and re-divide different numbers of high-quality grids to achieve grid independence verification, determine the number of grids used in the final calculation, and obtain the calculation grid. GCI is an indicator with a 95% confidence interval that represents the distance between the denser grid and the asymptotic value of the two contrasting grids, and predicts the impact of further grid refinement on the solution. The GCI grid independence verification requires three sets of grids with different densities, namely fine grids (Fine), medium grids (Medium) and coarse grids (Coarse). The approximate relative error e calculated is a , and the relative error e ext And the grid convergence index GCI calculation formula is as follows:

[0050]

[0051] Among them, φ is the key variable selected for calculation, r is the mesh refinement factor, and p is the apparent order calculated by the fixed-point iteration method. The subscripts 1, 2, and 3 correspond to the calculation variables of the meshes Fine, Medium, and Coarse, respectively, and the superscripts 21 and 32 represent the error values ​​of the mesh Fine relative to Medium and the mesh Medium relative to Coarse, respectively.

[0052] Furthermore, during the numerical simulation, the operating characteristics of the unit including the runner speed, runner torque, thrust in all directions of the runner, pressure distribution on the front and back sides of the bucket blades and dynamic response of the runner solid components are monitored in real time.

[0053] By real-time monitoring of the jet impact force on the bucket blade surface and the wheel torque, the rotor speed is calculated using the rotor dynamics torque balance equation.

[0054]

[0055] Wherein, M is the rotor torque, including the dynamic torque of the rotor impacted by the jet and the resistance torque caused by mechanical friction and airflow resistance. Since the resistance torque is extremely small, it has little effect on the increase of the rotor speed, so it is usually ignored in numerical calculations, unit: Nm; J is the moment of inertia of the rotor, unit: kg / m 2 ; ω is the angular velocity of rotation, unit: rad / s; t is the time, unit: s.

[0056] After discretization, it becomes the equation:

[0057]

[0058] Among them, ω i and ω i+1 is the current and next moment wheel rotation angular velocity, unit: rad / s; M i is the current wheel torque, unit: Nm; J is the moment of inertia, unit: kg / m 2 ;t i and t i+1 The physical time between the current and next solution time step, unit: s.

[0059] The specific calculation process is as follows: Figure 2 shown.

[0060] Furthermore, the discretized form of the rotor dynamics torque balance equation is added to the sliding mesh control option of the runner domain to achieve real-time updating of the runner speed and position during the no-load operation of the impulse turbine.

[0061] Furthermore, Figure 3 The law of the change of the runner torque over time during the no-load operation of the impulse turbine is shown in the figure. According to the operating characteristics of the unit, it can be divided into four stages. The first stage is 0s-0.1s. In order to ensure the accuracy of subsequent numerical calculations, it is first operated for 0.1s under the optimal working conditions to obtain a stable initial flow field. In this stage, the runner torque is stable and always fluctuates around the design value. The second stage is 0.1s-0.4s. In this stage, the runner torque shows a cliff-like decline, and the runner torque drops to 0.16Topt in a very short time. The third stage is 0.4s-2.0s. In this stage, the runner torque decrease rate slows down, and the runner torque value shows a small decrease. The fourth stage is 2.0s-3.0s. In this stage, the runner torque fluctuates slowly and shows a relatively stable state for a long time.

[0062] Furthermore, Figure 4 The evolution characteristics of the impeller speed over time when the impulse turbine reaches the no-load operation process can be divided into four stages corresponding to the impeller torque according to the unit operation characteristics. Stage I is 0s-0.1s, and the impeller speed is stable at the rated value. Stage II is 0.1s-0.4s, and the impeller speed rises sharply, and the impeller speed rises to 1.32Nopt in a short time. Stage III is 0.4s-2.0s, and the impeller speed rises gradually slows down in this stage. Stage IV is 2.0s-3.0s, the impeller speed has reached a large value, and the impeller speed slowly rises to the maximum speed.

[0063] Furthermore, Figure 5 (a)-(e) show the flow distribution characteristics of the front and back sides of seven adjacent buckets in the runner area of ​​the impulse turbine at different stages of no-load operation. Among them, the jet surface is given as an isosurface with a water-gas volume fraction of 0.7 and is highlighted with velocity characteristics; the flow distribution on the bucket surface is shown with water-gas distribution characteristics. OP01 is in the first stage of the numerical simulation, the runner speed is the optimal speed, the jet diffuses evenly on the bucket surface, and the energy conversion efficiency is high. OP02 is in the second stage, the runner speed is 1.21Nopt. Due to the increase in speed, the jet action position moves to the bucket tip section, the water film flow at the root of the bucket continues to decrease, and the interference between the jet and the bucket is enhanced, resulting in a large amount of splash flow distributed on the back of the bucket. OP03 and OP04 are in the third stage, the runner speeds are 1.38Nopt and 1.55Nopt respectively, the jet only acts on the bucket tip, and as the speed increases, the jet effect gradually weakens. OP05 is in stage IV, with a runner speed of 1.65 Nopt. The interference between the bucket and the jet is extremely strong, and the jet cannot enter the bucket to convert energy.

[0064] Furthermore, according to the evolution law of the impact force on the runner in all directions monitored and recorded during the simulation calculation, the dynamic stress distribution characteristics of the bucket blades, the displacement distribution characteristics of the bucket blades, etc., the running stability and fatigue characteristics of the impulsive turbine during no-load operation can be quantitatively analyzed.

[0065] Furthermore, the analysis methods of the unit operation characteristics and internal hydraulic excitation include:

[0066] Load the wheel speed information and analyze the speed increase law during the process of reaching the no-load operation state;

[0067] Load the wheel torque information and analyze the fluctuation characteristics of the wheel torque during the process of reaching the no-load operation state;

[0068] Load the force information of the runner in all directions and analyze the fluctuation characteristics of the radial force and axial force of the runner during the process of reaching the no-load operation state;

[0069] Load the bucket surface pressure information and analyze the pressure distribution and evolution characteristics on the front and back sides of the bucket during the no-load operation process;

[0070] Load the dynamic stress information of the bucket blade surface and analyze the stress distribution and evolution law of the runner during the no-load operation process;

[0071] The displacement information of the bucket surface is loaded, and the displacement fluctuation law of the runner is analyzed during the process of reaching the no-load operation state.

[0072] The present invention discloses a high-precision numerical prediction method for the no-load characteristic curve of an impulse turbine. In the numerical simulation, the jet impact force distribution and the torque characteristics of the impeller on the bucket surface are monitored, and the impeller speed is calculated in real time using the rotor dynamics equation, and the no-load operation process of the impulse turbine is successfully simulated. The stress-strain characteristics of the impeller are analyzed by the one-way fluid-solid coupling method, and the fatigue damage characteristics of the flow-through components are quantitatively evaluated. Based on the computational fluid dynamics method, the present invention fully simulates the growth process of the impeller speed of the impulse turbine by relying on the rotor dynamics torque balance equation, and solves the dynamic response characteristics of the solid area of ​​the impeller in combination with the one-way fluid-solid coupling technology, and realizes the transient numerical simulation of the no-load operation process of the impulse turbine with high accuracy. The numerical calculation structure can clarify the internal flow field structure of the flow channel during the no-load operation of the impulse turbine, the force distribution on the bucket surface, and the stress-strain characteristics of the flow-through components, so as to fully predict the fatigue damage of the impeller and the stable operation of the unit. The connection between the speed growth law of the impulse turbine during no-load operation and the flow state evolution in the flow channel, the force distribution of the bucket blades and the dynamic response of the flow components is established. The present invention provides a scientific and reliable research method to improve the prediction accuracy of the no-load operation process of the impulse turbine and explore the unstable flow characteristics induced by the speed growth. The method can predict the stable operation of the unit under extreme working conditions of the impulse turbine and provide a reference for the optimal design of the impulse turbine.

[0073] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A high-precision numerical prediction method for the no-load characteristic curve of an impulse turbine, characterized in that: include: S1. Construct a full three-dimensional numerical calculation model of the impulse turbine; S2. Formulate a grid division strategy according to the different characteristics of each calculation area, perform grid division on the fluid area of ​​the full three-dimensional numerical calculation model of the impulse turbine, and obtain a grid division result; S3, solving the flow distribution characteristics in the flow channel according to the grid division result; S4, loading the fluid area load information onto the solid area surface of the flow-through component through the fluid-solid interface; S5. Solve the dynamic response characteristics of the solid region of the runner during no-load operation of the turbine using the one-way fluid-solid coupling technology, and use the rain flow method to predict the fatigue life and operating stability of the runner components; S6. Adjust the opening of the injection mechanism and repeat the numerical simulation process of S1-S5 to solve the no-load operation characteristics of the turbine under different opening conditions and complete the high-precision numerical prediction of the no-load characteristic curve.

2. The high-precision numerical prediction method for the no-load characteristic curve of an impulse turbine according to claim 1, characterized in that: The fluid area of ​​the full three-dimensional numerical calculation model of the impulse turbine includes: water distribution ring pipe, injection mechanism, runner disc and bucket blades in the form of cantilever beams.

3. The high-precision numerical prediction method for the no-load characteristic curve of an impulse turbine according to claim 1, characterized in that: The solid parts in the fluid area of ​​the full three-dimensional numerical calculation model of the impulse turbine are the runner disk in the runner area and the bucket blades in the form of cantilever beams.

4. The high-precision numerical prediction method for the no-load characteristic curve of an impulse turbine according to claim 1, characterized in that: The meshing of the fluid region of the full three-dimensional numerical calculation model of the impulse turbine includes: using polyhedral meshing for the fluid calculation domain of the water distribution ring pipe, the injection mechanism and the fluid part of the runner; and using unstructured tetrahedral meshing for the solid region of the runner.

5. The high-precision numerical prediction method for the no-load characteristic curve of an impulse turbine according to claim 1, characterized in that: The meshing results include: The SST k-ωDES and VOF turbulence models are adopted, the annular pipe inlet is set as a full-pressure inlet, and the injection mechanism outlet is an atmospheric pressure outlet. Under the conditions of optimal opening and optimal speed, the grid independence of the computational grid is verified to obtain a verification result; based on the verification result, the grid division result is obtained.

6. The high-precision numerical prediction method for the no-load characteristic curve of an impulse turbine according to claim 5, characterized in that: Verifying the grid independence of the computational grid includes: The mesh division results of three different densities are constructed respectively, and the approximate relative error of the numerical simulation is e a , relative error e ext And the grid convergence index GCI, calculated as follows: Among them, φ is the key variable selected for calculation, r is the grid refinement factor, p is the apparent order calculated by the fixed-point iteration method, subscripts 1, 2 and 3 are the calculation variables of the grid division results of three different densities, and superscripts 21 and 32 are the error values ​​of the fine grid relative to the medium grid and the medium grid relative to the coarse grid, respectively.

7. The high-precision numerical prediction method for the no-load characteristic curve of an impulse turbine according to claim 1, characterized in that: The dynamic response characteristics include the rotor speed, the rotor torque, the forces in all directions of the rotor, the pressure distribution on the front and back sides of the bucket blades, and the dynamic response characteristics of the rotor solid parts.

8. The high-precision numerical prediction method for the no-load characteristic curve of an impulse turbine according to claim 7, characterized in that: Calculate the wheel speed as: Wherein, J is the moment of inertia of the runner, ω is the angular velocity of rotation, t is the time, and M is the dynamic torque of the runner impacted by the jet and the resistance torque caused by mechanical friction and airflow resistance.

Images