Cavitation analysis method for hydroelectric generating set under specific working condition

Abstract

The invention discloses a cavitation analysis method for a hydroelectric generating set under a specific working condition. Belongs to the field of hydraulic turbine set simulation. The method comprises the following steps of: firstly, checking parameters of a selected hydroelectric generating set model according to set specific water head and load conditions; then, various runner cone structures are considered, including perforated runner cones, non-porous runner cones and traditional runner cones. And by applying a cavitation calculation model based on a Rayleigh-Plesset equation, performing simulation calculation on the hydroelectric generating set under each runner cone configuration. And after a calculation result is deeply analyzed and compared, evaluating the cavitation performance of the hydroelectric generating set under a set working condition by utilizing a cavitation volume fraction contour surface and an actual observation result. According to the method, the similarity of model calculation can be ensured, the cavitation performance of the water turbine under the specific working condition can be accurately evaluated, an important basis is provided for hydroelectric generating set design and engineering transformation, and the method has high practicability and wide application prospects.

Description

Technical Field

[0001] The invention belongs to the field of hydraulic turbine simulation, and more specifically relates to a cavitation analysis method under specific working conditions of a hydraulic turbine. Background Art

[0002] The hydroelectric unit is the most important operating equipment in the process of hydroelectric power generation, and its operating status is directly related to the overall operating efficiency, safety and stability of the hydropower station. However, due to the characteristics of the turbine's own structure and working principle, as well as the variable operating conditions, idling, load fluctuations, head changes and other operating conditions often occur, resulting in bubbles in the turbine. This phenomenon is what we often call cavitation, which is the phenomenon of local vaporization caused by a sudden drop in water pressure.

[0003] The occurrence of cavitation will not only reduce turbine efficiency and increase noise, but also cause cavitation damage and damage equipment. Therefore, cavitation analysis is an important part of the design, operation and maintenance of hydropower units, and targeted research is needed to reduce the occurrence of cavitation.

[0004] Commonly used cavitation analysis methods for hydropower units are mainly based on computational fluid dynamics (CFD) technology. By calculating and simulating the behavior of water flow in the turbine, the conditions and locations where cavitation may occur can be accurately predicted, providing a basis for formulating cavitation prevention and control measures.

[0005] However, existing methods often focus on turbine design and adjustment of operating parameters when dealing with cavitation problems under specific operating conditions, and do not specifically aim to determine and evaluate the impact of the drain cone on cavitation performance. In addition, in actual operation, due to the influence of various uncertain factors, the prediction and analysis of cavitation performance under specific operating conditions also presents certain challenges.

[0006] In addition, for different types of spillway cones, such as perforated spillway cones, holeless spillway cones and traditional spillway cones, their structural characteristics and action mechanisms are quite different. How to accurately evaluate the impact of different spillway cone structures on the cavitation performance of hydropower units and provide a basis for the optimal design and operation of hydropower units is an important topic of current research. Summary of the invention

[0007] The present invention mainly aims at the cavitation analysis problem of hydropower units under specific working conditions and proposes a new evaluation method. At present, although the research on cavitation phenomenon has received widespread attention, how to accurately evaluate the influence of different types of water discharge cones on cavitation performance is still a technical difficulty. Therefore, the present invention aims to solve this technical problem and provide a systematic analysis method. Under certain head and load conditions, the influence of three types of water discharge cones, namely perforated water discharge cones, non-perforated water discharge cones and traditional water discharge cones on cavitation performance is studied through numerical calculation simulation, in order to provide a basis for the optimal design and operation of the power station.

[0008] In order to achieve the above object, the present invention is implemented by adopting the following technical scheme: the method comprises:

[0009] Set specific head and load conditions to check the model and real machine parameters of the selected hydropower unit;

[0010] Different forms of spill cone structures are adopted, including perforated spill cones, non-perforated spill cones and traditional spill cones;

[0011] The cavitation calculation model, mainly based on the Rayleigh-Plesset equation, is used to simulate the hydropower unit under each discharge cone configuration;

[0012] Analyze the calculated results and compare the cavitation performance of different drain cones;

[0013] Finally, by observing the cavitation volume fraction isosurface and combining it with the actual observation results, the cavitation performance of the hydropower unit under the set operating conditions is evaluated.

[0014] In one embodiment, the method uses the Rayleigh-Plesset equation for cavitation calculations and uses a model scale for calculations.

[0015]

[0016] Where R' represents the rate of change of radius, R" represents the acceleration of radius change, ρ is the liquid density, Pv is the saturated vapor pressure, P∞ is the liquid static pressure, and σ is the surface tension.

[0017] In one embodiment, the model is scaled to a diameter of 400 mm.

[0018] In one embodiment, the method uses real head and load conditions for calculation, and calculates the cavitation coefficient of the device based on the installation elevation and suction height of the real machine.

[0019] In one scheme, the tailwater pipe outlet pressure is set to ensure that the cavitation coefficient of the model device is the same as that of the real machine, thereby ensuring the similarity of the model calculation.

[0020] In one solution, the cavitation performance of the turbine is evaluated by observing the isosurface of the cavitation volume fraction in the CFX post-processing.

[0021] In one scenario, if no obvious cavitation phenomenon is found throughout the entire passage of the turbine, it is considered that the discharge cone has no significant effect on the cavitation performance under this operating condition.

[0022] In one embodiment, the different forms of spillway cones include existing perforated spillway cones, non-perforated spillway cones, and traditional types of spillway cones.

[0023] Beneficial effects of the present invention:

[0024] By comparing and analyzing the cavitation performance of three different types of spill cones, a scientific basis can be provided for the optimal design and selection of spill cones for hydropower units, which will help improve the working efficiency of hydropower units, improve energy utilization efficiency, and reduce equipment maintenance costs.

[0025] By setting specific water head and load conditions, the cavitation analysis is made more consistent with the actual operating conditions, the accuracy of the cavitation analysis is improved, and the prevention and elimination of problems that may be caused by cavitation are more targeted.

[0026] Using real head and load conditions for calculation can more accurately predict the possible situations in which cavitation may occur, which helps to promptly detect and prevent problems that may arise during the operation of hydropower units and improve the operational safety and stability of power stations.

[0027] The present invention can also effectively predict the influence of different water discharge cones on the cavitation performance of hydropower units under specific working conditions, provide a basis for optimizing equipment design and adjusting operating parameters, further prevent and reduce the influence of cavitation phenomena, and improve the operating safety and stability of the equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a flow chart of the method of the present invention;

[0029] Figure 2 Schematic diagrams of three types of water discharge cones;

[0030] Figure 3 This is a diagram of cavitation occurring at the water outlet of the blade for a similar project;

[0031] Figure 4 The critical force isosurface corresponding to the cavitation coefficient of 1.5 times the opening drain cone;

[0032] Figure 5 The critical force isosurface corresponding to the cavitation coefficient of 1.5 times the device without a hole for the drain cone;

[0033] Figure 6 The critical force isosurface corresponding to the cavitation coefficient of the traditional water discharge cone is 1.5 times;

[0034] Figure 7 To analyze the effect of the water discharge cone on cavitation performance;

[0035] Figure 8 This paper analyzes the influence of water discharge cone on cavitation performance. DETAILED DESCRIPTION

[0036] In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. Typical embodiments of the present invention are given in the drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described in the present invention. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive.

[0037] Unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used in the present invention in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. Typical embodiments of the present invention are shown in the drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described in the present invention. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive.

[0038] like Figure 1 As shown, the cavitation analysis method under specific working conditions of a hydropower unit includes:

[0039] S1. Set specific head and load conditions and check the model and real machine parameters of the selected hydropower unit.

[0040] First, specific head and load conditions need to be set. Head refers to the potential energy of the liquid, usually expressed as height or pressure; load refers to the power that the hydropower unit needs to provide or the stress it needs to withstand.

[0041] Then, a suitable hydropower unit model is selected according to the set conditions. This model should be as close to the real hydropower unit as possible, while also being computationally simple enough. The validity of the model is tested by comparing the consistency between the model predictions and the actual observations.

[0042] Next, the model parameters need to be checked, which may include the physical properties of the fluid (such as density and viscosity), the dimensions of the unit (such as the diameter of the blades and the thickness of the casing), and the operating conditions of the unit (such as speed and flow).

[0043] Finally, after the verification is completed, if the model prediction results and the actual data are in good agreement, it is considered that the model can be used for the subsequent cavitation calculation; if not, the model needs to be modified and optimized, or replaced with other more suitable models.

[0044] S2. Use different forms of spill cone structures, including perforated spill cones, holeless spill cones and traditional spill cones.

[0045] In a hydropower unit, the discharge cone is a key component. Its main function is to divert water from the turbine while minimizing the flow resistance of the water as much as possible, improving efficiency, and preventing and inhibiting the occurrence of cavitation.

[0046] 1. Perforated drain cone: A perforated drain cone has a series of holes on its surface. These holes allow some water to be released in advance, thereby reducing the flow resistance of the drain cone and increasing the speed and efficiency of the fluid after passing through the turbine. However, at the same time, perforation will generate some small vortices, which may promote the occurrence of cavitation.

[0047] 2. Holeless sluice cone: Holeless sluice cone is a traditional structure with no holes on the surface. Holeless sluice cone has a simple structure and low manufacturing and maintenance costs. However, compared with perforated sluice cone, holeless sluice cone may be less efficient under certain working conditions.

[0048] 3. Traditional water discharge cone: The traditional water discharge cone is a common design, and its shape and size are mainly determined based on experience. In this design, the shape and size of the water discharge cone need to be selected and optimized according to the specific hydropower unit and operating conditions.

[0049] Through the study of three different spillway cone structures, we can better understand the advantages and disadvantages of each structure, thus providing a basis for the design and selection of spillway cones.

[0050] S3. Using the cavitation calculation model, mainly based on the Rayleigh-Plesset equation, the hydropower unit under each discharge cone configuration is simulated and calculated.

[0051] The method described uses the Rayleigh_Plesset equation for cavitation calculations and adopts a model scale for calculations.

[0052]

[0053] Where R' represents the rate of change of radius, R" represents the acceleration of radius change, ρ is the liquid density, Pv is the saturated vapor pressure, P∞ is the liquid static pressure, and σ is the surface tension.

[0054] Firstly, according to the operating conditions set in step S1 and the discharge cone structure selected in step S2, the hydropower unit model is preprocessed, such as meshing and boundary condition setting.

[0055] Next, the Rayleigh-Plesset equation is applied for cavitation calculation. The Rayleigh-Plesset equation is a dynamic equation that describes the expansion or contraction of liquid bubbles. The parameters in the equation have been given in detail in the previous description, including the rate of change of radius, acceleration of radius change, liquid density, saturated vapor pressure, liquid static pressure and surface tension.

[0056] Based on the Rayleigh-Plesset equation, the process of liquid swelling or shrinkage under each discharge cone structure in the hydropower unit model is simulated and calculated. In this process, it may be necessary to adjust some calculation parameters or use some numerical methods to improve the accuracy and efficiency of the calculation according to the actual situation.

[0057] Finally, record and save the simulation calculation results, including the cavitation volume fraction, pressure distribution, flow field structure, etc. under each discharge cone configuration, to prepare for the analysis and evaluation of the subsequent S4 step.

[0058] S4. Analyze the calculated results and compare the cavitation performance of different drain cones.

[0059] First, the cavitation volume fraction under each taper cone configuration needs to be calculated and compared. The cavitation volume fraction is an important parameter for evaluating the degree of cavitation, usually represented by α. It is defined as: α = cavitation volume / total volume.

[0060] The cavitation volume refers to the volume of the area where cavitation occurs during the simulation calculation process, and the total volume refers to the total volume of the hydropower unit model.

[0061] Then, the pressure distribution and velocity distribution of the flow field under different discharge cone configurations are analyzed and compared. Since the occurrence of cavitation is directly related to the distribution of pressure and velocity, comparing the pressure and velocity distribution under various conditions will help us to have a deeper understanding of the relationship between cavitation and various parameters.

[0062] The calculation of pressure distribution and velocity distribution requires solving the Navier-Stokes equation and the Bernoulli equation:

[0063] Navier-Stokes equations:

[0064] Bernoulli equation: p1 / ρ+v1 2 / 2+gz1=p2 / ρ+v2 2 / 2+gz2

[0065] Here, ρ is the fluid density, u is the flow velocity, p is the pressure, μ is the dynamic viscosity, g is the gravitational acceleration, v is the flow speed, and z is the height.

[0066] Then, based on the simulation results of parameters such as cavitation volume fraction, pressure distribution, and flow velocity distribution, the cavitation performance of different drain cones was evaluated to find the drain cone configuration with the best cavitation performance.

[0067] Finally, by comparing and analyzing the above results, we can gain a deeper understanding of the cavitation performance and optimization schemes under different discharge cone configurations, and provide a basis for making reasonable conclusions in step S5.

[0068] S5. Finally, by observing the cavitation volume fraction isosurface and combining it with the actual observation results, the cavitation performance of the hydropower unit under the set operating conditions is evaluated.

[0069] In step S5, the implementer starts the final evaluation stage. At this time, the calculation results obtained in steps S3 and S4 will be integrated, and the cavitation volume fraction isosurface (isosurface means that the cavitation volume fraction of each point on the same surface is equal) and other actual observation results will be observed to comprehensively evaluate the cavitation performance of the hydropower unit under specific working conditions.

[0070] These observations include not only simulation results from computational models, but also observations from actual operations, which together provide information for the assessment.

[0071] Observe the cavitation volume fraction isosurface. If there is no obvious area with a large cavitation volume fraction inside the isosurface, it is considered that there is no significant cavitation phenomenon. Alternatively, by comparing the cavitation volume fractions obtained under different discharge cone configurations, if the cavitation volume fraction under a certain configuration is significantly smaller than that of other configurations, then it can be judged that the cavitation performance of this configuration is better.

[0072] Example:

[0073] In order to study the abnormal vibration of a power station unit at a head of 127-133m and a load of about 700MW, this calculation analyzed the influence of different types of spillage cones on the cavitation performance under specific working conditions. A total of three types of spillage cones were analyzed, namely the long spillage cone with holes used in the Wudongde Power Station, the same as the existing spillage cone but without holes, and the traditional type of spillage cone. The three types of spillage cones are as follows Figure 2 As shown:

[0074] The cavitation calculation adopts the Rayleigh_Plesset equation. Since the calculation error occurred when the real machine scale was used for cavitation calculation and the calculation could not be completed, we used the model scale for cavitation calculation, and the model diameter was 400mm.

[0075] The calculation condition is 131mm head and 700MW output of the real machine. In order to ensure the similarity of the model calculation, according to the installation elevation of the real machine of 803m and the suction height of the real machine of -18.8m, the cavitation coefficient of the device under the calculation condition can be calculated to be 0.213. When calculating the model, the cavitation coefficient of the device of the model can be ensured to be the same as that of the real machine by setting the outlet pressure of the tailwater pipe, thereby ensuring the similarity of the model calculation.

[0076] Calculation results

[0077] By observing the cavitation volume fraction isosurface in CFX post-processing, the cavitation characteristics of the turbine can be judged. Figure 3 A similar project is the cavitation that occurs at the water outlet of the blade.

[0078] In similar projects, cavitation occurs at the water outlet edge of blades, and the cavitation volume fraction is 0.5. However, in this calculation condition, under the cavitation coefficient of the power plant device, no obvious cavitation is found in the entire passage of the turbine using three types of water discharge cones. In order to further analyze the influence of different types of water discharge cones on cavitation performance, the critical force isosurface corresponding to 1.5 times the device cavitation coefficient is made. By observing the isosurface, the cavitation margin can be roughly estimated. Figure 4 There is no obvious cavitation in the open-hole drain cone. Figure 5 There is no obvious cavitation when the drain cone is not drilled. Figure 6 Traditional drain cone, no obvious cavitation

[0079] like Figure 7-Figure 8 From the above analysis results, it can be seen that the three types of water discharge cones do not show obvious cavitation phenomenon under the cavitation coefficient of the device, and the isosurfaces of different low pressures are basically equivalent. It can be considered that under this working condition, the water discharge cone has no significant effect on the cavitation performance.

[0080] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiments can be implemented by instructing the relevant hardware through a computer program, and the program can be stored in a computer-readable storage medium, and when the program is executed, it can include the processes of the embodiments of the above-mentioned methods. The storage medium can be a disk, an optical disk, a read-only memory (ROM) or a random access memory (RAM).

[0081] It should be understood that the detailed description of the technical solutions of the present invention by means of the preferred embodiments is illustrative rather than restrictive. A person skilled in the art may modify the technical solutions described in the embodiments, or replace some of the technical features by equivalents, based on reading the specification of the present invention; and these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A cavitation analysis method for a hydropower unit under specific operating conditions, characterized by: The method includes: Set specific head and load conditions to check the model and real machine parameters of the selected hydropower unit; Different forms of spill cone structures are adopted, including perforated spill cones, non-perforated spill cones and traditional spill cones; The cavitation calculation model, mainly based on the Rayleigh-Plesset equation, is used to simulate the hydropower unit under each discharge cone configuration; Analyze the calculated results and compare the cavitation performance of different drain cones; Finally, by observing the cavitation volume fraction isosurface and combining it with the actual observation results, the cavitation performance of the hydropower unit under the set operating conditions is evaluated.

2. The cavitation analysis method for a hydropower unit under specific operating conditions according to claim 1 is characterized by: The method described uses the Rayleigh_Plesset equation for cavitation calculations and adopts model scale for calculations. Where R' represents the rate of change of radius, R" represents the acceleration of radius change, ρ is the liquid density, Pv is the saturated vapor pressure, P∞ is the static pressure of the liquid, and σ is the surface tension.

3. The cavitation analysis method for a hydropower unit under specific operating conditions according to claim 1 is characterized in that: The model scale is set to a diameter of 400 mm.

4. The cavitation analysis method for a hydropower unit under specific operating conditions according to claim 1 is characterized by: The method uses real water head and load conditions for calculation, and calculates the cavitation coefficient of the device according to the installation elevation and suction height of the real machine.

5. A cavitation analysis method for a hydropower unit under specific operating conditions according to claim 4, characterized in that: The tailwater pipe outlet pressure is set to ensure that the cavitation coefficient of the model device is the same as that of the real machine, thereby ensuring the similarity of the model calculation.

6. The cavitation analysis method for a hydropower unit under specific operating conditions according to claim 1 is characterized by: The cavitation performance of the turbine is evaluated by observing the isosurface of the cavitation volume fraction in the CFX post-processing.

7. A cavitation analysis method for a hydropower unit under specific operating conditions according to claim 6, characterized in that: If no obvious cavitation phenomenon is found in the entire passage of the turbine, it is considered that under this operating condition, the discharge cone has no significant effect on the cavitation performance.

8. The cavitation analysis method for a hydropower unit under specific operating conditions according to claim 1 is characterized by: The different forms of water spill cones include existing perforated water spill cones, non-perforated water spill cones, and traditional water spill cones.

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