Method for evaluating steam flow excitation of cooling pipe of condenser of nuclear power unit

Abstract

The invention particularly relates to a nuclear power unit condenser cooling pipe steam flow excitation assessment method which comprises the following steps: solving a condenser throat steam motion equation for a condenser throat geometric model of a coupling turbine exhaust hood to obtain condenser throat steam flow distribution, and outputting condenser throat outlet section flow velocity distribution; dispersing a flow area where the condenser throat outlet section is in butt joint with a downstream cooling pipe to obtain a plurality of discrete areas; calculating the allowable maximum span of the downstream cooling pipe of each discrete area by adopting a steam flow excitation evaluation formula of the condenser cooling pipe; and according to the actual span and the allowable maximum span of the downstream cooling pipe of each discrete area, steam flow excitation evaluation is carried out on the downstream cooling pipe of each discrete area. The method considers nuclear power unit condenser throat outlet flow velocity distribution, and is suitable for scenes with special geometric structures of a nuclear power unit steam turbine exhaust hood and a condenser throat and complex arrangement of internal parts of the condenser throat.

Description

Technical Field

[0001] The invention relates to the technical field of steam flow excitation vibration assessment of a condenser cooling pipe of a nuclear power unit, and in particular to a method for assessing steam flow excitation vibration of a condenser cooling pipe of a nuclear power unit. Background Art

[0002] The condenser is an important auxiliary equipment of the nuclear power unit. The failure of the condenser will directly affect the reliability and safety of the operation of the nuclear power unit. The leakage of the cooling pipe is a common failure of the condenser. If the cooling water enters the steam side of the condenser due to the leakage of the cooling pipe, the condensate quality on the steam side of the condenser will deteriorate rapidly. In severe cases, it may even cause the nuclear power unit to reduce power and shut down.

[0003] In addition to the corrosion of the cooling pipe material, the cause of condenser cooling pipe leakage may also be caused by the vibration of the cooling pipe caused by local steam flow excitation during the operation of the condenser. Steam flow excitation is a common form of cooling pipe vibration induced by high-speed steam flow. It is generally believed that when a fluid with a certain flow rate flows through a cooling pipe with a large span, the cooling pipe will be instantly displaced from its original equilibrium position, and the balance of the adjacent cooling pipe will be destroyed, causing it to also be in a vibrating state. If in each vibration cycle, the energy absorbed by the cooling pipe from the fluid is greater than the energy consumed by the cooling pipe due to damping, steam flow excitation will occur.

[0004] In order to avoid the occurrence of steam flow excitation, the world-recognized American Heat Transfer Society's HEI standard for condenser design has specific design requirements for the condenser partition spacing, that is, the cooling pipe span. However, the HEI standard determines the maximum span of the cooling pipe under the assumption that the turbine exhaust velocity distribution is uniform, and does not take into account the situation where the steam flow rate may be too high locally. In addition, researchers have also proposed empirical formulas to evaluate whether the cooling pipe steam flow excitation is vibrated based on experimental research and actual measurement analysis, which is used to prevent high-speed steam flow from causing vibration damage to the cooling pipe. For example, Sebald JF proposed to keep the maximum deflection of the cooling pipe within 1 / 4 of the cooling pipe arrangement hole bridge, and thus derived the Sebald JF formula for calculating the maximum allowable span of the cooling pipe; Coit RL proposed a concept to characterize the severity of cooling pipe vibration, namely the vibration degree, which is considered to be proportional to the steam flow load per unit length of the cooling pipe and inversely proportional to the stiffness and logarithmic decay rate of the cooling pipe span, and thus derived another Coit RL formula for calculating the maximum allowable span of the cooling pipe; Peaker CC, based on the Coit RL formula, increased the limit on the vibration degree from 0.0432m to 0.0254m, and thus obtained the Peaker CC formula. However, these empirical formulas also use the average steam flow rate to evaluate the cooling pipe steam flow excitation when calculating the maximum allowable span of the cooling pipe, and do not take into account the specific situation of the steam flow field distribution at the condenser throat outlet.

[0005] In fact, due to the particularity of the geometric structure of the turbine exhaust cylinder and condenser throat and the complexity of the layout of the internal components of the condenser throat, the steam flow velocity distribution at the condenser throat outlet cannot be uniformly distributed, and there is a possibility of excessive local flow velocity. Therefore, it is necessary to conduct a steam flow excitation evaluation of the cooling pipe based on the condenser throat outlet flow velocity distribution. Summary of the invention

[0006] The purpose of the present invention is to provide a method for evaluating the steam flow excitation vibration of the cooling pipe of the condenser of a nuclear power unit, taking into account the flow velocity distribution at the outlet of the condenser throat of the nuclear power unit, and is suitable for the scenarios where the geometric structures of the turbine exhaust cylinder and the condenser throat of the nuclear power unit are special and the internal components of the condenser throat are complexly arranged.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] A method for evaluating steam flow excitation vibration of a condenser cooling tube of a nuclear power unit comprises the following steps:

[0009] Step 1, solving the condenser throat steam motion equation for the condenser throat geometric model coupled to the turbine exhaust cylinder, obtaining the condenser throat steam flow distribution, and outputting the condenser throat outlet cross-sectional flow velocity distribution;

[0010] Step 2, discretizing the flow area where the throat outlet section of the condenser is connected to the downstream cooling pipe to obtain multiple discrete areas;

[0011] Step 3, using the steam flow excitation evaluation formula for condenser cooling pipes, calculate the maximum allowable span of the downstream cooling pipes in each discrete area;

[0012] Step 4: Based on the actual span and the maximum allowable span of the cooling pipe downstream of each discrete area, the steam flow excitation vibration of the cooling pipe downstream of each discrete area is evaluated.

[0013] As one of the achievable methods, step 1, solving the condenser throat steam motion equation for the condenser throat geometric model coupled to the turbine exhaust cylinder, obtaining the condenser throat steam flow distribution, and outputting the condenser throat outlet cross-sectional flow velocity distribution, includes the following steps:

[0014] Step 11, using geometric modeling software to establish a condenser throat geometry model coupled to a steam turbine exhaust cylinder;

[0015] Step 12, based on the geometric model of the condenser throat, extending the flow area from the condenser throat outlet section toward the downstream;

[0016] Step 13: Based on the law of conservation of mass, the law of conservation of momentum and the characteristics of steam flow, the motion equation of the steam at the throat of the condenser is constructed;

[0017] Step 14: Solve the condenser throat steam motion equation for the condenser throat geometric model coupled to the turbine exhaust cylinder, obtain the condenser throat steam flow distribution, and output the condenser throat outlet cross-sectional flow velocity distribution.

[0018] As one of the feasible ways, in step 11, a geometric model of the condenser throat coupled to the exhaust cylinder of the steam turbine is established by using geometric modeling software according to the shape, size and internal components of the condenser throat coupled to the exhaust cylinder of the steam turbine.

[0019] As one achievable method, in step 12, the distance by which the flow area of ​​the condenser outlet cross section is extended is 10 to 20 times the equivalent diameter of the condenser outlet cross section; the equivalent diameter of the condenser outlet cross section is calculated according to the following formula:

[0020]

[0021] Where A is the cross-sectional area of ​​the condenser throat outlet, and X is the circumference of the condenser throat outlet cross-sectional area.

[0022] As one possible implementation method, in step 13, the general form of the steam motion equation at the condenser throat is:

[0023]

[0024] Where, ρ is the steam density; is a universal variable; for the continuity equation, For the momentum equations in the three coordinate directions, μi is the steam velocity, i = 1, 2, 3; for the standard ki turbulence model, and are diffusion coefficient and source term respectively, according to It depends.

[0025] As one of the achievable methods, in step 14, the numerical calculation software is Fluent or CFX; the condenser throat geometric model coupled to the turbine exhaust cylinder is solved to obtain the condenser throat steam flow distribution, and the condenser throat outlet cross-sectional flow velocity distribution is output, including the following steps:

[0026] Meshing the geometric model of the throat of the condenser coupled to the exhaust cylinder of the steam turbine to obtain a mesh; performing mesh independence verification on the mesh to obtain a mesh that passes the mesh independence verification;

[0027] In the numerical calculation software, the physical properties of saturated steam, the inlet boundary conditions and outlet boundary conditions of the flow area of ​​the condenser throat outlet section are set, the standard turbulence model, the second-order upwind discrete format and the SIMPLEC algorithm are selected, the condenser throat steam motion equation is iteratively solved, the condenser throat steam flow distribution is obtained, and the flow velocity distribution of the condenser throat outlet section is output;

[0028] The saturated steam physical property parameters are the saturated steam physical property parameters corresponding to the pressure under the condenser working condition; the saturated steam physical property parameters include the density and dynamic viscosity of saturated steam.

[0029] As one of the feasible ways, step 2 is to discretize the flow area where the throat outlet section of the condenser is connected with the downstream cooling pipe to obtain multiple discrete areas, including the following steps:

[0030] Step 21, according to the condenser throat outlet size and the condenser shell side cooling pipe position, determine the flow area that interfaces with the downstream cooling pipe on the condenser throat outlet section;

[0031] Step 22, according to the middle partition span and the end plate span on the condenser shell side, the flow area that is determined on the condenser throat outlet section and that interfaces with the downstream cooling pipe is discretized along the axial direction of the cooling pipe to obtain a plurality of first discrete areas;

[0032] Step 23, according to the spacing between the cooling tubes of the condenser, take n times the spacing between the tubes, discretize the flow area connected to the downstream cooling tube on the throat outlet section of the condenser in the axial direction of the cooling tube, and discretize each first discrete area into multiple second discrete areas; n is between 3 and 7.

[0033] As one of the feasible methods, step 3, using the steam flow excitation vibration evaluation formula of the condenser cooling pipe, calculates the maximum allowable span of the downstream cooling pipe of each discrete area, including the following steps:

[0034] Step 31, extracting the steam velocity of each second discrete region according to the velocity field distribution of the condenser throat outlet section, and calculating the average steam velocity of each first discrete region;

[0035] Step 32: Taking the average steam flow rate of each first discrete area as the steam flow rate characteristic value, the condenser cooling pipe steam flow excitation evaluation formula is used to calculate the allowable maximum span of the downstream cooling pipe for each first discrete area one by one.

[0036] As one of the feasible ways, the steam flow excitation vibration evaluation formula for the condenser cooling tube is a safer one, and the Peaker CC formula is preferred; the Peaker CC formula is:

[0037]

[0038] Where E is the elastic modulus of the cooling pipe; I is the moment of inertia of the cooling pipe section; d 2 is the outer diameter of the cooling pipe; ρ is the steam density; V is the vertical downward flow velocity of the steam.

[0039] As one of the achievable methods, step 4, based on the actual span and the maximum allowable span of the cooling pipe downstream of each discrete area, performs steam flow excitation vibration evaluation on the cooling pipe downstream of each discrete area, including the following steps:

[0040] Step 41, define the risk coefficient as the ratio of the actual span of the cooling pipe to the maximum allowable span of the cooling pipe; calculate the risk coefficient for each first discrete area one by one;

[0041] Step 42: Perform steam flow excitation assessment on the cooling pipe downstream of each first discrete area according to the risk coefficient of each first discrete area;

[0042] When the risk factor of the first discrete sub-region is greater than the risk factor safety margin, it is assessed that there is a risk of steam flow excitation in the cooling pipe downstream of the first discrete sub-region;

[0043] The risk factor safety margin is determined based on theoretical value or engineering experience, and the theoretical value of the risk factor safety margin is 1.

[0044] Beneficial technical effects of the present invention:

[0045] The method for evaluating steam flow excitation vibration of condenser cooling pipes of a nuclear power unit of the present invention obtains the steam flow distribution in the condenser throat by using numerical calculation software through coupling the condenser throat geometric model of the turbine exhaust cylinder; the condenser throat outlet cross-section is regionally discretized according to the condenser throat outlet size and the condenser shell side cooling pipe position, and the condenser cooling pipe is evaluated for steam flow excitation vibration based on the calculation of the maximum allowable span of the cooling pipe in each discrete area. The method is suitable for scenarios with special geometric structures of the turbine exhaust cylinder and condenser throat of nuclear power units and complex layout of internal components of the condenser throat. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 A flow chart of an embodiment of a method for evaluating steam flow excitation vibration of a condenser cooling tube of a nuclear power plant unit according to the present invention;

[0047] Figure 2 A schematic structural view of an embodiment of a geometric model of a condenser throat coupled to a steam turbine exhaust casing.

[0048] Figure 3 A schematic diagram of meshing of a geometric model of a condenser throat coupled to a steam turbine exhaust casing according to an embodiment;

[0049] Figure 4 A schematic diagram of an embodiment of the velocity field distribution of the condenser throat outlet section;

[0050] Figure 5 A schematic diagram of an embodiment of a flow area at a throat outlet section of a condenser that interfaces with a downstream cooling pipe;

[0051] Figure 6 A schematic diagram of an embodiment of performing regional discretization along the axial direction of the cooling pipe on the flow area of ​​the outlet section of the throat of the condenser that interfaces with the downstream cooling pipe;

[0052] Figure 7 A schematic diagram of an embodiment of discretizing the region in the vertical cooling pipe axial direction of the flow region at the throat outlet section of the condenser that interfaces with the downstream cooling pipe;

[0053] Figure 8 is a schematic diagram of an embodiment of an average steam flow rate in a first discrete region;

[0054] Fig. 9 A schematic diagram of an embodiment of an allowable maximum span of a cooling pipe downstream of a first discrete region;

[0055] Fig.10 FIG. 4 is a schematic diagram of an embodiment of a first discrete region risk coefficient. DETAILED DESCRIPTION

[0056] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by technicians in the technical field of this application; the terms used in the specification of the application are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned drawings are intended to cover non-exclusive inclusions. The terms "first", "second", etc. in the specification and claims of this application or the above-mentioned drawings are used to distinguish different objects, not to describe a specific order.

[0057] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0058] The technical solution of the present invention is clearly and completely described below in conjunction with the accompanying drawings and specific embodiments.

[0059] See also Figure 1-10 This embodiment provides a method for evaluating steam flow excitation vibration of a condenser cooling pipe of a nuclear power unit, comprising the following steps:

[0060] Step 1, solving the condenser throat steam motion equation for the condenser throat geometric model coupled to the turbine exhaust cylinder, obtaining the condenser throat steam flow distribution, and outputting the condenser throat outlet cross-sectional flow velocity distribution;

[0061] Step 2, discretizing the flow area where the throat outlet section of the condenser is connected to the downstream cooling pipe to obtain multiple discrete areas;

[0062] Step 3, using the steam flow excitation evaluation formula for condenser cooling pipes, calculate the maximum allowable span of the downstream cooling pipes in each discrete area;

[0063] Step 4: Based on the actual span and the maximum allowable span of the cooling pipe downstream of each discrete area, the steam flow excitation vibration of the cooling pipe downstream of each discrete area is evaluated.

[0064] In this embodiment, step 1, solving the condenser throat steam motion equation for the condenser throat geometric model coupled to the turbine exhaust cylinder, obtaining the condenser throat steam flow distribution, and outputting the condenser throat outlet cross-sectional flow velocity distribution, includes the following steps:

[0065] Step 11, using geometric modeling software to establish a condenser throat geometry model coupled to a steam turbine exhaust cylinder;

[0066] Step 12, based on the geometric model of the condenser throat, extending the flow area from the condenser throat outlet section toward the downstream;

[0067] Step 13: Based on the law of conservation of mass, the law of conservation of momentum and the characteristics of steam flow, the motion equation of the steam at the throat of the condenser is constructed;

[0068] Step 14: Solve the condenser throat steam motion equation for the condenser throat geometric model coupled to the turbine exhaust cylinder, obtain the condenser throat steam flow distribution, and output the condenser throat outlet cross-sectional flow velocity distribution.

[0069] In this embodiment, in step 11, geometric modeling software is used to establish a geometric model of the condenser throat coupled to the exhaust cylinder of the steam turbine according to the shape, size and internal components that have a significant impact on the steam flow of the condenser throat coupled to the exhaust cylinder of the steam turbine; the components inside the condenser throat coupled to the exhaust cylinder of the steam turbine that have a significant impact on the steam flow include the turbine air inlet duct, the extraction duct, the low-pressure heater, the drain manifold, the bypass diffuser and the high-pressure drain expansion tank connecting port.

[0070] In this embodiment, in step 12, the distance of the flow area of ​​the condenser outlet cross section extension is 10 times the equivalent diameter of the condenser outlet cross section; the equivalent diameter of the condenser outlet cross section d e =12.56mm, calculated according to the following formula:

[0071]

[0072] Where A is the cross-sectional area of ​​the condenser throat outlet, and X is the circumference of the condenser throat outlet cross-sectional area.

[0073] In this embodiment, in step 13, the condenser throat steam motion equation is the control equation of the three-dimensional, steady-state flow of the condenser throat steam; in order to facilitate the solution and analysis of the condenser throat steam motion equation, the general form of the condenser throat steam motion equation is constructed as follows:

[0074]

[0075] Where, ρ is the steam density; is a universal variable; for the continuity equation, For the momentum equations in the three coordinate directions, μi is the steam velocity, i = 1, 2, 3; for the standard k-ε turbulence model, and are diffusion coefficient and source term respectively, according to It depends.

[0076] In this embodiment, in step 14, the numerical calculation software is Fluent; solving the condenser throat steam motion equation for the condenser throat geometric model coupled to the turbine exhaust cylinder, obtaining the condenser throat steam flow distribution, and outputting the condenser throat outlet cross-sectional flow velocity distribution, including the following steps:

[0077] The geometric model of the throat of the condenser coupled to the exhaust cylinder of the steam turbine is meshed to obtain a mesh; the mesh independence is verified to obtain a mesh that passes the mesh independence verification, and the number of meshes that pass the mesh independence verification is 6.95 million;

[0078] The density of saturated steam is set to 0.0367 kg / m in the numerical calculation software. 3 , dynamic viscosity is 2.71872e - 4 m 2 / s; set the inlet boundary condition of the flow area of ​​the condenser throat outlet section to a mass flow rate of 421.05kg / s, and set the outlet boundary condition of the flow area of ​​the condenser throat outlet section to a relative pressure of 0Pa; select the standard k-ε turbulence model, the second-order upwind discrete format and the SIMPLEC algorithm, iteratively solve the condenser throat steam motion equation, obtain the condenser throat steam flow distribution, and output the condenser throat outlet section flow velocity distribution;

[0079] The saturated steam physical property parameters are the saturated steam physical property parameters corresponding to the pressure under the condenser working condition. The saturated steam physical property parameters include the density and dynamic viscosity of saturated steam. The condenser working condition is VWO working condition. The pressure of the condenser under VWO working condition is 5180Pa, and the corresponding saturated steam density is 0.0367kg / m 3 , dynamic viscosity is 2.71872e -4 m 2 / s.

[0080] In this embodiment, step 2, discretizing the flow area where the throat outlet section of the condenser is connected to the downstream cooling pipe to obtain multiple discrete areas, includes the following steps:

[0081] Step 21, according to the condenser throat outlet size and the condenser shell side cooling pipe position, determine the flow area that interfaces with the downstream cooling pipe on the condenser throat outlet section;

[0082] Step 22, according to the middle partition span l=0.7145m and the end plate span l'=0.5605m on the condenser shell side, the flow area determined on the condenser throat outlet section and connected to the downstream cooling pipe is discretized along the axial direction of the cooling pipe to obtain a plurality of first discrete areas; there are 22 first discrete areas in total;

[0083] Step 23, based on the condenser cooling tube spacing s=31mm, take 6 times the tube spacing, discretize the flow area connected to the downstream cooling tube determined on the condenser throat outlet section in the axial direction of the cooling tube, and discretize each first discrete area into 17 second discrete areas.

[0084] In this embodiment, step 3, using the steam flow excitation vibration evaluation formula of the condenser cooling pipe, calculates the allowable maximum span of the downstream cooling pipe of each first discrete area, including the following steps:

[0085] Step 31, extracting the steam velocity of each second discrete region according to the velocity field distribution of the condenser throat outlet section, and calculating the average steam velocity of each first discrete region;

[0086] Step 32: Taking the average steam flow rate of each first discrete area as the steam flow rate characteristic value, the condenser cooling pipe steam flow excitation evaluation formula is used to calculate the allowable maximum span of the downstream cooling pipe for each first discrete area one by one.

[0087] In this embodiment, the evaluation formula for steam flow excitation of the condenser cooling tube is the Peaker CC formula; the Peaker CC formula is:

[0088]

[0089] Where E is the elastic modulus of the cooling pipe; I is the moment of inertia of the cooling pipe section; d 2 is the outer diameter of the cooling pipe; ρ is the steam density; V is the vertical downward flow velocity of the steam.

[0090] In this embodiment, step 4, based on the actual span and the maximum allowable span of the cooling pipe downstream of each first discrete area, performs steam flow excitation vibration evaluation on the cooling pipe downstream of each first discrete area, including the following steps:

[0091] Step 41: define the risk factor β as the difference between the actual span L of the cooling pipe and the maximum allowable span L of the cooling pipe. P Calculate the risk coefficient β for each first discrete area one by one;

[0092] Step 42: Perform steam flow excitation assessment on the cooling pipe downstream of each first discrete area according to the risk coefficient β of each first discrete area;

[0093] When the risk coefficient β of the first discrete area is greater than the risk coefficient safety margin, it is assessed that there is a risk of steam flow excitation in the cooling pipe downstream of the first discrete area; the risk coefficient safety margin is taken as 0.8 based on engineering experience.

[0094] In this embodiment, the throat outlet of the condenser is the shell side inlet of the condenser.

[0095] In this embodiment, the parameters of the condenser cooling pipe are shown in Table 1.

[0096] Table 1 Parameters of condenser cooling tubes of nuclear power units

[0097] parameter unit Numeric Throat outlet size m×m 15.411×10.59 Cooling pipe middle span m 0.7145 Cooling pipe end span m 0.5605 Cooling pipe spacing m 0.031 Cooling pipe material -- TA2 Cooling pipe specifications mm φ25×0.5 Elastic modulus of cooling pipe MPa 107800

[0098] The method for evaluating steam flow excitation vibration of condenser cooling pipes of a nuclear power unit of the present invention obtains the steam flow distribution in the condenser throat by using numerical calculation software through coupling the condenser throat geometric model of the turbine exhaust cylinder; the condenser throat outlet cross-section is regionally discretized according to the condenser throat outlet size and the condenser shell side cooling pipe position, and the condenser cooling pipe is evaluated for steam flow excitation vibration based on the calculation of the maximum allowable span of the cooling pipe in each discrete area. The method is suitable for scenarios with special geometric structures of the turbine exhaust cylinder and condenser throat of nuclear power units and complex layout of internal components of the condenser throat.

[0099] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for evaluating steam flow excitation of a condenser cooling tube of a nuclear power unit, characterized in that: The following steps are involved: Step 1, solving the condenser throat steam motion equation for the condenser throat geometric model coupled to the turbine exhaust cylinder, obtaining the condenser throat steam flow distribution, and outputting the condenser throat outlet cross-sectional flow velocity distribution; Step 2, discretizing the flow area where the throat outlet section of the condenser is connected to the downstream cooling pipe to obtain multiple discrete areas; Step 3, using the steam flow excitation vibration evaluation formula of the condenser cooling pipe, calculate the maximum allowable span of the downstream cooling pipe of each discrete area; Step 4: Based on the actual span and the maximum allowable span of the cooling pipe downstream of each discrete area, the steam flow excitation vibration of the cooling pipe downstream of each discrete area is evaluated.

2. The method for evaluating steam flow excitation vibration of a condenser cooling pipe of a nuclear power plant according to claim 1, characterized in that: Step 1, solving the condenser throat steam motion equation for the condenser throat geometric model coupled to the turbine exhaust cylinder, obtaining the condenser throat steam flow distribution, and outputting the condenser throat outlet cross-sectional flow velocity distribution, including the following steps: Step 11, using geometric modeling software to establish a condenser throat geometry model coupled to a steam turbine exhaust cylinder; Step 12, based on the geometric model of the condenser throat, extending the flow area from the condenser throat outlet section toward the downstream; Step 13: Based on the law of conservation of mass, the law of conservation of momentum and the characteristics of steam flow, the motion equation of the steam at the throat of the condenser is constructed; Step 14: Solve the condenser throat steam motion equation for the condenser throat geometric model coupled to the turbine exhaust cylinder, obtain the condenser throat steam flow distribution, and output the condenser throat outlet cross-sectional flow velocity distribution.

3. The method for evaluating steam flow excitation vibration of a condenser cooling pipe of a nuclear power unit according to claim 2, characterized in that: In step 11, a geometric model of the condenser throat coupled to the exhaust cylinder of the steam turbine is established by using geometric modeling software according to the shape, size and internal components of the condenser throat coupled to the exhaust cylinder of the steam turbine.

4. The method for evaluating steam flow excitation vibration of a condenser cooling pipe of a nuclear power plant according to claim 2, characterized in that: In step 12, the distance of the extension of the flow area of ​​the condenser outlet cross section is 10 to 20 times the equivalent diameter of the condenser outlet cross section; the equivalent diameter of the condenser outlet cross section is calculated according to the following formula: Where A is the cross-sectional area of ​​the condenser throat outlet, and X is the circumference of the condenser throat outlet cross-sectional area.

5. The method for evaluating steam flow excitation vibration of a condenser cooling pipe of a nuclear power unit according to claim 2, characterized in that: In step 13, the general form of the equation of motion for the steam at the condenser throat is: Where, ρ is the steam density; is a universal variable; for the continuity equation, For the momentum equations in the three coordinate directions, μ i is the steam velocity, i = 1, 2, 3; for the standard k-ε turbulence model, take and are diffusion coefficient and source term respectively, according to It depends.

6. The method for evaluating steam flow excitation vibration of a condenser cooling pipe of a nuclear power plant according to claim 2, characterized in that: In step 14, the numerical calculation software is Fluent or CFX; the condenser throat geometric model coupled to the turbine exhaust cylinder is solved to obtain the condenser throat steam flow distribution, and the condenser throat outlet cross-sectional flow velocity distribution is output, including the following steps: Meshing the geometric model of the throat of the condenser coupled to the exhaust cylinder of the steam turbine to obtain a mesh; performing mesh independence verification on the mesh to obtain a mesh that passes the mesh independence verification; In the numerical calculation software, the physical properties of saturated steam, the inlet boundary conditions and outlet boundary conditions of the flow area of ​​the condenser throat outlet section are set, the standard turbulence model, the second-order upwind discrete format and the SIMPLEC algorithm are selected, the condenser throat steam motion equation is iteratively solved, the condenser throat steam flow distribution is obtained, and the flow velocity distribution of the condenser throat outlet section is output; The saturated steam physical property parameters are the saturated steam physical property parameters corresponding to the pressure under the condenser working condition; the saturated steam physical property parameters include the density and dynamic viscosity of saturated steam.

7. The method for evaluating steam flow excitation vibration of a condenser cooling pipe of a nuclear power plant according to claim 1, characterized in that: Step 2, discretizing the flow area where the condenser throat outlet section and the downstream cooling pipe meet to obtain multiple discrete areas, including the following steps: Step 21, according to the condenser throat outlet size and the condenser shell side cooling pipe position, determine the flow area that interfaces with the downstream cooling pipe on the condenser throat outlet section; Step 22, according to the middle partition span and the end plate span on the condenser shell side, the flow area that is determined on the condenser throat outlet section and that interfaces with the downstream cooling pipe is discretized along the axial direction of the cooling pipe to obtain a plurality of first discrete areas; Step 23, according to the spacing between the cooling tubes of the condenser, take n times the spacing between the tubes, discretize the flow area connected to the downstream cooling tube on the throat outlet section of the condenser in the axial direction of the cooling tube, and discretize each first discrete area into multiple second discrete areas; n is between 3 and 7.

8. The method for evaluating steam flow excitation vibration of a condenser cooling pipe of a nuclear power plant according to claim 7, characterized in that: Step 3, using the steam flow excitation vibration evaluation formula of the condenser cooling pipe, calculates the maximum allowable span of the downstream cooling pipe of each discrete area, including the following steps: Step 31, extracting the steam velocity of each second discrete region according to the velocity field distribution of the condenser throat outlet section, and calculating the average steam velocity of each first discrete region; Step 32: Taking the average steam flow rate of each first discrete area as the steam flow rate characteristic value, the condenser cooling pipe steam flow excitation evaluation formula is used to calculate the allowable maximum span of the downstream cooling pipe for each first discrete area one by one.

9. The method for evaluating steam flow excitation vibration of a condenser cooling pipe of a nuclear power plant unit according to claim 8, characterized in that: The evaluation formula for steam flow excitation of condenser cooling tubes is the Peaker CC formula; the Peaker CC formula is: Wherein, E is the elastic modulus of the cooling pipe material; I is the moment of inertia of the cooling pipe section; d2 is the outer diameter of the cooling pipe; ρ is the steam density; and V is the vertical downward flow velocity of the steam.

10. The method for evaluating steam flow excitation vibration of a condenser cooling pipe of a nuclear power plant unit according to claim 7, characterized in that: Step 4, based on the actual span and the maximum allowable span of the cooling pipe downstream of each discrete area, the steam flow excitation vibration evaluation is performed on the cooling pipe downstream of each discrete area, including the following steps: Step 41, define the risk factor as the ratio of the actual span of the cooling pipe to the maximum allowable span of the cooling pipe; calculate the risk factor for each first discrete area one by one; Step 42: Perform steam flow excitation assessment on the cooling pipe downstream of each first discrete area according to the risk coefficient of each first discrete area; When the risk factor of the first discrete sub-region is greater than the risk factor safety margin, it is assessed that there is a risk of steam flow excitation in the cooling pipe downstream of the first discrete sub-region; The risk factor safety margin is determined based on theoretical value or engineering experience, and the theoretical value of the risk factor safety margin is 1.

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