Flow stability evaluation method for extreme variable working conditions of marine steam turbine

Abstract

The invention discloses a flow stability evaluation method for a marine steam turbine under extreme variable working conditions, which comprises the following steps of: establishing a three-dimensional geometric model and a flow field model, introducing a multi-layer grid division technology, and combining an Euler multiphase flow model and a turbulence model to evaluate the flow stability of the marine steam turbine under extreme variable working conditions. An internal steady-state and transient-state flow field numerical calculation method suitable for the turbine from the rated working condition to the shaftless work output working condition is provided; steam turbine variable working condition flow field disturbance characteristic analysis is carried out with actual working condition working parameters of a steam turbine as boundary conditions, internal flow characteristics of the steam turbine under the minimum flow working condition are revealed, and an unstable flow threshold value and a whole machine stable operation power flow threshold value are determined; based on high-precision flow field distribution basic data, a turbine last-stage one-way fluid-solid coupling calculation method and process are provided, a static and dynamic stress change rule of a key part of a last-stage blade is obtained through calculation, flow-induced vibration excitation source recognition and instability characteristic analysis of the last-stage blade are achieved, blade flutter characteristic evaluation is completed, and a final-stage blade flutter characteristic evaluation result is obtained. Powerful technical support is provided for safe and stable operation of the steam turbine.

Description

Technical Field

[0001] The invention belongs to the technical field of marine steam turbines, and in particular relates to a flow stability evaluation method for marine steam turbines under extreme variable operating conditions. Background Art

[0002] The marine nuclear-powered steam turbine propulsion device is constantly developing in the direction of low noise, high power, high safety and high power density, and is often operated under extremely variable conditions, which will lead to extremely complex internal flow of the steam turbine, and there are practical problems such as small volume flow leading to flow separation and backflow, unstable flow inducing steam flow excitation, and some stages of blasting power consumption and heat generation, which will directly lead to adverse effects such as excessive force on the turbine blades and flutter. To this end, high-precision calculation of internal flow characteristics of steam turbines and research on the mechanism of fluid-solid coupling are the key to identifying the boundary of stable operation of steam turbines under special conditions and ensuring safe and reliable operation of steam turbines under extremely variable conditions.

[0003] In recent years, more and more attention has been paid to the flow characteristics of steam turbines under special working conditions at home and abroad. When the operating conditions are small flow and low load, the flow field will become very complicated. At present, the flow characteristics of steam turbines under small flow conditions are mainly analyzed by numerical simulation methods. In the research on small flow conditions of the last stage of steam turbines, there are currently three main directions: changes in internal steam flow characteristics under extreme conditions, the impact caused by multiphase flow phase change, and the corresponding performance analysis and structural optimization on the internal flow characteristics of steam turbines. Summary of the invention

[0004] The purpose of the present invention is to solve the above technical problems and provide a flow stability evaluation method for extreme variable operating conditions of a marine steam turbine, so as to provide strong technical support for the safe and stable operation of the steam turbine.

[0005] To achieve the above object, the present invention provides a method for evaluating flow stability of a marine steam turbine under extreme variable operating conditions as follows:

[0006] Step 1: Based on the given design drawing, the turbine cascade channel including the integral moving and stator blades of the turbine is taken as a whole, a three-dimensional geometric model and a flow field model are established, and the cascade channel geometric model is meshed; according to the boundary condition design parameters under the characteristic working conditions of the turbine, the inlet flow rate, temperature, enthalpy value and outlet pressure are determined as the boundary condition design parameters;

[0007] Step 2: Set the working parameters under different working conditions as boundary conditions, perform steady-state calculations and transient calculations on the blade flow field, and obtain the overall performance from rated working conditions to no-shaft power output conditions, as well as the power, efficiency, post-stage stiffness, reaction, enthalpy drop, pressure, and axial thrust of each stage; at the same time, set constraint boundary conditions to simulate the actual fixed condition of the blades, perform modal analysis on the last-stage moving blades, calculate the first n-order modes of the last-stage moving blades, and obtain the static frequency and dynamic frequency of the blades;

[0008] Step 3: First, assume that the change of enthalpy drop and power between the two working conditions is linear, and estimate the value of the intermediate working condition according to the straight line passing through the two points; through the linear interpolation approximation simulation of the boundary conditions of adjacent working conditions, finally obtain the accurate critical instability volume flow threshold of the last-stage blade based on the enthalpy drop blast standard and the accurate zero power flow threshold of the whole machine based on the reverse pressure stall standard;

[0009] Step 4: The surface pressure field data obtained by transient calculation in step 2 is used as a load and imported into the blade structure model through the ANSYS Workbench fluid-solid coupling module for mechanical analysis, and static stress calculation and dynamic stress unidirectional fluid-solid coupling calculation are performed on the blade structure domain;

[0010] Since the distribution and position of the blade surface grid nodes are not consistent, the solid domain grid surface pressure is obtained from the fluid domain pressure distribution through interpolation, and the blade time domain and frequency domain data, blade modal frequency data, vibration displacement and other data are calculated;

[0011] Step five: Compare and analyze the time domain and frequency domain data of the transient flow field inside the turbine flow passage with the blade modal frequency data, vibration displacement and other data to determine the flow stability under wide load variable conditions.

[0012] Furthermore, the specific process of step one is as follows: a full-circle model of the blade is modeled according to the profile lines of multiple sections provided by each stage of blades and the number of blades around the whole circumference, and the blade cascade channel is meshed using a structured grid; at the same time, a symmetrical 5-star layout is adopted in the leading edge and trailing edge areas, and the grid blocks are all quadrilaterals; the geometric shape and topological structure of the grid area blocks are optimized by making the edges of the blocks close to the blade perpendicular to the blade surface and the four corners of each block approach 90°.

[0013] Furthermore, the characteristic operating conditions in step 1 include typical operating conditions including the same speed but different steam consumption, and the same steam consumption but different speeds.

[0014] Furthermore, the specific process of the steady-state calculation of the flow field in step 2 is as follows: according to the actual situation, the flow field medium is set to wet steam, and the thermodynamic properties of water are described by the IAPWS-IF97 formula group. The Euler multiphase flow model and the κ-ωSST turbulence model are combined in the CFD steady-state calculation to achieve a large range of flow field calculations from rated conditions to non-axial power output conditions, and the flow field streamlines, steam velocity, humidity and internal pressure state parameters are obtained to describe the distribution characteristics of the flow field in the blade, including the outlet pressure and velocity of a single-stage full-circle blade, and the meridian velocity vector distribution cloud map of the exhaust chamber, and the steady-state results are input as the initial file for transient calculations. The κ-ωSST turbulence model is introduced when solving the NS equations, and the calculation formula for its turbulent viscosity is as follows:

[0015]

[0016] Where:

[0017]

[0018] Where ρ is the fluid density, α1 and β' are model constants, κ is the turbulent kinetic energy, ω is the specific dissipation rate, y is the distance from the first layer of grid nodes on the wall to the wall, and v is the molecular kinematic viscosity of the fluid;

[0019] Transient flow field calculation: Through CFD transient calculation, the transient results of the blade flow field are obtained, and the blade surface pressure field data is obtained.

[0020] Furthermore, in the step 2: based on the blade structure field model, taking the actual working condition of the steam turbine blade as a constraint condition, a fixed support is applied to the bottom of the blade, and a displacement constraint condition is set at the blade root to calculate the upper line frequency of the first n-order modes.

[0021] Furthermore, the specific process of step three is: according to the positive and negative of the last-stage blade power and the whole machine power, the working condition intervals of the blast working condition and the zero power output working condition are determined, and the inlet flow of the last-stage blade power is zero and the whole machine power is zero is gradually approached by linear interpolation of the inlet flow of the two adjacent working conditions. The blade-level power formula is as follows:

[0022]

[0023] Among them, τ is the turbine blade torque; n is the blade speed, that is, the number of revolutions per minute.

[0024] Furthermore, the specific process of step four is: using the transient pressure field on the blade surface as the aerodynamic force of steam to be applied to the solid blade, and then analyzing the influence of pressure fluctuations on the blade stress.

[0025] Furthermore, in the step 5, the fluctuation of four blades arranged at equal intervals within one rotation cycle is selected;

[0026] The transient flow field data inside the turbine flow passage include the steam flow force variation curves over time on four blade surfaces, exhaust chamber end face, and the steam flow excitation spectrum variation curve obtained by FFT transformation.

[0027] Furthermore, the data comparison analysis in step 5 includes comparing the blade natural frequency f0 obtained by calculation with the low-frequency exciting force f excited by the steam flow. l , high-frequency exciting force f of wake disturbance h By comparison, the frequency avoidance rate is used to determine whether the blade is running stably, whether the blade will resonate, and the degree of danger of blade flutter; and the deformation of the leading and trailing edges of the blade root and the maximum displacement of the blade δ are analyzed. max , through the blade relative amplitude A = δ max The peak value and change of / h can be used to judge the operating stability of the blade under the action of airflow.

[0028] Frequency avoidance rate = (f0-f1) or (f h -f0) / f0

[0029] If (f0-f1)≮15% or (f h -f0) / f0≮15% means stable operation, otherwise unstable operation;

[0030] h is the ratio of the maximum displacement of the blade to the blade height.

[0031] Compared with the prior art, the beneficial effects of the present invention are: the present invention has high precision and high efficiency, can accurately determine the flow threshold of critical instability of the last-stage blade and the flow threshold of zero power output of the whole machine, and provides strong technical support for the safe and stable operation of the turbine. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic diagram of the process of the present invention;

[0033] Figure 2 It is a schematic diagram of the leading edge, inter-lobe, and trailing edge regions;

[0034] Figure 3 This is the distribution diagram of the four leaves at level eight;

[0035] Figure 4 This is the structured grid diagram of the eighth-stage moving blade cascade channel;

[0036] Figure 5 is the numerical model and boundary condition diagram;

[0037] Figure 6a6a is a cross-sectional view of 10% leaf height, 6b is a cross-sectional view of 50% leaf height, and 6c is a cross-sectional view of 90% leaf height;

[0038] Figure 7 The velocity vector distribution of the upper half of the exhaust chamber meridian and the boundary line of the recirculation area;

[0039] Figure 8 It is to iteratively solve the critical instability flow threshold diagram of the last stage blade;

[0040] Fig. 9 is the sixth-order mode diagram of the blade;

[0041] Fig.10 is the blade displacement constraint diagram;

[0042] Fig.11 It is the distribution diagram of blade measurement points;

[0043] Fig.12 This is the dynamic stress variation diagram of the blade measuring point;

[0044] Fig.13 is the dynamic stress amplitude-frequency diagram;

[0045] Fig.14 This is the maximum displacement versus time graph. DETAILED DESCRIPTION

[0046] This method aims at the unsteady flow field problem of the steam turbine under working conditions. The Euler multiphase flow model and the κ-ωSST turbulence model are used to analyze the flow field disturbance characteristics of a compact steam turbine under small flow conditions. The internal flow characteristics of the steam turbine under small flow conditions are revealed, and the instability flow threshold and the stable operation power flow threshold of the whole machine are determined. The fluid-solid coupling calculation of the last three stages-exhaust cavity coupling model of the steam turbine under special conditions is carried out, and the flow-induced vibration excitation of the exhaust end is obtained to judge the flow stability under extreme variable load conditions.

[0047] The flow stability evaluation method for the extreme variable operating conditions of a marine steam turbine includes the following steps. Figure 1 As shown:

[0048] Step 1: Based on the given design drawing, the turbine cascade channel including the integral moving and stator blades of the turbine is taken as a whole, a three-dimensional high-precision geometric model and a flow field model are established, and the cascade channel geometric model is meshed; the boundary condition design parameters under the characteristic working conditions of the turbine are determined, and the inlet flow rate, temperature, enthalpy value and outlet pressure are determined as the boundary condition design parameters; wherein the characteristic working conditions include typical working conditions including the same speed but different steam consumption, and the same steam consumption but different speed;

[0049] The specific process is as follows: the blade full-circle model is modeled according to the multiple cross-section lines provided by each stage of blades and the number of blades around the whole circumference, and the cascade channel is meshed using a structured grid, such as Figure 2 As shown; in order to ensure the mesh quality, a symmetrical 5-star layout is adopted in the leading edge and trailing edge areas, and the mesh blocks are all quadrilaterals; in order to ensure the boundary layer mesh quality and mesh orthogonality, the geometric shape and topological structure of the mesh area blocks are optimized by making the edges of the blocks close to the blade perpendicular to the blade surface and the four corners of each block approach 90°.

[0050] Step 2: Set the working parameters under different working conditions as boundary conditions, perform steady-state calculations and transient calculations on the blade flow field, and obtain the overall performance from rated working conditions to no-shaft power output conditions, as well as the power, efficiency, post-stage stiffness, reaction, enthalpy drop, pressure, and axial thrust of each stage; at the same time, set constraint boundary conditions to simulate the actual fixed condition of the blades, perform modal analysis on the last-stage moving blades, calculate the first n-order modes of the last-stage moving blades, and obtain the static frequency and dynamic frequency of the blades;

[0051] The specific process of steady-state calculation of flow field is as follows: according to the actual situation, the flow field medium is set as wet steam, and the thermodynamic properties of water are described by the IAPWS-IF97 formula group. In the CFD steady-state calculation, the Euler multiphase flow model and the κ-ωSST turbulence model are combined to realize the large-scale flow field calculation from rated conditions to no shaft power output conditions, and the flow field streamlines, steam velocity, humidity and internal pressure state parameters are obtained to describe the distribution characteristics of the flow field in the blade, including the outlet pressure and velocity of a single-stage full-circle blade, and the meridian velocity vector distribution cloud map of the exhaust chamber. The steady-state results are input as the initial file for transient calculation. The κ-ωSST turbulence model is introduced when solving the NS equation. The calculation formula of its turbulent viscosity is as follows:

[0052]

[0053] Where:

[0054]

[0055] Where ρ is the fluid density, α1 and β' are model constants, κ is the turbulent kinetic energy, ω is the specific dissipation rate, y is the distance from the first layer of grid nodes on the wall to the wall, and v is the molecular kinematic viscosity of the fluid;

[0056] Transient flow field calculation: Through CFD transient calculation, the transient results of the blade flow field are obtained, and the blade surface pressure field data is obtained to provide data support for subsequent blade fluid-solid coupling calculation;

[0057] Based on the blade structure field model, with the actual working conditions of the steam turbine blades as constraints, a fixed support is applied to the bottom of the blade, and a displacement constraint is set at the blade root. The upper frequency of the calculated first n-order modes is generally 10kHz, and can be taken as below 2kHz for low-frequency vibration analysis.

[0058] Step 3: First, assume that the change of enthalpy drop and power between the two working conditions is linear, and estimate the value of the intermediate working condition according to the straight line passing through the two points; through the linear interpolation approximation simulation of the boundary conditions of adjacent working conditions, finally obtain the accurate critical instability volume flow threshold of the last-stage blade based on the enthalpy drop blast standard and the accurate zero power flow threshold of the whole machine based on the reverse pressure stall standard;

[0059] The specific process is: according to the positive and negative of the last-stage blade power and the whole machine power, the working condition intervals of the blast condition and the zero power output condition are determined, and the inlet flow rate of the last-stage blade power of zero and the whole machine power of zero is gradually approached by linear interpolation of the inlet flow rate of the two adjacent working conditions. The blade-level power formula is as follows:

[0060]

[0061] Among them, τ is the turbine blade torque; n is the blade speed, that is, the number of revolutions per minute.

[0062] Step 4: The surface pressure field data obtained by transient calculation in step 2 is used as a load to be imported into the blade structure model through the ANSYS Workbench fluid-solid coupling module for mechanical analysis, and static stress calculation and dynamic stress unidirectional fluid-solid coupling calculation are performed on the blade structure domain; since the distribution and position of the blade surface grid nodes are not consistent, the solid domain grid surface pressure is obtained from the fluid domain pressure distribution by interpolation, and the blade time domain, frequency domain data, blade modal frequency data, vibration displacement and other data are calculated;

[0063] The specific process is: using the transient pressure field on the blade surface as the aerodynamic force of steam to exert on the solid blade, and then analyzing the impact of pressure fluctuations on the blade stress.

[0064] Step five: Compare and analyze the time domain and frequency domain data of the transient flow field inside the turbine flow passage with the blade modal frequency data, vibration displacement and other data to determine the flow stability under wide load variable conditions.

[0065] Select the fluctuation of four blades arranged at equal intervals within a rotation cycle, such as Figure 3 As shown;

[0066] The transient flow field data inside the turbine flow passage include the steam flow force variation curves over time on four blade surfaces, exhaust chamber end face, and the steam flow excitation spectrum variation curve obtained by FFT transformation.

[0067] The data comparison analysis includes the calculated blade natural frequency f0 and the low-frequency exciting force f excited by steam flow. l , high-frequency exciting force f of wake disturbance h By comparison, the frequency avoidance rate is used to determine whether the blade is running stably, whether the blade will resonate, and the degree of danger of blade flutter; and the deformation of the leading and trailing edges of the blade root and the maximum displacement of the blade δ are analyzed. max , through the blade relative amplitude A = δ max The peak value and change of / h can be used to judge the operating stability of the blade under the action of airflow.

[0068] Frequency avoidance rate = (f0-f1) or (f h -f0) / f0

[0069] If (f0-f1)≮15% or (f h -f0) / f0≮15% means stable operation, otherwise unstable operation;

[0070] h is the ratio of the maximum displacement of the blade to the blade height.

[0071] Example

[0072] A three-dimensional high-precision geometric model and flow field model of the turbine cascade channel including the integral moving and stationary blades of a certain type of marine steam turbine are established, and the cascade channel structure area and flow field area are meshed. This method uses the same structured meshing method to mesh the second to eighth level cascade channels. Each blade is interpolated with at least 11 blade height section lines to ensure the accuracy of mesh restoration geometry. The blade structured mesh is as follows: Figure 4 shown.

[0073] According to the calculated turbine state parameters under different conditions, the calculation setting operating condition state parameters, numerical model and boundary conditions are set as follows Figure 5 The flow field is calculated in a steady state to obtain the streamline distribution of different blade height sections, as shown in Figure 6 (a, b, c). It can be seen that the reduction in flow rate causes the last-stage stationary blades and moving blades to have negative inlet attack angles, and a large shedding vortex is generated on the pressure surface of the last-stage moving blades. The work of the last stage of the turbine changes from positive to negative, and enters the blast condition. Figure 7 The velocity vector distribution of the upper half of the exhaust chamber meridian plane is further given and the boundary of the recirculation zone is depicted. It can be seen from the figure that the boundary of the recirculation zone starts from the vicinity of the eighth-stage blade root, which means that there is no recirculation inside the moving blade under this working condition or the recirculation zone is extremely small.

[0074] Taking the solution of the flow threshold of the blast condition as an example, in order to obtain the accurate critical instability volume flow threshold of the last-stage blade, the inlet flow of condition 2 and condition 3 is used for linear interpolation to gradually approach the inlet flow when the power of the last-stage blade is zero. The correspondence between the number of linear interpolation iterations and the power of the last-stage blade in the steady state is shown in Figure 8 From the figure, the critical instability flow threshold of the last stage blade can be obtained by interpolation iteration. The same method is used to gradually approximate the working conditions near the inlet flow when the whole machine power is zero by linear interpolation iteration to obtain the specific flow threshold of zero whole machine power.

[0075] Modal analysis is performed on the last stage moving blades. The sixth-order mode of the last stage moving blades is as follows: Fig. 9 The blade position constraint boundary conditions are all applied to the blade root, and a fixed support is applied to the bottom of the blade. At the same time, a single-direction fixed displacement of the Y axis and a free displacement constraint condition of the XZ axis are set at the blade root to simulate the fixation of the blade on the turbine rotor, as shown in Fig.10 The static frequency and dynamic frequency of the last stage moving blade are calculated by modal analysis.

[0076] The steady-state results were further simulated in a transient state to obtain the corresponding velocity field and pressure field. The above transient pressure field was used to apply a starting force to the blade to detect the static and dynamic stresses on the blade. Since the air inlet side, air outlet side, and blade root connection of the blade will produce large stress under the action of aerodynamic force and centrifugal force, five measuring points were placed at the root of the blade to extract and analyze the stress and deformation data during the fluid-solid coupling calculation. The five measuring points are distributed as follows: Fig.11 As shown in Figure 1, the fluctuation of the axial force and axial moment of the four blades of the eighth stage obtained by unidirectional fluid-structure coupling calculation within one rotation cycle. Fig.12 shown.

[0077] The axial force of the blade is changed by FFT to obtain Fig.13 The axial force amplitude-frequency diagram is shown in the figure. The main and secondary frequencies of stress fluctuation avoid the high-frequency and low-frequency excitation frequencies of steam flow, and the frequency avoidance rate is 88.9%. This proves that the blade is not prone to fatigue and breakage due to flutter under this working condition. The maximum displacement of the blade depends on the displacement of the trailing edge of the blade tip, such as Fig.14 As shown, the relative amplitude of the blade trailing edge is 0.59%. It can be further judged that the vibration of the blade has not diverged and the blade has not fluttered.

Claims

1. A method for evaluating flow stability of a marine steam turbine under extreme variable operating conditions, characterized in that: The evaluation method is as follows: Step 1: Based on the given design drawing, the turbine cascade channel including the integral moving and stator blades of the turbine is taken as a whole, a three-dimensional geometric model and a flow field model are established, and the cascade channel geometric model is meshed; according to the boundary condition design parameters under the characteristic working conditions of the turbine, the inlet flow rate, temperature, enthalpy value and outlet pressure are determined as the boundary condition design parameters; Step 2: Set the working parameters under different working conditions as boundary conditions, perform steady-state calculations and transient calculations on the blade flow field, and obtain the overall performance from rated working conditions to no-shaft power output conditions, as well as the power, efficiency, post-stage stiffness, reaction, enthalpy drop, pressure, and axial thrust of each stage; at the same time, set constraint boundary conditions to simulate the actual fixed condition of the blades, perform modal analysis on the last-stage moving blades, calculate the first n-order modes of the last-stage moving blades, and obtain the static frequency and dynamic frequency of the blades; Step 3: First, assume that the change of enthalpy drop and power between the two working conditions is linear, and estimate the value of the intermediate working condition according to the straight line passing through the two points; through the linear interpolation approximation simulation of the boundary conditions of adjacent working conditions, finally obtain the accurate critical instability volume flow threshold of the last-stage blade based on the enthalpy drop blast standard and the accurate zero power flow threshold of the whole machine based on the reverse pressure stall standard; Step 4: The surface pressure field data obtained by transient calculation in step 2 is used as a load and imported into the blade structure model through the ANSYS Workbench fluid-solid coupling module for mechanical analysis, and static stress calculation and dynamic stress unidirectional fluid-solid coupling calculation are performed on the blade structure domain; Since the distribution and position of the blade surface grid nodes are not consistent, the solid domain grid surface pressure is obtained from the fluid domain pressure distribution through interpolation, and the blade time domain and frequency domain data, blade modal frequency data, vibration displacement and other data are calculated; Step five: Compare and analyze the time domain and frequency domain data of the transient flow field inside the turbine flow passage with the blade modal frequency data, vibration displacement and other data to determine the flow stability under wide load variable conditions.

2. The method for evaluating flow stability of a marine steam turbine under extreme variable operating conditions according to claim 1, characterized in that: The specific process of step one is as follows: a full-circle model of the blade is modeled according to the profiles of multiple sections provided by each stage of blades and the number of blades around the blade, and the cascade channel is meshed using a structured grid; at the same time, a symmetrical 5-star layout is adopted in the leading edge and trailing edge areas, and the grid blocks are all quadrilaterals; the geometric shape and topological structure of the grid area blocks are optimized by making the edges of the blocks close to the blade perpendicular to the blade surface and the four corners of each block approach 90°.

3. The method for evaluating flow stability of a marine steam turbine under extreme variable operating conditions according to claim 1, characterized in that: The characteristic operating conditions in step 1 include typical operating conditions including the same speed but different steam consumption and the same steam consumption but different speed.

4. The method for evaluating flow stability of a marine steam turbine under extreme variable operating conditions according to claim 1, characterized in that: The specific process of the steady-state calculation of the flow field in step 2 is as follows: according to the actual situation, the flow field medium is set to wet steam, and the thermodynamic properties of water are described by the IAPWS-IF97 formula group. In the CFD steady-state calculation, the Euler multiphase flow model and the κ-ωSST turbulence model are combined to realize a large range of flow field calculations from rated conditions to non-axial power output conditions, and the flow field streamlines, steam velocity, humidity and internal pressure state parameters are obtained to describe the distribution characteristics of the flow field in the blade, including the outlet pressure and velocity of a single-stage full-circle blade, and the meridian velocity vector distribution cloud map of the exhaust chamber, and the steady-state results are input as the initial file for transient calculations. When solving the NS equation, the κ-ωSST turbulence model is introduced, and the calculation formula of its turbulent viscosity is as follows: Where: Where ρ is the fluid density, α1 and β' are model constants, κ is the turbulent kinetic energy, ω is the specific dissipation rate, y is the distance from the first layer of grid nodes on the wall to the wall, and v is the molecular kinematic viscosity of the fluid; Transient flow field calculation: Through CFD transient calculation, the transient results of the blade flow field are obtained, and the blade surface pressure field data is obtained.

5. The method for evaluating flow stability of a marine steam turbine under extreme variable operating conditions according to claim 1, characterized in that: In the step 2: based on the blade structure field model, taking the actual working condition of the steam turbine blade as the constraint condition, a fixed support is applied to the bottom of the blade, and a displacement constraint condition is set at the blade root to calculate the upper line frequency of the first n-order modes.

6. The method for evaluating flow stability of a marine steam turbine under extreme variable operating conditions according to claim 1, characterized in that: The specific process of step three is: according to the positive and negative of the last-stage blade power and the whole machine power, the working condition intervals of the blast working condition and the zero power output working condition are determined, and the inlet flow of the last-stage blade power is zero and the whole machine power is zero is gradually approached by linear interpolation of the inlet flow of the two adjacent working conditions. The blade-level power formula is as follows: Among them, τ is the turbine blade torque; n is the blade speed, that is, the number of revolutions per minute.

7. The method for evaluating flow stability of a marine steam turbine under extreme variable operating conditions according to claim 1, characterized in that: The specific process of step 4 is: using the transient pressure field on the blade surface as the aerodynamic force of steam to be applied to the solid blade, and then analyzing the influence of pressure fluctuations on the blade stress.

8. The method for evaluating flow stability of a marine steam turbine under extreme variable operating conditions according to claim 1, characterized in that: In the step 5, the fluctuation of four blades arranged at equal intervals within one rotation cycle is selected; The transient flow field data inside the turbine flow passage include the steam flow force variation curves over time on four blade surfaces, exhaust chamber end face, and the steam flow excitation spectrum variation curve obtained by FFT transformation.

9. The method for evaluating flow stability of a marine steam turbine under extreme variable operating conditions according to claim 8, characterized in that: The data comparison analysis in step 5 includes the calculated blade natural frequency f0 and the low-frequency exciting force f excited by steam flow. l , high-frequency exciting force f of wake disturbance h By comparison, the frequency avoidance rate is used to determine whether the blade is running stably, whether the blade will resonate, and the degree of danger of blade flutter; and the deformation of the leading and trailing edges of the blade root and the maximum displacement of the blade δ are analyzed. max , through the blade relative amplitude A = δ max The peak value and change of / h can be used to judge the operating stability of the blade under the action of airflow. Frequency avoidance rate = (f0-f1) or (f h -f0) / f0 If (f0-f1)≮15% or (f h -f0) / f0≮15% means stable operation, otherwise unstable operation; h is the ratio of the maximum displacement of the blade to the blade height.

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