A method for calculating turbine blade loss

Abstract

The present invention belongs to the field of energy and power energy conservation, and specifically relates to a method for calculating turbine blade loss. Through the method for calculating turbine blade loss proposed by the present invention, the size of blade loss in the turbine stage under variable working conditions can be accurately calculated. This technical solution is based on a finite element simulation analysis method for the blade flow field. According to the blade structure and working fluid parameters under different flow conditions, a flow channel geometry model including stage static blades and moving blades is created, and the blade pressure field distribution is calculated by flow field simulation, and the transient distribution pressure on the blade surface under different working conditions and the average pressure of the blade under different loads are solved. Finally, a polyvariable process function is used to obtain the average polyvariable process function value of the blade under variable load, and the average polyvariable process function value is used to reflect the blade loss, and it is used to perform a qualitative trend analysis on the blade loss.

Description

Technical Field

[0001] The invention belongs to the field of energy and electric power energy conservation, and in particular relates to a method for calculating turbine blade cascade losses. Background Art

[0002] Whether in large thermal power plants or in new energy power plants such as nuclear power, solar thermal, gas-steam combined cycle and distributed power generation, steam turbines are the main energy conversion equipment, so improving the thermal efficiency of steam turbines is of great significance for achieving energy conservation and efficiency improvement and reducing carbon emissions. The thermal efficiency of a steam turbine indicates the efficiency of converting steam energy into mechanical energy. The relative internal efficiency of the stage is generally used to judge the degree of perfection of energy conversion. Through experimental research on the composition of losses within the steam turbine stage, it was found that blade losses account for about 60% of the total stage losses. Therefore, accurately calculating the blade loss of steam turbine blades and reducing blade losses is the main method to improve the efficiency of the flow section.

[0003] In recent years, there are two methods for calculating turbine blade loss: detailed thermal calculation within the stage and experimental measurement. Both methods calculate the blade loss of the stage under design conditions or a specific condition. Experimental measurement is mostly used to study the mechanism of loss generation and provide a theoretical basis for blade design. Detailed thermal calculation within the stage is to obtain the blade loss under the corresponding condition based on the thermal parameters of the working fluid passing through the stage. Among the calculation methods of intra-stage losses related to blade loss, the detailed thermal calculation method within the stage is not completely based on the mechanism of blade flow loss, and does not consider the change in working fluid flow and the pressure distribution on the blade surface. Instead, it is only based on the thermal parameters of the working fluid passing through the blade surface, that is, the change in the enthalpy value of the working fluid. However, when the unit is 75%-100% loaded, the temperature before and after the stage and the enthalpy drop of the stage are basically unchanged. The result obtained by the detailed thermal calculation of the stage is that the proportion of blade loss to total loss is basically unchanged. However, from the mechanism of blade loss, it can be known that the size of blade loss depends on the pressure distribution on the blade surface, and the pressure distribution is not only related to the working fluid parameters passing through the blade, but also has a great relationship with the working fluid flow rate passing through. Therefore, when the turbine is running at variable load, if the working fluid flow rate passing through the blade changes, even if the thermal parameters do not change, the blade loss will also change. It can be seen that using the traditional detailed thermal calculation method within the stage, when the temperature before and after the stage and the enthalpy drop of the stage are basically unchanged, even if the flow rate changes, the calculated blade loss is basically unchanged, which is inconsistent with the actual situation of the blade loss of the stage, and cannot truly reflect the changing trend of the blade loss under these working conditions. Therefore, it is necessary to find a method to accurately calculate the blade loss to provide a reference for improving the efficiency of the flow-through part and tapping the energy-saving potential of the turbine. Summary of the invention

[0004] In view of the above problems, the present invention provides a method for calculating turbine blade loss, which uses a variable process function value of a stage to distinguish the blade loss of the stage under different working conditions, thereby solving the problem of inaccurate blade loss obtained by traditional thermal calculation. The method comprises the following steps:

[0005] Step 1: Construct a three-dimensional model of turbulent flow in the blade cascade;

[0006] Step 2: Mesh the three-dimensional model of turbulent flow in the blade cascade, and set the working fluid physical parameters, boundary conditions, solver model and solution algorithm;

[0007] Step 3: Solve the blade pressure field to obtain the pressure values ​​around the blade under different loads, and calculate the average pressure value of the blade under different loads based on these pressure values;

[0008] Step 4: Calculate the blade loss value S at the rated load level of the steam turbine 额定 ;

[0009] Step 5: Calculate the ratio of blade loss to total loss at rated load using the detailed thermal calculation method within the stage H 额定 , and obtain the proportionality coefficient

[0010] Step 6: Use the proportionality factor K to re-correct the ratio of the blade loss calculated by the detailed thermal calculation method within the stage to the total loss, and then correct the blade loss value.

[0011] Furthermore, the three-dimensional turbulent flow model in step (1) adopts the standard k-ε model, specifically: as shown in formulas (1) and (2):

[0012]

[0013]

[0014] Among them, G k is the generation term of turbulent kinetic energy k, caused by velocity gradient; G b is the generation term of turbulent kinetic energy k caused by buoyancy; Y M represents the expansion generation term of pulsating flow; C 1ε , C 2ε and C 3ε is the empirical coefficient; σ ε is the Prandtl number of the dissipation rate ε, σ k is the Prandtl number of the turbulent kinetic energy k; S k and S ε Two items are source items, which are defined by the user; u i represents the average speed; μ t represents turbulent viscosity.

[0015] Furthermore, in step (2), the physical property parameters of density, specific heat and thermal conductivity are selected as constants at the qualitative temperature, and the physical property parameters at the control volume interface are obtained using the second-order upwind difference format.

[0016] Furthermore, in step (2), the solver model adopts a segregated solver, and the algorithm adopts a fluid pressure-velocity coupled SIMPLE algorithm.

[0017] Furthermore, in step (2), given the blade inlet pressure and temperature, outlet pressure and temperature and working fluid flow rate, the distribution of the inlet and outlet parameters along the blade pitch direction satisfies the periodic boundary conditions, and the dynamic and static interfaces adopt the sliding surface mesh technology.

[0018] Furthermore, according to the geometric shape of the blade grid and the pressure, temperature and flow value of the working fluid entering the blade grid, the fluid calculation domain is simplified and the blade grid is modeled in full physical size.

[0019] Furthermore, in step 4, a polynomial process function is used to represent the turbine stage blade loss S 额定 , the multivariate process function is defined as:

[0020] S=pv n (3)

[0021] Where p is the average pressure value (Pa) of the cascade under different loads obtained in step (3); v is the specific volume of the working fluid (m 3 / kg); n is the variable process index, is the nozzle or moving blade velocity coefficient; k is the constant entropy index.

[0022] The technical effect of the present invention is that the method proposed by the present invention to distinguish the blade loss of the stage under different working conditions by using the variable process function value of the stage can solve the problem that the blade loss obtained by traditional thermal calculation is inaccurate when the temperature and pressure before the stage do not change, but only the flow rate changes. The present invention uses the change of the average variable process function value to reflect the change of the blade loss. The calculation result is better than the change of the blade loss obtained by the thermal calculation of the stage, and can better reflect the change trend of the blade loss, thereby providing a reference basis for analyzing the change law of the blade loss in the stage when the steam turbine is running under non-design conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the trend diagram of blade cascade loss change under variable operating conditions;

[0024] Figure 2 The nozzle and moving blade pressure distribution diagram under 75% working condition;

[0025] Figure 3This is the nozzle and moving blade pressure distribution diagram under 100% working condition;

[0026] Figure 4 The comparison chart of blade loss change trend calculated by two methods; DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] The traditional detailed thermal calculation method within the stage is to obtain the blade loss under the corresponding working conditions based on the thermal parameters of the working fluid passing through the stage. The calculation method of the stage loss related to the blade loss is as follows:

[0029] 1. Nozzle loss:

[0030]

[0031] in, ——Nozzle velocity coefficient; Δh n * ——Ideal specific enthalpy drop of nozzle stagnation, kJ / kg.

[0032] 2. Moving blade loss:

[0033]

[0034] Where, φ is the speed coefficient of moving blades; ——Ideal specific enthalpy drop when moving blades are stationary, kJ / kg.

[0035] 3. Leaf height loss:

[0036]

[0037] Among them, a is the experimental coefficient; Δh u ——effective specific enthalpy drop around the wheel, kJ / kg; l——blade height, mm. Where a is the coefficient, determined by experimental analysis, and a is related to the type of stage. For a single-row stage, a=1.2 (excluding fan-shaped losses) or a=1.6 (including fan-shaped losses), and for a double-row stage, a=2.

[0038] 4. Fan-shaped loss:

[0039]

[0040] Where lb d b ——Height and average diameter of moving blades, mm; E 0 ——Ideal enthalpy drop of blade cascade, kJ / kg.

[0041] From the above formulas 1-4, it can be seen that the traditional detailed thermal calculation method within the stage is not completely based on the mechanism of blade flow loss, and does not consider the change of working fluid flow and the pressure distribution on the blade surface, but is only based on the thermal parameters of the working fluid passing through the blade surface, that is, the change of working fluid enthalpy value. However, when the unit is 75%-100% loaded, the temperature before and after the stage and the enthalpy drop of the stage are basically unchanged. The result obtained by the detailed thermal calculation of the stage is that the proportion of blade loss to total loss is basically unchanged, as shown in Table 1:

[0042] Table 1 Calculation results of thermal losses in the stage under variable operating conditions

[0043]

[0044] The change trend of the proportion of blade loss to total loss obtained through detailed calculation is as follows: Figure 1 shown.

[0045] However, according to the blade loss mechanism, the size of the blade loss depends on the pressure distribution on the blade surface, and the pressure distribution is not only related to the working fluid parameters passing through the blade, but also has a great relationship with the working fluid flow rate passing through the blade. Therefore, when the turbine is running at variable load, if the working fluid flow rate passing through the blade changes, even if the thermal parameters do not change, the blade loss will also change. By performing finite element analysis on the blade, the blade pressure distribution obtained is Figure 2-Figure 3 It can be seen that, especially under 75%-100% load, the blade pressure distribution is different. According to the principle of blade aerodynamic characteristics, the blade loss at this time is changed.

[0046] Therefore, the present invention provides a method for calculating turbine blade loss, which uses a variable process function value of a stage to distinguish blade loss of a stage under different working conditions, thereby solving the problem of inaccurate blade loss obtained by traditional thermal calculation. The method comprises the following steps:

[0047] Step (1) According to the unit structure and operating conditions, the fluid calculation domain is simplified, the blade is modeled in full physical size, and a three-dimensional model of turbulent flow within the stage is established. The model is a standard k-ε model, as shown in formulas (1) and (2):

[0048]

[0049]

[0050] In the formula, G kis the generation term of turbulent kinetic energy k, which is caused by velocity gradient; G b is the generation term of turbulent kinetic energy k caused by buoyancy; Y M represents the expansion generation term of pulsating flow; C 1ε , C 2ε and C 3ε is a series of coefficients, obtained empirically; σ ε is the Prandtl number of the dissipation rate ε, σ k is the Prandtl number of the turbulent kinetic energy k; S k and S ε Two of the two items are source items, which are defined by the user; finally, u i represents the average speed; μ t represents turbulent viscosity.

[0051] Step (2) uses a structured grid for discretization; sets a phase change model, a turbulence model, boundary conditions and numerical solution parameters.

[0052] Step (3) performs simulation calculations on the blade cascade to obtain pressure distribution diagrams around the stationary blades and the moving blades, the blade cascade flow field distribution and the average pressure under different working conditions.

[0053] Step (4) uses a polyvariable process function to represent the blade loss of the turbine stage. The polyvariable process function is defined as:

[0054] S=pv n (3)

[0055] Where, p is static pressure (Pa); v is steam specific volume (m 3 / kg); n—variable process index, —Nozzle or blade velocity coefficient; k—constant entropy index.

[0056] Step (5) Calculate the ratio of blade loss to total loss at rated load using traditional thermal calculation method Compare this value with the multi-variable process function value S at rated load calculated in step (4) 额定 Calibrate and get the proportional coefficient

[0057] Step (6) uses K obtained in step (5) to re-correct the ratio H of the blade loss to the total loss calculated by the traditional thermal calculation method, and then corrects the blade loss value δh 叶栅 .

[0058] The turbine blade loss calculation method proposed in the present invention can accurately calculate the size of the blade loss in the turbine stage under variable working conditions. This technical solution is based on the finite element simulation analysis method of the blade flow field. According to the blade structure and working fluid parameters under different flow conditions, a flow channel geometry model including the stage static blades and moving blades is created, and the blade pressure field distribution is calculated by flow field simulation, and the transient distribution pressure on the blade surface under different working conditions and the average pressure of the blade under different loads are solved. Finally, a multi-variable process function is used to obtain the average multi-variable process function value of the blade under variable load, and the average multi-variable process function value is used to reflect the blade loss, and it is used to perform a qualitative trend analysis on the blade loss.

[0059] This method uses the average polytropic process function value to represent the blade loss, comprehensively considering the working fluid flow and thermal parameter changes flowing through the blade surface. When the unit load is lower than 75%, the blade loss represented by the average polytropic process function value is not only consistent with the changing trend of the proportion of blade loss to total loss obtained by traditional stage thermal calculation, but also can accurately reflect the changing trend of blade loss when the unit load is higher than 75%. This method solves the problem that the blade loss calculated solely by the change of working fluid thermal parameters cannot truly reflect the changing trend of blade loss under these working conditions. The blade loss change represented by the average polytropic process function value is better than the blade loss change obtained by stage thermal calculation, and can better reflect the actual changing trend of blade loss.

[0060] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, 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 method for calculating turbine blade loss, characterized in that: The following steps are included: Step 1: Construct a three-dimensional model of turbulent flow in the blade cascade; Step 2: Mesh the three-dimensional model of turbulent flow in the blade cascade, and set the working fluid physical parameters, boundary conditions, solver model and solution algorithm; Step 3: Solve the blade pressure field to obtain the pressure values ​​around the blade under different loads, and calculate the average pressure value of the blade under different loads based on these pressure values; Step 4: Calculate the blade loss value S at the rated load level of the steam turbine 额定 ; Step 5: Calculate the ratio of blade loss to total loss at rated load using the detailed thermal calculation method within the stage H 额定 , and obtain the proportionality coefficient Step 6: Use the proportionality factor K to re-correct the ratio of the cascade loss calculated by the detailed thermal calculation method within the stage to the total loss, and then correct the cascade loss value; The three-dimensional turbulent flow model in step (1) adopts the standard k-ε model, specifically: as shown in formulas (1) and (2): As shown: Among them, G k is the generation term of turbulent kinetic energy k, caused by velocity gradient; G b is the generation term of turbulent kinetic energy k caused by buoyancy; Y M represents the expansion generation term of pulsating flow; C 1ε , C 2ε and C 3ε is the empirical coefficient; σ ε is the Prandtl number of the dissipation rate ε, σ k is the Prandtl number of the turbulent kinetic energy k; S k and S ε Two items are source items, which are defined by the user; u i represents the average speed; μ t represents turbulent viscosity; In step 4, a polyvariable process function is used to represent the turbine stage blade loss S 额定 , the multivariate process function is defined as: S=pv n (3) Where p is the average pressure value (Pa) of the cascade under different loads obtained in step (3); v is the specific volume of the working fluid (m 3 / kg); n is the variable process index, is the nozzle or moving blade velocity coefficient; k is the constant entropy index.

2. A method for calculating turbine blade loss according to claim 1, characterized in that: In step (2), the physical property parameters of density, specific heat and thermal conductivity are selected as constants at the qualitative temperature, and the physical property parameters at the control volume interface are obtained using the second-order upwind difference format.

3. A method for calculating turbine blade loss according to claim 1, characterized in that: In step (2), the solver model adopts a segregated solver, and the algorithm adopts the fluid pressure-velocity coupled SIMPLE algorithm.

4. A method for calculating turbine blade loss according to claim 1, characterized in that: In step (2), the blade inlet pressure and temperature, outlet pressure and temperature and working fluid flow rate are given, the distribution of the inlet and outlet parameters along the blade pitch direction satisfies the periodic boundary conditions, and the sliding surface mesh technology is used for the dynamic and static interfaces.

5. A method for calculating turbine blade loss according to claim 1, characterized in that: According to the geometric shape of the blade and the pressure, temperature and flow value of the working fluid entering the blade, the fluid calculation domain is simplified and the blade is modeled in full physical size.

Images