Calculation method of working medium flow rate of boiler water-cooled wall and prediction method of boiler wall temperature

Abstract

The present invention discloses a method for calculating the working fluid flow of a boiler water-cooled wall and a method for predicting the boiler wall temperature, which belongs to the field of boiler hydrodynamic calculation. By constructing a water-cooled wall system model and optimally designing a method for calculating the working fluid flow of a water-cooled wall system, the water-cooled wall system is divided into multiple loops, and each loop is divided into multiple calculation modules. By sequentially iteratively calculating each calculation module, the flow in each loop can be calculated, and the working fluid flow distribution in the water-cooled wall system can be obtained. The method for calculating the working fluid flow of a boiler water-cooled wall in the present invention has simple steps and convenient calculations. It can accurately obtain the working fluid flow distribution, working fluid temperature and heat transfer coefficient of the water-cooled wall on the working fluid side, provide a basis for predicting the boiler wall temperature on the combustion side, fully realize the calculation cycle iteration of the combustion side and the working fluid side until convergence, realize the coupled calculation of the combustion side and the working fluid side, ensure the accuracy of the prediction and calculation of the boiler wall temperature distribution, and provide a guarantee for the safe operation of the boiler.

Description

Technical Field

[0001] The present invention belongs to the field of boiler hydrodynamic calculation, and in particular relates to a method for calculating the working medium flow of a boiler water-cooled wall and a method for predicting the boiler wall temperature. Background Art

[0002] In recent years, the installed capacity and power generation of renewable energy have been growing rapidly, playing an increasingly important role in the power generation sector. However, due to the strong volatility and intermittent nature of renewable energy, thermal power units are required to participate in peak load regulation in actual applications to support the large-scale integration of renewable energy power generation. The premise of deep peak load regulation is that the boiler maintains safe and stable operation under low-load conditions. However, in reality, under low-load conditions, problems such as uneven flame distribution, insufficient power, and low pressure in the furnace may lead to large steam temperature deviations on the heating surface, resulting in large fluctuations in the physical properties of the working fluid in the water-cooled wall tubes and complex changes in the working fluid flow state, which can easily lead to large wall temperature deviations between water-cooled walls, water-cooled wall drying, and even over-temperature tube bursts.

[0003] Under normal circumstances, a detailed and accurate wall temperature distribution is helpful in preventing pipeline degradation and leakage accidents. For this reason, in the applicant's prior patent application CN117648879A, a method for predicting the wall temperature of the boiler heating surface based on the integrated coupling modeling of Fluent and UDF was proposed. This method can achieve the prediction of the wall temperature of the boiler heating surface to a certain extent, and then provide a basis for the optimization of the heating surface to a certain extent, ensuring the safe operation of the boiler. When performing boiler combustion simulation calculations, it is necessary to pay attention to the working conditions of the combustion side and the working fluid side respectively, and simulate the actual conditions on both sides separately. However, due to the large size and harsh working environment of actual power plant boilers, it is difficult to accurately know the water-cooled wall flow distribution. As a result, existing boiler numerical simulation research mainly focuses on the furnace combustion situation, and the flow and heat transfer of the working fluid are simplified. Most of them assume a constant working fluid temperature or tube wall temperature or uniform flow distribution, which is exactly the case in the applicant's prior patent application.

[0004] It is precisely because of the existence of the above-mentioned situation that the technical solution in the applicant's prior patent has certain deficiencies and limitations. During the actual operation of the boiler, the distribution of the water-cooled wall flow is the result of the combined effects of the combustion side and the working fluid side. In traditional hydrodynamic calculation methods, the graphical method and the loop node method are usually used. Among them, the graphical method is only applicable to single-condition calculations and the calculation process is complex and has large errors. The loop point method fits the three-dimensional heat flow on the combustion side into a one-dimensional heat flux density through the thermal deviation coefficient during the flow distribution calculation, and does not feed back the calculation results on the working fluid side to the combustion side. In the simulation, it is impossible to simultaneously reflect the effects of flue gas side combustion and working fluid side heat transfer on the water-cooled wall flow distribution, resulting in the inability to accurately know and control the wall temperature of the boiler heating surface, making it difficult to timely detect local overheating of the boiler heating surface and propose solutions in a timely manner. Because of this, the existing methods are difficult to achieve accurate calculation of the working fluid flow in the boiler water-cooled wall system and cannot guarantee the accuracy of the prediction and calculation of the boiler wall temperature distribution. Summary of the Invention

[0005] In response to one or more of the above-mentioned defects or improvement needs in the prior art, the present invention provides a method for calculating the working fluid flow rate of the boiler water-cooled wall and a method for predicting the boiler wall temperature, which can accurately calculate the flow distribution on the boiler working fluid side and provide a basis and conditions for accurate prediction of the boiler wall temperature.

[0006] To achieve the above-mentioned object, one aspect of the present invention provides a method for calculating the working medium flow rate of a boiler water wall, which is used for calculating the flow distribution on the working medium side of the boiler, and comprises the following steps:

[0007] S1: Construct a geometric model of the boiler, calculate the combustion values ​​of the entire furnace, and obtain the heat flux density distribution on the boiler combustion side; establish a water-cooled wall system model for the boiler model and call the heat flux density distribution of the boiler heating surface;

[0008] S2: Set the inlet working fluid temperature, working fluid pressure, working fluid enthalpy value of each circuit in the water-cooled wall system and the total flow of the water-cooled wall system, and assume the outlet working fluid pressure, initial flow value and relative error value of the outlet working fluid pressure of each circuit, as well as the relative error value of the total flow;

[0009] S3: Calculate each circuit in turn along the working fluid flow direction to calculate the working fluid flow distribution of the water-cooled wall system. The calculation process includes:

[0010] S31: Divide each circuit into multiple calculation modules along the height direction of the boiler, and determine the phase state of the working fluid in each calculation module in turn;

[0011] S32: Starting from the inlet end of each loop, the working fluid pressure, working fluid enthalpy and working fluid temperature at the outlet of each calculation module are calculated in sequence;

[0012] S33: Input the outlet parameters of the previous calculation module as the inlet parameters to the next calculation module, and repeat steps S31 and S32 until the last calculation module of a single loop is calculated. The outlet working fluid pressure of the last calculation module is the outlet pressure value of the loop;

[0013] S34: Compare the calculated loop outlet working fluid pressure with the assumed outlet working fluid pressure. If the relative error between the two is greater than the assumed relative error of the outlet working fluid pressure, update the loop flow value. Repeat steps S31 to S33 until the relative error is less than the relative error of the outlet working fluid pressure, thereby completing the calculation of the single loop flow value.

[0014] S35: After all loop calculations converge, add up the flow rates of all loops to obtain the total flow rate of the water-cooled wall system. Compare the calculated total flow rate of the water-cooled wall system with the total flow rate of the water-cooled wall system set in S2. If the relative error between the two is greater than the relative error value of the total flow rate, update the assumed outlet working fluid pressure. Repeat steps S31 to S34 until the relative error is less than the relative error value of the assumed outlet working fluid pressure. The calculation converges. The updated assumed outlet working fluid pressure is the final working fluid outlet pressure. The flow values ​​of each loop are obtained, i.e., the flow distribution is obtained.

[0015] As a further improvement of the present invention, in step S2, the working fluid pressure at the loop inlet is recorded as P1, and the working fluid pressure at the outlet is preferably assumed to be (P1-2.0)±0.1Mpa;

[0016] and / or

[0017] Let the total flow of the water-cooled wall system be M and the total number of loops be N, then the initial flow value of each loop is M / N;

[0018] and / or

[0019] The relative error ratio of the outlet working fluid pressure is assumed to be no greater than 0.01, and the relative error ratio of the total flow rate is assumed to be no greater than 0.05.

[0020] As a further improvement of the present invention, in step S1, the geometric model of the boiler is constructed by Fluent, and the water-cooled wall system model is established and the heat flux density distribution of the heating surface is called by the user-defined function UDF in Fluent; and

[0021] The water-cooled walls include a spiral water-cooled wall located at the lower part of the boiler and a vertical water-cooled wall located at the upper part of the boiler, and the boiler water-cooled walls are expanded into two dimensions in the UDF.

[0022] As a further improvement of the present invention, in step S31, the phase state of the working medium is determined by calculating the working medium dryness, and the calculation formula is as follows:

[0023]

[0024] Where, x is the working fluid dryness; h is the enthalpy of the working fluid, kj / kg; Working fluid pressure p The corresponding saturated water enthalpy is kj / kg; For pressure p The corresponding saturated steam enthalpy is kj / kg; if the working fluid dryness is x =0 or x =1, the working medium is in the single-phase region; if 0< x <1, the working fluid is in the two-phase region.

[0025] As a further improvement of the present invention, in step S32, the working fluid pressure at the outlet of each calculation module is obtained by calculating the working fluid pressure drop, and the working fluid pressure drop is represented by the following formula:

[0026]

[0027] Where, is the total pressure drop, MPa; is the local resistance pressure drop, MPa; is the friction resistance pressure drop, MPa; is the pressure drop at the gravity position, MPa; is the accelerating pressure drop, MPa.

[0028] As a further improvement of the present invention, when the working fluid is single-phase, the working fluid pressure drop formula is 、 、 、 Calculated by the following formulas:

[0029]

[0030]

[0031]

[0032]

[0033] Where, is the local resistance coefficient; G is the working fluid mass flow rate, kg / (m 2 ·s); is the working fluid density, kg / m 3 ; l is the calculation module length, m; D is the inner diameter of the tube, m; It is the arithmetic mean of the import and export density of the calculation module; is the friction resistance coefficient; is the calculation module height, m; g is the acceleration due to gravity, m / s 2 ; is the specific volume of the working fluid at the outlet of the calculation module, m 3 / kg; is the specific volume of the working fluid at the inlet of the calculation module, m 3 / kg.

[0034] As a further improvement of the present invention, when the working fluid is two-phase, the working fluid pressure drop formula is 、 、 、 Respectively characterized as 、 、 、 , the calculation process is as follows:

[0035]

[0036]

[0037] Where, is the local resistance coefficient corresponding to the two-phase fluid; is the density of saturated water, kg / m 3 ; is the density of saturated steam, kg / m 3 ; is the average mass gas fraction of the calculation module, is the mass gas fraction of the working fluid at the inlet of the calculation module; if the working fluid at the outlet is a steam-water mixture, the working fluid at the inlet is in the unsaturated zone, then =0; To calculate the gas content of the working fluid at the module outlet;

[0038]

[0039] Where, is the friction loss correction factor, which is calculated as follows:

[0040] =1;( G =1000kg / (m 2 s));

[0041] ; ( G >1000kg / (m 2 s))

[0042] ; ( G<1000kg / (m 2 s))

[0043]

[0044]

[0045]

[0046]

[0047] Where, is the average cross-sectional air content of the calculation module; is the calculated average value of the volumetric gas fraction in the calculation module; S is the slip ratio; is the circulation velocity of the working fluid in the calculation module, m / s; p is the absolute pressure of the working fluid in the calculation module, MPa;

[0048]

[0049] Where: is the specific volume of saturated water vapor, m 3 / kg; is the specific volume of saturated water, m 3 / kg.

[0050] As a further improvement of the present invention, the working fluid outlet enthalpy and working fluid outlet temperature are calculated by the following formula:

[0051]

[0052]

[0053] Where, Q The heat flux absorbed by the calculation module; m is the flow rate of the working fluid flowing through the calculation module; and are the inlet enthalpy and outlet enthalpy of the calculation module respectively; T is the outlet temperature of the working fluid; dimensionless pressure , =1Mpa; is the working fluid pressure, MPa; dimensionless enthalpy ,in, ; h is the working fluid enthalpy, kj / kg ; ; 、 and are all constants.

[0054] Another aspect of the present invention provides a method for predicting boiler wall temperature, which includes the above-mentioned method for calculating the working medium flow rate of the boiler water-cooled wall, and further includes the following steps:

[0055] S4: After obtaining the flow distribution, the flow heat transfer calculation is performed on each loop in turn by inputting the corresponding flow value of the loop to obtain the working fluid temperature and heat transfer coefficient. The wall temperature at the corresponding position is calculated based on the working fluid temperature, heat transfer coefficient and heat flux density, realizing the prediction of the boiler wall temperature;

[0056] S5: Feedback the obtained working fluid temperature and heat transfer coefficient to the combustion side as boundary conditions to achieve coupled calculation of the combustion side and the working fluid side; and the heat balance relationship between the combustion side and the working fluid side is as follows:

[0057]

[0058] Where, Q is the heat flow; is the radiation emissivity; is the Stefen-Boltzmann constant; is the incident radiation, W / m 2 ; is the convective heat transfer, W / m 2 ; and Solve within Fluent; is the wall temperature, K; k is the heat transfer coefficient, W / (m 2 K); is the working fluid temperature, K.

[0059] As a further improvement of the present invention, in step S4, the formula for calculating the wall temperature of the heated surface according to the working fluid temperature and the heat flow is as follows:

[0060]

[0061] Where, is the wall temperature; is the working fluid temperature; Q is the heat flow; is the convection heat transfer coefficient, W / (m 2 K); is the pipe wall thickness; is the thermal conductivity of the pipe.

[0062] The above-mentioned improved technical features can be combined with each other as long as they do not conflict with each other.

[0063] In general, the above technical solutions conceived by the present invention have the following beneficial effects compared with the prior art:

[0064] (1) The method for calculating the working fluid flow rate of the boiler water-cooled wall of the present invention constructs a water-cooled wall system model and optimally designs a method for calculating the working fluid flow rate of the water-cooled wall system, divides the water-cooled wall system into multiple loops, and divides each loop into multiple calculation modules. By using the sequential iterative calculation of each calculation module, the flow rate in each loop can be calculated, and then the working fluid flow rate distribution in the water-cooled wall system can be obtained, thereby realizing the visualization of the calculation on the boiler working fluid side and providing a basis for the prediction and calculation of the wall temperature on the combustion side.

[0065] (2) The method for calculating the working fluid flow rate of the boiler water-cooled wall of the present invention uses the two-dimensional heat flux density obtained by real-time calculation of the three-dimensional model as the boundary condition for the working fluid side calculation. Compared with the traditional working fluid flow rate calculation method that fits the boiler's one-dimensional heat flow curve into the two-dimensional heat flux density of the heating surface, this method can better consider the influence of uneven heating of the heating surface and effectively improve the accuracy of the working fluid flow rate calculation.

[0066] (3) The method for calculating the working fluid flow rate of the boiler water-cooled wall of the present invention can effectively improve the accuracy of the calculation by optimizing the assumed values ​​of each parameter in step S2, reduce the trial and error and iterative calculation process in the working fluid flow distribution calculation process, reduce the calculation workload, and improve the efficiency and accuracy of the working fluid flow rate calculation in the water-cooled wall system.

[0067] (4) The method for calculating the working fluid flow rate of the boiler water-cooled wall of the present invention can meet the working fluid flow rate calculation of a single circuit and the entire water-cooled wall system by optimizing the design of the calculation process of the working fluid flow rate in each circuit and utilizing the corresponding design of S31~S35, thereby ensuring the accuracy of the working fluid flow rate distribution calculation and providing a reliable basis for the wall temperature calculation on the combustion side. Compared with the traditional method of keeping the working fluid temperature constant and distributing the flow rate evenly in the wall temperature calculation process, it can realize the accurate calculation of the working fluid temperature at different positions and the working fluid pressure changes at different positions, and realize the visualization of the entire working fluid flow rate calculation process.

[0068] (5) The boiler wall temperature prediction method of the present invention has simple steps and is easy to operate. By calculating the working fluid flow rate on the working fluid side of the boiler water-cooled wall system, the flow distribution on the working fluid side can be accurately obtained, and this is used as the basis for predicting the wall temperature on the combustion side. It effectively realizes the two-way coupling between the working fluid side and the combustion side, ensures the accuracy of the calculation results, provides a basis for the optimization of the heating surface, and ensures the safe operation of the boiler.

[0069] (6) The method for calculating the working fluid flow rate of the boiler water-cooled wall in the present invention has simple steps and convenient calculations. It can accurately obtain the working fluid flow rate distribution, working fluid temperature and heat transfer coefficient of the water-cooled wall on the working fluid side, and use them as boundary conditions on the combustion side to provide sufficient basis and conditions for the prediction of the boiler wall temperature on the combustion side. It fully realizes the calculation cycle iteration of the combustion side and the working fluid side until convergence, realizes the coupled calculation of the combustion side and the working fluid side, ensures the accuracy of the boiler wall temperature distribution prediction and calculation, and provides protection for the safe operation of the boiler. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0071] Figure 1 1 is a flow chart of a method for calculating the working medium flow rate of a boiler water wall according to an embodiment of the present invention;

[0072] Figure 2 Schematic diagram of heat flux density distribution on the heating surface of a boiler according to an embodiment of the present invention;

[0073] Figure 3 Schematic diagram of the circuit and calculation module division of the boiler water wall system in an embodiment of the present invention;

[0074] Figure 4 1 is a schematic diagram of a calculation flow of a single calculation module in a working fluid flow calculation method according to an embodiment of the present invention;

[0075] Figure 5 Schematic diagram showing the change of the working fluid physical parameters of the circuit along the height direction of a certain circuit in an embodiment of the present invention;

[0076] Figures 6 to 8 It is a schematic diagram of the working medium pressure distribution in the lower water-cooled wall tube, the working medium temperature distribution in the lower water-cooled wall tube, and the lower water-cooled wall flow distribution in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0077] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only intended to illustrate the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0078] In the description of the present invention, it should be understood that, unless otherwise expressly specified and limited, the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like to indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore should not be understood as limiting the present invention.

[0079] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature. In the description of the present invention, "plurality" means at least two, such as two, three, etc., unless otherwise specifically defined.

[0080] In the present invention, unless otherwise specified or limited, the terms "installed," "connected," "connect," "fixed," etc. should be understood in a broad sense. For example, they can refer to fixed connection, detachable connection, or integration; mechanical connection, electrical connection; direct connection, or indirect connection through an intermediate medium; internal communication between two components, or interaction between two components, unless otherwise specified. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0081] In the present invention, unless otherwise expressly specified or limited, when a first feature is "above" or "below" a second feature, it may mean that the first and second features are in direct contact, or that the first and second features are in indirect contact through an intermediary. Furthermore, when a first feature is "above," "above," or "above" a second feature, it may mean that the first feature is directly above or diagonally above the second feature, or simply means that the first feature is at a higher level than the second feature. When a first feature is "below," "below," or "below" a second feature, it may mean that the first feature is directly below or diagonally below the second feature, or simply means that the first feature is at a lower level than the second feature.

[0082] Example:

[0083] See also Figure 1 The working fluid flow calculation method of the boiler water-cooled wall in the preferred embodiment of the present invention is mainly used for the distribution calculation of the working fluid flow in the working fluid side of the combustion boiler. Through the calculation of the working fluid side flow distribution, it provides a basis for the prediction and calculation of the boiler wall temperature on the boiler combustion side.

[0084] Specifically, the method for calculating the working medium flow rate of the boiler water wall in the preferred embodiment includes the following steps:

[0085] S1: Construct a geometric model of the boiler, calculate the combustion values ​​of the entire furnace, and obtain the heat flux density distribution on the boiler combustion side; establish a water-cooled wall system model for the boiler model and call the heat flux density distribution of the boiler heating surface;

[0086] In a preferred embodiment, the establishment of the two models is preferably carried out according to the boiler dimensions in the boiler design manual. The entire boiler structure includes at least a furnace body, a water-cooled wall system, a burner, a cold ash hopper, a superheater and a reheater. The heating surface of the boiler is preferably simplified, and the boundary condition of the heating surface is further preferably a convection boundary.

[0087] Preferably, the heat flux density distribution on the combustion side of the boiler is preferably performed in the commercial software Fluent. After the iteration is completed, a schematic diagram of the heat flux density distribution on the heating surface is obtained, such as Figure 2 At the same time, in the modeling, it is preferred to adopt the method of regional grid division, use high-quality hexahedral grids, and encrypt the grid in the burner area where the physical and chemical reactions are intense.

[0088] Accordingly, it is preferred to establish the water-cooled wall system model and call the heat flux density distribution of the heated surface through the user-defined function (UDF) in Fluent.

[0089] More specifically, when dividing the calculation area, it is preferred to unfold the boiler water-cooled wall into two dimensions in the UDF, and it is preferred to simplify the boiler water-cooled wall into a plane with zero thickness, and the water-cooled wall preferably includes a spiral water-cooled wall (lower water-cooled wall) located at the lower part of the boiler and a vertical water-cooled wall (upper water-cooled wall) located at the upper part of the boiler.

[0090] At the same time, it is preferred to divide the water-cooled wall into multiple loops in the horizontal direction, and further preferably to divide each loop into multiple computing modules in the vertical direction. In this case, the computing modules in the spiral water-cooled wall are preferably parallelograms, and the computing modules in the vertical water-cooled wall are preferably rectangular. The division diagram is as follows: Figure 3 shown.

[0091] S2: Set the inlet working fluid temperature, working fluid pressure, working fluid enthalpy value of each circuit in the water-cooled wall system and the total flow of the water-cooled wall system, and assume the outlet working fluid pressure, initial flow value and relative error value of the outlet working fluid pressure of each circuit, as well as the relative error value of the total flow.

[0092] The inlet working fluid temperature, working fluid pressure, working fluid enthalpy, number of circuits in the water-wall system, and total flow rate of the water-wall system are parameters used during actual boiler operation and can be selected based on actual setup conditions. Furthermore, the outlet working fluid pressure, initial flow rate, relative error in outlet working fluid pressure, and relative error in total flow rate for each circuit are assumed values. In actual operations, to conserve computing resources, these assumed values ​​are generally selected based on empirical experience, and their setting does not deviate from natural laws or common knowledge.

[0093] More specifically, in actual application, the pressure drop of the working fluid in the boiler water-cooled wall system generally does not exceed 2 MPa. The working fluid pressure at the loop inlet is denoted as P1, and the working fluid pressure at the outlet is preferably assumed to be (P1-2.0)±0.1 MPa. At the same time, the initial flow value of the loop is preferably given by the average value of the total flow of the water-cooled wall system. The total flow of the water-cooled wall system is denoted as M, and the total number of loops is N. The initial flow value of each loop is M / N. In addition, the outlet working fluid pressure and the total flow are both major factors affecting the working fluid temperature. The assumptions of the relative error value of the outlet working fluid pressure and the relative error value of the total flow should save computing resources as much as possible and ensure the accuracy of the working fluid temperature calculation. To this end, the relative error ratio of the outlet working fluid pressure is preferably not greater than 0.01, and the relative error ratio of the total flow is preferably assumed to be not greater than 0.05. The selection of the relative error ratio can realize the determination of the corresponding relative error value.

[0094] In a specific preferred embodiment, the working fluid pressure at the circuit inlet is 35.24 MPa, the total flow rate of the water-cooled wall system is 697.77 kg / s, the total number of circuits is 712, and the relative error ratio of the outlet working fluid pressure is set to 0.01, and the relative error ratio of the total flow rate is set to 0.05. In this case, the initial flow rate value of each circuit is 0.98 kg / s, and the working fluid pressure at the outlet can be assumed to be 33.24 MPa, that is, the relative error is 0.3 MPa, and the corresponding working fluid temperature is 721.3 ± 0.7 K, which is within the acceptable range of working fluid temperature error. Correspondingly, the relative error value of the total flow rate is 34.8 kg / s, and the corresponding working fluid temperature is 721.3 ± 0.5 K, which is also within the acceptable range of working fluid temperature error.

[0095] S3: Calculate each circuit in turn along the working medium flow direction to calculate the working medium flow distribution of the water wall system;

[0096] Specifically, the flow rate distribution calculation process in S3 is as follows: Figure 4 As shown in , it preferably includes the following process:

[0097] S31: Determine the phase of the working fluid in the calculation module;

[0098] In actual calculation, it is preferred to calculate the working fluid dryness from the working fluid inlet pressure and the working fluid inlet enthalpy of the calculation module. The working fluid phase is determined according to the working fluid dryness, and is divided into a single-phase region and a two-phase region. The working fluid dryness calculation formula is as follows:

[0099]

[0100] Where, x is the working fluid dryness; h is the enthalpy of the working fluid, kj / kg; Working fluid pressure p The corresponding saturated water enthalpy is kj / kg; For pressure p The corresponding saturated steam enthalpy is kj / kg.

[0101] If the working fluid dryness x =0 or x =1, the working fluid is in the single-phase region; if 0< x <1, the working fluid is in the two-phase region.

[0102] S32: Starting from the inlet of each loop, the corresponding formula is used to calculate the working fluid pressure, working fluid enthalpy and working fluid temperature at the outlet of each calculation module;

[0103] In a preferred embodiment, the above parameters are measured using the physical properties of the working fluid in combination with corresponding formulas. The physical properties mentioned here include pressure, temperature, density, specific heat capacity, dryness, etc.

[0104] Specifically, the working fluid pressure at the module outlet is calculated by calculating the working fluid pressure drop between the module inlet and outlet, which is represented by the following pressure drop formula:

[0105]

[0106] Where, is the total pressure drop, MPa; is the local resistance pressure drop, MPa; is the friction resistance pressure drop, MPa; is the pressure drop at the gravity position, MPa; is the accelerating pressure drop, MPa.

[0107] However, depending on the phase of the working fluid at the circuit outlet, the calculation process of the circuit outlet pressure drop is different, which is specifically reflected in the different calculation methods of various pressure drops in the above total pressure drop formula.

[0108] (1) When the working fluid is single-phase, for the above pressure drop formula, the pressure drop caused by local resistance is It is preferably characterized by the following formula:

[0109]

[0110] Where, is the local resistance coefficient; G is the working fluid mass flow rate, kg / (m 2 ·s); is the working fluid density, kg / m 3 .

[0111] Friction resistance pressure drop It is preferably characterized by the following formula:

[0112]

[0113] Where, l is the calculation module length, m; D is the inner diameter of the tube, m; It is the arithmetic mean of the import and export density of the calculation module; is the friction resistance coefficient.

[0114] Heavy pressure drop It is preferably characterized by the following formula:

[0115]

[0116] Where, is the calculation module height, m; g is the acceleration due to gravity, m / s 2 .

[0117] Accelerated pressure drop It is preferably characterized by the following formula:

[0118]

[0119] Where, is the specific volume of the working fluid at the outlet of the calculation module, m 3 / kg; is the specific volume of the working fluid at the inlet of the calculation module, m 3 / kg; G is the working fluid mass flow rate, kg / (m 2 ·s).

[0120] (2) When the working fluid is two-phase, the total pressure drop at the circuit outlet is expressed as follows:

[0121]

[0122] At this time, the pressure drop caused by local resistance It is preferably characterized by the following formula:

[0123]

[0124] Where, is the local resistance coefficient corresponding to the two-phase fluid; is the saturated water density, kg / m 3 ; is the saturated steam density, kg / m 3 ; To calculate the average mass gas content of the module, it can be calculated as follows:

[0125]

[0126] Where, is the mass gas fraction of the working fluid at the inlet of the calculation module; if the working fluid at the outlet is a steam-water mixture, the working fluid at the inlet is in the unsaturated zone, then ; It is the gas content of the working fluid at the outlet of the calculation module.

[0127] At the same time, the friction resistance pressure drop It is preferably characterized by the following formula:

[0128]

[0129] Where, is the friction loss correction coefficient, when G =1000kg / (m 2 ·s), =1;

[0130] when G >1000kg / (m 2 ·s), then calculate as follows:

[0131]

[0132] when G <1000kg / (m 2 ·s), calculate as follows:

[0133]

[0134] In addition, when the working fluid is two-phase, the gravity pressure drop It is preferably characterized by the following formula:

[0135]

[0136]

[0137]

[0138]

[0139] Where, is the average cross-sectional air content of the calculation module; is the calculated average value of the volumetric gas fraction in the calculation module; S is the slip ratio; is the circulation velocity of the working fluid in the calculation module, m / s; p To calculate the absolute pressure of the working fluid in the module, MPa.

[0140] Furthermore, the accelerated pressure drop when the working fluid is two-phase It is preferably characterized by the following formula:

[0141]

[0142] Where: is the specific volume of saturated water vapor, m 3 / kg; is the specific volume of saturated water, m 3 / kg.

[0143] Furthermore, the enthalpy value of the working fluid at the outlet of the calculation module can be calculated through the heat flux density and the enthalpy value of the working fluid at the inlet.

[0144] Under the premise of knowing the heat flow rate corresponding to the calculation module and the working fluid inlet temperature, the inlet and outlet enthalpy rise of each calculation module can be calculated by energy conservation. The working fluid outlet enthalpy value can be obtained by the enthalpy rise. The preferred calculation formula is as follows:

[0145]

[0146] Where, Q The heat flux absorbed by the calculation module; m is the flow rate of the working fluid flowing through the calculation module; and are the inlet enthalpy and outlet enthalpy of the calculation module respectively.

[0147] In addition, after completing the above calculations, that is, when the working fluid outlet pressure and working fluid outlet enthalpy are known, the working fluid outlet temperature can be calculated through the functional relationship, and the calculation formula is preferably as follows:

[0148]

[0149] Where, T is the outlet temperature of the working fluid; dimensionless pressure , =1Mpa; is the working fluid pressure, MPa; dimensionless enthalpy ,in, ; h is the working fluid enthalpy, kj / kg ; ; 、 and are all constants.

[0150] S33: Input the outlet parameters of the previous calculation module as the inlet parameters into the next calculation module, and repeat steps S31 and S32 until the last calculation module of a single loop is calculated. The outlet working fluid pressure of the last calculation module is the outlet pressure value of this loop.

[0151] S34: Compare the calculated loop outlet working fluid pressure with the assumed outlet working fluid pressure. If the relative error between the two is greater than the assumed relative error value of the outlet working fluid pressure, update the loop flow value and repeat steps S31 to S33 until the relative error is less than the relative error value of the outlet working fluid pressure.

[0152] When actually updating the loop flow value, if the outlet working fluid pressure is less than the assumed outlet working fluid pressure, it proves that the total pressure drop of the working fluid is too large. At this time, the loop flow is reduced; if the outlet working fluid pressure is greater than the assumed outlet working fluid pressure, it proves that the total pressure drop of the working fluid is too small. At this time, the loop flow is increased.

[0153] S35: After all loop calculations converge, add up the flow rates of all loops to obtain the total flow rate of the water-cooled wall system. Compare the calculated total flow rate of the water-cooled wall system with the total flow rate of the water-cooled wall system set in S2. If the relative error between the two is greater than the relative error value of the total flow rate, update the assumed outlet working fluid pressure. Repeat steps S31 to S34 until the relative error is less than the relative error value of the assumed outlet working fluid pressure. The calculation converges. The updated assumed outlet working fluid pressure is the final working fluid outlet pressure. The flow values ​​of each loop are obtained, i.e., the flow distribution is obtained.

[0154] When actually updating the outlet working fluid pressure, if the total flow of the water-cooled wall system is less than the set total flow of the water-cooled wall system, it proves that the total flow of the water-cooled wall system is too small, and the assumed working fluid outlet pressure needs to be reduced; if the total flow of the water-cooled wall system is greater than the set total flow of the water-cooled wall system, it proves that the total flow of the water-cooled wall system is too large, and the assumed working fluid outlet pressure needs to be increased.

[0155] In the working fluid flow distribution calculation method of the preferred embodiment, the calculation of the flow of each loop is completed by converting it into the calculation of multiple small calculation modules, which can accurately calculate the changes in the various physical properties of the working fluid in a single loop and realize the visualization of the entire calculation.

[0156] At the same time, in order to simplify the calculation process during actual calculations, it is preferred to define multiple circuits with the same or similar working fluid delivery conditions as a circuit group. During actual calculations, only a single circuit in each circuit group needs to be calculated, and then the flow calculations of other circuits in the circuit group are completed in an equivalent representation manner.

[0157] In a specific preferred embodiment, the physical parameters of the working fluid in the circuit change along a certain circuit height direction as follows: Figure 5 As shown; the working medium pressure distribution in the lower water-cooled wall tube is preferably as follows Figure 6 As shown; the working medium temperature distribution in the lower water-cooled wall tube is preferably as follows Figure 7 As shown; the lower water wall flow distribution diagram is as follows Figure 8 shown.

[0158] As another aspect of the present invention, after completing the calculation of the working medium flow distribution of the boiler water wall, a prediction calculation of the heating surface wall temperature can be further completed on this basis. Based on the above steps, it further includes the following process:

[0159] S4: After obtaining the flow distribution, the flow heat transfer calculation is performed on each loop in turn by inputting the corresponding flow value of the loop to obtain the working fluid temperature and heat transfer coefficient. The wall temperature at the corresponding position can be calculated based on the working fluid temperature, heat transfer coefficient and heat flux density, thus realizing the prediction of the boiler wall temperature.

[0160] When performing actual heat transfer calculations, it is preferable to separately calculate the wall temperature for each calculation module in each loop. By calculating the wall temperature for multiple loops and multiple calculation modules in each loop, the wall temperature for each zone of the boiler's heating surface can be calculated.

[0161] Furthermore, during the numerical simulation process, the wall temperature of the heated surface is preferably calculated based on the calculated working fluid temperature and heat flow rate, and the calculation formula is preferably as follows:

[0162]

[0163] Where, is the wall temperature; is the working fluid temperature; Q is the heat flow; is the convection heat transfer coefficient, W / (m 2 K); is the pipe wall thickness; is the thermal conductivity of the pipe.

[0164] S5: Feedback the obtained working fluid temperature and heat transfer coefficient to the combustion side as boundary conditions to achieve coupled calculations on the combustion side and the working fluid side.

[0165] More specifically, in a preferred embodiment, the heat balance relationship between the combustion side and the working medium side is as follows:

[0166]

[0167] Where, Q is the heat flow; is the radiation emissivity; is the Stefen-Boltzmann constant; is the incident radiation, W / m 2 ; is the convective heat transfer, W / m 2 ; and Solve within Fluent; is the wall temperature, K; k is the heat transfer coefficient, W / (m 2 K); is the working fluid temperature, K.

[0168] When the heat transfer coefficient is given k and working fluid temperature The heat balance equation can be solved. Based on the above, the coupled simulation strategy uses the average of the outlet and inlet working fluid temperatures obtained in the current iteration, as well as the calculated heat transfer coefficient, as the boundary conditions on the combustion side. The combustion side recalculates the pulverized coal combustion process based on the updated boundary conditions to obtain a new heat flux density distribution. The iterative calculation process in S2-S4 is repeated under the updated heat flux conditions, thereby accurately calculating the boiler wall temperature.

[0169] The method for calculating the working fluid flow rate of the boiler water-cooled wall in the present invention has simple steps and convenient calculations. It can accurately obtain the water-cooled wall working fluid flow distribution, working fluid temperature and heat transfer coefficient on the working fluid side, and use them as boundary conditions on the combustion side to provide sufficient basis and conditions for predicting the boiler wall temperature on the combustion side. It fully realizes the calculation cycle iteration of the combustion side and the working fluid side until convergence, realizes the coupled calculation of the combustion side and the working fluid side, ensures the accuracy of the boiler wall temperature distribution prediction and calculation, and provides protection for the safe operation of the boiler.

[0170] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for calculating the working medium flow rate of a boiler water wall, used for calculating the flow distribution on the working medium side of the boiler, characterized in that: The steps include: S1: Construct a geometric model of the boiler, calculate the combustion values ​​of the entire furnace, and obtain the heat flux density distribution on the boiler combustion side; establish a water-cooled wall system model for the boiler model and call the heat flux density distribution of the boiler heating surface; S2: Set the inlet working fluid temperature, working fluid pressure, working fluid enthalpy value of each circuit in the water-cooled wall system and the total flow of the water-cooled wall system, and assume the outlet working fluid pressure, initial flow value and relative error value of the outlet working fluid pressure of each circuit, as well as the relative error value of the total flow; S3: Calculate each circuit in turn along the working fluid flow direction to calculate the working fluid flow distribution of the water-cooled wall system. The calculation process includes: S31: Divide each circuit into multiple calculation modules along the height direction of the boiler, and determine the phase state of the working fluid in each calculation module in turn; S32: Starting from the inlet end of each loop, the working fluid pressure, working fluid enthalpy and working fluid temperature at the outlet of each calculation module are calculated in sequence; S33: Input the outlet parameters of the previous calculation module as the inlet parameters to the next calculation module, and repeat steps S31 and S32 until the last calculation module of a single loop is calculated. The outlet working fluid pressure of the last calculation module is the outlet pressure value of the loop; S34: Compare the calculated loop outlet working fluid pressure with the assumed outlet working fluid pressure. If the relative error between the two is greater than the assumed relative error of the outlet working fluid pressure, update the loop flow value. Repeat steps S31 to S33 until the relative error is less than the relative error of the outlet working fluid pressure, thereby completing the calculation of the single loop flow value. S35: After all loop calculations converge, add up the flow rates of all loops to obtain the total flow rate of the water-cooled wall system. Compare the calculated total flow rate of the water-cooled wall system with the total flow rate of the water-cooled wall system set in S2. If the relative error between the two is greater than the relative error value of the total flow rate, update the assumed outlet working fluid pressure. Repeat steps S31 to S34 until the relative error is less than the relative error value of the assumed outlet working fluid pressure. The calculation converges. The updated assumed outlet working fluid pressure is the final working fluid outlet pressure. The flow values ​​of each loop are obtained, i.e., the flow distribution is obtained.

2. The method for calculating the working medium flow rate of the boiler water wall according to claim 1, characterized in that: In step S2, the working fluid pressure at the loop inlet is recorded as P1, and the working fluid pressure at the outlet is (P1-2.0)±0.1Mpa; and / or Let the total flow of the water-cooled wall system be M and the total number of loops be N, then the initial flow value of each loop is M / N; and / or The relative error ratio of the outlet working fluid pressure is assumed to be no greater than 0.01, and the relative error ratio of the total flow rate is assumed to be no greater than 0.

05.

3. The method for calculating the working medium flow rate of the boiler water wall according to claim 2, characterized in that: In step S1, the geometric model of the boiler is constructed using Fluent, and the water-cooled wall system model is established and the heat flux density distribution of the heating surface is called through the user-defined function (UDF) in Fluent; and The water-cooled walls include a spiral water-cooled wall located at the lower part of the boiler and a vertical water-cooled wall located at the upper part of the boiler, and the boiler water-cooled walls are expanded into two dimensions in the UDF.

4. The method for calculating the working medium flow rate of a boiler water wall according to any one of claims 1 to 3, characterized in that: In step S31, the phase of the working fluid is determined by calculating the working fluid dryness, and the calculation formula is as follows: Where, x is the working fluid dryness; h is the enthalpy of the working fluid, kj / kg; Working fluid pressure p The corresponding saturated water enthalpy is kj / kg; For pressure p The corresponding saturated steam enthalpy is kj / kg; if the working fluid dryness is x =0 or x =1, the working medium is in the single-phase region; if 0< x <1, the working fluid is in the two-phase region.

5. The method for calculating the working medium flow rate of the boiler water wall according to claim 4, characterized in that: In step S32, the working fluid pressure at the outlet of each calculation module is obtained by calculating the working fluid pressure drop, which is represented by the following formula: Where, is the total pressure drop, MPa; is the local resistance pressure drop, MPa; is the friction resistance pressure drop, MPa; is the pressure drop at the gravity position, MPa; is the accelerating pressure drop, MPa.

6. The method for calculating the working medium flow rate of the boiler water wall according to claim 5, characterized in that: When the working fluid is single-phase, the working fluid pressure drop formula 、 、 、 Calculated by the following formulas: Where, is the local resistance coefficient; G is the working fluid mass flow rate, kg / (m 2 ·s); is the working fluid density, kg / m 3 ; l is the calculation module length, m; D is the inner diameter of the tube, m; It is the arithmetic mean of the import and export density of the calculation module; is the friction resistance coefficient; is the calculation module height, m; g is the acceleration due to gravity, m / s 2 ; is the specific volume of the working fluid at the outlet of the calculation module, m 3 / kg; is the specific volume of the working fluid at the inlet of the calculation module, m 3 / kg.

7. The method for calculating the working medium flow rate of a boiler water wall according to claim 6, characterized in that: When the working fluid is two-phase, the working fluid pressure drop formula 、 、 、 Respectively characterized as 、 、 、 , the calculation process is as follows: Where, is the local resistance coefficient corresponding to the two-phase fluid; is the saturated water density, kg / m 3 ; is the saturated steam density, kg / m 3 ; is the average mass gas fraction of the calculation module, is the mass gas fraction of the working fluid at the inlet of the calculation module; if the working fluid at the outlet is a steam-water mixture, the working fluid at the inlet is in the unsaturated zone, then =0; To calculate the gas content of the working fluid at the module outlet; Where, is the friction loss correction factor, which is calculated as follows: =1; G =1000kg / (m 2 ·s); ; G >1000kg / (m 2 ·s) ; ; G <1000kg / (m 2 ·s) ; Where, is the average cross-sectional gas fraction of the calculation module; is the calculated average value of the volumetric gas fraction in the calculation module; S is the slip ratio; is the circulation velocity of the working fluid in the calculation module, m / s; p is the absolute pressure of the working fluid in the calculation module, MPa; Where: is the specific volume of saturated water vapor, m 3 / kg; is the specific volume of saturated water, m 3 / kg.

8. The method for calculating the working medium flow rate of a boiler water wall according to any one of claims 1 to 3 and 5 to 7, characterized in that: The working fluid outlet enthalpy and working fluid outlet temperature are calculated using the following formula: Where, Q The heat flux absorbed by the calculation module; m is the flow rate of the working fluid flowing through the calculation module; and are the inlet enthalpy and outlet enthalpy of the calculation module respectively; T is the outlet temperature of the working fluid; dimensionless pressure , =1Mpa; is the working fluid pressure, MPa; dimensionless enthalpy ,in, ; h is the working fluid enthalpy, kj / kg ; ; 、 and are all constants.

9. A method for predicting boiler wall temperature, comprising the method for calculating the working medium flow rate of a boiler water-cooled wall according to any one of claims 1 to 8, characterized in that: The following steps are also included: S4: After obtaining the flow distribution, the flow heat transfer calculation is performed on each loop in turn by inputting the corresponding flow value of the loop to obtain the working fluid temperature and heat transfer coefficient. The wall temperature at the corresponding position is calculated based on the working fluid temperature, heat transfer coefficient and heat flux density, realizing the prediction of the boiler wall temperature; S5: Feedback the obtained working fluid temperature and heat transfer coefficient to the combustion side as boundary conditions to achieve coupled calculation of the combustion side and the working fluid side; and the heat balance relationship between the combustion side and the working fluid side is as follows: Where, Q is the heat flow; is the radiation emissivity; is the Stefen-Boltzmann constant; is the incident radiation, W / m 2 ; is the convective heat transfer, W / m 2 ; and Solve within Fluent; is the wall temperature, K; k is the heat transfer coefficient, W / (m 2 K); is the working fluid temperature, K.

10. The method for predicting boiler wall temperature according to claim 9, characterized in that: In step S4, the formula for calculating the wall temperature of the heated surface according to the working fluid temperature and heat flow is as follows: Where, is the wall temperature; is the working fluid temperature; Q is the heat flow; is the convection heat transfer coefficient, W / (m 2 K); is the pipe wall thickness; is the thermal conductivity of the pipe.

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