A three-dimensional wall temperature coupling calculation method for a boiler water wall

Abstract

The application discloses a kind of boiler water-cooled wall three-dimensional wall temperature coupling calculation methods, belong to boiler hydrodynamic calculation field, it is by constructing boiler and water-cooled wall model, carries out full furnace combustion numerical simulation and exports the heat flux density distribution of furnace water-cooled wall, convective heat transfer coefficient and working medium temperature;Again by for each calculation module design wall surface temperature distribution calculation method, can accurately calculate to obtain the water-cooled wall circumferential temperature field in calculation module, determine the water-cooled wall of calculation region and fire side temperature, back fire side temperature and pipe inner wall temperature, complete the calculation of water-cooled wall three-dimensional wall temperature.The boiler water-cooled wall three-dimensional wall temperature coupling calculation method in the application, its steps are simple, convenient to calculate, can accurately obtain the detailed distribution of water-cooled wall pipe wall temperature of boiler under different loads and water-cooled wall pipe circumferential temperature distribution, realize the accurate prediction of boiler heating surface wall temperature, provide basis for the optimization of heating surface, and provide guarantee for the safe operation of boiler.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of boiler water dynamics calculation, and particularly relates to a three-dimensional wall temperature coupling calculation method for a boiler water cooling wall. BACKGROUND

[0002] With the continuous development of renewable energy technology, the installed capacity and power generation of new energy are increasing year by year, and it plays an increasingly important role in the field of power generation. However, due to its own volatility and intermittency, in its actual operation process, the thermal power plant with good stability needs to participate in peak shaving.

[0003] In the process of deep peak shaving, coal-fired boilers are increasingly frequently operated under supercritical, low load and other complex conditions. In a high-temperature environment, the water cooling wall is prone to thermal fatigue and over-temperature. In addition, under low load conditions, the stability of the flame in the furnace is poor, and it is easy to cause the flameout accident. In addition, the steam flow in the pipe is reduced, the cooling effect is poor, and the maximum pipe temperature is higher, which is more prone to pipe explosion danger. This has a negative impact on the safe and stable operation of the boiler.

[0004] Under normal circumstances, detailed and accurate wall temperature distribution is conducive to preventing the occurrence of boiler pipe overheating problems. Boiler pipe wall over-temperature will cause deformation and shedding of the pipe wall material and accelerate the wear and aging of the pipe wall, seriously affecting the safety and service life of the boiler. However, due to the large size of the actual coal-fired boiler and the high temperature in the furnace, it is inefficient and uneconomical to monitor the heating surface temperature by installing a large number of thermocouples on the boiler pipe. The existing numerical simulation research of the boiler mainly focuses on the combustion in the furnace. For example, in the applicant's prior patent application CN117648879A, a method for predicting the wall temperature of the boiler heating surface based on the integration and coupling of Fluent and UDF is proposed, which can to some extent realize the prediction of the wall temperature of the boiler heating surface, and provide a certain basis for the optimization of the heating surface. However, the existing prediction method simplifies the flow of the working medium and the heat transfer of the water cooling wall, and mostly sets a constant wall temperature distribution or regards the heat transfer process from the combustion side to the working medium side as one-dimensional flat wall heat conduction. The simulation cannot simultaneously reflect the combined effect of the flue gas side combustion, the working medium side flow and the water cooling wall pipe geometry on the wall temperature of the heating surface, and cannot accurately obtain the wall temperature distribution of the boiler heating surface and the circumferential temperature field of the pipe, so it is difficult to timely discover the local over-temperature phenomenon of the boiler water cooling wall and timely propose a solution. SUMMARY

[0005] In response to one or more of the above-mentioned defects or improvement needs in the prior art, the present invention provides a three-dimensional wall temperature coupling calculation method for boiler water-cooled walls, which can simultaneously consider the comprehensive influence of flue gas side, working fluid side and water-cooled wall heat transfer on the wall temperature, improve the prediction accuracy of water-cooled wall temperature, obtain a three-dimensional detailed distribution of wall temperature, and provide a basis and support for the discovery and resolution of local overheating of the water-cooled wall.

[0006] To achieve the above object, the present invention provides a coupled calculation method for three-dimensional wall temperature of a boiler water wall, which comprises the following steps:

[0007] S1: Construct a geometric model based on the actual size of the boiler, select the physical and chemical model required for the numerical calculation of the boiler, set the boundary conditions and assume the initial calculation conditions of the heating surface;

[0008] S2: Perform numerical simulation of the entire furnace combustion to obtain and output the heat flux density distribution of the furnace water-cooled wall;

[0009] S3: Divide the water-cooled wall calculation module, calculate the heat flow of each module according to the heat flux density distribution of the furnace water-cooled wall, and calculate the flow and heat transfer of the working fluid in the tube to obtain the convection heat transfer coefficient and working fluid temperature of each calculation module;

[0010] S4: Calculating the wall temperature distribution of each calculation module based on the heat flux density distribution, convection heat transfer coefficient and working fluid temperature; the calculation process includes:

[0011] S41: Read the heat flux density on the combustion side of the first calculation module at the inlet of the water-cooled wall working medium tube, as well as the convection heat transfer coefficient and working medium temperature on the working medium side:

[0012] S42: Create a grid system in the UDF based on the actual size of the membrane water wall, and divide the calculation area by determining the coordinates of each grid center point:

[0013] S43: Establish the control equation of the calculation area, and discretize the control equation of each node, determine the discrete control equation of each calculation node in the calculation area, and obtain the equation group corresponding to the calculation area:

[0014] S44: Solve the equation group in S43 to obtain the temperature of each calculation node in the current calculation area, and then obtain the circumferential temperature field of the water-cooled wall in the calculation module;

[0015] S45: Count the temperatures of each node in the calculation area to determine the fire-facing side temperature, the back-fire side temperature, and the pipe inner wall temperature of the calculation area;

[0016] S46: Calculate the next calculation module along the working medium flow direction, and repeat the process of S42 to S45 until the calculation of all calculation modules is completed, and then combine them to obtain the calculated wall temperature field of the current water-cooled wall;

[0017] S5: through multiple iterations of numerical simulation, and comparing the temperature of the fire side calculated by two adjacent iterations; if the maximum temperature difference of the same region is greater than a preset value, updating the wall temperature boundary condition of the boiler heating surface, repeating S2-S5 until the maximum temperature difference is less than the preset value, at this time, the calculation converges, the iteration is terminated, and the converged wall temperature field is obtained.

[0018] As a further improvement of the present application, in step S42, specifically includes the following process:

[0019] First, a rectangular region is established in UDF corresponding to each calculation module, then the grid system is established by grid division according to the defined minimum size, and then the grid division is carried out by the inner node method, each grid is a control region, and various physical property parameters of the grid are stored on the grid point;

[0020] The grid system is divided into a calculation region and an extended region through the geometric coordinates of each grid center point and the geometric shape of the water wall, and whether each grid is in the calculation region is determined according to the judgment of the grid point position.

[0021] As a further improvement of the present application, in step S43, the discrete control equation of each calculation node in the calculation region is obtained by discretizing the heat conduction differential equation, and for the case that the nodes around the calculation node in each direction are all in the calculation region, the discrete control equation is:

[0022] (1)

[0023] In the formula, N, S, W, E respectively represent the four direction nodes around the calculation node P ; , , , and represent the temperatures of the four direction nodes and the calculation node P ; , , , , are the coefficients required by the equation corresponding to the calculation node P , and

[0024]

[0025]

[0026]

[0027] Among them, , , , is the computational node P The thermal conductivity of the four direction interfaces; , , , is the thermal conductivity of the four nodes, which is obtained from the physical parameters of the water wall.

[0028] As a further improvement of the present application, in step S43, for the case that at least one side of the computational node P is a second type of boundary condition, the discrete control equation (1) is transformed into:

[0029] (2)

[0030] (3)

[0031] wherein the discrete control equation (2) corresponds to the case that the computational node P is subjected to a one-way heat flow, and ; the discrete control equation (3) corresponds to the case that the computational node P is subjected to a two-way heat flow, and ; B , C denote different direction nodes, and are any one of the direction nodes N, S, W, E ; are the coefficients and temperatures corresponding to the direction nodes B , respectively; is the heat flow acting on the computational node P from the direction node B ; is the length of the intersection between the direction node B and the computational node P ; are the coefficients and temperatures corresponding to the direction nodes C , respectively; is the heat flow acting on the computational node P from the direction node C ; is the length of the intersection between the direction node C and the computational node P .

[0032] As a further improvement of the present application, in step S43, for the case that at least one side of the computational node P is a third type of boundary condition, the discrete control equations (2), (3) are respectively transformed into discrete control equations (4), (5):

[0033] (4)

[0034] (5)

[0035] wherein the discrete control equation (4) is for a calculation node P a discrete control equation for a node in one direction affected by convection, in which ; the discrete control equation (5) is for a calculation node P a discrete control equation for a node in two directions affected by convection, in which as above, ; a direction node D , F denote different direction nodes, and are respectively N, S, W, E a node in the direction directly opposite to the calculation node P ; , are respectively direction nodes D , F corresponding direction fluid temperature; , are respectively direction nodes D , F corresponding direction convection heat transfer coefficient between the boundary and the fluid; , are respectively distances between the calculation node P and the direction node D , F ; and are respectively equation coefficients, temperature, boundary length and heat flow of the direction node D , F corresponding to the calculation node P .

[0036] As a further improvement of the present application, in step S44, the internal nodes in the calculation region are calculated using the discrete control equation (1);

[0037] the boundary nodes on the fire side wall surface are calculated using the discrete control equation (2), (3) under the second type of boundary condition, and the heat flux density thereof is determined through the combustion side model;

[0038] the boundary nodes on the backfire side wall surface are calculated using the discrete control equation (2), (3) under the second type of boundary condition, and the heat flux density thereof is 0;

[0039] the boundary nodes on the pipe inner wall surface are calculated using the discrete control equation (4) under the third type of boundary condition, and the working medium temperature and the convection heat transfer coefficient are determined through the working medium side model.

[0040] As a further improvement of the present application, in step S44, the solving process of the equation set adopts Gauss-Seidel iteration, and the specific process of solving the temperature field is as follows:

[0041] (1) Assume an initial temperature field;

[0042] (2) Determine the heat conductivity coefficient according to the physical parameters of the water-cooled wall system;

[0043] (3) Calculate the control equation of each calculation node to obtain a new temperature distribution;

[0044] (4) Calculate the temperature difference between the new and old temperature fields at each node, and if the maximum temperature difference is less than a set threshold value, it is considered that the iteration converges, and the obtained temperature field is the real temperature field, otherwise, replace the old temperature field with the new temperature field and repeat the processes (2) and (3) until the result converges.

[0045] As a further improvement of the present application, the set threshold value is 1x10 -6 K.

[0046] As a further improvement of the present application, in step S45, the average temperature of each boundary node of the fire side wall surface is taken as the fire side temperature, the average temperature of each boundary node of the backfire side wall surface is taken as the backfire side temperature, and the average temperature of each boundary node of the inner wall surface of the tube is taken as the inner wall surface temperature; the fire side temperature and the inner wall surface temperature of the tube are transmitted to the combustion side and working medium side models of the current calculation module.

[0047] As a further improvement of the present application, in step S5, the fire side temperature obtained in the current calculation is used as the boundary condition for the next iteration, and the iteration calculation process in S2-S5 is repeated under the updated boundary condition.

[0048] The above improvement technical features can be combined with each other as long as they do not conflict with each other.

[0049] Overall, compared with the prior art, the above technical solutions conceived by the present application have the following beneficial effects:

[0050] (1) The boiler water-cooled wall three-dimensional wall temperature coupling calculation method of the present application can accurately calculate the circumferential temperature field of the water-cooled wall in the calculation module, determine the fire side temperature, backfire side temperature and inner wall surface temperature of the water-cooled wall in the calculation region, complete the calculation of the three-dimensional wall temperature of the water-cooled wall, provide accurate basis for the prediction of the boiler wall temperature, avoid the occurrence of local overheating of the water-cooled wall, and improve the reliability of the boiler operation.

[0051] (2) The boiler water wall three-dimensional wall temperature coupling calculation method of the present application accurately realizes the determination of the calculation region and the expansion region by optimizing the grid division method of each calculation module in combination with the geometric shape and coordinates of the water wall pipeline, provides an accurate basis for the selection of the subsequent calculation mode, and guarantees the accuracy of the calculation process and the calculation result.

[0052] (3) The boiler water wall three-dimensional wall temperature coupling calculation method of the present application accurately realizes the rapid calculation of the calculation node temperature under different conditions by designing different discrete control equations according to the environmental conditions of the calculation node, guarantees the reliability of the temperature acquisition of each calculation node, provides a basis for obtaining the three-dimensional wall temperature distribution of the water wall, and indirectly improves the accuracy of the boiler wall temperature prediction.

[0053] (4) The boiler water wall three-dimensional wall temperature coupling calculation method of the present application has simple steps and convenient calculation, can accurately simulate the flow combustion process of the boiler flue gas side, the flow heat transfer process of the boiler working medium side, and the heat transfer process of the water wall, accurately realizes the data interaction between the flue gas side and the working medium side through the water wall heat transfer model, fully considers the main factors in the pipe wall temperature calculation process, provides the detailed distribution of the water wall pipe wall temperature and the circumferential temperature distribution of the water wall pipeline of the boiler under different loads, realizes the accurate prediction of the wall temperature of the boiler heating surface, provides a basis for the optimization of the heating surface, and provides protection for the safe operation of the boiler. BRIEF DESCRIPTION OF DRAWINGS

[0054] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0055] Figure 1 is a flowchart of the boiler water wall three-dimensional wall temperature coupling calculation method in the embodiments of the present application;

[0056] Figure 2 is a schematic diagram of the structure of the boiler in the embodiments of the present application;

[0057] Figure 3 is a schematic diagram of the grid of the furnace hearth and the bottom layer of the primary air center cross section of the boiler in the embodiments of the present application;

[0058] Figure 4 is a two-dimensional expansion diagram of the furnace water wall surface in the embodiments of the present application;

[0059] Figure 5 is a schematic diagram of the heat flux density distribution of the furnace water wall surface in the embodiments of the present application;

[0060] Figure 6 is a schematic diagram of temperature distribution of working medium in the water-cooled wall tube in the embodiment of the present application;

[0061] Figure 7 is a schematic diagram of distribution of convective heat transfer coefficient in the water-cooled wall tube in the embodiment of the present application;

[0062] Figure 8 is a schematic diagram of the membrane water-cooled wall in the embodiment of the present application;

[0063] Figure 9 is a network system diagram in the rectangular coordinate system in the embodiment of the present application;

[0064] Figure 10 is a network system diagram of the water-cooled wall pipeline in the embodiment of the present application;

[0065] Figure 11 is a schematic diagram of temperature distribution of the water-cooled wall tube wall on the fire side in the embodiment of the present application;

[0066] Figure 12 is a schematic diagram of temperature distribution of the water-cooled wall tube wall on the back fire side in the embodiment of the present application;

[0067] Figure 13 is a schematic diagram of temperature distribution of the water-cooled wall tube inner wall in the embodiment of the present application;

[0068] Figure 14 is a schematic diagram of temperature distribution of the water-cooled wall tube circumferential direction in the embodiment of the present application. DETAILED DESCRIPTION

[0069] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not used to limit the present application. In addition, the technical features involved in the various embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0070] In the description of the present application, it should be understood that, unless otherwise explicitly specified and limited, the orientation or positional relationship indicated by the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential" are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and therefore cannot be understood as indicating or implying that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application.

[0071] In addition, the terms "first", "second", "third", etc. are used only for descriptive purposes and do not connote or imply any relative importance or any meaning pertaining to the quantity of the features indicated. Thus, a feature defined with "first", "second", etc. can include at least one of the features, explicitly or implicitly.

[0072] In the present application, unless otherwise explicitly specified and limited, the terms "mounting", "connecting", "connecting", "fixing" and the like should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or it can be integrated; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium; it can be the internal communication of two elements or the interaction relationship between two elements, unless otherwise explicitly limited. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0073] In the present application, unless otherwise explicitly specified and limited, the first feature is "on" or "under" the second feature, which can be direct contact between the first and second features, or indirect contact between the first and second features through an intermediate medium. Moreover, the first feature "above", "over" and "on" the second feature can be directly above or obliquely above the first feature, or it can only mean that the horizontal height of the first feature is higher than that of the second feature. The first feature "below", "under" and "under" the second feature can be directly below or obliquely below the first feature, or it can only mean that the horizontal height of the first feature is less than that of the second feature.

[0074] Embodiment:

[0075] Please refer to Figure 1 The three-dimensional wall temperature coupling calculation method of the boiler water wall in the preferred embodiment of the present application includes simulation calculation processes of the combustion side, the water wall and the working medium side, and data interaction and iteration in the three regions are performed.

[0076] Specifically, the coupling calculation method in the preferred embodiment preferably includes the following processes:

[0077] S1: According to the actual size of the boiler, a geometric model is constructed, the physical and chemical model required for numerical calculation of the boiler is selected, the boundary conditions are set and the initial calculation conditions of the heating surface are assumed;

[0078] Specifically, in actual operation, the modeling is preferably performed according to the boiler size in the boiler design specification, the boiler structure at least includes a water wall, a burner, a cold ash hopper, a superheater and a reheater, and the heating surface thereof is preferably simplified.

[0079] In the preferred embodiment, the boiler heating surface constructed in step S1 is divided into two types, namely, water wall and superheater. Among them, the water wall is preferably simplified as a plane with a thickness of zero, and the superheater is simplified as a cavity with a thickness of the outer diameter of the tube. The boundary condition of the heating surface is preferably a temperature boundary, and the initial working medium temperature and heat transfer coefficient of the water wall are calculated by the given water wall surface temperature.

[0080] At the same time, in the modeling, the method of dividing the grid by region is preferably adopted, high-quality hexahedral grids are used, and the grid is encrypted in the burner area where the physical and chemical reactions are intense.

[0081] In real-time modeling, the boiler structure, the furnace center cross-section grid and the bottom layer of primary air center cross-section grid are preferably as shown in Figure 2 、 Figure 3 .

[0082] Further, in the process of pulverized coal combustion in the boiler, suitable models are preferably selected to simulate each sub-process. Among them, the Realizable κ-ε turbulence model is used to simulate the gas phase turbulence, the component transport model is used to simulate the gas phase combustion, the Rosin-rammler distribution is used for the particle size of the coal powder, the single-rate model is used to simulate the coal pyrolysis, the dynamic / diffusion-controlled reaction rate model is used to simulate the coke combustion, and the P1 model is used to simulate the radiation heat transfer.

[0083] When setting the boundary conditions, the primary air nozzle at the inlet is preferably a velocity inlet, and the rest of the nozzles are mass flow inlets. The outlet is set as a pressure outlet. In actual operation, due to the presence of the induced draft fan, the outlet will be in a negative pressure state, and the pressure is set to -60 Pa. The discrete phase type at the outlet is set to escape.

[0084] More specifically, the furnace wall is set as a non-slip boundary condition, the water wall and the superheater are set as a temperature boundary condition, and the initial wall temperature is calculated according to the empirical value.

[0085] S2: Perform full-furnace combustion numerical simulation to obtain and output the heat flux density distribution of the furnace water wall;

[0086] In the preferred embodiment, the commercial software FLUENT is used to perform numerical simulation calculation of the furnace combustion, and after the iteration is completed, the heat flux density distribution of the heating surface is obtained. Then, by calling the user-defined function (UDF) in FLUENT, the heat flux density distribution of the water wall surface can be calculated.

[0087] For example, in a specific preferred embodiment, the heat flux density distribution of the furnace water wall surface is as shown in Figure 5 .

[0088] S3: dividing the water-cooled wall calculation module, according to the heat flux density distribution of the furnace water-cooled wall, counting the heat flow of each module, and calculating the flow and heat transfer of the working medium in the pipe to obtain the convective heat transfer coefficient and the working medium temperature of each calculation module;

[0089] In actual operation, different module division methods are adopted according to the position of the water-cooled wall. Among them, the water-cooled wall is preferably divided into spiral water-cooled wall (lower water-cooled wall) and vertical water-cooled wall (upper water-cooled wall); the spiral water-cooled wall is divided into calculation modules along the inclined angle direction and the horizontal direction, and the inclined angle is arbitrarily changed according to the actual spiral angle; and the vertical water-cooled wall is preferably considered as a special form of the spiral water-cooled wall, that is, the inclined angle thereof is considered as 90°.

[0090] The boiler water-cooled wall is unfolded into two dimensions in the UDF to obtain a two-dimensional unfolded diagram of the water-cooled wall, the coordinates and heat flux density information of each grid of the water-cooled wall surface are called, the x or y coordinate in the extracted grid surface coordinate is regarded as the X coordinate in the two-dimensional diagram, and the z coordinate in the grid surface coordinate is regarded as the Y coordinate in the two-dimensional diagram, so as to realize the correspondence between the three-dimensional and two-dimensional wall surfaces, the water-cooled two-dimensional wall surface is divided according to the flow direction of the working medium to obtain a plurality of calculation modules, the position information is used to judge the calculation module of the two-dimensional wall surface corresponding to the wall grid in the three-dimensional model, the parameters of each module are counted, the heat flux density of each calculation module is calculated, and the heat flux density distribution of the water-cooled wall surface is obtained. In the preferred embodiment, the two-dimensional unfolded diagram of the water-cooled wall is as shown in Figure 4 , and the heat flux density distribution of the water-cooled wall surface is as shown in Figure 5 .

[0091] More specifically, the lower water-cooled wall is a spiral water-cooled wall, and the actual inclination angle of the pipeline needs to be considered when the region is divided, and the horizontal direction of the water-cooled wall is preferably divided into multiple sections, for example, in the preferred embodiment, it is divided into 50 sections, and finally the unit calculation module is a parallelogram; at the same time, the upper water-cooled wall is a vertical water-cooled wall, which is divided along the horizontal and vertical directions of the water-cooled wall, and finally the unit calculation module is a rectangle. Further, the inlet working medium temperature of the water-cooled wall inlet calculation region is preferably given according to the thermal calculation specification, and along the flow direction, the downstream inlet working medium temperature is preferably equal to the upstream outlet working medium temperature.

[0092] Given the heat flow corresponding to the calculation module and the inlet working medium temperature, the inlet and outlet enthalpy rise of each calculation module is further calculated by energy conservation, and the outlet working medium enthalpy can be obtained by the enthalpy rise, and the calculation formula is preferably as follows:

[0093]

[0094] In the formula, Q is the heat flow absorbed by the calculation region; m is the working medium flow through the calculation region; h in andh out respectively the inlet enthalpy and outlet enthalpy of the calculation region.

[0095] Given the feedwater pressure and the separator pressure, the pressure of the working medium linearly decreases during the flow process, based on the known outlet enthalpy value and the pressure, the outlet temperature of the module is obtained by using the back-calculation equation of IAPWS-IF97, and the calculation formula is preferably as follows:

[0096]

[0097] wherein, T out is the outlet temperature; p out is the outlet pressure; h out is the outlet enthalpy.

[0098] Further, the outlet working medium physical property parameters in the preferred embodiment are calculated by the formula provided by IAPWS-IF97, and the outlet working medium physical property parameters in the preferred embodiment at least include temperature, density, specific heat capacity, and isothermal expansion coefficient. At the same time, the temperature distribution of the working medium in the downcomer wall tube is preferably as shown in Figure 6 According to the calculated physical property parameters, the viscosity and the thermal conductivity coefficient are obtained, and finally a series of dimensionless numbers are obtained, including Reynolds number, Prandtl number and Nusselt number.

[0099] More specifically, the viscosity calculation formula and the thermal conductivity coefficient calculation formula used in the preferred embodiment are preferably as follows:

[0100]

[0101]

[0102] wherein, represents the limitation of the viscosity by the rarefied gas state, represents the influence of the finite density on the viscosity; represents the effect of the critical state on the viscosity enhancement, which is usually set to 1 in engineering applications; , , respectively represent the influence of the rarefied gas state, the finite density and the critical state on the thermal conductivity coefficient.

[0103] Further, the dimensionless quantities such as Reynolds number and Prandtl number are calculated by the obtained quantities, the phase state of the working medium is determined by the dryness, and then the heat transfer coefficient of the working medium is calculated according to the pressure and the phase state of the working medium, which is divided into four parts: subcritical single phase, supercritical single phase, boiling heat transfer and two-phase heat transfer deterioration.

[0104] The subcritical single phase and supercritical single phase are preferably calculated using the improved Dittus-Boelter model.

[0105] The boiling heat transfer is preferably calculated using the following calculation method:

[0106]

[0107]

[0108]

[0109] Where, and denote the contribution of bubbling boiling and convection, respectively, is the mass flow rate, is the inner diameter of the pipe, and are the densities of saturated water and saturated steam, 、 、 、 are the constant pressure specific heat capacity, thermal conductivity, viscosity and surface tension of saturated water, is the latent heat of soda.

[0110] Accordingly, the two-phase heat transfer deterioration formula is preferably calculated using the following formula:

[0111]

[0112] Where, is the Nusselt number, is the Reynolds number, is the Prandtl number, For dryness, is the density of saturated water, is the density of saturated water.

[0113] In the preferred embodiment, the working medium convection heat transfer coefficient of the water-cooled wall bare tube is obtained through the above calculation process, and its distribution in the preferred embodiment is as follows: Figure 7 As shown in .

[0114] S4: Based on the heat flux density of the furnace heating surface on the combustion side of each calculation module of the water-cooled wall and the convective heat transfer coefficient and working fluid temperature calculated on the working fluid side, the wall temperature of the corresponding calculation module is calculated through the heat transfer model to obtain the furnace water-cooled wall surface temperature distribution;

[0115] by Figure 8Taking the pipe structure shown in the figure as an example, it shows two parallel pipes in a membrane water wall. As shown in the figure, the side of the water wall pipe and fins closest to the boiler combustion area is the fire side wall surface, and the side away from it is the fire side wall surface. The inner circular surface of the pipe that contacts the working medium is the inner wall surface of the tube.

[0116] The temperature distribution across the membrane water wall cross section is called the circumferential temperature distribution of the water wall. By statistically analyzing the circumferential temperature distribution and mapping it onto a two-dimensional unfolded diagram of the water wall, the wall surface temperature distribution is ultimately obtained. The wall surface temperature includes the temperatures of the fire side, the back side, and the inner wall of the tube. The wall surface temperature distribution includes the temperatures of the fire side, the back side, and the inner wall of the tube. In other words, for the wall surface temperature distribution ultimately mapped onto the two-dimensional unfolded diagram of the water wall, the temperature value corresponding to a single point is a value group, which at least includes the temperatures of the fire side, the back side, and the inner wall of the tube.

[0117] Furthermore, in a preferred embodiment, the fire-facing side is treated as a given heat load boundary, corresponding to the second-class boundary condition in heat transfer theory, with the heat load calculated from the combustion side. The fire-facing side is typically insulated to create an adiabatic boundary with a heat flux of zero. The inner tube wall is treated as a given convection heat transfer coefficient and the surrounding fluid (working fluid) temperature boundary, corresponding to the third-class boundary condition in heat transfer theory, with the convection heat transfer coefficient and working fluid temperature derived from working fluid side calculations. Furthermore, the heat conduction problem of a membrane water wall can be treated as a two-dimensional, steady-state problem without an internal heat source.

[0118] In S3, the water wall surface is divided into multiple modules, and the heat flux density on the combustion side and the convective heat transfer coefficient and working fluid temperature on the working fluid side are calculated for each calculation module. In actual calculations, the heat transfer model is preferably calculated starting with the first calculation module in the lower left corner of the 2D expanded diagram of the water wall. Each calculation module is calculated sequentially along the working fluid flow direction. The calculation module currently performing the heat transfer model calculation is referred to as the current calculation module.

[0119] Before performing calculations for each calculation module, it is preferred to first establish a grid system in the UDF heat transfer model and obtain the calculation area. The parameters obtained from the current calculation module are read and attached to the boundary nodes at the corresponding positions as boundary conditions. The temperature field is obtained by iterative calculation. The statistically calculated temperatures on the fire side, back-fire side, and inner wall surface are then returned as the boundary conditions for the working fluid and combustion sides in the next iteration of the current calculation module. The heat transfer model then repeats the above process to calculate each calculation module one by one until all calculation modules are traversed.

[0120] More specifically, the calculation of the circumferential temperature field of the membrane water wall preferably includes the following steps:

[0121] S41: Read the heat flux density on the combustion side of the first calculation module at the inlet of the water-cooled wall working medium tube, as well as the convection heat transfer coefficient and working medium temperature on the working medium side:

[0122] In the preferred embodiment, the first calculation module at the inlet of the water wall working tube is located at the lower left corner of the two-dimensional development of the water wall, as shown in Figure 4 .

[0123] S42: Establish a grid system in the UDF according to the actual size of the membrane water wall, and divide the calculation region by judging the coordinates of the center points of each grid:

[0124] In actual operation, a rectangular region corresponding to each calculation module is preferably established in the UDF, which can be regarded as a top view of the calculation module. The grid system is established by grid division according to the defined minimum size, and the grid division is performed by the internal node method. The grid system is a rectangular coordinate system. In this process, a physical model and a grid are not established. Each grid is a control region. The various physical property parameters of the grid are stored at the grid points, and the parameter information of each grid point is stored in a matrix.

[0125] For example, in the preferred embodiment shown in Figure 9 , the P in the grid system is the current position grid point, N, S, W, E are the upper and lower and left and right position grid points, n, s, w, e represent the current position grid point and the four direction interfaces.

[0126] More specifically, the grid system is divided into a calculation region and an extended region by the geometric coordinates of each grid center point and the geometric shape of the water wall. Whether the grid is in the calculation region is determined according to the judgment of the position of the grid point. The grids in the calculation region constitute the geometric model of the membrane water wall and are subjected to subsequent calculations, while the extended region does not participate, as shown in Figure 10 .

[0127] S43: Establish the control equation of the calculation region, and discretize the control equation of each node to determine the discrete control equation of each calculation node in the calculation region, and further obtain an equation group corresponding to the calculation region:

[0128] In the rectangular coordinate system, the grid nodes are set by the internal node method, and the control equation is discretized by the finite volume method. The basic idea of the finite volume method is to divide the calculation region into grids, and to have each grid point surrounded by a control volume that does not overlap. The differential equation (control equation) to be solved is integrated for each control volume, thereby obtaining a group of discrete equations.

[0129] In the preferred embodiment, the control equation of the calculation region is the heat conduction differential equation:

[0130]

[0131] wherein:Lambda is the thermal conductivity of the material; T is the temperature; S is the internal heat source intensity, in the case of the present application, S = 0.

[0132] Further, for the case that each control volume around the calculation region is the calculation region, it is preferred to discretize the control equation as:

[0133] (1)

[0134] In the formula, N, S, W, E respectively represent the four direction nodes P around the calculation node; , , , and represent the temperatures of the four direction nodes and the calculation node P ; , , , , are the coefficients required by the equation corresponding to the calculation node P , and wherein,

[0135]

[0136]

[0137] wherein, , , , are the thermal conductivities of the four direction interfaces of the calculation node P ; , , , are the thermal conductivities of the four nodes, which are obtained from the physical parameters of the water wall.

[0138] Further, for the boundary nodes, the situation is different from the foregoing cases, and it is necessary to use the additional source term method to discuss separately. The additional source term method is to regard the heat entering or exiting the calculation region as the equivalent source term of the control volume adjacent to the boundary. In the calculation process, the temperature of the unknown boundary node is excluded, and the temperature of the boundary node is obtained through the boundary condition after the internal temperature field is obtained.

[0139] For the second type of boundary condition, it mainly has two cases: one is that one direction of the calculation node P is affected by the heat flow; the other is that the calculation node Ptwo directions are affected by heat flow, the aforementioned discrete control equation (1) is transformed into the following discrete control equations (2) and (3):

[0140] (2)

[0141] (3)

[0142] wherein the discrete control equation (2) corresponds to the calculation node P affected by one-way heat flow, and the discrete control equation (3) corresponds to the calculation node affected by two-way heat flow. P ; ; B , C denote different directional nodes, and are any one of the directional nodes N, S, W, E ; are the coefficients and temperatures corresponding to the directional nodes B ; is the heat flow acting on the calculation node B from the directional node P ; is the length of the interface between the directional node B and the calculation node P ; are the coefficients and temperatures corresponding to the directional nodes C ; is the heat flow acting on the calculation node C from the directional node P ; is the length of the interface between the directional node C and the calculation node P .

[0143] More preferably, taking the calculation node P affected by N directional heat flow q as an example, the discrete control equation is selected as equation (2) and is simplified as shown below

[0144]

[0145]

[0146] Similarly, for the third type of boundary condition, the situation affected by convective heat transfer is similar to the aforementioned second type of boundary condition, which is divided into two situations: one direction is affected by convection and two directions are affected by convection. In the two situations, the directional nodes affecting the calculation node P by convective heat transfer are the directional nodes P and the calculation node PNot directly connected, at this time, the discrete control equations (2), (3) are respectively transformed into the following equations (4), (5):

[0147] (4)

[0148] (5)

[0149] Wherein, the discrete control equation (4) is the calculation node P The discrete control equation of one direction affected by convection, in which, The discrete control equation (5) is the calculation node P The discrete control equation of two directions affected by convection, in which, As before, The direction node D , F Denote different direction nodes, and are respectively N, S, W, E The node opposite to the calculation node P in the direction; , The direction node D , F Corresponding direction fluid temperature; , The direction node D , F Corresponding direction boundary and fluid convection heat transfer coefficient; , The distance between the calculation node P and the direction node D , F ; And The equation coefficient, temperature, boundary length and heat flow of the direction node D , F Corresponding to the calculation node P .

[0150] Preferably, E The direction temperature T f The convection heat transfer coefficient h For example, at this time, the discrete control equation is as follows:

[0151]

[0152] .

[0153] When four control bodies around a control body have multiple control bodies outside the calculation area, additional source term method needs to be used to process the boundaries of different directions of the control body respectively.

[0154] S44: solve the equation set in S43 to obtain the temperature of each calculation node in the current calculation region, and further obtain the circumferential temperature field of the water cooling wall in the calculation module;

[0155] In actual calculation, the internal nodes in the calculation region are calculated by using the discrete control equation (1);

[0156] The boundary nodes on the fire side wall surface are calculated by using the discrete control equation (2) and (3) under the second type of boundary condition, and the heat flux density is determined by the combustion side model;

[0157] The boundary nodes on the backfire side wall surface are calculated by using the discrete control equation (2) and (3) under the second type of boundary condition, and the heat flux density is 0;

[0158] The boundary nodes on the inner wall surface of the tube are calculated by using the discrete control equation (4) under the third type of boundary condition, and the working medium temperature and the convective heat transfer coefficient are determined by the working medium side model.

[0159] More specifically, the foregoing solving process preferably adopts Gauss-Seidel iteration, and the specific process of solving the temperature field is preferably as follows:

[0160] (1) Assume an initial temperature field;

[0161] (2) Determine the thermal conductivity according to the physical property parameters of the water cooling wall system;

[0162] (3) Calculate the control equation of each calculation node to obtain a new temperature distribution;

[0163] (4) Calculate the temperature difference of each node between the new and old temperature fields. If the maximum temperature difference is less than a set threshold, it is considered that the iteration converges, and the obtained temperature field is the real temperature field. Otherwise, replace the old temperature field with the new temperature field and repeat processes (2) and (3) until the result converges.

[0164] More specifically, the threshold value is preferably 1×10 -6 K.

[0165] S45: Statistically calculate the temperature of each node in the calculation region to determine the fire side temperature, backfire side temperature and inner wall surface temperature of the calculation region;

[0166] In the preferred embodiment, the average temperature of each boundary node on the fire side wall surface is taken as the fire side temperature, the average temperature of each boundary node on the backfire side wall surface is taken as the backfire side temperature, and the average temperature of each boundary node on the inner wall surface of the tube is taken as the inner wall surface temperature; the fire side temperature and the inner wall surface temperature of the tube are transmitted to the combustion side and working medium side models of the current calculation module.

[0167] S46: Calculate the next calculation module in the flow direction of the working medium, repeat the process of S42-S45 until the calculation of all calculation modules is completed, and the wall temperature combination of each calculation module obtains the calculated wall temperature field of the current water cooling wall, and the fire side temperature of each calculation module is obtained by coordinate corresponding backstepping to obtain the wall temperature boundary of the three-dimensional combustion model.

[0168] In the preferred embodiment, the water cooling wall pipe circumferential temperature field obtained by the above calculation process is as shown in Figure 14 On this basis, the fire side, backfire side and pipe inner wall temperature distribution of the entire furnace water cooling wall are respectively obtained, as shown in Figures 11-13

[0169] S5: Through multiple iterations of numerical simulation, the fire side temperature obtained by adjacent two iterations is compared; if the maximum temperature difference of the same region is greater than the preset value, the wall temperature boundary condition of the boiler heating surface is updated, and the process of S2 to S5 is repeated until the maximum temperature difference is less than the preset value, and it is considered that the calculation converges at this time, and the iteration is terminated to obtain the converged wall temperature field.

[0170] In the preferred embodiment, the preset value of the outlet working medium temperature difference of the two iterations is preferably 1K.

[0171] More specifically, in the preferred embodiment, the fire side temperature obtained by the current calculation is used as the boundary condition for the next iteration, and the iteration calculation process in S2-S5 is repeated under the updated boundary condition.

[0172] In the process of predicting the wall temperature of the boiler heating surface based on the integrated coupling modeling of Fluent and UDF, the main function is to reflect the combined action of the three parts of the flue gas side heat release, the working medium side heat absorption and the water cooling wall pipe heat transfer. The complex temperature distribution obtained by the furnace combustion simulation reflects the uneven heating of the pipe, while the water dynamics calculation reflects the influence of the working medium convective heat transfer on the pipe wall temperature, and the water cooling wall heat transfer part further considers the influence of its geometric shape on the heat transfer. The coupling surface of the flue gas side and the heat transfer model is the fire side wall of the water cooling wall of the heating surface, and the data interaction of the heat flux and the fire side temperature is realized through the data interaction of the working medium side and the heat transfer model. The coupling surface of the working medium side and the heat transfer model is the inner wall of the water cooling wall of the heating surface, and the data interaction of the working medium temperature and the inner wall temperature is realized through the data interaction of the working medium side and the heat transfer model. The flue gas side and the working medium side realize data interaction through heat flux, so as to realize coupled calculation.

[0173] ​The boiler water wall three-dimensional wall temperature coupling calculation method in the application has simple steps, convenient calculation, can accurately simulate the flow combustion process of the boiler flue gas side, the flow heat transfer process of the boiler working medium side and the heat transfer process of the water wall, accurately realizes the data interaction between the flue gas side and the working medium side through the water wall heat transfer model, fully considers the main factors in the pipe wall temperature calculation process, provides the detailed distribution of the water wall pipe wall temperature of the boiler under different loads and the circumferential temperature distribution of the water wall pipe, realizes the accurate prediction of the boiler heating surface wall temperature, provides the basis for the optimization of the heating surface, and provides the guarantee for the safe operation of the boiler.

[0174] Those skilled in the art will easily understand that the above description is only the preferred embodiment of the present application, and is not intended to limit the present application, and any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A three-dimensional coupled calculation method for boiler water wall temperature, characterized in that: The steps include: S1: Construct a geometric model based on the actual size of the boiler, select the physical and chemical model required for the numerical calculation of the boiler, set the boundary conditions and assume the initial calculation conditions of the heating surface; S2: Perform numerical simulation of the entire furnace combustion to obtain and output the heat flux density distribution of the furnace water-cooled wall; S3: Divide the water-cooled wall calculation module, calculate the heat flow of each module according to the heat flux density distribution of the furnace water-cooled wall, and calculate the flow and heat transfer of the working fluid in the tube to obtain the convection heat transfer coefficient and working fluid temperature of each calculation module; S4: Calculating the wall temperature distribution of each calculation module according to the heat flux density distribution, the convection heat transfer coefficient and the working fluid temperature; The calculation process includes: S41: Read the heat flux density on the combustion side of the first calculation module at the inlet of the water-cooled wall working medium tube, as well as the convection heat transfer coefficient and working medium temperature on the working medium side: S42: Create a grid system in the UDF based on the actual size of the membrane water wall, and divide the calculation area by determining the coordinates of each grid center point: S43: Establish the control equation of the calculation area, and discretize the control equation of each node, determine the discrete control equation of each calculation node in the calculation area, and obtain the equation group corresponding to the calculation area: S44: Solve the equation group in S43 to obtain the temperature of each calculation node in the current calculation area, and then obtain the circumferential temperature field of the water-cooled wall in the calculation module; S45: Count the temperatures of each node in the calculation area to determine the fire-facing side temperature, the back-fire side temperature, and the pipe inner wall temperature of the calculation area; S46: Calculate the next calculation module along the working medium flow direction, and repeat the process of S42 to S45 until the calculation of all calculation modules is completed, and then combine them to obtain the calculated wall temperature field of the current water-cooled wall; S5: Perform multiple iterations of the numerical simulation and compare the fire-facing temperatures obtained from two adjacent iterations. If the maximum temperature difference in the same area is greater than the preset value, update the wall temperature boundary conditions of the boiler heating surface and repeat S2 to S5 until the maximum temperature difference is less than the preset value. At this point, the calculation converges, the iteration is terminated, and a converged wall temperature field is obtained.

2. The three-dimensional coupled calculation method for boiler water wall temperature according to claim 1 is characterized in that: In step S42, the following process is specifically included: First, a rectangular area is created for each calculation module in the UDF. Then, a grid system is established by meshing based on the specified minimum size. The grid is then divided using the internal node method. Each grid is a control area, and the physical properties of the grid are stored at the grid points. The grid system is divided into a calculation area and an expansion area by the geometric coordinates of each grid center point and the geometric shape of the water-cooled wall. Whether each grid is in the calculation area is determined by judging the position of the grid point.

3. The boiler water wall three-dimensional wall temperature coupled calculation method according to claim 1 or 2, characterized in that: In step S43, the discrete control equation of each computing node in the computing area is obtained by discretizing the heat conduction differential equation. For the case where all nodes in all directions around the computing node are computing areas, the discrete control equation is: （1） Where, N,S,W,E Represents computing nodes P Four directional nodes around; , , , and Represents four direction nodes and computing nodes P temperature; , , , , For computing nodes P The coefficients required for the corresponding equation, and in, , , , For computing nodes P Thermal conductivity of interfaces in four directions; , , , is the thermal conductivity of the four nodes, which is obtained from the physical parameters of the water-cooled wall.

4. The three-dimensional coupled calculation method for boiler water wall temperature according to claim 3 is characterized in that: In step S43, for the computing node P When at least one side of is a second-type boundary condition, the discrete control equation (1) is transformed into: （2） （3） Among them, the discrete control equation (2) corresponds to the computation node P Under the action of unidirectional heat flow, ; The discrete control equation (3) corresponds to the computational node P In the case of bidirectional heat flow, ; B 、 C Refers to different direction nodes and is a direction node N,S,W,E Any of; Direction nodes B The corresponding coefficient and temperature; Direction node B Acting on computing nodes P heat flow; Direction node B Compute nodes P the length of the junction; Direction nodes C The corresponding coefficient and temperature; Direction node C Acting on computing nodes P heat flow; Direction node C Compute nodes P Length of the junction.

5. The boiler water wall three-dimensional wall temperature coupled calculation method according to claim 4 is characterized in that: In step S43, for the computing node P When at least one side of the boundary condition is the third type, the discrete control equations (2) and (3) are transformed into discrete control equations (4) and (5) respectively: （4） （5） Among them, the discrete control equation (4) is the computation node P The discrete governing equation for convection in one direction is: ; Discrete control equation (5) is the computation node P The discrete governing equations for convection in two directions, in which Same as above. ; Direction node D 、 F Refers to different direction nodes, and are N,S,W,E Direction and computing nodes P A node directly opposite; 、 Direction nodes D 、 F Fluid temperature in the corresponding direction; 、 Direction nodes D 、 F Convective heat transfer coefficient between the boundary and the fluid in the corresponding direction; 、 Compute nodes P With direction node D 、 F the distance between them; and Direction nodes D 、 F Corresponding to the computing node P The equation coefficients, temperature, boundary length, and heat flux.

6. The boiler water wall three-dimensional wall temperature coupled calculation method according to claim 5 is characterized in that: In step S44, the internal nodes in the calculation area are calculated using the discrete control equation (1); The boundary nodes of the fire side wall are calculated using the discrete control equations (2) and (3) under the second type of boundary conditions, and their heat flux density is determined by the combustion side model; The boundary nodes of the back-fire side wall are calculated using the discrete control equations (2) and (3) under the second type of boundary conditions, and their heat flux density is 0; The boundary nodes of the inner wall of the tube are calculated using the discrete governing equation (4) under the third kind of boundary conditions, and the working fluid temperature and convective heat transfer coefficient are determined by the working fluid side model.

7. The boiler water wall three-dimensional wall temperature coupled calculation method according to claim 5 or 6, characterized in that: In step S44, the equation group is solved using Gauss-Seidel iteration, and the specific process of obtaining the temperature field is as follows: (1) Assume an initial temperature field; (2) Determine the thermal conductivity based on the physical parameters of the water-cooled wall system; (3) Calculate the control equations of each computing node and find the new temperature distribution; (4) Calculate the temperature difference between each node of the new and old temperature fields. If the maximum temperature difference is less than the set threshold, the iteration is considered to have converged and the obtained temperature field is the true temperature field. Otherwise, the new temperature field is used to replace the old temperature field and repeat processes (2) and (3) until the result converges.

8. The three-dimensional coupled calculation method for boiler water wall temperature according to claim 7 is characterized in that: The threshold is set to 1×10 -6 K.

9. The method for coupled calculation of three-dimensional wall temperature of boiler water wall according to any one of claims 1, 2, 4 to 6, and 8, characterized in that: In step S45, the average temperature of each boundary node on the fire-oriented side wall is taken as the fire-oriented side temperature, the average temperature of each boundary node on the back-of-fire side wall is taken as the back-of-fire side temperature, and the average temperature of each boundary node on the inner wall of the tube is taken as the inner wall temperature; the fire-oriented side temperature and the inner wall temperature of the tube are passed to the combustion side and working fluid side models of the current calculation module.

10. The method for coupled calculation of three-dimensional wall temperature of boiler water wall according to any one of claims 1, 2, 4 to 6, and 8, characterized in that: In step S5, the fire-facing side temperature calculated this time is used as the boundary condition for the next iteration, and the iterative calculation process in S2 to S5 is repeated under the updated boundary condition.

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