Method and system for calculating heat transfer and hydrodynamic characteristics of vertical evaporation pipe of once-through boiler

Abstract

The invention discloses a method and a system for calculating heat transfer and hydrodynamic characteristics of a vertical evaporation tube of a once-through boiler. The method comprises the following steps: firstly, collecting boiler operation data and boiler equipment data; according to different boiler types, the vertical evaporation pipes are divided into a plurality of different pipe sections according to the connection modes of the vertical evaporation pipes; and based on the obtained boiler operation data, the enthalpy value of each pipe section is calculated, and then the average specific volume of the working medium in the pipe is deduced through the enthalpy values, so that the average specific volume value of each pipe section is obtained. And further analyzing by utilizing boiler equipment data and the average specific volume of each pipe section to obtain the dynamic characteristics of each pipe section. And finally, boiler operation data and equipment data are integrated, and the heat transfer distribution condition of each pipe section is evaluated and obtained. According to the method, an effective calculation means is provided for accurately analyzing the thermal hydraulic performance of the vertical evaporation pipe of the once-through boiler, a scientific basis is provided for safe operation and performance improvement of the boiler, boiler configuration can be adjusted according to heat transfer and dynamic characteristics, and safe operation of the boiler is guaranteed.

Description

Technical Field

[0001] The invention belongs to the technical field of thermal power plant boilers and relates to a method and system for calculating heat transfer and hydrodynamic characteristics of a vertical evaporation tube of a once-through boiler. Background Art

[0002] With the continuous growth of global energy demand and the increasing awareness of environmental protection, the thermal power generation industry, as an important part of energy conversion, is facing unprecedented challenges and opportunities. In my country, coal, as the main primary energy source, occupies a core position in power production. Its economic efficiency and abundant resource reserves make coal-fired power generation the main mode of power supply in my country in the foreseeable future. However, the environmental pollution, high energy consumption and low operating efficiency generated in the process of traditional coal-fired power generation have become the key factors restricting the sustainable development of the industry.

[0003] To meet these challenges, the power industry is actively transforming and upgrading towards large-scale, high-efficiency and environmentally friendly directions. Large-scale, efficient and environmentally friendly thermal power units have become the main trend of future development. Among them, ultra-supercritical power generation technology is regarded as one of the key technologies to improve thermal power generation efficiency and reduce emissions due to its advantages such as high power generation, strong load capacity, low energy consumption, stable operation and low environmental pollution. Supercritical units significantly improve thermal efficiency by increasing steam parameters (such as pressure and temperature). At the same time, their complex boiler structure and working environment also put forward higher requirements for design, operation and maintenance.

[0004] As one of the core equipment of supercritical power plants, the design of the furnace water-cooled wall of a direct-current boiler is directly related to the safe and stable operation of the unit. As the main heating surface of the boiler, the flow characteristics, hydrodynamic characteristics and heat transfer characteristics of the steam-water mixture in the water-cooled wall directly affect the temperature distribution of the tube wall metal, and thus affect the operating efficiency and safety of the entire boiler. Since the steam-water mixture of a supercritical direct-current boiler undergoes a complex phase change process from supercooled water to superheated steam in the water-cooled wall, the density difference between steam and water is significant. This density difference will cause complex behavior of the working fluid during the flow process, resulting in unstable hydrodynamic characteristics, which will affect the safety of boiler operation. Summary of the invention

[0005] The purpose of the present invention is to solve the technical problem that the hydrodynamic instability of a direct-flow boiler in the prior art affects the safe operation of the boiler, and to provide a method and system for calculating the heat transfer and hydrodynamic characteristics of a vertical evaporation tube of a direct-flow boiler.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions: A first aspect of the present invention provides a method for calculating heat transfer and hydrodynamic characteristics of a vertical evaporation tube of a once-through boiler, comprising the following steps: Obtain boiler operation data and boiler equipment data; Based on the different types of boilers, the vertical evaporation tubes are divided into different sections according to different connection methods; Based on the boiler operation data corresponding to different pipe sections, the enthalpy value of each pipe section is obtained; Based on the enthalpy value of each pipe segment, calculate the average specific volume of each pipe segment; Based on the boiler equipment data and the average specific volume of each pipe section, the dynamic characteristics of each pipe section are obtained; Based on the boiler operation data and boiler equipment data, the heat transfer distribution of each pipe section is obtained.

[0007] Furthermore, based on the enthalpy value of each pipe section, the average specific volume of the working fluid in the pipe is calculated to obtain the average specific volume of the working fluid in each pipe section. The specific method is: When the working fluid pressure is less than the critical pressure, the average specific volume of the pipe section is calculated according to the corresponding average enthalpy value. h To calculate as:

[0008] When the average enthalpy of the supercritical pressure working fluid is in the range of 1674.4~2720.9kJ / kg, the average specific volume of the pipe section is:

[0009] When the total enthalpy of the pipe section is lower than 209.3 kJ / kg or the average enthalpy is outside the range of 1674.2~2720.9 kJ / kg, the average specific volume of the pipe section is:

[0010] in, Enthalpy of the working fluid at the tube inlet; is the enthalpy of the working fluid at the tube outlet; is the specific volume of the working fluid on the cross section at the tube outlet, is the specific volume of the working fluid in the cross section at the tube inlet.

[0011] Furthermore, the dynamic characteristic is described as the total pressure drop of each pipe section, and the total pressure drop of each pipe section is:

[0012] in, is the weight resistance pressure drop, To accelerate the pressure drop, is the flow resistance.

[0013] Furthermore, the flow resistance includes friction resistance and local resistance, and the calculation method of the friction resistance is: For a single-phase fluid, the frictional resistance is:

[0014] For a two-phase fluid, the frictional resistance is:

[0015] The calculation method of the local resistance is: For a single-phase fluid, the local resistance is:

[0016] For a two-phase fluid, the local resistance is:

[0017] Among them, among them, is the friction coefficient per unit pipe length, is the friction coefficient, is the average specific heat capacity of the fluid along the length of the tube; is the length of the pipe section; D is the inner diameter of the tube; g is the acceleration due to gravity; is the working fluid density; is the working fluid flow rate; is the pipe roughness; is the friction loss correction factor; W 0 is the circulation velocity in the calculation tube, It is expressed as the density value of saturated water under pressure P; It is expressed as the density value of saturated steam at pressure P; is the mass vapor content; is the local resistance coefficient.

[0018] Furthermore, the gravity resistance pressure drop is:

[0019] in, is the average density of the fluid inlet and outlet in the tube, g is the acceleration due to gravity, is the length of the pipe section.

[0020] Furthermore, the calculation method of the acceleration pressure drop is: For a single-phase fluid, the accelerating pressure drop is:

[0021] For a two-phase fluid, the accelerating pressure drop is:

[0022] in, Specific volume of the working fluid in the cross section at the tube outlet; are the specific volumes of the working fluid on the cross section at the tube inlet; is the dryness of the working fluid on the cross section at the pipe inlet; is the dryness of the working fluid on the cross section at the pipe outlet; is the working fluid density; is the working fluid flow rate; is the specific volume of saturated water working medium; is the specific volume of saturated steam working fluid.

[0023] Furthermore, based on the boiler operation data and boiler equipment data, the heat transfer distribution of each pipe section is obtained, specifically: Under supercritical pressure, the heat transfer coefficient of the vertical evaporation tube is calculated and is:

[0024] Under subcritical pressure, the vertical evaporation tube is divided into different zones, and the corresponding heat transfer coefficient is calculated according to different zones, specifically: For the forced convection heat transfer zone, the heat transfer coefficient is:

[0025] For the subcooled boiling heat transfer zone, the heat transfer coefficient is:

[0026] For the two-phase flow heat exchange zone, the heat transfer coefficient is:

[0027] in, is the heat transfer coefficient of single-phase water; is the pressure value corresponding to the current working fluid; is the critical pressure value, is the saturated water Reynolds number corresponding to the working fluid temperature; G is the mass flow rate of the working fluid in the tube; Indicates dryness; is the saturated water Reynolds number; , is the Prandtl number of water corresponding to the working fluid temperature, is the Prandtl number of water corresponding to the inner wall temperature of the tube, is the saturated water Reynolds number corresponding to the inner wall temperature of the tube; is the saturated water Reynolds number; Based on boiler operating data, boiler equipment data and heat transfer coefficient, the wall temperature of each tube section is calculated.

[0028] Furthermore, the wall temperature of each pipe segment includes the inner wall temperature of the pipe segment and the outer wall temperature of the pipe segment.

[0029] Furthermore, the inner wall temperature of the pipe section is:

[0030] The outer wall temperature of the pipe section is:

[0031] in, is the working fluid temperature in the tube at the tube wall temperature point, is the heat flow coefficient of the inner wall of the tube front surface, It is the ratio of the outside diameter to the inside diameter of the pipe; is the heat release coefficient between the inner wall of the pipe and the medium in the pipe at the pipe wall temperature calculation point; is the external wall radiation heat load at the wall temperature calculation point; is the tube wall thickness; is the thermal conductivity of the tube.

[0032] A second aspect of the present invention provides a system for calculating heat transfer and hydrodynamic characteristics of a once-through boiler vertical evaporation tube, comprising: Data acquisition module: used to obtain boiler operation data and boiler equipment data; Pipe segment division module: used to divide the vertical evaporation pipes into different pipe segments according to different connection methods based on different boiler types; Enthalpy calculation module: used for different pipe sections to obtain the enthalpy value of each pipe section based on boiler operation data; A specific volume calculation module, used for calculating the average specific volume of each pipe section based on the enthalpy value of each pipe section; Dynamic characteristics calculation module: used to obtain the total pressure drop of each pipe section based on boiler equipment data and the average specific volume of each pipe section; Heat transfer characteristic calculation module: used to obtain the wall temperature distribution of each pipe section based on boiler operation data and boiler equipment data.

[0033] Compared with the prior art, the present invention has the following beneficial effects: The present invention discloses a method for calculating the heat transfer and hydrodynamic characteristics of a vertical evaporator tube of a direct current boiler. By collecting boiler operation data and equipment data in detail, and subdividing the vertical evaporator tube into different tube sections according to the boiler type, this method can more accurately reflect the actual working status of each tube section. At the same time, based on the calculation of enthalpy value and average specific volume, the comprehensiveness and accuracy of the dynamic characteristics and heat transfer distribution analysis are further ensured. It can clearly point out the bottleneck of boiler performance, provide strong guidance for boiler design optimization, operation adjustment and maintenance; it helps to timely discover and deal with potential operational safety hazards, such as local overheating, water hammer phenomenon, etc., thereby improving the operational safety and reliability of the boiler. Once a potential safety hazard is detected, the method can quickly provide detailed diagnostic information, including the specific location of the problem, the severity and possible causes. This information provides strong support for targeted treatment plans, thereby minimizing the impact of safety hazards on boiler operation. BRIEF DESCRIPTION OF THE DRAWINGS In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0034] Figure 1 It is a flow chart of the method for calculating the heat transfer and hydrodynamic characteristics of the vertical evaporation tube of a once-through boiler of the present invention; Figure 2 This is the heat transfer and flow relationship diagram in the vertically rising heated tube; Figure 3 The changes of physical properties of water near the critical zone at 23Mpa; Figure 4 This is the relationship between the specific heat capacity of water at constant pressure and the enthalpy value; Figure 5 This is a diagram showing the effect of the mass flow rate of the water-cooled wall on various loads; Figure 6 This is a block diagram of the heat transfer and dynamic characteristics calculation system of the vertical evaporation tube of the direct current boiler of the present invention. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. The components of the embodiments of the present invention described and marked in the drawings here can be arranged and designed in various different configurations.

[0036] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0038] In the description of the embodiments of the present invention, it should be noted that if the terms "upper", "lower", "horizontal", "inner", etc. indicate an orientation or positional relationship based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of the invention is usually placed when in use, it is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance.

[0039] In addition, if the term "horizontal" appears, it does not mean that the component must be absolutely horizontal, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal than "vertical", which does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0040] In the description of the embodiments of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal connection of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0041] The present invention is further described in detail below in conjunction with the accompanying drawings: See also Figure 1 The present invention discloses a method for calculating the wall temperature and hydrodynamic characteristics of a vertical evaporation tube of a once-through boiler, which is characterized by comprising the following steps: S1, obtaining boiler operation data and boiler equipment data; The equipment parameters of the boiler are as follows:

[0042] The operating parameters of the boiler are as follows:

[0043] S2, based on the different boiler types, the vertical evaporation tubes are divided into different sections according to different connection methods; According to the boiler design, the vertical evaporation tube is divided into three independent tube sections (sections A, B, and C) according to the connection mode (such as parallel connection, series connection, etc.), and each section has the same length. According to the different connection modes of the loop and each tube group, the present invention divides the vertical evaporation tube into different tube sections according to the different connection modes: (1) simple tube group (2) primary parallel tube group, the parallel tubes in the tube group have different structures and heating characteristics, but have the same inlet pipe and outlet pipe (3) secondary parallel tube group, which is composed of an inlet pipe, multiple parallel pipes and multiple outlet pipes.

[0044] S3, based on the boiler operation data, the outlet enthalpy of each pipe section is obtained; The outlet enthalpy of each pipe section is calculated by using the law of conservation of energy, combining the working fluid state (temperature, pressure, mass flow) at the inlet of each pipe section and the heat exchange efficiency in the pipe section.

[0045] For example, if the loop pipe number is i The calculation is performed on a single vertical evaporator tube, which can be divided into two parts along the height direction. j According to the law of conservation of energy, the relationship between the inlet and outlet enthalpy of each pipe section can be listed as:

[0046] in, The outlet enthalpy of the jth section of the ith tube; The outlet enthalpy of the j-1th section of the ith tube; s is the cross-sectional area of ​​the pipe segment, l is the length of the pipe section, For the i Root canal j The heat of the working fluid in the section; is the dryness of the ith tube.

[0047] S4, based on the outlet enthalpy of each pipe section, calculating the average specific volume of the working fluid in the pipe, and obtaining the average specific volume of each pipe section; When calculating the average specific heat capacity of the fluid along the length of the pipe, for working fluid pressure less than the critical pressure, the average specific heat capacity of water and steam According to the corresponding average enthalpy h To calculate, ; For supercritical pressure working fluid ( h = Average specific heat capacity in the range of 1674.4~2720.9 kJ / kg exist When the total enthalpy of the pipe section is lower than 209.3 kJ / kg or h When the value is outside the range of 1674.2~2720.9kJ / kg, the average specific heat capacity According to the corresponding average enthalpy h Find the solution. In this, , is the enthalpy of the working fluid at the inlet and outlet of the pipe.

[0048] S5, based on the boiler equipment data and the average specific volume of each pipe section, the total pressure drop of each pipe section is obtained; The total pressure drop for each pipe section is:

[0049] in, is the weight resistance pressure drop, To accelerate the pressure drop, is the flow resistance.

[0050] The flow resistance includes friction resistance and local resistance: The calculation method of single-phase fluid friction resistance is:

[0051] in, , is the friction coefficient per unit pipe length, is the average specific heat capacity of the fluid along the length of the tube; To calculate the mass flow rate of the working fluid in the tube; is the length of the pipe section; is the cross-sectional diameter of the tube; D is the inner diameter of the tube; g is the acceleration due to gravity; is the working fluid density; is the working fluid flow rate; is the pipe roughness.

[0052] The calculation method of the friction resistance of two-phase fluid is:

[0053] In the formula: is the friction loss correction factor; W 0 is the circulation velocity in the calculation tube, It is expressed as the density value of saturated water under pressure P; It is expressed as the density value of saturated steam at pressure P; is the mass vapor content.

[0054] Correction factor for friction loss : when hour, ; when hour: ; when hour: .

[0055] Local resistance refers to the energy loss caused by collision and eddy current of fluid particles due to changes in flow cross-section or flow direction when the fluid flows.

[0056] Local resistance of single-phase fluid:

[0057] In the formula: is the local resistance coefficient, which can be obtained through experiments or by looking up the table using the power calculation method for boiler water heating in power stations.

[0058] Two-phase flow local resistance:

[0059] In the formula: is the local resistance coefficient of two-phase flow, which can be obtained through experiments or by looking up the table using the power calculation method for boiler water heating in power stations.

[0060] Calculation of gravity resistance pressure drop: Gravity resistance pressure drop of single-phase fluid:

[0061] In the formula: is the arithmetic mean density of the fluid inlet and outlet in the pipe; To calculate the length of the pipe section.

[0062] Gravity resistance pressure drop of two-phase flow: The calculation formula is the same as that of single-phase flow:

[0063] The average density can be expressed by the formula Calculated; is the average cross-sectional area vapor content in the pipe section; , where is the correction factor of the relative horizontal inclination, ; Sliding speed ratio ; It is to calculate the steam content rate of the pipe section. is the conversion factor; when , ;otherwise, .in, is the horizontal tilt angle; is the saturated water Reynolds number, , is the inner diameter of the pipe, which is the maximum value of the imaginary inner diameter. , Pick and DThe minimum value between the two; is the surface tension coefficient of saturated water.

[0064] When the steam-water mixture flows in the tube, the momentum change caused by heat is called acceleration pressure drop. The calculation of acceleration pressure drop is always based on homogeneous incompressible fluid.

[0065] The accelerating pressure drop of a single-phase fluid is:

[0066] In the formula: Specific volume of the working fluid in the cross section at the tube outlet; are the specific volumes of the working fluid on the cross section at the pipe inlet.

[0067] The accelerating pressure drop of a two-phase fluid is:

[0068] In the formula: is the dryness of the working fluid on the cross section at the pipe inlet; It is the dryness of the working fluid in the cross section at the pipe outlet.

[0069] Based on the above method, the dynamic characteristics are calculated:

[0070] It can be seen that due to the increase in boiler load, the flow resistance and gravity resistance pressure drop both increase with the increase. It can also be seen that the rising trend of gravity resistance pressure drop is relatively low, and the increase is not so large. This is because the size of gravity resistance pressure drop will not change much with the increase of steam-water mixture in the tube. It will also be gentle due to the increase in boiler load. Although there is no significant increase, the increase is not large. Therefore, the increase in gravity resistance pressure drop is not very prominent; while the flow resistance pressure drop is the opposite. The change in load is greater than the change in gravity resistance pressure drop, because the influence of flow resistance pressure drop is due to the mass flow rate changing with the increase in boiler load ratio. Therefore, the total pressure drop change of the boiler water-cooled wall is due to the increase in its mass flow rate.

[0071] See also Figure 5 , is the influence of the mass flow of the water-cooled wall on each load. When the flow is close to 1800t / h, the gravity resistance pressure drop changes slowly. In the tubes that absorb more heat, the density of the steam-water mixture decreases, and the decrease in gravity resistance pressure drop is higher than the change in friction pressure drop. As a result, natural circulation will occur between the tubes that absorb more heat and the tubes that absorb less heat, and more flow will flow through the tubes that absorb more heat.

[0072] S5, based on boiler operation data and boiler equipment data, calculate the wall temperature of each pipe section. Figure 2-4 When discussing the convective boiling heat transfer characteristics of saturated water in a heated vertical evaporator tube, we first need to pay attention to how the flow structure of the steam-water mixture in the tube affects the heat transfer performance. In the flow of steam-water mixture or pure water, especially in the subcooled boiling or low vapor content region, the fluid usually exhibits fine bubble flow. Under low load conditions, the steam-water mixture undergoes nucleate boiling, and the contact between the tube wall and the water effectively reduces the wall temperature.

[0073] However, as the heat load increases, a large number of bubbles are generated on the tube wall surface and gradually form a continuous bubble layer, resulting in a reduction in direct contact between the fluid and the wall surface, forming the so-called "evaporation point". This phenomenon marks the beginning of modal boiling (or deviation from nucleate boiling), also known as the first type of heat transfer deterioration, which causes the wall temperature to rise rapidly.

[0074] However, in systems such as once-through boilers, the second type of heat transfer degradation is more commonly encountered. This occurs when the water on the tube wall is completely evaporated and the tube wall is in direct contact with the steam, causing the wall temperature to rise sharply and the heat transfer coefficient to drop significantly.

[0075] In addition, the physical properties of water change significantly when approaching the critical point (such as the state under 23MPa). Near the phase transition point, the specific volume of water reaches its maximum value and undergoes a sharp change; at the same time, its dynamic viscosity and density both drop significantly; and the thermal conductivity decreases significantly after the phase transition point. These changes in physical properties further affect the flow and heat transfer characteristics of the steam-water mixture in the heated tube.

[0076] Based on the above analysis, the calculation method of the heat transfer characteristics of the vertical evaporation tube of the present invention is: When operating under supercritical pressure, calculate the heat transfer coefficient of the vertical evaporator; the heat transfer calculation formula of the vertical evaporator is:

[0077] in, , is the Prandtl number of water corresponding to the working fluid temperature, is the Prandtl number of water corresponding to the inner wall temperature of the tube, is the saturated water Reynolds number corresponding to the inner wall temperature of the tube.

[0078] Under subcritical pressure, the vertical evaporator tube is divided into zones, and the corresponding heat transfer coefficients are calculated according to different zones; according to the fluid state, the vertical evaporator tube is divided into forced convection heat transfer zone, subcooled boiling heat transfer zone and two-phase flow heat transfer zone.

[0079] In the enhanced convection of single-phase water, the formula for the convection heat transfer coefficient of the vertical evaporator tube is:

[0080] In the calculation formula of the heat transfer coefficient of a single-phase tube, when the working fluid in the tube is in the form of subcooled boiling heat exchange, the calculation formula of the heat transfer coefficient is:

[0081] When the pressure in the tube is 13~20.5MPa, the formula for the convection heat transfer coefficient in the two-phase flow heat transfer zone is:

[0082] In the formula: is the heat transfer coefficient of single-phase water; is the pressure value corresponding to the current working fluid; is the critical pressure value, is the saturated water Reynolds number corresponding to the working fluid temperature; G is the mass flow rate of the working fluid in the pipe; Indicates dryness.

[0083] Based on the boiler operation data, boiler equipment data and heat transfer coefficient, the wall temperature of each pipe section is calculated. The wall temperature of each pipe section includes the inner wall temperature of the pipe section and the outer wall temperature of the pipe section.

[0084] The calculation formula of the inner wall temperature of the pipe is:

[0085] In the formula: is the inner wall temperature of the water-cooled wall tube; is the working fluid temperature in the tube at the tube wall temperature point, is the heat flow coefficient of the inner wall of the tube front surface, It is the ratio of the outside diameter to the inside diameter of the pipe; is the heat release coefficient between the inner wall of the pipe and the medium in the pipe at the pipe wall temperature calculation point; is the external wall radiation heat load at the wall temperature calculation point.

[0086]

[0087] In the formula: is the outer wall temperature of the tube, °C; is the average heat distribution coefficient of the tube, is the tube wall thickness, m; is the thermal conductivity of the pipe, W / (m·℃).

[0088] The average temperature of the tube is calculated as:

[0089] In the formula: is the average temperature of the tube; is the temperature of the inner wall of the tube; is the outer wall temperature of the tube.

[0090] Based on the above method, the heat transfer distribution is obtained: Under BMCR conditions, the influence of thermal deviation on water wall pressure drop and thermal deviation on outlet temperature of boiler lower radiation zone is shown as follows:

[0091] Under 50% BMCR conditions, the influence of thermal deviation on water wall pressure drop and thermal deviation on boiler lower radiation zone outlet temperature is shown as follows:

[0092] From the above results, it can be seen that under BMCR conditions and 50% BMCR conditions, the change in the total pressure drop of the water-cooled wall increases with the increase of thermal deviation. When the heat absorption of the deviation tube increases, the specific heat capacity of the steam-water mixture in the tube will increase and the mass flow rate will increase. This will cause its density to decrease, which will lead to a decrease in the gravity pressure drop in the tube. The size of the flow resistance in the tube will increase with the increase in mass flow rate. When the load of the boiler is higher, the ratio of its flow resistance in the total pressure drop will greatly exceed the value of the gravity pressure drop, but the total pressure drop is increased, which will also lead to a small flow rate. Therefore, when the boiler load is higher, the flow compensation characteristics in the water-cooled wall tube will also increase.

[0093] It can be seen from this that when the mass flow rate is small, the flow resistance will be lower than the gravity pressure drop as the mass flow rate decreases, and the ratio in the total pressure drop will also decrease. At this time, in the descending tube with more heat absorption, the ratio of the increase in flow resistance is also much lower than the ratio of the decrease in gravity pressure drop. At this time, the total pressure drop is reduced, and the working fluid in the tube will also lose more flow due to the flow compensation characteristics.

[0094] No matter how large the boiler load is, the increase in the thermal deviation value will always lead to an increase in the outlet temperature of the lower radiation zone. If the thermal deviation value decreases, the opposite is true. When the boiler load increases, it can be seen from the physical properties of the steam-water mixture that the temperature of the steam-water mixture depends on how much heat the steam-water mixture absorbs. In this case, if the thermal deviation changes, it will trigger a series of changes. This also causes the temperature of the steam-water mixture to change with the change of the thermal deviation under high load. And because the change in the working fluid temperature will cause hydrodynamic instability, so controlling the thermal deviation under high load is the key to ensure the safe operation of the boiler unit.

[0095] The present invention discloses a heat transfer and hydrodynamic characteristics calculation system for a once-through boiler vertical evaporation tube, comprising: Data acquisition module: used to obtain boiler operation data and boiler equipment data; Pipe segment division module: used to divide the vertical evaporation pipes into different pipe segments according to different connection methods based on different boiler types; Enthalpy calculation module: used to obtain the enthalpy value of each pipe section based on the boiler operation data corresponding to different pipe sections; A specific volume calculation module, used for calculating the average specific volume of each pipe section based on the enthalpy value of each pipe section; Dynamic characteristics calculation module: used to obtain the total pressure drop of each pipe section based on boiler equipment data and the average specific volume of each pipe section; Heat transfer characteristic calculation module: used to obtain the wall temperature distribution of each pipe section based on boiler operation data and boiler equipment data.

[0096] The present invention can accurately simulate the heat transfer and flow process of the fluid in the vertical evaporator tube by collecting boiler operation data and equipment parameters in detail and combining with advanced calculation models. By accurately calculating the enthalpy value, total pressure drop and wall temperature distribution of each tube section, the thermal hydraulic behavior of the fluid inside the boiler can be deeply understood. This helps to optimize the operating parameters of the boiler, such as fuel quantity, feed water flow, etc., so as to ensure that the boiler operates under the best working conditions and improve the overall operating efficiency. The calculation of wall temperature distribution is crucial for evaluating the safety performance of the boiler. Excessive wall temperature may cause material failure and cause safety accidents. This method can timely discover and warn potential overheating areas, providing strong guarantee for the safe operation of the boiler. In the boiler design stage, this method can be used to simulate and calculate different design schemes and evaluate their heat transfer and dynamic characteristics. Through comparative analysis, the optimal design scheme can be selected, the number of design iterations can be reduced, the design cost can be reduced, and the design quality can be improved. When the boiler fails, such as reduced heat transfer efficiency, abnormal pressure drop, etc., this method can quickly locate the problem. By comparing the normal operation data with the data in the fault state, the fault type and cause can be accurately determined, providing strong support for rapid troubleshooting. Based on the calculation results of this method, a more scientific and reasonable maintenance plan can be formulated. Pipe sections prone to overheating, abnormal pressure drop and other problems can be inspected and maintained to reduce unplanned downtime and improve maintenance efficiency.

[0097] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for calculating heat transfer and hydrodynamic characteristics of vertical evaporation tubes of a once-through boiler, characterized in that: The following steps are involved: Obtain boiler operation data and boiler equipment data; Based on the different types of boilers, the vertical evaporation tubes are divided into different sections according to different connection methods; Based on the boiler operation data corresponding to different pipe sections, the enthalpy value of each pipe section is obtained; Based on the enthalpy value of each pipe segment, calculate the average specific volume of each pipe segment; Based on the boiler equipment data and the average specific volume of each pipe section, the dynamic characteristics of each pipe section are obtained; Based on the boiler operation data and boiler equipment data, the heat transfer distribution of each pipe section is obtained.

2. The method for calculating heat transfer and hydrodynamic characteristics of vertical evaporation tubes of a once-through boiler according to claim 1 is characterized in that: Based on the enthalpy value of each pipe section, the average specific volume of the working fluid in the pipe is calculated to obtain the average specific volume of the working fluid in each pipe section. The specific method is: When the working fluid pressure is less than the critical pressure, the average specific volume of the pipe section is calculated according to the corresponding average enthalpy value. h To calculate as: When the average enthalpy of the supercritical pressure working fluid is in the range of 1674.4~2720.9kJ / kg, the average specific volume of the pipe section is: When the total enthalpy of the pipe section is lower than 209.3 kJ / kg or the average enthalpy is outside the range of 1674.2~2720.9 kJ / kg, the average specific volume of the pipe section is: in, Enthalpy of the working fluid at the tube inlet; is the enthalpy of the working fluid at the tube outlet; is the specific volume of the working fluid on the cross section at the tube outlet, is the specific volume of the working fluid in the cross section at the tube inlet.

3. The method for calculating heat transfer and hydrodynamic characteristics of vertical evaporation tubes of a once-through boiler according to claim 1 is characterized in that: The hydrodynamic characteristics are described as the total pressure drop of each pipe section, and the total pressure drop of each pipe section is: in, is the weight resistance pressure drop, To accelerate the pressure drop, is the flow resistance.

4. The method for calculating heat transfer and hydrodynamic characteristics of vertical evaporation tubes of a once-through boiler according to claim 3 is characterized in that: The flow resistance, friction resistance and local resistance, the calculation method of the friction resistance is: For a single-phase fluid, the frictional resistance is: For a two-phase fluid, the frictional resistance is: The calculation method of the local resistance is: For a single-phase fluid, the local resistance is: For a two-phase fluid, the local resistance is: Among them, among them, is the friction coefficient per unit pipe length, is the friction coefficient, is the average specific heat capacity of the fluid along the length of the tube; is the length of the pipe section; D is the inner diameter of the tube; g is the acceleration due to gravity; is the density of the working fluid; is the working fluid flow rate; is the pipe roughness; is the friction loss correction factor; W 0 is the circulation velocity in the calculation tube, It is expressed as the density value of saturated water under pressure P; It is expressed as the density value of saturated steam at pressure P; is the mass vapor content; is the local resistance coefficient.

5. The method for calculating heat transfer and hydrodynamic characteristics of vertical evaporation tubes of a once-through boiler according to claim 3 is characterized in that: The gravity resistance pressure drop is: in, is the average density of the fluid inlet and outlet in the tube, g is the acceleration due to gravity, is the length of the pipe section.

6. The method for calculating heat transfer and hydrodynamic characteristics of vertical evaporation tubes of a once-through boiler according to claim 3 is characterized in that: The calculation method of the acceleration voltage drop is: For a single-phase fluid, the accelerating pressure drop is: For a two-phase fluid, the accelerating pressure drop is: in, Specific volume of the working fluid in the cross section at the tube outlet; are the specific volumes of the working fluid on the cross section at the tube inlet; is the dryness of the working fluid on the cross section at the pipe inlet; is the dryness of the working fluid on the cross section at the pipe outlet; is the density of the working fluid; is the working fluid flow rate; is the specific volume of saturated water working medium; is the specific volume of saturated steam working fluid.

7. The method for calculating heat transfer and hydrodynamic characteristics of vertical evaporation tubes of a once-through boiler according to claim 1 is characterized in that: Based on the boiler operation data and boiler equipment data, the heat transfer distribution of each pipe section is obtained, specifically: Under supercritical pressure, the heat transfer coefficient of the vertical evaporation tube is calculated and is: Under subcritical pressure, the vertical evaporation tube is divided into different zones, and the corresponding heat transfer coefficient is calculated according to different zones, specifically: For the forced convection heat transfer zone, the heat transfer coefficient is: For the subcooled boiling heat transfer zone, the heat transfer coefficient is: For the two-phase flow heat exchange zone, the heat transfer coefficient is: in, is the heat transfer coefficient of single-phase water; is the pressure value corresponding to the current working fluid; is the critical pressure value, is the saturated water Reynolds number corresponding to the working fluid temperature; G is the mass flow rate of the working fluid in the tube; Indicates dryness; is the saturated water Reynolds number; , is the Prandtl number of water corresponding to the working fluid temperature, is the Prandtl number of water corresponding to the inner wall temperature of the tube, is the saturated water Reynolds number corresponding to the inner wall temperature of the tube; is the saturated water Reynolds number; Based on boiler operating data, boiler equipment data and heat transfer coefficient, the wall temperature of each tube section is calculated.

8. The method for calculating heat transfer and hydrodynamic characteristics of vertical evaporation tubes of a once-through boiler according to claim 7 is characterized in that: The wall temperature of each pipe segment includes the inner wall temperature of the pipe segment and the outer wall temperature of the pipe segment.

9. The method for calculating heat transfer and hydrodynamic characteristics of vertical evaporation tubes of once-through boilers according to claim 8 is characterized in that: The inner wall temperature of the pipe section is: The outer wall temperature of the pipe section is: in, is the working fluid temperature in the tube at the tube wall temperature point, is the heat flow coefficient of the inner wall of the tube front surface, It is the ratio of the outside diameter to the inside diameter of the pipe; is the heat release coefficient between the inner wall of the pipe and the medium in the pipe at the pipe wall temperature calculation point; is the external wall radiation heat load at the wall temperature calculation point; is the tube wall thickness; is the thermal conductivity of the tube.

10. A heat transfer and hydrodynamic characteristics calculation system for vertical evaporation tubes of a once-through boiler, characterized in that: include: Data acquisition module: used to obtain boiler operation data and boiler equipment data; Pipe segment division module: used to divide the vertical evaporation pipes into different pipe segments according to different connection methods based on different boiler types; Enthalpy calculation module: used for different pipe sections to obtain the enthalpy value of each pipe section based on boiler operation data; A specific volume calculation module, used for calculating the average specific volume of each pipe section based on the enthalpy value of each pipe section; Dynamic characteristics calculation module: used to obtain the total pressure drop of each pipe section based on boiler equipment data and the average specific volume of each pipe section; Heat transfer characteristic calculation module: used to obtain the wall temperature distribution of each pipe section based on boiler operation data and boiler equipment data.

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