Transient heat transfer coefficient calculation method based on immersion boundary method

Abstract

The invention discloses a transient heat transfer coefficient calculation method based on an immersion boundary method. At present, in the transient starting and stopping process of a steam turbine system, thermal parameters such as the flow, the pressure and the flowing direction of fluid need to be dynamically adjusted. The method comprises the following steps that a computational domain model needing to be studied by the adjusting mechanism in the fluid calculation process is established; establishing a fluid domain grid model and a solid domain grid model of the adjusting mechanism; according to the start-stop curve, in combination with a stroke flow curve of the adjusting mechanism, thermal parameters of different working conditions under the variable load are determined; setting boundary conditions of the fluid domain; according to the basic principle of fluid heat exchange and based on an immersion boundary method, the transient heat exchange coefficient of the opening degree of a movement module of the adjusting mechanism in the changing process from large to small is calculated; determining a heat exchange coefficient extraction form, and extracting a transient heat exchange coefficient result; selecting typical points on the wall surface of the fluid-solid coupling research object, observing the time-domain change track of the heat exchange coefficient, and performing comparative analysis. The method is used for calculating the transient heat transfer coefficient.

Description

Technical Field

[0001] The invention relates to the technical field of computational fluid dynamics, and in particular to a method for calculating a transient heat transfer coefficient based on an immersed boundary method. Background Art

[0002] For a research object like a steam turbine regulating mechanism with a complex external structure and moving boundaries, generating an accurate grid is a difficult task, so it is also very challenging to calculate the heat transfer coefficient using this grid as a prerequisite. In the technical route of obtaining the heat transfer coefficient, a complex high-quality grid is often required when the boundary of the object moves, and a new grid must be regenerated after each time step to adapt to the instantaneous boundary changes. It is precisely with a highly reliable grid that the transient heat transfer coefficient can be successfully obtained, thereby achieving the expected calculation goal.

[0003] As a method for dealing with fluid-solid boundary problems, the immersed boundary method can be used to deal with the calculation of different fluids and solids for fluid-solid coupling problems. Mechanistically, the immersed boundary method is to achieve the coupling between fluid and solid by introducing additional force source terms in the momentum equation, and to divide the fluid domain using a fixed orthogonal grid. In the calculation process, no deformation or reconstruction of the grid is required, which is more suitable for large deformation problems and can effectively reduce the calculation cost, thus being more conducive to the calculation of the transient heat transfer coefficient of the fluid. At the same time, as a numerical method that can effectively simulate flow fields with complex dynamic boundaries, the immersed boundary algorithm of fluids has achieved great success in engineering fluid mechanics, gas-liquid-solid multiphase flow and other practices. The simulation results of the heat transfer coefficient are quite consistent with the experimental data and the calculation results of other body-fitting grid algorithms. Therefore, the immersed boundary method has become a hot spot in the study of heat transfer coefficient algorithms in the field of computational fluid dynamics. The fluid analysis method based on the immersed boundary method of the present invention can successfully solve various problems related to transient heat transfer coefficients raised in industrial production and engineering projects. Summary of the invention

[0004] The present invention aims to solve the problem of dynamically adjusting the flow rate, pressure, flow direction and other thermal parameters of the fluid during the transient start and stop of the steam turbine system, and provides a method for calculating the transient heat transfer coefficient based on the immersed boundary method.

[0005] The above purpose is achieved through the following technical solutions: A method for calculating transient heat transfer coefficient based on an immersed boundary method, the method comprising the following steps: Step 1: Establish the computational domain model required for the regulation mechanism to be studied during the fluid calculation process; Step 2: Establish the fluid domain and solid domain mesh models of the regulating mechanism; Step 3: According to the start-stop curve and the stroke flow curve of the regulating mechanism, determine the thermal parameters of different working conditions under variable load; Step 4: Set the boundary conditions of the fluid domain, including inlet temperature, inlet pressure, and outlet flow rate; Step 5: According to the basic principle of fluid heat transfer and based on the immersed boundary method, calculate the transient heat transfer coefficient when the opening of the motion module of the regulating mechanism changes from large to small; Step 6: Determine the heat transfer coefficient extraction form and extract the transient heat transfer coefficient results; Step 7: Select typical points on the wall of the fluid-solid coupling research object, observe the time domain variation trajectory of its heat transfer coefficient, and conduct comparative analysis.

[0006] The method for calculating transient heat transfer coefficient based on the immersed boundary method, the specific process of step 2 is: establishing a solid model of the turbine regulating mechanism, determining the solid domain and fluid domain required in the fluid calculation process, the model consists of the internal fluid domain of the regulating mechanism and the solid domains of two motion modules, and the grid is divided using tetrahedral solid units.

[0007] The transient heat transfer coefficient calculation method based on the immersed boundary method, the start-stop curve determined in the step three includes the start-up and shutdown process of the unit, and the start-up and shutdown time is determined by the actual situation.

[0008] In the transient heat transfer coefficient calculation method based on the immersed boundary method, the thermodynamic parameters in step three include temperature, pressure, and flow rate.

[0009] The transient heat transfer coefficient calculation method based on the immersed boundary method, the process of calculating the heat transfer coefficient based on the immersed boundary method in step 5 is as follows: The heat transfer coefficient refers to the amount of heat transferred per square meter per hour when the temperature difference between the heat flux and the metal wall of the regulating mechanism is 1 degree. Set the boundary conditions of each calculation domain of the regulating mechanism and calculate the convective heat transfer coefficient of the regulating mechanism under different load conditions; Convective heat transfer is a dynamic process of interaction between heat flow and heat transfer. According to the basic theory of fluid machinery, the heat transfer coefficient is calculated as follows:

[0010] is the heat transfer coefficient; is the experimental constant; is the fluid Reynolds number; is the fluid Prandtl number; is the thermal conductivity of the fluid; The inner diameter of the steam inlet pipe of the regulating mechanism; The average velocity of the fluid in the steam inlet pipe of the regulating mechanism; is the kinematic viscosity of the fluid.

[0011] The method for calculating transient heat transfer coefficient based on the immersed boundary method, the specific process of step seven is: according to the distribution of heat transfer coefficient of the regulating mechanism, select any point on the inner wall of the heat exchange surface of the regulating mechanism, use time as the horizontal coordinate and the heat transfer coefficient of the point as the vertical coordinate, draw the time domain change curve of the heat transfer coefficient of the point, and take out different points at different positions. Beneficial Effects

[0012] 1. The present invention calculates the transient heat transfer coefficient of the turbine regulating mechanism based on the immersed boundary method of fluid, which can effectively simulate the entity model of complex geometric shapes, simplify the grid division of the moving boundary, and introduce virtual forces in the fluid grid to make the simulation results more accurate. In particular, it can quickly and accurately complete the grid model of the regulating mechanism corresponding to different openings, so as to obtain the heat transfer coefficient. The fluid analysis method based on the immersed boundary method can directly extract the heat transfer coefficient field and realize the fluid-heat-solid multi-field coupling. The unique advantage of the immersed boundary method in simulating complex objects enables it to achieve unprecedented breakthroughs in the simulation of large deformation and large motion problems, fluid-solid coupling and other problems. The heat transfer coefficient calculation method described in the present invention is standardized and reliable, and is suitable for fluid analysis of turbine regulating mechanisms, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a flow chart of the heat transfer coefficient calculation method based on the immersed boundary method; Figure 2 It is the calculation domain model of the steam turbine regulating mechanism; Figure 3 It is a three-dimensional fluid computational grid model; Figure 4 is a schematic diagram of the grid for the immersed boundary method; Figure 5 It is a schematic diagram of the non-body-fitting grid (Cartesian grid) of the steam turbine regulating mechanism; Figure 6 is the start-stop curve of the steam turbine unit; Figure 7 It is a dynamic diagram of the immersed boundary process of the steam turbine regulating mechanism; Figure 8 It is the heat transfer coefficient cloud diagram of the steam turbine regulating mechanism under different loads; Fig. 9 It is the three-dimensional coordinate form of the extracted transient heat transfer coefficient; Fig.10 It is the time domain variation curve of the transient heat transfer coefficient of the point on the wall surface of fluid-heat coupling. DETAILED DESCRIPTION

[0014] A method for calculating transient heat transfer coefficient based on the immersed boundary method comprises the following steps: establishing an immersed boundary model of a steam turbine regulating mechanism in a three-dimensional solid modeling software, determining calculation domains such as a fluid domain and a solid domain; performing Cartesian grid division on the fluid according to a preset grid type division method based on the immersed boundary method; importing the divided grid model into a fluid analysis software and setting boundary conditions; establishing a dynamic function relationship between thermal parameters and time to express transient calculation results; combining the start-stop curve and the immersed boundary method based on the fluid to calculate and obtain the transient heat transfer coefficient of the steam turbine regulating mechanism; extracting the transient heat transfer coefficient result based on the immersed boundary method, and verifying the result by drawing the time domain change curve of any point on the wall during the fluid-heat coupling process of the research object. The present invention avoids the use of a dynamic grid algorithm, saves the update generation time when drawing traditional grids, overcomes the occurrence of traditional heat transfer coefficient calculation errors from the source, optimizes calculation data, simplifies the calculation process, greatly reduces the workload, and refreshes a new era of transient heat transfer coefficient calculation work.

[0015] Reference Figure 2 , establish the solid model of the turbine regulating mechanism, determine the solid domain and fluid domain required in the fluid calculation process, the model consists of the internal fluid domain of the regulating mechanism and the solid domain of two motion modules, the grid is divided by tetrahedral solid units, and the three-dimensional fluid calculation grid model after division is shown in Figure 3 The immersed boundary method uses a specific fluid grid to represent the flow field, and uses a set of solid grid points to represent the solid boundary in the immersed fluid. The influence of the solid boundary is transferred to the momentum equation as a force source term. The schematic diagram of the immersed boundary method grid is shown in Figure 4 The basic idea is to regard the physical boundary as a deformable boundary with great rigidity. Small deformation or small movement of the boundary will generate a force that drives the boundary to return to its original shape or position, namely, the restoring force. The restoring force on the boundary is distributed to the surrounding fluid grid nodes in turn, and then the flow field with force source terms is solved on the entire fluid domain including inside and outside the immersed body. The immersed boundary method is used to process the boundaries of the moving modules of the research object. Generally speaking, when solving the flow of fluids through complex shapes, a body-fitting mesh is usually generated in the entire fluid domain, and the drawing of the body-fitting mesh requires repeated size adjustment and regeneration verification. For moving boundary problems, in addition to constantly changing the mesh to adapt to boundary changes during the calculation process, the body-fitting mesh also needs to exchange various data between the old and new meshes, which not only increases the amount of calculation, but also reduces the accuracy and stability of the calculation results. The immersed boundary rule is to simulate the fluid with arbitrary shape transformations under a simple background grid, and a non-body-fitting mesh is generated. The non-body-fitting mesh does not need to be generated along the surface of a complex shape and has a simple structure. It is easy to handle moving boundaries, does not require moving or regenerating the mesh, and does not need to update the mesh at each different time step, which saves mesh generation time and improves calculation efficiency. See the schematic diagram of the non-body-fitting mesh of the research object based on the fluid immersed boundary method. Figure 5 Compared with the traditional body-fitting mesh algorithm, the immersed boundary method is simpler to handle complex boundaries, has higher computational efficiency and can directly derive the heat transfer coefficient field for subsequent multi-field coupling. The advantage of non-body-fitting mesh over body-fitting mesh is that it is easy to generate and simple to calculate.

[0016] The core of the immersed boundary method is to calculate the fluid and solid separately, and use the solid grid to solve the flow problem in the irregular boundary area. It is also called the continuous force method. Its advantage is that it is simpler and faster to process the fluid-solid boundary grid while ensuring the calculation accuracy. The immersed boundary method also directly adds the force source term to the NS equation, solves the immersed boundary in the fluid domain, and introduces the Dirac delta function to couple the Euler and Lagrangian variables. Its control equation can be expressed as the following formula:

[0017] Among them, equations (E-1) and (E-2) are the fluid control equations (NS equations), and the force source term is added to the momentum equation , used to reflect the effect of the boundary on the fluid; equation (E-3) is the distribution function of the force source term; equation (E-4) reflects the no-slip boundary condition at the immersed boundary. In the formula, A is the two-dimensional curve or three-dimensional surface where the immersed boundary method is located, v is the flow field velocity; z is the position coordinate in Euler coordinates; t is time; p is the flow field pressure; ρ is the fluid density; μ is the viscosity coefficient; n is used to describe the interaction between the flow field and the elastic body; I is the entire calculation domain.

[0018] Determine the start-stop curve of the steam turbine unit, which includes the start-up and shutdown process of the unit. The start-up and shutdown time is determined by the actual situation. Figure 6As shown in the figure, the start-up and shutdown process is divided into 8 intervals in total. The first interval is stable operation under rated load conditions, the second interval is from stable operation to shutdown, the third interval is from shutdown to 5% load conditions, the fourth interval is from 5% load conditions to 15% load conditions, the fifth interval is stable operation from 15% load conditions, the sixth interval is from 15% load conditions to 60% load conditions, the seventh interval is from 60% load conditions to rated load conditions, and the eighth interval is stable operation under rated load conditions.

[0019] Specific implementation method 5: During the dynamic immersion boundary of the turbine regulating mechanism, the stroke corresponding to the load can be adjusted according to the actual working conditions, such as Figure 7 As shown, from process ① to process ⑥, the process of the opening of the motion module of the regulating mechanism from fully open to fully closed is shown. L1, L2, L3, L4, L5, and L6 represent the stroke of the motion module. The stroke from L1 to L6 gradually decreases, reflecting the process change of the solid domain being immersed in the fluid domain little by little.

[0020] Specific implementation method six: According to the operating point in the start-stop curve, the thermal parameters of the steam turbine regulating mechanism, including temperature, pressure, flow, etc., are determined. The heat transfer coefficient refers to the amount of heat transferred per square meter per hour when the temperature difference between the heat flow and the metal wall of the regulating mechanism is 1 degree. The boundary conditions of each calculation domain of the regulating mechanism are set to calculate the convective heat transfer coefficient of the regulating mechanism under different load conditions.

[0021] Convective heat transfer is a dynamic process of interaction between heat flow and heat transfer. According to the basic theory of fluid machinery, the heat transfer coefficient is calculated as follows: ; is the heat transfer coefficient; is the experimental constant; is the fluid Reynolds number; is the fluid Prandtl number; is the thermal conductivity of the fluid; The inner diameter of the steam inlet pipe of the regulating mechanism; The average velocity of the fluid in the steam inlet pipe of the regulating mechanism; is the kinematic viscosity of the fluid.

[0022] When using the traditional empirical formula to calculate the heat transfer coefficient, it is necessary to calculate the relevant aerodynamic parameters of the flow through the regulating mechanism, such as the Reynolds number, Planck number, or steam flow rate, etc. When performing one-dimensional calculations, these steam parameters can be given but the accuracy cannot be further guaranteed. In addition, the heat transfer calculation method is relatively rough, which will have a certain impact on the distribution of the heat transfer coefficient field. Therefore, using the immersed boundary method to calculate the heat transfer coefficient will be closer to the actual operating conditions.

[0023] Through the three-dimensional flow field analysis inside the regulating mechanism, we can see the flow of the main steam inside it under various load conditions. Through the flow field analysis, we can get a more accurate heat transfer coefficient field inside the regulating mechanism. Figure 8 The heat transfer coefficient cloud diagram of the regulating mechanism under different load conditions shows that the average heat transfer coefficient at 100% load is 8529W / (m 2 K), the average heat transfer coefficient at 60% load is 12501W / (m 2 K), the average heat transfer coefficient at 15% load is 11223W / (m 2 K), the average heat transfer coefficient at 5% load is 7548W / (m 2 K), it can be seen that the heat transfer capacity of the regulating mechanism is not the strongest at full load. The heat transfer coefficient value is the largest under 60% load conditions. As the load decreases, the heat transfer coefficient value decreases. This is related to the flow state of the working fluid in the flow field and the pressure loss under different working conditions is different. The heat transfer coefficient field obtained by the three-dimensional flow field analysis is output in the form of (x, y, z, heat transfer coefficient) coordinates and parameters, and coupled loaded on the inner surface of the regulating mechanism to perform transient analysis of the regulating mechanism. The three-dimensional coordinate form of the extracted heat transfer coefficient is shown in Fig. 9 The fluid analysis method of the immersed boundary method is used to accurately calculate the heat transfer coefficient on the surface of the regulating mechanism. This provides favorable technical support for ensuring the accuracy of subsequent temperature field, stress field or strain field calculations, and is also an upgrade of the simulation calculation of the turbine regulating mechanism.

[0024] According to the distribution of heat transfer coefficient of the regulating mechanism, select any point on the inner wall of the heat transfer surface of the regulating mechanism, take time as the horizontal coordinate and the heat transfer coefficient of the point as the vertical coordinate, draw the time domain variation curve of the heat transfer coefficient of the point, and take different points at different positions. Fig.10 Point A is located at the center of one side of the inner wall of the regulating mechanism, point B is located near the outlet of the regulating mechanism, point C is located near the inlet of the regulating mechanism, and point D is located near the position before the regulating mechanism flows into the motion module. According to the time domain variation curve of the heat transfer coefficient of the four points, it can be seen that when the opening of the regulating mechanism motion module changes from full open to full closed, that is, when the stroke changes from maximum to minimum, the heat transfer coefficient of the inner wall of the regulating mechanism, the fluid inlet and before entering the motion module is 5000-8000W / (m 2 K) range, and the fluctuation over time is relatively stable, while the heat transfer coefficient value of the outlet area after the fluid flows through the regulating mechanism motion module increases rapidly, and the maximum value can reach nearly 30000W / (m 2K), and its trend of change over time is obvious. This may be because there is a large vortex here. In the process of gradually reducing the stroke of the motion module of the regulating mechanism, the pressure loss of the fluid flow will have a process of first decreasing and then increasing, so the heat exchange capacity will increase and decrease accordingly. In addition, the transient flow of the regulating mechanism is affected by the airflow parameters and the physical structure, and the transient heat transfer characteristics of the regulating mechanism are affected by the flow distribution. Therefore, when the fluid flows through the spherical chamber of the regulating mechanism and then passes through the throat of the motion module, the oscillation of the wall jet and backflow may occur. The increase in jet increases the local temperature and enhances the heat transfer capacity, so the heat transfer coefficient increases sharply. It can also be inferred that the turbulent distribution of the flow field has a direct impact on its heat transfer characteristics.

[0025] The present invention adopts the immersed boundary method to calculate the transient heat transfer coefficient of the turbine regulating mechanism. According to the start-stop curve operating conditions of the turbine unit, the transient heat transfer coefficient under any load is simply and quickly calculated, effectively avoiding the time-consuming and laborious grid drawing and the problem that the heat transfer coefficient cannot be changed in real time in special research. The calculation method is standardized and reliable, and provides a method for processing fluid-solid dynamic boundary problems for research objects such as turbine units with complex external structures and moving boundaries, and also provides important theoretical and technical support for the solution algorithm coupling different fluids and solids.

Claims

1. A method for calculating transient heat transfer coefficient based on the immersed boundary method, characterized in that: The method comprises the following steps: Step 1: Establish the computational domain model required for the regulation mechanism to be studied during the fluid calculation process; Step 2: Establish the fluid domain and solid domain mesh models of the regulating mechanism; Step 3: According to the start-stop curve and the stroke flow curve of the regulating mechanism, determine the thermal parameters of different working conditions under variable load; Step 4: Set the boundary conditions of the fluid domain, including inlet temperature, inlet pressure, and outlet flow rate; Step 5: According to the basic principle of fluid heat transfer and based on the immersed boundary method, calculate the transient heat transfer coefficient when the opening of the motion module of the regulating mechanism changes from large to small; Step 6: Determine the heat transfer coefficient extraction form and extract the transient heat transfer coefficient results; Step 7: Select typical points on the wall of the fluid-solid coupling research object, observe the time domain variation trajectory of its heat transfer coefficient, and conduct comparative analysis.

2. The method for calculating transient heat transfer coefficient based on the immersed boundary method according to claim 1 is characterized in that: The specific process of step 2 is: establish a solid model of the turbine regulating mechanism, determine the solid domain and fluid domain required in the fluid calculation process, the model consists of the internal fluid domain of the regulating mechanism and the solid domains of two motion modules, and the grid is divided using tetrahedral solid units.

3. The method for calculating transient heat transfer coefficient based on the immersed boundary method according to claim 1 is characterized in that: Determining the start-stop curve in the step three includes the start-up and shutdown process of the unit, and the start-up and shutdown time is determined by the actual situation.

4. The method for calculating transient heat transfer coefficient based on the immersed boundary method according to claim 3 is characterized by: The thermal parameters in step three include temperature, pressure and flow rate.

5. The method for calculating transient heat transfer coefficient based on the immersed boundary method according to claim 1 is characterized in that: The process of calculating the heat transfer coefficient based on the immersed boundary method in step 5 is as follows: The heat transfer coefficient refers to the amount of heat transferred per square meter per hour when the temperature difference between the heat flux and the metal wall of the regulating mechanism is 1 degree. Set the boundary conditions of each calculation domain of the regulating mechanism and calculate the convective heat transfer coefficient of the regulating mechanism under different load conditions; Convective heat transfer is a dynamic process of interaction between heat flow and heat transfer. According to the basic theory of fluid machinery, the heat transfer coefficient is calculated as follows: α is the heat transfer coefficient; C v4 is the experimental constant; Re is the fluid Reynolds number; Pr is the fluid Prandtl number; λ is the fluid thermal conductivity; d2 is the inner diameter of the steam inlet pipe of the regulating mechanism; ω is the average velocity of the fluid in the steam inlet pipe of the regulating mechanism; v is the kinematic viscosity of the fluid.

6. The method for calculating transient heat transfer coefficient based on the immersed boundary method according to claim 1 is characterized in that: The specific process of step seven is as follows: according to the heat transfer coefficient distribution of the regulating mechanism, select any point on the inner wall of the heat transfer surface of the regulating mechanism, use time as the horizontal coordinate and the heat transfer coefficient of the point as the vertical coordinate, draw the time domain change curve of the heat transfer coefficient of the point, and take out different points at different positions.

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