A CFD-based simulation and analysis method for the falling film flow characteristics of viscous fluids

Abstract

The present invention relates to the field of chemical engineering and discloses a CFD-based simulation and analysis method for analyzing the falling film flow characteristics of viscous fluids. This method can simulate and calculate the flow of a viscous fluid along a falling film flow field structure. It can effectively obtain flow field information such as velocity distribution, phase distribution, component distribution, temperature distribution, viscosity distribution, and residence time distribution during the falling film flow process. This method provides reference and guidance for analyzing the falling film flow characteristics of viscous fluids and for the design, scale-up, and process optimization of industrial equipment. The simulation and analysis method provided by the present invention is universal and also has reference value for other flow systems using viscous fluids as the working medium.

Description

Technical Field

[0001] The present invention relates to the field of chemical engineering, and in particular to a CFD-based simulation analysis method for falling film flow characteristics of a viscous fluid. Background Art

[0002] Viscous fluids, including but not limited to organic matter, polymer solutions, or melts, exhibit significant viscous stresses and are prone to shear deformation when subjected to force. Due to their high dynamic viscosity, viscous fluids must overcome greater resistance during flow, resulting in flow behaviors that differ from those of conventional fluids.

[0003] Falling film flow refers to the process by which a fluid, under the influence of its own gravity, moves downward along a falling film flow field structure, expanding into a liquid film. Depending on the fluid's physical properties and flow conditions, falling film flow can exhibit different flow states, such as turbulent falling film and laminar stable falling film. Falling film flow has a wide range of industrial applications, such as in chemical production for devolatilization, absorption, condensation, and evaporation; in heat exchangers for improving heat transfer efficiency and reducing energy consumption; and in food processing for coating, spraying, and packaging.

[0004] CFD stands for Computational Fluid Dynamics. CFD uses the theory of numerical solutions to partial differential equations to discretize the governing equations in fluid dynamics. It then conducts numerical experiments, computer simulations, and analytical research on various fluid dynamics problems to solve various practical problems. CFD simulations typically include pre-processing, solution, and post-processing.

[0005] Flow characteristics refer to the various physical properties and behavioral patterns exhibited during fluid flow, including but not limited to film thickness variation, velocity distribution, residence characteristics, mixing and renewal characteristics, etc. These flow characteristics are of great significance in understanding and optimizing fluid devices and guiding production applications.

[0006] Currently, two common methods for analyzing falling film flows are cold model experiments and numerical simulations. Cold model experiments use simulated media to simulate fluid motion and observe flow behavior, but these experiments are generally difficult to implement, have limited observational tools, have low flow field resolution, have large data errors, and require long experimental cycles. CFD simulations are a relatively unpopular research method, particularly for viscous falling film flows, and are plagued by deficiencies in the research foundation, insufficient research, and a limited number of case studies.

[0007] A search of existing technologies revealed no comprehensive reports on CFD-based methods for simulating and analyzing the falling film flow characteristics of viscous fluids. Therefore, there is an urgent need for a method that can quickly and effectively obtain flow field information such as velocity distribution, phase distribution, composition distribution, temperature distribution, viscosity distribution, and residence time distribution during the falling film flow process. This method can provide reference and guidance for analyzing the falling film flow characteristics of viscous fluids and for the design, scale-up, and process optimization of industrial equipment. Summary of the Invention

[0008] To address the aforementioned technical issues, the present invention provides a CFD-based simulation and analysis method for the falling film flow characteristics of viscous fluids. This method simulates the flow of viscous fluids along a falling film flow field structure, effectively obtaining flow field information during the falling film flow process. This method provides a reference and guidance for analyzing the falling film flow characteristics of viscous fluids, as well as for the design, scale-up, and process optimization of industrial equipment.

[0009] The specific technical solution of the present invention is: a CFD-based simulation and analysis method for the falling film flow characteristics of a viscous fluid. The present invention uses CFD to pre-process, solve, and post-process the falling film flow field of a given structure to obtain flow field characteristic information. The specific steps include:

[0010] 1) Pre-processing: Model the falling film flow field structure through pre-processing software.

[0011] 2) Solution: Determine the solution equation, solution method and boundary conditions in the CFD software, and run the CFD software calculation module to obtain the viscous fluid falling film flow field information, and is not limited to performing secondary or higher CFD calculations on the flow field.

[0012] 3) Post-processing: The viscous fluid falling film flow field information obtained by CFD solution is analyzed, synthesized and visualized through post-processing software.

[0013] Preferably, the falling film flow field structure includes a falling film element or a falling film element combination structure that supports the flow of viscous fluid, a film distribution structure that provides assistance for the fluid to enter the falling film element, a positioning structure that fixes the falling film element, and a component structure that changes the surface structure of the falling film element.

[0014] Preferably, the pre-processing includes: determining the size of the flow field structure, geometrically modeling the falling film flow field structure, determining the calculation domain, meshing the calculation domain, verifying the independence of the mesh, and verifying the independence of the time step.

[0015] Further preferably, step 1) specifically includes:

[0016] 1.1) Determine the dimensions of the falling film flow field structure and perform geometric modeling of the falling film flow field structure: Use modeling software including but not limited to UG NX, Solidworks, Pro / E, SpaceClaim, DesignModel, etc. to establish a two-dimensional or three-dimensional model of the device according to the structural dimensions.

[0017] 1.2) Determine the computational domain: Based on the 3D model, extract the fluid domain in modeling software including but not limited to SpaceClaim, DesignModel, etc., or directly model the fluid domain in the modeling software based on the device structure dimensions.

[0018] 1.3) Meshing the computational domain: Use meshing software such as ANSYS Mesh, Fluent Meshing, ICEM CFD, and Gambit to spatially discretize the established computational domain model. Use structured or unstructured meshing methods to create two-dimensional or three-dimensional meshes to obtain meshes of varying sizes and quantities.

[0019] 1.4) Verify the independence of the grid: Use grids of different sizes, densities, and numbers to perform calculations under the same conditions, verify the impact of the number of grids on the simulation results, and determine a set of grids with sufficient simulation accuracy.

[0020] 1.5) Verify the independence of the time step: For simulation calculations using the transient time format, the time step independence verification is performed under the same conditions to eliminate the influence of the time step on the simulation results; this is not required for simulation calculations using the steady-state time format.

[0021] Preferably, in step 1), the viscous fluid exhibits significant viscous stress and is susceptible to shear deformation when subjected to force, including but not limited to various organic substances, polymer solutions, or melts. The dynamic viscosity of the viscous fluid can be a constant, steady fluid or a constant, unsteady fluid.

[0022] Preferably, the solution includes: selecting or setting the solver type and time format, selecting a single-phase flow or multiphase flow model, selecting a laminar flow or turbulence model, selecting an energy equation, selecting a component transport equation and / or a scalar transport equation, setting fluid physical properties or physical property equations, selecting a boundary type and initial conditions, selecting a coupling method and discrete format, setting a relaxation factor, setting a report residual and monitoring output, performing initialization, and setting iteration parameters.

[0023] Further preferably, step 2) specifically includes:

[0024] 2.1) Select the solver type and time format: Select the corresponding 2D or 3D solver according to the grid type, select single-precision or double-precision solver, select pressure-based solver or density-based solver for the solver type, and select steady-state or transient time format.

[0025] 2.2) Select a single-phase or multiphase flow model: Select a single-phase or multiphase flow model based on the number of phases and flow characteristics in the flow system and physical scenario. Multiphase flow models include but are not limited to the VOF model, Mixture model, and WetSteam model.

[0026] 2.3) Select laminar or turbulent flow model: Select laminar or turbulent flow calculation according to the falling film Reynolds number. Generally speaking, laminar flow calculation is suitable when the falling film Reynolds number is less than 1000.

[0027] 2.4) Select Energy Equation: Enable or disable the energy equation based on whether energy issues need to be considered.

[0028] 2.5) Select component transport equation and / or scalar transport equation: Depending on whether you need to calculate multiple components or virtual components, choose whether to add component transport equation or scalar transport equation.

[0029] 2.6) Set fluid physical property parameters or physical property equations: Call and set physical property parameters in the software's built-in physical property database, or use mathematical equations to express the physical property information and bring it into the solution software.

[0030] 2.7) Select boundary type and initial conditions: Based on the actual boundary type of the fluid system, select boundary types including but not limited to velocity inlet, mass flow inlet, pressure inlet, pressure outlet, velocity outlet, mass outlet, wall, symmetry plane, and axis. Set the corresponding initial conditions for each boundary type.

[0031] 2.8) Select coupling mode and discretization format: Based on experience and the convergence of the actual calculation process, SIMPLE, SIMPLEC, PISO, Coupled and other coupling methods can be selected for pressure-velocity coupling. Appropriate discretization formats can be selected for spatial discretization of physical quantities such as gradient, pressure, momentum, volume fraction, energy, and composition.

[0032] 2.9) Set the relaxation factor: To adjust the convergence of the discrete equation, set the appropriate sub-relaxation factor.

[0033] 2.10) Set up reports, residuals, and monitoring outputs: Set the information and locations that need to be tested based on actual needs to better judge the reliability of the calculation.

[0034] 2.11) Initialization: Select global initialization or local initialization to assign initial flow field information, or assign a calculation starting point based on actual conditions.

[0035] 2.12) Set iteration parameters: Set the number of iterations as needed or specify the iteration parameters required for transient calculation.

[0036] Preferably, step 3) specifically includes:

[0037] 3.1) Analyze the flow field information.

[0038] 3.2) Synthesize flow field information: Perform secondary data calculations based on the original flow field information according to mathematical principles or specified models to obtain one or more results including but not limited to specified position types, cloud maps, vector maps, trace maps, distribution maps, histograms, animations, reports, custom functions and scalars.

[0039] 3.3) Visualization of flow field: Use post-processing software or visualization software including but not limited to Fluent, CFD Post, Tecplot, Matlab, Origin, etc. to visualize the flow field information.

[0040] Preferably, the flow field information includes but is not limited to one or more of velocity distribution, temperature distribution, phase distribution, viscosity distribution, component distribution and residence time distribution.

[0041] Further preferably:

[0042] The velocity distribution includes but is not limited to one or more of velocity magnitude and direction at any position, velocity gradient, surface flow velocity, and surface update frequency.

[0043] The temperature distribution includes but is not limited to one or more of the thermodynamic temperature distribution, enthalpy distribution, and energy distribution obtained in the simulation considering energy factors; when considering energy factors, the flow field includes heating, and the heating form includes but is not limited to single-sided heating, double-sided heating, volume heat source, etc.; the heating method includes but is not limited to uniform heating and non-uniform heating, among which non-uniform heating includes spatial non-steady heating and temporal non-steady heating.

[0044] The phase distribution includes but is not limited to one or more of the phase volume fraction of each phase in the multiphase flow, the position of the free surfaces of two adjacent phases, the film forming area and film forming efficiency of the free surface, the film thickness of a certain phase and its distribution.

[0045] The component distribution includes but is not limited to one or more of the concentration distribution of each component and virtual component in a single-phase flow, and the concentration distribution of components in each phase in a multiphase flow; wherein the concentration distribution includes but is not limited to mass concentration, volume concentration, molar concentration, mass distribution, volume fraction, and mole fraction.

[0046] The viscosity distribution can be a uniform distribution or a non-uniform distribution in each phase of a single-phase flow or a multi-phase flow.

[0047] The residence time distribution includes residence time density distribution, residence time distribution curve, and the first-order moment, second-order moment and higher-order moment of the distribution; the first-order moment is the average residence time, and the second-order moment is the variance of the residence time distribution.

[0048] Preferably, the viscosity distribution is uniform or non-uniform, specifically:

[0049] The viscous fluid is a fluid with a constant viscosity characteristic or a fluid with a variable viscosity characteristic;

[0050] In a single-phase flow, or within each phase of a multiphase flow, a fluid with constant viscosity characteristics obtains a uniform viscosity distribution, while a fluid with variable viscosity characteristics obtains a non-uniform distribution;

[0051] The variable viscosity characteristic can have a variety of viscosity change rules. In this case, the viscosity generally refers to the viscosity of the fluid micro-group;

[0052] The physical forms of variable viscosity laws include but are not limited to time-viscosity law, concentration-viscosity law, temperature-viscosity law, stress-strain-viscosity law and various combinations thereof;

[0053] The mathematical form of the variable viscosity law includes, but is not limited to, a linear equation form, a polynomial form, a power law form, an exponential form, a logarithmic form, and various combinations thereof.

[0054] Preferably, the residence time distribution is divided into two categories: transient residence time distribution and steady-state residence time distribution:

[0055] Transient residence time distribution: mainly refers to how long it takes for fluid particles entering the flow field at the same time to reach the specified position, or the residence time when leaving the flow field, which is used to evaluate the mixing effect in the flow field; it is mainly calculated using the transient format, including the pulse method and the step method, among which the step method includes the rising method and the falling method. The difference lies in the different concentration expressions, but both can obtain the same residence time distribution density function and residence time distribution function.

[0056] Steady-state residence time distribution: mainly refers to the residence time of fluid clusters that reach the specified position at the same time since entering the flow field, which is used to evaluate the degree of mixing of fluid clusters at the specified position; it is mainly obtained by steady-state format calculation, and the concentration expression is similar to the pulse method. The average residence time at any position and the average residence time distribution in the entire flow field can be obtained, which is applicable to both single-phase flow and multiphase flow; if the transient format is used for calculation, the average residence time distribution within each steady-state time step can be obtained.

[0057] Further preferably, a pulse method is used to simulate the transient residence time distribution:

[0058] a) Enable component transport equations in the flow field, set tracer components, and set report definitions, residual standards, and monitoring outputs at specified locations;

[0059] b) Injecting the tracer component into the flow field in a very short time. Specifically, the tracer component is set to a concentration value and a time step is calculated. After that, the tracer concentration is adjusted back to 0 to complete the pulse injection. Generally, the time step length is required to be much shorter than the residence time of the tracer in the flow field.

[0060] c) continuing the calculation until the tracer completely passes through the designated location, during which a relationship between the tracer concentration at the designated location and time is obtained;

[0061] d) Based on the relationship between the tracer change over time and the pulse method tracer concentration relationship, the residence time distribution density function and the residence time distribution function are obtained;

[0062] Simulation of steady-state residence time distribution using the pulse method:

[0063] A) Derive the average residence time scalar transport equation for steady-state calculations based on the pulse method concentration expression and write a user-defined function;

[0064] B) Enable the scalar transport equation in the simulation, set the user-defined scalar, and compile and load the user-defined function;

[0065] C) Set the boundary type, initial conditions, discretization format, and sub-relaxation factor of the scalar transport equation, initialize, and solve the transport equation;

[0066] D) Based on the mathematical relationship between the scalar value and the average residence time, the solution results are analyzed, synthesized and visualized.

[0067] Compared with the prior art, the present invention has the following beneficial effects:

[0068] (1) The present invention simulates and calculates the falling film flow field of viscous fluid, which can effectively and quickly obtain the flow behavior of viscous fluid in the falling film flow field, as well as the relationship between the flow behavior and the fluid physical properties and flow conditions. It can provide reference and guidance for analyzing the falling film flow characteristics of viscous fluid and the design, scale-up and process optimization of industrial equipment.

[0069] (2) The present invention provides a comprehensive and reliable simulation analysis method for the falling film flow characteristics of viscous fluids, including pre-processing, simulation calculation and solution, and result post-processing, which can save research and development costs and has good application prospects.

[0070] (3) The simulation analysis method provided by the present invention is universal and has reference value for other flow systems using viscous fluid as the working medium. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Figure 1 This is a flow chart for the simulation and analysis of the falling film flow characteristics of viscous fluids;

[0072] Figure 2 This is a three-dimensional model diagram of the falling film flow of viscous fluid along a smooth vertical tube;

[0073] Figure 3 Schematic diagram of mesh division;

[0074] Figure 4 is the velocity distribution diagram of the axial plane of the falling film flow field of viscous fluid;

[0075] Figure 5 is the phase distribution diagram of the viscous fluid falling film flow field in the axial plane;

[0076] Figure 6 The velocity distribution diagram of the falling film flow field of variable viscosity fluid in the axial plane;

[0077] Figure 7 This is the phase distribution diagram of the falling film flow field of variable viscosity fluid in the axial plane;

[0078] Figure 8 is the temperature distribution diagram of the axial plane of the falling film flow field of viscous fluid;

[0079] Figure 9 is the steady-state residence time distribution diagram of the viscous fluid falling film flow field in the axial plane;

[0080] Figure 10 is the density function of the transient residence time distribution at the observation point in the falling film direction of the viscous fluid falling film flow field;

[0081] Figure 11 Simulate workflow diagram for transient residence time distribution;

[0082] Figure 12 Workflow diagram for steady-state residence time distribution simulation;

[0083] Figure 13 This is the workflow diagram for simulating variable viscosity fluid. DETAILED DESCRIPTION

[0084] To make the technical solutions, features, and beneficial effects of the present invention more clearly understood, the following describes the present invention with reference to specific embodiments. However, the embodiments of the present invention are not limited to the scope of the embodiments.

[0085] Example 1

[0086] This example uses CFD software to simulate the falling film flow process of a viscous fluid with a dynamic viscosity of 20 Pa·s along a smooth vertical pipe. The simulation work is mainly divided into three parts: pre-processing, solution, and post-processing. Figure 1 .

[0087] The ultimate goal of pre-processing is to obtain a grid that accurately describes the physical scene and has appropriate spatial discretization.

[0088] 1) First refer to Figure 2 The diameter of the smooth falling film tube is determined to be 25 mm and the length is 100 mm. Based on experience, the calculation domain width is selected to be 12 to 30 mm to form the space required for fully developed falling film flow. Based on actual requirements, the width of the annular gap at the falling film inlet is determined to be 3 mm.

[0089] 2) Due to the simple structure of the flow field, UG NX can be used to directly model the fluid domain; preferably, the modeling function of the meshing software ICEM CFD can also be used to model the fluid domain.

[0090] 3) Import the fluid domain geometry model into ICEM CFD software for meshing. According to the flow characteristics of the falling film unilateral wall flow forming a velocity gradient, an encrypted mesh is used near the wall to obtain high-resolution flow field information. In ICEM CFD, the mesh is divided in a way that the nodes are connected to each other. Set the number of inlet annular nodes to 37, arrange them at intervals according to the second-order relationship, and the maximum spacing is 0.1mm; according to the actual size of the calculation domain, set the number of uniformly distributed nodes in the film thickness direction to 120-300, with a spacing of 0.1mm; set the number of uniformly distributed nodes in the falling film direction to 1001, with a spacing of 0.1mm; set the number of uniformly distributed nodes in the circumferential direction to 1440, with a spacing of 0.2° at the central angle; preferably, in order to save computational overhead, a periodic meshing method can be used to reduce the number of meshes, and a fluid domain with a central angle of 30° can be used instead of the entire fluid domain for calculation. The simulation results remain unchanged, see Figure 3 .

[0091] 3) The mesh independence of the initially divided grid needs to be verified. The initially divided grid is encrypted and thinned several times to obtain several sets of grids with different numbers, such as 240,000, 360,000, 480,000, and 600,000. These are imported into Fluent and simulated under the same conditions. The obtained results (such as the position of the free surface of two-phase flow) are compared. The minimum grid number critical value at which the calculation results no longer change significantly when the grid density is further increased is 480,000. In other words, the range of optional grid numbers that can ensure both calculation accuracy and computational cost savings in the current simulation is greater than or equal to 480,000.

[0092] 4) The grid involved in transient flow calculations also needs to be verified for time step independence. The grid determined through grid independence verification is used again under the same conditions for calculations. The results obtained at the same flow moment under transient calculations with different time steps are observed. The maximum time step critical value at which the calculation results no longer change significantly with further reduction of the time step is selected. The final range of optional time steps that is not affected by the calculation time step and saves computational overhead in the current simulation is ≤ 0.001.

[0093] Import the mesh obtained through the above steps into Fluent for solution settings.

[0094] 5) Select 3D mode, double-precision solver, 40-core parallel processor, and start Fluent; read the mesh and check and report the mesh quality, adjust the scale; select the steady-state solution format and pressure-based solver.

[0095] 6) Set up a VOF two-phase flow model, with the primary phase being the viscous fluid phase and the secondary phase being the gas phase; use the implicit volume formula to calculate the volume fraction parameter; set the two-phase interaction surface tension coefficient to 0.02 N / m based on actual conditions, and add a continuous surface tension model.

[0096] 7) Select the viscous laminar flow model; open the energy equation; and do not calculate the transport equations for components and scalars for now.

[0097] 8) Set the fluid viscosity to a constant of 20 Pa·s, and set physical parameters such as density, specific heat, thermal conductivity, molecular weight, and standard enthalpy according to actual conditions; set various physical properties of the gas phase fluid according to actual conditions.

[0098] 9) Set the inlet boundary condition to velocity inlet, the converted equivalent flow rate to 15 kg / h, the initial gauge pressure to zero, the inlet fluid temperature to 260°C, and the secondary phase volume fraction to 0; set the outlet boundary condition to pressure outlet, the back pressure to 0, the reflux temperature to 260°C, and the secondary phase reflux volume fraction to 1; set the remaining boundaries to fixed no-slip adiabatic wall conditions, and set the periodic surface to periodic boundary conditions; set the reference pressure position and size.

[0099] 10) Select SIMPLE pressure-velocity coupling mode, and the spatial discretization methods for gradient, pressure, momentum, volume fraction, and energy are Green-Gauss Cell Based, PRESTO!, second-order upwind, Compressive, and second-order upwind, respectively.

[0100] 11) Adjust the relaxation factor within the range of 0 to 1 based on experience. Initially, use the default relaxation factors of 0.3, 1, 1, 0.7, and 0.5. If convergence is poor, reduce the relaxation factor appropriately. In this example, the flow conditions are favorable for film formation, so the default influence factors are sufficient to obtain falling film flow field information.

[0101] 12) To better monitor convergence, use inlet and outlet flow monitoring to assist in judgment. First, define inlet and outlet mass flow monitoring using the mass flow report integrated along the surface. Next, define a plot report to output the inlet and outlet mass flow data in real time to the console and plot window. Convergence can be determined by whether the inlet and outlet flows are stable and equal. Additionally, set the "Contour Plot" option to display a volume fraction contour plot at the central axis plane and output the contour plot in image format to assist in monitoring the calculation process.

[0102] 13) Initialize the fluid domain using standard initialization starting from the inlet, assigning the flow field an initial pressure value of 0 Pa, a velocity of 0, a gas phase volume fraction of 1, etc.

[0103] 14) Set the iteration parameters for the solution. In this example, the steady-state calculation is set to 50,000 iterations and a reporting interval of 1. The unsteady-state calculation is set to a fixed time marching format, 10,000 time steps, a time step length of 0.001 s, and a maximum number of iterations per time step of 100.

[0104] The flow field data calculated by Fluent are written into case files and data files, and the post-processing software CFDPost is used to analyze, synthesize and visualize the results.

[0105] 15) Create the falling film tube axial plane in CFD Post and create the velocity cloud map on the axial plane as shown below: Figure 4 、 Figure 6 As shown; create a phase distribution diagram as shown Figure 5 、 Figure 7 As shown; create a temperature distribution map as shown Figure 8 As shown; in the steady-state residence time simulation, the local average residence time data can be imported into the three-dimensional drawing software Origin to obtain the steady-state residence time distribution three-dimensional diagram as shown Figure 9 As shown; in the transient residence time simulation, the tracer concentration history information recorded at the observation point can be processed, and the residence time distribution density function diagram at each observation point can be drawn in Origin as shown Figure 10 shown.

[0106] The above embodiment of the present invention provides a simulation and analysis method for the falling film flow characteristics of a viscous fluid. Preferably, in the case where most of the steps are the same, transient residence time distribution simulation, steady-state residence time distribution simulation, and falling film simulation of a fluid with variable viscosity can also be performed. Figure 11 、 Figure 12 and Figure 13 .

Claims

1. A CFD-based simulation and analysis method for the falling film flow characteristics of a viscous fluid, characterized by: For the viscous fluid flowing downward along the falling film flow field structure, CFD is used to perform pre-processing, solution and post-processing on the falling film flow field of a given structure to obtain flow field characteristic information; The specific steps include: 1) Pre-processing: Model the falling film flow field structure through pre-processing software; 2) Solution: Determine the solution equation, solution method and boundary conditions in the CFD software, and run the CFD software calculation module to obtain the viscous fluid falling film flow field information, and is not limited to performing secondary or higher CFD calculations on the flow field; 3) Post-processing: Analyze, synthesize and visualize the viscous fluid falling film flow field information obtained through CFD solution through post-processing software; The falling film flow field structure includes a falling film element or a falling film element combination structure that supports the flow of viscous fluid, a film distribution structure that assists the fluid in entering the falling film element, a positioning structure that fixes the falling film element, and a component structure that changes the surface structure of the falling film element. Step 2) specifically includes: 2.1) Select the solver type and time format: Select the corresponding 2D or 3D solver according to the grid type, select single-precision or double-precision solver, select pressure-based solver or density-based solver for the solver type, and select steady-state or transient time format; 2.2) Selecting a single-phase or multiphase flow model: Select a single-phase or multiphase flow model based on the number of phases and flow characteristics in the flow system and physical scenario. 2.3) Select laminar or turbulent flow model: Select laminar or turbulent flow calculation according to the falling film Reynolds number; 2.4) Select Energy Equation: Enable or disable the energy equation based on whether energy issues need to be considered. 2.5) Select component transport equation and / or scalar transport equation: Depending on whether you need to calculate multiple components or virtual components, choose whether to add component transport equation or scalar transport equation; 2.6) Set fluid physical property parameters or physical property equations: call and set physical property parameters in the software's built-in physical property database, or use mathematical equations to express the physical property information and bring it into the solution software; 2.7) Select boundary type and initial conditions: Based on the actual boundary type of the fluid system, select at least one boundary type from the following: velocity inlet, mass flow inlet, pressure inlet, pressure outlet, velocity outlet, mass outlet, wall, symmetry plane, and axis. Set the corresponding initial conditions for each boundary type. 2.8) Select the coupling method and discretization format: Select the pressure-velocity coupling method and physical space discretization format based on the convergence of the actual calculation process; 2.9) Setting the relaxation factor: To adjust the convergence of the discrete equation, set the sub-relaxation factor; 2.10) Set up reports, residuals, and monitoring outputs: Set the information and locations to be tested; 2.11) Initialization: Select global initialization or local initialization to assign initial flow field information, or assign a calculation starting point based on actual conditions; 2.12) Set iteration parameters: Set the number of iterations or specify the iteration parameters required for transient calculation.

2. The method according to claim 1, wherein: The pre-processing includes: determining the size of the flow field structure, geometrically modeling the falling film flow field structure, determining the calculation domain, meshing the calculation domain, verifying the independence of the mesh, and verifying the independence of the time step; The solution includes: selecting or setting the solver type and time format, selecting a single-phase flow or multiphase flow model, selecting a laminar flow or turbulence model, selecting an energy equation, selecting a component transport equation and / or a scalar transport equation, setting fluid physical properties or physical property equations, selecting a boundary type and initial conditions, selecting a coupling method and discrete format, setting a relaxation factor, setting a report residual and monitoring output, performing initialization, and setting iteration parameters.

3. The simulation analysis method according to claim 1, wherein: Step 1) specifically includes: 1.1) Determine the structural dimensions of the falling film flow field and perform geometric modeling of the falling film flow field structure: Use modeling software to create a two-dimensional or three-dimensional model of the device based on the structural dimensions; 1.2) Determine the computational domain: Extract the fluid domain in the modeling software based on the 3D model, or directly model the fluid domain in the modeling software based on the device structure dimensions; 1.3) Meshing the computational domain: Use meshing software to spatially discretize the established computational domain model. Use structured or unstructured meshing methods to create two-dimensional or three-dimensional grids to obtain grids of different sizes and numbers. 1.4) Verify the independence of the grid: Use grids of different sizes, densities, and numbers to perform calculations under the same conditions, verify the impact of the number of grids on the simulation results, and determine a grid with sufficient simulation accuracy; 1.5) Verify the independence of the time step: For simulation calculations using the transient time format, the time step independence verification is performed under the same conditions to eliminate the influence of the time step on the simulation results; this is not required for simulation calculations using the steady-state time format.

4. The simulation analysis method according to claim 1, wherein: Step 3) specifically includes: 3.1) Analyze flow field information; 3.2) Synthesize flow field information: Perform secondary data calculations based on the original flow field information according to mathematical principles or specified models to obtain one or more results of the specified position type, cloud map, vector map, trace map, distribution map, histogram, animation, report, custom function and scalar; 3.3) Visualization of flow field: Use post-processing software or visualization software to visualize the flow field information.

5. The simulation analysis method according to claim 1 or 4, characterized in that: The flow field information includes one or more of velocity distribution, temperature distribution, phase distribution, viscosity distribution, component distribution and residence time distribution.

6. The simulation analysis method according to claim 5, characterized in that: The velocity distribution includes one or more of velocity magnitude and direction, velocity gradient, surface flow velocity, and surface update frequency at any position; The temperature distribution includes one or more of a thermodynamic temperature distribution, an enthalpy distribution, and an energy distribution obtained in a simulation taking energy factors into account; when considering energy factors, the flow field includes heating, and the heating forms include single-sided heating, double-sided heating, and a volume heat source; the heating methods include uniform heating and non-uniform heating, wherein non-uniform heating includes spatial unsteady heating and temporal unsteady heating; The phase distribution includes one or more of the phase volume fraction of each phase in the multiphase flow, the position of the free surfaces of two adjacent phases, the film forming area and film forming efficiency of the free surface, the film thickness of a certain phase and its distribution; The component distribution includes one or more of the concentration distribution of each component and virtual component in a single-phase flow and the concentration distribution of components in each phase in a multiphase flow; wherein the concentration distribution includes mass concentration, volume concentration, molar concentration, mass distribution, volume fraction, and mole fraction; The viscosity distribution is a uniform distribution or a non-uniform distribution in each phase of a single-phase flow or a multi-phase flow; The residence time distribution includes residence time density distribution, residence time distribution curve, and the first-order moment, second-order moment and higher-order moment of the distribution; the first-order moment is the average residence time, and the second-order moment is the variance of the residence time distribution.

7. The simulation analysis method according to claim 6, characterized in that: The viscosity distribution is uniform or non-uniform, specifically: The viscous fluid is a fluid with a constant viscosity characteristic or a fluid with a variable viscosity characteristic; In a single-phase flow, or within each phase of a multiphase flow, a fluid with constant viscosity characteristics obtains a uniform viscosity distribution, while a fluid with variable viscosity characteristics obtains a non-uniform distribution; There are many viscosity change laws for variable viscosity characteristics. Viscosity refers to the viscosity of fluid microclusters; The physical forms of variable viscosity law include time-viscosity law, concentration-viscosity law, temperature-viscosity law, stress-strain-viscosity law and their various combination laws; The mathematical forms of variable viscosity laws include linear equation form, polynomial form, power law form, exponential form, logarithmic form and their various combinations.

8. The simulation analysis method according to claim 6, characterized in that: The residence time distribution is divided into two categories: transient residence time distribution and steady-state residence time distribution: Transient residence time distribution: refers to the time it takes for fluid particles entering the flow field at the same time to reach a specified position, or the residence time when leaving the flow field, which is used to evaluate the mixing effect in the flow field; Steady-state residence time distribution: refers to the residence time of each fluid cluster that reaches a specified position at the same time since entering the flow field, which is used to evaluate the mixing degree of the fluid clusters at the specified position; Simulation of transient residence time distribution: calculated using transient format, including pulse method and step method, where the step method includes rising method and falling method; Simulation of steady-state residence time distribution: The steady-state format is used for calculation, which can obtain the average residence time at any position and the average residence time distribution in the entire flow field, applicable to both single-phase and multiphase flows; the transient format is used for calculation to obtain the average residence time distribution within each steady-state time step.

9. The simulation analysis method according to claim 8, characterized in that: Simulation of transient residence time distribution using the pulse method: a) Enable component transport equations in the flow field, set tracer components, and set report definitions, residual standards, and monitoring outputs at specified locations; b) Injecting the tracer component into the flow field, specifically by setting the tracer component to a concentration value and calculating a time step, then adjusting the tracer concentration back to 0 to complete the pulse injection, where the time step length is required to be less than the residence time of the tracer in the flow field; c) continuing the calculation until the tracer completely passes through the designated location, during which the relationship between the tracer concentration at the designated location and time is obtained; d) Based on the relationship between the tracer change over time and the pulse method tracer concentration relationship, the residence time distribution density function and residence time distribution function are obtained; Simulation of steady-state residence time distribution using the pulse method: A) Derive the average residence time scalar transport equation for steady-state calculations based on the pulse method concentration expression and write a user-defined function; B) Enable the scalar transport equation in the simulation, set the user-defined scalar, and compile and load the user-defined function. C) Set the boundary type, initial conditions, discretization format, and sub-relaxation factor of the scalar transport equation, initialize, and solve the transport equation; D) Based on the mathematical relationship between the scalar value and the average residence time, the solution results are analyzed, synthesized and visualized.

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