CFD-based viscous fluid falling film flow characteristic simulation analysis method

Abstract

The invention relates to the field of chemical engineering, and discloses a CFD-based viscous fluid falling film flow characteristic simulation analysis method. The method provided by the invention can be used for carrying out simulation calculation on the flowing process of the viscous fluid to the falling film along the falling film flow field structure, so that flow field information such as speed distribution, phase distribution, component distribution, temperature distribution, viscosity distribution and residence time distribution in the falling film flowing process can be effectively obtained; and reference and guidance are provided for analysis of viscous fluid falling film flow characteristics and design, amplification and process optimization of industrial devices. The simulation analysis method provided by the invention has universality and also has reference value for other flow systems taking viscous fluid as a working medium.

Description

Technical Field

[0001] The 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 have significant viscous stress and are prone to shear deformation when subjected to force, including but not limited to various organic substances, polymer solutions or melts. Due to their large dynamic viscosity, viscous fluids need to overcome greater resistance during flow, resulting in more flow behaviors that are different from common fluids.

[0003] Falling film flow refers to the flow process in which a fluid moves downward along the falling film flow field structure under the action of its own gravity to form a liquid film. Depending on the physical properties of the fluid and the 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 application values. For example, it is used for devolatilization, absorption, condensation, evaporation, etc. in chemical production; it is used in heat exchangers to improve heat transfer efficiency and reduce energy consumption; it is used for coating, spraying and packaging in food processing, etc.

[0004] CFD stands for Computational Fluid Dynamics. CFD uses the theory of numerical solution of partial differential equations to discretize the control equations in fluid mechanics, conduct numerical experiments, computer simulations and analytical research on various problems in fluid mechanics to solve various practical problems. CFD simulation usually includes pre-processing, solution, post-processing and other steps.

[0005] Flow characteristics refer to the various physical properties and behavioral laws exhibited during the flow of fluid, 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] At present, the commonly used methods for falling film flow analysis are cold model experiment and numerical simulation. In cold model experiments, simulated media are used to simulate fluid movement and observe flow behavior, but there are generally defects such as difficulty in implementing experiments, limited observation methods, low flow field resolution, large data test errors, and long experimental cycles. CFD simulation is a research method that is not yet popular, especially in the field of viscous fluid falling film flow, and there are defects such as insufficient research foundation, insufficient research, and few research cases.

[0007] Through the search of the prior art, it is found that there is no comprehensive report on the simulation and analysis method of viscous fluid falling film flow characteristics based on CFD. Therefore, there is an urgent need for a method that can quickly and 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, so as to provide reference and guidance for analyzing the falling film flow characteristics of viscous fluids and the design, scale-up and process optimization of industrial equipment. Summary of the invention

[0008] In order to solve the above technical problems, the present invention provides a simulation analysis method of viscous fluid falling film flow characteristics based on CFD. The present invention can simulate and calculate the flow process of viscous fluid along the falling film flow field structure to the falling film, and can effectively obtain the flow field information in the falling film flow process, which can provide reference and guidance for analyzing the viscous fluid falling film flow characteristics and the design, amplification and process optimization of industrial equipment.

[0009] The specific technical scheme of the present invention is: a simulation and analysis method of falling film flow characteristics of viscous fluid based on CFD. The present invention uses CFD to pre-process, solve and post-process the falling film flow field of a given structure for the viscous fluid to flow downward along the falling film flow field 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, 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, integrated 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 supporting the flow of viscous fluid, a film distribution structure providing assistance for the fluid to enter the falling film element, a positioning structure fixing the falling film element, and a component structure changing 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 structural dimensions of the falling film flow field and perform geometric modeling on 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 calculation domain: Based on the three-dimensional 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 according to the device structure size.

[0018] 1.3) Meshing the computational domain: Use meshing software including but not limited to ANSYS Mesh, Fluent Meshing, ICEM CFD, Gambit, etc. to spatially discretize the established computational domain model, and use structured or unstructured meshing methods to divide the two-dimensional or three-dimensional grids to obtain grids of different sizes and numbers.

[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 independence of the time step is verified 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 has significant viscous stress and is prone to shear deformation when subjected to force, including but not limited to various organic substances, polymer solutions or melts. The dynamic viscosity value of the viscous fluid can be a constant steady fluid or a constant unsteady fluid.

[0022] Preferably, the solving 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 property parameters or physical property equations, selecting a boundary type and initial conditions, selecting a coupling method and a 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 as the solver type, and select steady-state or transient format as the time format.

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

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

[0027] 2.4) Select the energy equation: Choose to turn on or off the energy equation depending 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 physical property database provided by the software, or express the physical property information with custom mathematical equations and bring it into the solving software.

[0030] 2.7) Select boundary type and initial conditions: According to 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, axis, etc., and set the corresponding initial conditions for each boundary type.

[0031] 2.8) Select coupling mode and discrete format: According to experience and the convergence of the actual calculation process, SIMPLE, SIMPLEC, PISO, Coupled and other coupling methods are selected for pressure-velocity coupling mode, and appropriate discrete formats are selected for spatial discretization of physical quantities such as gradient, pressure, momentum, volume fraction, energy, and component.

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

[0033] 2.10) Set reports, residuals and monitoring outputs: According to actual needs, set the information and locations that need to be detected 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 the 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 and the like; the heating method includes but is not limited to uniform heating and non-uniform heating, wherein 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 distribution or non-uniform distribution, specifically:

[0049] Viscous fluids are fluids with constant viscosity characteristics or fluids with variable viscosity characteristics;

[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. At this time, 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 the time it takes for fluid particles that enter the flow field at the same time to reach the specified position, or the residence time they have 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, where the step method includes the rise method and the decline 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 each fluid micro-cluster that reaches the specified position at the same time since entering the flow field, which is used to evaluate the degree of mixing of the fluid micro-cluster 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 in each steady-state time step can be obtained.

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

[0058] a) Open the component transport equation in the flow field, set the tracer components, set the report definition, residual standard and monitoring output at the specified location;

[0059] b) Injecting the tracer component into the flow field in a very short time, specifically setting the tracer component to a concentration value and calculating a time step, and then adjusting the tracer concentration to 0 again 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 position, during which the relationship between the tracer concentration at the designated position and time is obtained;

[0061] d) According to the relationship between the tracer and the 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 under steady-state calculation 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, 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 beneficial effects of the present invention are:

[0068] (1) The present invention simulates and calculates the falling film flow field of viscous fluid, and 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 physical properties and flow conditions of the fluid, which 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 and 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 also has reference value for other flow systems using viscous fluid as the working medium. BRIEF DESCRIPTION OF THE DRAWINGS

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

[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 This is a schematic diagram of grid division;

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

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

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

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

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

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

[0080] Fig.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] Fig.11 Simulate workflow diagram for transient residence time distribution;

[0082] Fig.12 Simulate the workflow diagram for steady-state residence time distribution;

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

[0084] In order to make the technical solutions, features and beneficial effects of the present invention more clearly understood, the following is an illustration of specific implementation methods, but the implementation methods of the present invention are not limited to the scope of the examples.

[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 25mm and the length is 100mm. Based on experience, the calculation domain width is selected to be 12 to 30mm 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 3mm.

[0089] 2) Since the flow field structure is simple, 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 the ICEM CFD software for meshing. According to the flow characteristics of the falling film unilateral wall flow to form a velocity gradient, use an encrypted grid near the wall to obtain high-resolution flow field information. In ICEM CFD, the grid is divided in a way that the nodes are connected to each other. Set the number of inlet annular seam 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 grids, and a fluid domain with a central angle of 30° can be used instead of the entire fluid domain for calculation, and the simulation results remain unchanged, see Figure 3 .

[0091] 3) The grids that have been initially divided need to be verified for grid independence. The initially divided grids are encrypted and scaled several times to obtain several sets of grids of varying numbers, such as 240,000, 360,000, 480,000, and 600,000. They are imported into Fluent, and simulation calculations are performed under the same conditions. The results (such as the position of the free surface of the two-phase flow) are compared, and the minimum critical value of the number of grids that continues to increase the grid density without a significant change in the calculation results is 480,000. That is, the range of the number of optional grids that can ensure both calculation accuracy and save calculation overhead under 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 by the grid independence verification will continue to be calculated under the same conditions, and the results obtained at the same flow moment under transient calculations with different time steps will be observed. The maximum time step critical value at which the calculation results no longer change significantly when the time step is further reduced will be screened out, and finally the optional time step range that is not affected by the calculation time step and saves calculation overhead under the current simulation is less than or equal to 0.001.

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

[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 the VOF two-phase flow model, with the main phase being the viscous fluid phase and the secondary phase being the gas phase; use the implicit volume formula to calculate the volume fraction parameters; set the two-phase interaction surface tension coefficient to 0.02 N / m according to actual conditions, and add a continuous surface tension model.

[0096] 7) Select the viscous laminar flow model; turn on the energy equation; 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 the physical parameters such as density, specific heat, thermal conductivity, molecular weight and standard state 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, set the equivalent flow rate after conversion to 15kg / h, the initial gauge pressure to zero, the inlet fluid temperature to 260℃, 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℃, and the secondary phase reflux volume fraction to 1; set the other 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 modes of 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 in the range of 0 to 1 based on experience. In the initial stage of calculation, the default relaxation factors of 0.3, 1, 1, 0.7, 0.5, etc. are used for calculation. If the convergence is poor, the relaxation factor can be appropriately reduced. In this embodiment, the flow conditions are easy to form a film, and the falling film flow field information can be obtained using the default influence factor.

[0101] 12) In order to better monitor the convergence, the import and export flow monitoring method is used to assist in judgment. First, the mass flow report along the surface integral is used to define the import and export mass flow monitoring, and then the drawing report is defined to output the import and export mass flow data in real time in the console and drawing window. The calculation can be judged whether the calculation is converged based on whether the import and export flows are stable and equal. In addition, the volume fraction cloud map at the central axis plane is set in the "contour map" option, and the volume fraction cloud map is set to be output in picture format to assist in monitoring whether the calculation process is normal.

[0102] 13) Initialize the fluid domain, use standard initialization with the inlet as the starting point, and assign the flow field an initial table pressure value of 0Pa, a velocity of 0, a gas phase volume fraction of 1, etc.

[0103] 14) Set the iteration parameters for solution calculation. In this embodiment, the steady-state calculation part sets the number of iterations to 50,000 and the report interval to 1; the unsteady-state calculation part sets the fixed time advancement format, 10,000 time steps, time step length 0.001s, and the maximum number of iterations per time step to 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 a velocity cloud map on the axial plane as shown in Figure 4 , Figure 6 As shown; create a phase distribution diagram as shown Figure 5 , Figure 7 As shown; create a temperature distribution diagram 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 a three-dimensional diagram of the steady-state residence time distribution as shown Fig. 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 Fig.10 shown.

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

Claims

1. A CFD-based simulation and analysis method for the falling film flow characteristics of viscous fluids, characterized by: For the downward falling film flow of viscous fluid along the falling film flow field structure, CFD is used 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: 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, 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 supporting the flow of viscous fluid, a film distribution structure providing assistance for the fluid to enter the falling film element, a positioning structure fixing the falling film element, and a component structure changing the surface structure of the falling film element.

2. The method according to claim 1, characterized in that: 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 property parameters or physical property equations, selecting a boundary type and initial conditions, selecting a coupling method and a discrete format, setting a relaxation factor, setting a report residual and monitoring output, initializing, and setting iteration parameters.

3. The simulation analysis method according to claim 1, characterized in that: 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 establish a two-dimensional or three-dimensional model of the device according to the structural dimensions; 1.2) Determine the calculation 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 size; 1.3) Mesh the computational domain: Use meshing software to spatially discretize the established computational domain model, and use structured or unstructured meshing methods to divide the 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 set of grids with sufficient simulation accuracy; 1.5) Verify the independence of the time step: For simulation calculations using the transient time format, the independence of the time step is verified 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, characterized in that: 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 the single-precision or double-precision solver, the pressure-based solver or the density-based solver, and the steady-state or transient time format; 2.2) Select single-phase or multiphase flow model: Select single-phase or multiphase flow model according to the number of phases and flow characteristics in the flow system and physical scenario; 2.3) Select laminar or turbulent model: Select laminar or turbulent calculation according to the falling film Reynolds number; 2.4) Select the energy equation: Choose to turn on or off the energy equation based on whether the energy problem needs 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 physical property database provided by the software, 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: Select at least one boundary type from velocity inlet, mass flow inlet, pressure inlet, pressure outlet, velocity outlet, mass outlet, wall, symmetry plane, and axis according to the actual boundary type of the fluid system, and set the corresponding initial conditions for each boundary type; 2.8) Select coupling mode and discrete format: According to the convergence of the actual calculation process, select the pressure-velocity coupling mode and the physical space discrete format; 2.9) Set the relaxation factor: To adjust the convergence of the discrete equation, set the sub-relaxation factor respectively; 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 according to actual conditions; 2.12) Set iteration parameters: Set the number of iterations or specify the iteration parameters required for transient calculations.

5. The simulation analysis method according to claim 1, characterized in that: Step 3) specifically includes: 3.1) Analyze the flow field information; 3.2) Synthesize the 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.

6. The simulation analysis method according to claim 1 or 5, 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.

7. The simulation analysis method according to claim 6, 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 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 forms include single-side heating, double-side heating, and volume heat source; the heating methods include uniform heating and non-uniform heating, wherein the non-uniform heating includes spatial non-steady heating and temporal non-steady 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.

8. The simulation analysis method according to claim 7, characterized in that: The viscosity distribution is uniform distribution or non-uniform distribution, specifically: Viscous fluids are fluids with constant viscosity characteristics or fluids with variable viscosity characteristics; 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 rules for variable viscosity characteristics. Viscosity generally refers to the viscosity of fluid micro-groups; 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.

9. The simulation analysis method according to claim 7, 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 that enter the flow field at the same time to reach the specified position, or the residence time when they leave 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 micro-group that reaches the specified position at the same time since entering the flow field, which is used to evaluate the mixing degree of the fluid micro-group 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 to calculate the average residence time at any position and the average residence time distribution in the entire flow field, which is applicable to both single-phase flow and multiphase flow; the transient format is used to calculate the average residence time distribution in each steady-state time step.

10. The simulation analysis method according to claim 9, 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, setting the tracer component to a concentration value and calculating a time step, and then adjusting the tracer concentration to 0 again to complete the pulse injection, requiring the time step length to be less than the residence time of the tracer in the flow field; c) continue calculating until the tracer completely passes through the designated position, during which the relationship between the tracer concentration and time at the designated position is obtained; d) Based on the relationship between the tracer and 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 under steady-state calculation 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, 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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