Method for measuring liquid film thickness in thin film evaporator internal flow field

Abstract

The present application relates to a method for measuring the thickness of liquid film in the flow field inside a thin film evaporator, which comprises the following steps: carrying out grid processing on the shape of three-dimensional vertical cylindrical tube of the flow field inside the thin film evaporator, dividing it into n three-dimensional vertical sub-cylindrical tubes which are coaxial, mutually nested and have a wall thickness of t; obtaining the gas-liquid two-phase distribution cloud picture of i three-dimensional vertical sub-cylindrical tubes; drawing the variation graph of the number of interface grid nodes with radial distance to find the liquid film area; then drawing the volume fraction average curve graph; in the liquid film area and the interactive area between liquid clusters and liquid film, carrying out equal-width segmentation on the obtained volume fraction average curve according to the segmentation width t; counting the volume fraction average of all grid nodes in the thin film evaporator corresponding to each segmentation width; and multiplying the segmentation width value to obtain the thickness of the liquid film. The method effectively solves the problem that the liquid film with complex spatial distribution cannot be identified and measured in the prior art.

Description

Technical Field

[0001] This invention belongs to the field of thin-film evaporator technology and relates to a method for measuring the thickness of the liquid film in the internal flow field of a thin-film evaporator. Background Technology

[0002] Thin-film evaporators use rotating blades to force fluid materials into a uniform liquid film, resulting in higher heat and mass transfer efficiency. They are used for rapid evaporation, dehydration, and concentration of materials, playing a crucial role in pharmaceuticals, chemicals, textiles, and food industries. The key to rapid evaporation lies in spreading the material into a thin film, increasing the heated and evaporation surface area, thus enabling rapid dehydration. Therefore, the thickness of the liquid film significantly impacts the evaporation efficiency of the thin-film evaporator and is decisive for the continuous and efficient evaporation of the fluid within the evaporator. The high efficiency of scraped-film evaporators relies on spreading the material on the wall surface into a thin film for rapid evaporation at high temperatures. Measuring the liquid film thickness is crucial for determining the evaporator's design.

[0003] Existing studies on the flow field inside thin-film evaporators are all based on the internal in-line scraper blade structure, and have undergone numerous simplifications, resulting in the most classic flow field morphology theory (such as...). Figures 1-2 As shown, it includes a liquid film 2, a circular wave 3, a scraper 4, a rotating shaft 5, and a cylinder wall 6. Figure 4 As shown, this theory simplifies the flow field inside a thin-film evaporator into a relatively regularly shaped circular wave (also known as a liquid slug) at the leading edge of the blades and a thin, uniform liquid film scraped out at the trailing edge of the blades, while neglecting the axial variation in the flow field morphology. Existing studies are all based on this model to explore its liquid film thickness characteristics, residence time characteristics, and heat and mass transfer characteristics, etc.

[0004] However, in practical thin-film evaporators, when applied to the production of high-viscosity materials, the scraper structure is no longer a single, straight scraper along the axial direction. Instead, it is a complex structure with interruptions or various shaped notches along the axial direction. This makes the flow field distribution characteristics inside the evaporator exceptionally complex. The flow field inside the evaporator then differs from the traditional theoretical descriptions, becoming complex and diverse, such as... Figure 3 As shown, it includes liquid clump 1 and liquid film 2, and its simulation results are as follows. Figure 5 As shown, at this point, the liquid film is no longer a uniform and flat film structure, and the liquid clump morphology is no longer the traditional circular wave, but an irregularly distributed shape. Furthermore, there is a very intense exchange behavior between the clumps and the film, manifested as an increased contact area between them, and significant deformation of both the clumps and the film at the contact points. At this point, quantitatively describing the liquid film flow field, such as measuring the liquid film thickness, becomes quite difficult.

[0005] Liquid film thickness measurement can generally be categorized into four types: electrical methods, optical methods, acoustic methods, and digital image processing methods. However, for common evaporator-type equipment, the evaporation structure is surrounded by a heating jacket. The superheated steam or other fluid media within the heating jacket can significantly interfere with the measurement of the liquid film thickness inside the evaporation structure. Whether it's an interventional electrical method or a non-contact optical, acoustic, or image processing method, it's difficult to accurately measure the liquid film thickness inside the evaporation structure through the heating jacket. The flow field inside the evaporator cannot be directly obtained through testing or photography; it can only be simulated through numerical simulation. Using the actual evaporator's structure and process parameters, a fluid simulation model of the evaporator's interior is constructed to obtain the complete internal flow pattern and statistically extract the liquid film thickness.

[0006] Reference 1 (Research on Gas-Liquid Helical Two-Phase Flow Pattern Based on Visualization and Image Processing [J]. Journal of Engineering Thermophysics, 2022, 43(11):2978-2985.) uses a high-speed camera to photograph the gas-liquid two-phase flow under the action of a vortex generator in a vertical tube, and performs batch image processing using MATLAB software to obtain the spatiotemporal distribution law of liquid film thickness in the helical flow pattern. Similar image processing methods can be applied to the simulation field to process the images obtained from the simulation to obtain relevant information such as liquid film thickness, but they are only suitable for processing two-dimensional cross-sectional images such as axial or circumferential sections. Reference 2 (Evaporation Effect on Thickness Distribution for Spin-Coated Films on Rectangular and Circular Substrates. Coatings 2021, 11, 1322.) simulates the process of spin-coating photoresist thin films using CFD technology. The simulation data is then processed to obtain the film thickness distribution map on the substrate. However, this study involves liquid film measurement from a single viewpoint, suitable only for situations where the gas-liquid interface is clear and intact without additional interference. In summary, the above two methods are only suitable for processing two-dimensional images or isoplethograms from a single viewpoint. Furthermore, the internal flow field distribution of the evaporator is extremely complex, making it difficult to characterize the film formation of the entire evaporator using liquid film distribution from a single cross-section or single viewpoint. This poses challenges to the evaluation of the overall evaporator performance and the optimization design of its structure.

[0007] Therefore, it is of great significance to study a new method for measuring the thickness of liquid film in a flow field, in order to solve the problem that existing technologies cannot identify and measure the thickness of liquid films with complex spatial distributions. Summary of the Invention

[0008] To address the problems existing in the prior art, the present invention provides a method for measuring the liquid film thickness in the internal flow field of a thin-film evaporator;

[0009] To achieve the above objectives, the present invention adopts the following solution:

[0010] A method for measuring the liquid film thickness in the internal flow field of a thin-film evaporator, wherein the viscosity of the material in the thin-film evaporator is not less than 50 Pa·s, the shape of the internal flow field of the thin-film evaporator is a three-dimensional vertical cylinder (the outer wall of the three-dimensional vertical cylinder is equivalent to the inner wall of the thin-film evaporator, and the inner wall of the three-dimensional vertical cylinder is equivalent to the cylindrical surface of the rotating shaft in the thin-film evaporator), and the gap width of the thin-film evaporator (i.e., the distance between the blades and the inner wall of the thin-film evaporator) is d, including the following steps:

[0011] (1) Mesh the three-dimensional vertical cylindrical tube;

[0012] (2) Divide the three-dimensional vertical cylindrical tube into n coaxial, interlocking three-dimensional vertical sub-cylinders with a wall thickness of t, where t≤d / 4;

[0013] (3) After obtaining the gas-liquid two-phase distribution cloud map of the i-th three-dimensional vertical sub-cylinder, count the total number B of the grid nodes corresponding to the gas-liquid interface. i And the total number A of mesh nodes of the i-th 3D vertical sub-cylinder. i The percentage of interface points C of the i-th three-dimensional vertical sub-cylinder was calculated. i C i =B i / A i ×100%, i=1,2,...,n, the larger i is, the closer it is to the central axis of the three-dimensional vertical cylinder;

[0014] (4) Using the parameters related to i as the x-axis, C i Using the vertical axis as the ordinate, plot a two-dimensional curve. Find the highest and second-highest peaks on the two-dimensional curve. The latest peak among the highest and second-highest peaks is denoted as peak P. The region between the three-dimensional vertical sub-cylinder corresponding to the starting point of peak P and the outer wall of the three-dimensional vertical cylinder is the liquid film region. The region between the three-dimensional vertical sub-cylinder corresponding to the ending point of peak P and the inner wall of the three-dimensional vertical cylinder is the liquid clump region. The three-dimensional vertical sub-cylinder corresponding to the starting point of peak P, the three-dimensional vertical sub-cylinder corresponding to the ending point of peak P, and the three-dimensional vertical sub-cylinder between these two three-dimensional vertical sub-cylinders together constitute the interaction region between the liquid clump and the liquid film.

[0015] (5) Calculate the volume fraction of all grid nodes in the gas-liquid two-phase distribution cloud map and perform mean processing to obtain the mean volume fraction D of all grid nodes of the i-th three-dimensional vertical sub-cylinder. ii = 1, 2, ..., n, the larger i is, the closer it is to the central axis of the three-dimensional vertical cylinder;

[0016] (6) Using the parameters related to i as the x-axis, D i Plot the volume fraction mean curve with the vertical axis as the ordinate;

[0017] (7) In the liquid film region and the interaction region between the liquid clump and the liquid film, the average volume fraction curve obtained in step (6) is divided into equal widths according to the division width t. The average volume fraction of all grid nodes in the thin film evaporator corresponding to each division width is calculated, and then multiplied by the division width value to obtain the thickness of the liquid film.

[0018] In steps (4) and (6), the parameter related to i is the distance between the outer wall of the i-th three-dimensional vertical sub-cylinder and the outer wall of the 1st three-dimensional vertical sub-cylinder.

[0019] As a preferred technical solution:

[0020] The method for measuring the liquid film thickness in the internal flow field of the thin-film evaporator as described above, where t≤d / 16.

[0021] As described above, the method for measuring the liquid film thickness in the internal flow field of a thin-film evaporator is t>s, where s is the maximum length of the line connecting any two points in the smallest grid node. This ensures that the three-dimensional vertical sub-cylinder wraps around the smallest grid node, avoiding data loss. There are many shapes of grid nodes. Two-dimensional grid nodes are commonly triangular and quadrilateral grids, while three-dimensional grid nodes are commonly tetrahedral, hexahedral, prism, pyramid, and polyhedral grids.

[0022] The method for measuring the liquid film thickness in the internal flow field of the thin-film evaporator described above uses grid nodes that correspond to pixels in the gas-liquid two-phase distribution cloud map.

[0023] As described above, the method for measuring the liquid film thickness in the internal flow field of a thin-film evaporator has corresponding volume fraction data for each pixel in the gas-liquid two-phase distribution cloud map. The volume fraction is the ratio of the volume of the liquid phase at each network node to the total volume of the liquid phase and the gas phase.

[0024] As described above, in the method for measuring the liquid film thickness in the internal flow field of the thin film evaporator, the grid node corresponding to the gas-liquid interface in step (3) is a pixel with a volume fraction of 0.4 to 0.6 in the gas-liquid two-phase distribution cloud map.

[0025] As described above, in the method for measuring the liquid film thickness in the internal flow field of the thin film evaporator, step (3) is a grid node corresponding to the gas-liquid interface, which is a pixel with a volume fraction of 0.45 to 0.55 in the gas-liquid two-phase distribution cloud map.

[0026] The method for measuring the liquid film thickness in the internal flow field of the thin-film evaporator described above is used, where the viscosity of the material in the thin-film evaporator is 50–20000 Pa·s.

[0027] The principle of this invention is:

[0028] (I) The principle of dividing the liquid film region is as follows:

[0029] This invention uses CFD simulation to recreate the flow inside an evaporator and identifies the internal flow field structure based on the simulation results. By processing and analyzing the calculation results of a large number of grid nodes in the three-dimensional flow field model, the interaction area between liquid clumps and liquid films is obtained, and then the distribution area of ​​liquid films and liquid clumps is obtained, thus achieving accurate division of the flow field state.

[0030] To delineate the liquid clump and liquid film regions, the location of their gas-liquid interface must first be identified. Gas-liquid two-phase distribution cloud maps can be used to capture the gas-liquid interface. This is relevant for the internal flow field of the thin-film evaporator (a three-dimensional vertical cylindrical tube, such as...). Figure 7 After meshing (as shown), the volume fraction of each mesh node can be determined based on the gas-liquid two-phase distribution cloud map. Since the volume fraction is an important parameter expressing the proportion of gas and liquid phases in a mesh node, the type of each mesh node (gas phase, liquid phase, gas-liquid interface) can be determined. Figure 6 The red area has a volume fraction of 1 for all grid nodes, indicating that these grid nodes are filled with liquid phase; the blue area has a volume fraction of 0, indicating that all grid nodes in this area are filled with gas phase; and the transition color area indicates that these grid nodes contain both gas and liquid phases, with a volume fraction between 0 and 1, and the color changes with the volume fraction value.

[0031] In the early stages of the research, the inventors studied the gas-liquid two-phase distribution cloud maps and the number of interface grid nodes (i.e., the grid nodes corresponding to the gas-liquid interface) as a function of radial distance for different internal flow fields of thin-film evaporators (see [link to research]). Figure 7 The variation graph of ) is as follows:

[0032] Example 1: The internal flow field of a thin-film evaporator has a simple structure, and the liquid film is a flat liquid film without liquid clumps; Figure 8 The left image is Figure 7The axial cross-section shows the gas-liquid two-phase distribution cloud map. On the left, near the wall, is the liquid film with a volume fraction of 1, displayed in red; on the right, away from the wall, is the gas phase with a volume fraction of 0, displayed in blue. These two parts lack a gas-liquid interface, therefore the number of interface grid nodes is 0. The middle transitional color area represents the gas-liquid interface. In this part, the number of interface grid nodes is not zero, and because the gas-liquid interface is vertically distributed, there are more interface grid nodes when counting along the radial direction. Therefore, after counting the number of interface grid nodes at three different radial positions, the following results are obtained: Figure 8 The graph on the right shows the variation of the number of interface grid nodes with radial distance. There is a peak in this graph, and the location of the peak is the gas-liquid interface.

[0033] Example 2: The internal flow field structure of a thin-film evaporator is relatively complex, with the liquid film consisting of a flat liquid film portion and a liquid film protrusion portion, and no liquid clumps; Figure 9 The left image is Figure 7 A cloud map showing the gas-liquid two-phase distribution along the axial cross section. Figure 9 The right figure shows the variation of the number of interface mesh nodes with radial distance. There is a peak in this graph, and the position of the peak is near the surface of the liquid film.

[0034] Example 3: The internal flow field of a thin-film evaporator has a complex structure, with the liquid film consisting of flat liquid film parts and liquid film protrusion parts, as well as liquid clumps; Figure 10 The left image is Figure 7 A cloud map showing the gas-liquid two-phase distribution along the axial cross section. Figure 10 The right figure shows the variation of the number of interface mesh nodes with radial distance. There are two peaks in this graph. One peak corresponds to the position near the liquid film surface, and the other peak corresponds to the interaction region between the liquid clump and the liquid film.

[0035] The internal flow field of a thin-film evaporator for high-viscosity materials has a complex structure. The liquid film includes a flat liquid film portion and a liquid film protrusion portion, as well as liquid clumps. Therefore, the variation spectrum of the number of interface grid nodes with radial distance will have two peaks. One peak corresponds to the position near the liquid film surface, and the other peak corresponds to the interaction region between the liquid clumps and the liquid film. This characteristic can be used to determine the interaction region between the liquid clumps and the liquid film in the internal flow field of the thin-film evaporator for high-viscosity materials.

[0036] However, the internal flow field structure of thin-film evaporators for high-viscosity materials is complex. Figure 7A cylindrical surface corresponding to a certain radial distance is essentially composed of multiple vertical strips. The number of interface mesh nodes for different vertical strips may not be the same, and may even differ significantly. Therefore, the number of interface mesh nodes for a single vertical strip cannot represent the total number of interface mesh nodes for that radial distance. Instead, the number of interface mesh nodes for the cylindrical surface corresponding to that radial distance should be used to represent the total number of interface mesh nodes for that radial distance. This ensures more accurate and reliable data. Assuming there are a total of 1100 mesh nodes on the cylindrical surface corresponding to that radial distance, the volume fraction data corresponding to different mesh nodes are shown in the table below:

[0037] Volume fraction 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Number of grid nodes 100 100 100 100 100 100 100 100 100 100 100

[0038] If only the grid nodes with a volume fraction of 0.5 are considered to be gas-liquid interfaces, and the other grid nodes are simply liquid or gas phases, then the number of interface grid nodes on this cylindrical surface is 100; if all grid nodes with a volume fraction between 0.4 and 0.6 are considered to be gas-liquid interfaces, then the number of interface grid nodes on this cylindrical surface is 300.

[0039] However, considering the internal flow field of a thin-film evaporator for high-viscosity materials as composed of multiple cylindrical surfaces, determining the number of interface mesh nodes for each cylindrical surface individually would result in an excessive number of cylindrical surfaces, making statistical calculations difficult. Therefore, the inventors attempted to divide the internal flow field (a three-dimensional vertical cylinder) of the thin-film evaporator into n coaxial, interconnected three-dimensional vertical sub-cylinders, each with a wall thickness of t, and then determine the number of interface mesh nodes for each of these sub-cylinders. However, this introduced new problems. On the one hand, if the internal flow field of the thin-film evaporator for high-viscosity materials is very large, the number of mesh nodes would also be excessive, leading to a large number of interface mesh nodes for each of the three-dimensional vertical sub-cylinders, making it difficult to draw the interface mesh. The graph shows the variation of the number of surface mesh nodes with radial distance. On the other hand, during the meshing process, the mesh size has a fixed growth rate along the radial direction. The change in size will lead to different total number of nodes in each cylinder, resulting in a large difference in the number of interface mesh nodes in different cylinders. This means that simply counting the number of interface mesh nodes in each three-dimensional vertical sub-cylinder cannot accurately measure the number of interface mesh nodes in that area. Therefore, the inventors proposed to use "the percentage of the number of interface mesh nodes in a certain three-dimensional vertical sub-cylinder to the total number of mesh nodes in a certain three-dimensional vertical sub-cylinder" instead of "the number of interface mesh nodes in a certain three-dimensional vertical sub-cylinder".

[0040] (II) Measurement of liquid film thickness:

[0041] The method of dividing the volume fraction mean curve into equal-width segments within the liquid film and interaction region is employed, such as... Figure 11The average volume fraction of all grid nodes is calculated within the thin-walled cylinder formed by each sampling width, and then multiplied by the sampling width value to obtain the weighted thickness under a single width; then the weighted thickness values ​​under all widths are summed to finally obtain the average weighted thickness of the liquid film;

[0042] The liquid film thickness is calculated using the following formula (1), and the calculation diagram is shown below. Figure 11 ;

[0043]

[0044] Where h is the film thickness, L i vf is the sampling width for the average volume fraction. i Given the average volume fraction of material within a specific sampling width, the film thickness calculation in this invention is sampled to the exchange zone, thereby yielding the average liquid film thickness of the liquid film zone and the exchange zone.

[0045] Furthermore, the calculated average liquid film thickness can also be used to calculate the liquid film volume and proportion in the evaporator. The liquid film volume is calculated using formula (2), and the liquid film volume proportion is calculated using formula (3):

[0046] v l =2*π*r*h*l (2);

[0047] c = v l / v (3);

[0048] Among them, v l Let r be the volume of the liquid film in the evaporator, h be the radius of the evaporator, l be the height of the evaporator, c be the proportion of the liquid film volume, and v be the total volume of the material.

[0049] Beneficial effects

[0050] (1) The method for measuring the liquid film thickness in the internal flow field of the thin film evaporator of the present invention is not based on data analysis of two-dimensional images, but on three-dimensional data processing, which can provide a true picture of the flow field inside the entire evaporator.

[0051] (2) The method for measuring the thickness of liquid film in the internal flow field of the thin film evaporator of the present invention first distinguishes the liquid film region, the liquid clump region and the liquid film-liquid clump interaction region, and then identifies the morphology of the liquid film according to the region of liquid film distribution and measures the thickness of the liquid film, which effectively solves the problem in the prior art that the liquid film with a complex spatial distribution cannot be identified and its thickness measured. Attached Figure Description

[0052] Figures 1-2 This is a schematic diagram of the theoretical distribution of the flow field morphology;

[0053] Figure 3This is a schematic diagram of the three-dimensional flow field structure inside the thin-film evaporator in this invention;

[0054] Figure 4 This is a cloud map showing the axial local gas-liquid two-phase distribution in existing technology.

[0055] Figure 5 This is a cloud map showing the axial local gas-liquid two-phase distribution inside the thin-film evaporator in this invention.

[0056] Figure 6 Schematic diagrams of different liquid film morphologies; (a) a flat liquid film, (b) a liquid film with air bubbles attached to the cylinder wall inside, and (c) a wavy liquid film.

[0057] Figure 7 This is a schematic diagram of radial distance; the arrows indicate the direction of scraper movement and the direction of circular wave rolling.

[0058] Figure 8 This is a schematic diagram of Example 1; where the arrow indicates the direction of the scraper's movement.

[0059] Figure 9 This is a schematic diagram of Example 2; where the arrow indicates the direction of the scraper's movement.

[0060] Figure 10 This is a schematic diagram of Example 3; where the arrow indicates the direction of the scraper's movement.

[0061] Figure 11 The principle for solving liquid film thickness under complex flow fields;

[0062] Figures 12-13 This is a graph showing the average volume fraction of the liquid film in a real thin-film evaporator.

[0063] Among them, 1-liquid mass, 2-liquid film, 3-circular wave, 4-scraper, 5-rotating shaft, 6-cylinder wall. Detailed Implementation

[0064] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0065] The specific steps for measuring the liquid film thickness in the internal flow field of a thin-film evaporator are as follows:

[0066] (1) The viscosity of the material in the thin-film evaporator is 50 to 20000 Pa·s, the shape of the internal flow field of the thin-film evaporator is a three-dimensional vertical cylindrical barrel (the outer side wall of the three-dimensional vertical cylindrical barrel is equivalent to the inner wall of the thin-film evaporator, and the inner side wall of the three-dimensional vertical cylindrical barrel is equivalent to the cylindrical surface of the rotating shaft in the thin-film evaporator), and the three-dimensional vertical cylindrical barrel is subjected to meshing processing;

[0067] (2) The three-dimensional vertical cylindrical barrel is divided into n coaxial, mutually nested three-dimensional vertical sub-cylindrical barrels each with a wall thickness of t, where s<t≤d / 16; d is the gap width of the thin-film evaporator (that is, the distance between the blade and the inner wall of the thin-film evaporator), and s is the maximum value of the length of the connecting line between any two points in the minimum grid nodes;

[0068] (3) After acquiring the gas-liquid two-phase distribution cloud map of the i-th three-dimensional vertical sub-cylindrical barrel, count the total number B of grid nodes corresponding to the gas-liquid interface i and the total number A of grid nodes of the i-th three-dimensional vertical sub-cylindrical barrel i , and calculate to obtain the interface point proportion C of the i-th three-dimensional vertical sub-cylindrical barrel i , C i =B i / A i ×100%, i=1,2,...,n, where a larger i indicates a position closer to the central axis of the three-dimensional vertical cylindrical barrel;

[0069] Herein, the grid nodes correspond to the pixel points of the gas-liquid two-phase distribution cloud map; each pixel point of the gas-liquid two-phase distribution cloud map has corresponding volume fraction data, and the volume fraction is the ratio of the volume of the liquid phase at each grid node to the total volume of the liquid phase and the gas phase; the grid nodes corresponding to the gas-liquid interface are the pixel points with a corresponding volume fraction of 0.45 to 0.55 in the gas-liquid two-phase distribution cloud map;

[0070] (4) Using the parameter related to i as the abscissa and C i as the ordinate, draw a two-dimensional curve graph, find the highest peak and the second highest peak on the two-dimensional curve graph, the peak that appears later among the highest peak and the second highest peak is denoted as peak P, the region between the three-dimensional vertical sub-cylindrical barrel corresponding to the starting point of peak P and the outer side wall of the three-dimensional vertical cylindrical barrel is a liquid film region, the region between the three-dimensional vertical sub-cylindrical barrel corresponding to the end point of peak P and the inner side wall of the three-dimensional vertical cylindrical barrel is a liquid mass region, and the three-dimensional vertical sub-cylindrical barrel corresponding to the starting point of peak P, the three-dimensional vertical sub-cylindrical barrel corresponding to the end point of peak P, and the three-dimensional vertical sub-cylindrical barrels between these two three-dimensional vertical sub-cylindrical barrels jointly form an interaction region of liquid mass and liquid film; the parameter related to i is the distance between the outer side wall of the i-th three-dimensional vertical sub-cylindrical barrel and the outer side wall of the first three-dimensional vertical sub-cylindrical barrel;

[0071] (5) Calculate the volume fraction of all grid nodes in the gas-liquid two-phase distribution cloud map and perform mean processing to obtain the mean volume fraction D of all grid nodes of the i-th three-dimensional vertical sub-cylinder. i i = 1, 2, ..., n, the larger i is, the closer it is to the central axis of the three-dimensional vertical cylinder;

[0072] (6) Using the parameters related to i as the x-axis, D i Plot the volume fraction mean curve with the vertical axis as the ordinate;

[0073] (7) In the liquid film region and the interaction region between the liquid clump and the liquid film, the average volume fraction curve obtained in step (6) is divided into equal widths according to the division width t. The average volume fraction of all grid nodes in the thin film evaporator corresponding to each division width is calculated, and then multiplied by the division width value to obtain the thickness of the liquid film.

[0074] Using the measurement method of this invention, when the zero-shear viscosity of the material in the thin-film evaporator is 38 Pa·s, this invention obtains a curve of the average volume fraction of the liquid film in the thin-film evaporator with d = 1 mm (e.g., Figure 12 ), t=0.05mm, its liquid film region and the interaction region between the liquid cluster and the liquid film are distributed in the range of 0~1.1mm, and the thickness of the liquid film is calculated to be 0.86mm.

[0075] Using the measurement method of this invention, when the zero-shear viscosity of the material in the thin-film evaporator is 4000 Pa·s, this invention obtained the average volume fraction curve of the liquid film in a thin-film evaporator with d = 4.5 mm (e.g., Figure 13 ), t=0.25mm, its liquid film region and the interaction region between the liquid cluster and the liquid film are distributed in the range of 0~4.9mm, and the thickness of the liquid film is calculated to be 3.6mm.

Claims

1. A method for measuring the liquid film thickness in the internal flow field of a thin-film evaporator, wherein the viscosity of the material in the thin-film evaporator is not less than 50 Pa·s, the shape of the internal flow field of the thin-film evaporator is a three-dimensional vertical cylindrical tube, and the gap width of the thin-film evaporator is d, characterized in that... Includes the following steps: (1) Mesh the three-dimensional vertical cylindrical tube; (2) Divide the three-dimensional vertical cylindrical tube into n coaxial, interlocking three-dimensional vertical sub-cylinders with a wall thickness of t, where t≤d / 4; (3) After obtaining the gas-liquid two-phase distribution cloud map of the i-th three-dimensional vertical sub-cylinder, count the total number B of the grid nodes corresponding to the gas-liquid interface. i And the total number A of mesh nodes of the i-th 3D vertical sub-cylinder. i The percentage of interface points C of the i-th three-dimensional vertical sub-cylinder was calculated. i C i =B i / A i ×100%, i=1,2,...,n, the larger i is, the closer it is to the central axis of the three-dimensional vertical cylinder; (4) Using the parameters related to i as the x-axis, C i Using the vertical axis as the ordinate, plot a two-dimensional curve. Find the highest and second-highest peaks on the two-dimensional curve. The latest peak among the highest and second-highest peaks is denoted as peak P. The region between the three-dimensional vertical sub-cylinder corresponding to the starting point of peak P and the outer wall of the three-dimensional vertical cylinder is the liquid film region. The region between the three-dimensional vertical sub-cylinder corresponding to the ending point of peak P and the inner wall of the three-dimensional vertical cylinder is the liquid clump region. The three-dimensional vertical sub-cylinder corresponding to the starting point of peak P, the three-dimensional vertical sub-cylinder corresponding to the ending point of peak P, and the three-dimensional vertical sub-cylinder between these two three-dimensional vertical sub-cylinders together constitute the interaction region between the liquid clump and the liquid film. (5) Calculate the volume fraction of all grid nodes in the gas-liquid two-phase distribution cloud map and perform mean processing to obtain the mean volume fraction D of all grid nodes of the i-th three-dimensional vertical sub-cylinder. i ; (6) Using the parameters related to i as the x-axis, D i Plot the volume fraction mean curve with the vertical axis as the ordinate; (7) In the liquid film region and the interaction region between the liquid clump and the liquid film, the average volume fraction curve obtained in step (6) is divided into equal widths according to the division width t. The average volume fraction of all grid nodes in the thin film evaporator corresponding to each division width is calculated, and then multiplied by the division width value to obtain the thickness of the liquid film. In steps (4) and (6), the parameter related to i is the distance between the outer wall of the i-th three-dimensional vertical sub-cylinder and the outer wall of the 1st three-dimensional vertical sub-cylinder.

2. The method for measuring the liquid film thickness in the internal flow field of a thin-film evaporator according to claim 1, characterized in that, t≤d / 16.

3. The method for measuring the liquid film thickness in the internal flow field of a thin-film evaporator according to claim 2, characterized in that, t>s, where s is the maximum length of the line connecting any two points in the smallest grid node.

4. The method for measuring the liquid film thickness in the internal flow field of a thin-film evaporator according to claim 1, characterized in that, The grid nodes correspond to the pixels in the gas-liquid two-phase distribution cloud map.

5. The method for measuring the liquid film thickness in the internal flow field of a thin-film evaporator according to claim 4, characterized in that, Each pixel in the gas-liquid two-phase distribution cloud map has corresponding volume fraction data. The volume fraction is the ratio of the volume of the liquid phase at each network node to the total volume of the liquid phase and the gas phase.

6. The method for measuring the liquid film thickness in the internal flow field of a thin-film evaporator according to claim 5, characterized in that, Step (3) The grid nodes corresponding to the gas-liquid interface are the pixels with a volume fraction of 0.4 to 0.6 in the gas-liquid two-phase distribution cloud map.

7. The method for measuring the liquid film thickness in the internal flow field of a thin-film evaporator according to claim 6, characterized in that, Step (3) The grid nodes corresponding to the gas-liquid interface are the pixels with a volume fraction of 0.45 to 0.55 in the gas-liquid two-phase distribution cloud map.

8. The method for measuring the liquid film thickness in the internal flow field of a thin-film evaporator according to claim 1, characterized in that, The viscosity of the material in the thin-film evaporator is 50–20000 Pa·s.

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