Identification method of liquid film state in the flow field inside thin film evaporator

Abstract

The invention relates to a method for identifying a liquid film state in an internal flow field of a thin film evaporator. The method comprises the following steps: gridding the shape of a three-dimensional vertical cylindrical tube in the internal flow field of the thin film evaporator, dividing the tube into n coaxial, mutually nested three-dimensional vertical sub-cylindrical tubes with a wall thickness of t, obtaining a gas-liquid two-phase distribution cloud map of i three-dimensional vertical sub-cylindrical tubes, drawing a graph of the change of the number of interface grid nodes with radial distance to find a liquid film area, and then drawing a volume fraction mean curve map to identify the liquid film state. The method of the invention can distinguish between a liquid film area, a liquid mass area and a liquid film and liquid mass interaction area, and on this basis can identify the liquid film morphology, can reflect all the characteristics of the liquid film in the evaporator, and can realize a quantitative description of the characteristics to a certain extent, thereby providing a basis for the structure and process optimization of the thin film evaporator.

Description

Technical Field

[0001] The invention belongs to the technical field of thin film evaporation, and relates to a method for identifying a liquid film state in an internal flow field of a thin film evaporator. Background Art

[0002] The internal structure of the scraped film evaporator is as follows: Figure 1 As shown, it is a new type of high-efficiency evaporator that can perform falling film evaporation under vacuum conditions by forced film formation through a rotating scraper. It has the characteristics of large production capacity, high efficiency, and short material heating time. It is suitable for distillation and concentration in the pharmaceutical, food, chemical and other industries.

[0003] As a new type of high-efficiency evaporator, the scraped film evaporator is favored by scholars at home and abroad, and researchers have conducted a lot of research on it. Among them, the flow, distribution and transmission mechanism of the fluid in the scraped film evaporator directly determine the evaporation efficiency and power consumption of the scraped film evaporator, and therefore become the focus of research. However, the existing research on the flow field inside the thin film evaporator is based on the internal in-line scraper blade structure (such as Fig.10 As shown in Figure 2), and at the same time, a lot of simplifications were made to obtain the most classic flow field morphology theory (as shown in Figure 2). Figures 11-12 As shown in the figure, it includes liquid film 2, circular wave 3, scraper 4, rotating shaft 5, and cylinder wall 6). This theory simplifies the flow field inside the thin film evaporator into a relatively regular circular wave (also called liquid mass) at the leading edge of the blade and a thin and uniform liquid film scraped by the trailing edge of the blade, and ignores the change of the flow field shape in the axial direction. Existing studies are based on this model to explore its liquid film thickness characteristics, residence time characteristics, heat and mass transfer characteristics, etc.

[0004] However, when the actual thin film evaporator is used in high viscosity material occasions, in order to promote the high viscosity material to form a good film on the evaporator wall and improve the production capacity, the blade shape on the rotor (such as Fig.13 The flow field inside the evaporator is different from the traditional theoretical form, and its flow pattern becomes complex and diverse, such as Fig.14 As shown in the left figure, it includes liquid mass 1 and liquid film 2. The simulation results are as follows Fig.14 At this time, the liquid film is no longer a uniform and flat film structure, and the liquid mass is no longer a traditional circular wave, but a periodically distributed irregular shape; and there is a very intense exchange behavior between the mass and the film, which is manifested in an increase in the contact area between the mass and the film, and a large deformation of the mass and the film at the contact position.

[0005] It can be seen that the internal flow field distribution characteristics of the thin film evaporator that processes high-viscosity materials are difficult to describe through an axial or radial cross-sectional diagram, and it is impossible to accurately distinguish which area is the liquid film and which area is the liquid mass. This brings great difficulties to the understanding of the evaporation and dissolution mechanism of such thin film evaporators, the evaluation of their performance, and the subsequent optimization design.

[0006] For a thin film evaporator, the liquid film formed inside it is the key to achieving efficient evaporation.

[0007] The scraped film evaporator achieves efficient evaporation of materials by forcing film formation on the inner wall surface through a rotating scraper. In the actual scraped film evaporator flow field, the liquid film is not in an ideal uniform and flat state, such as Figure 2 As shown in the figure, it is a schematic diagram of different forms of liquid film. Pure red represents pure liquid phase, blue represents pure gas phase, and the colored transition area represents the gas-liquid interface. Among them, (a) is a flat liquid film, which is only relatively flat, and the surface is also somewhat uneven; (b) is a liquid film with bubbles attached to the cylinder wall inside, and (c) is a wavy liquid film, and the surface of the liquid film presents relatively regular periodic fluctuations; under the forced disturbance of the scraper, the liquid film may appear regular or irregular wavy shapes, and may break. At this time, the thickness, surface area, surface curvature, uniformity and other factors of the liquid film itself will affect the evaporation efficiency of the evaporator. In addition, bubbles may appear in the liquid film close to the wall. The presence of wall-attached bubbles will greatly reduce the heat transfer performance inside the evaporator, thereby affecting the evaporation efficiency. For this reason, it is very important to determine the state of the liquid film.

[0008] Experimental research on liquid film can generally be carried out through electrical, optical, acoustic and digital image processing methods. However, for common evaporator equipment, the evaporation structure is covered with a heating jacket, which is opaque. The superheated steam or other fluid media in the jacket will cause great interference to the measurement of the liquid film thickness inside the evaporation structure, making it difficult to accurately capture the liquid film morphology inside the evaporation structure through the heating jacket in the actual production process.

[0009] For this reason, the flow field inside the evaporator can only be simulated through numerical simulation. With the help of the actual evaporator structure and process parameters, a fluid simulation model inside the evaporator is constructed to completely restore the liquid film distribution inside the evaporator, and then the different states of the liquid film are identified based on the simulation results. Similar working conditions can be identified through image processing methods, but this method can only be used for two-dimensional images. The actual flow field inside the evaporator is complex, and the information of several specific cross-sections cannot represent the overall flow field inside the evaporator. For example, wall-attached bubbles will appear in some screenshots, but not in others; and the wavy liquid film will have different overall heights and curvatures in different cross-sections.

[0010] For example, in document 1 (Application of digital imaging technology in liquid film thickness measurement. Northeast Electric Power Technology [J]·. 2007, 02): 44-47), based on the analysis of traditional liquid film thickness measurement technology, a non-contact measurement method of liquid film thickness based on CCD images was proposed, and the average liquid film thickness under different Reynolds conditions was measured. Patent CN107084669A provides a pure water liquid film thickness measurement system using the reflection principle. These two methods can obtain local detailed data in the flow field, but cannot obtain all the characteristics of the liquid film inside the evaporator.

[0011] Therefore, it is of great significance to study a method for identifying the state of the liquid film in the internal flow field of a thin-film evaporator in order to solve the problem that the liquid film identification method in the prior art cannot reflect all the characteristics of the liquid film inside the evaporator and cannot achieve a quantitative description of the characteristics. Summary of the invention

[0012] In order to solve the problems existing in the prior art, the present invention provides a method for identifying the state of a liquid film in an internal flow field of a thin film evaporator;

[0013] To achieve the above object, the scheme adopted by the present invention is as follows:

[0014] A method for identifying the state of a liquid film in an internal flow field of a thin film evaporator, wherein the viscosity of a 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 barrel (the outer wall of the three-dimensional vertical cylindrical barrel is equivalent to the inner wall of the thin film evaporator, and the inner 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 gap width of the thin film evaporator (i.e., the distance between the blade and the inner wall of the thin film evaporator) is d, comprising the following steps:

[0015] (1) Meshing the three-dimensional vertical cylinder;

[0016] (2) dividing the three-dimensional vertical cylindrical tube into n coaxial, mutually nested three-dimensional vertical sub-cylindrical tubes with a wall thickness of t, t≤d / 4;

[0017] (3) After obtaining the gas-liquid two-phase distribution cloud map of the i-th three-dimensional vertical sub-cylinder, count the total number of grid nodes corresponding to the gas-liquid interface B i and the total number of mesh nodes A of the i-th three-dimensional vertical sub-cylinder i , the interface point ratio C of the i-th three-dimensional vertical sub-cylinder is 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;

[0018] (4) Plot the horizontal axis as i or the parameter related to i and the vertical axis as C i A spectrum (line graph or bar graph) is obtained, and the highest peak and the second highest peak are found from the spectrum. The latest peak among the highest peak and the second highest peak is recorded as peak P. The area 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 area. The area between the three-dimensional vertical sub-cylinder corresponding to the end point of peak P and the inner wall of the three-dimensional vertical cylinder is the liquid mass area. The three-dimensional vertical sub-cylinder corresponding to the starting point of peak P, the three-dimensional vertical sub-cylinder corresponding to the end point of peak P and the three-dimensional vertical sub-cylinder between these two three-dimensional vertical sub-cylinders together constitute the interaction area between the liquid mass and the liquid film.

[0019] (5) Count the volume fractions of all grid nodes in the gas-liquid two-phase distribution cloud map and perform mean processing to obtain the volume fraction mean value 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;

[0020] (6) With i or a parameter related to i as the horizontal axis, D i As the ordinate, draw a volume fraction mean curve, combine the liquid film area and the interaction area between the liquid mass and the liquid film identified in step (4), and identify the state of the liquid film in the internal flow field of the thin film evaporator according to the volume fraction mean curve;

[0021] The horizontal coordinate corresponding to the spectrum in step (4) and the horizontal coordinate corresponding to the volume fraction mean curve in step (6) are the same parameter.

[0022] As the preferred technical solution:

[0023] In the above-mentioned method for identifying the state of the liquid film in the internal flow field of the thin film evaporator, in order to improve the accuracy, the segmentation width should preferably be one sixteenth of the gap width or less, t≤d / 16.

[0024] In the above-mentioned identification method of the liquid film state in the internal flow field of the thin film evaporator, t>s, s is the maximum value of the length of the line connecting any two points in the minimum grid node, so that the three-dimensional vertical sub-cylinder can wrap the minimum grid node to avoid data loss. There are many shapes of grid nodes. Two-dimensional grid nodes are commonly triangular and quadrilateral grids, and three-dimensional grid nodes are commonly tetrahedron, hexahedron, prism, pyramid and polyhedron grids.

[0025] In the above-mentioned method for identifying the state of the liquid film in the internal flow field of the thin film evaporator, the grid nodes correspond to the pixel points of the gas-liquid two-phase distribution cloud map.

[0026] As described above, in the method for identifying the state of the liquid film in the internal flow field of the thin film evaporator, 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 network node to the total volume of the liquid phase and the gas phase.

[0027] In the above-mentioned method for identifying the state of the liquid film in the internal flow field of the thin film evaporator, the grid node corresponding to the gas-liquid interface in step (3) is the pixel point with a volume fraction of 0.4 to 0.6 in the gas-liquid two-phase distribution cloud map.

[0028] In the above-mentioned method for identifying the state of the liquid film in the internal flow field of the thin film evaporator, the grid nodes corresponding to the gas-liquid interface in step (3) are the pixels with a volume fraction of 0.45 to 0.55 in the gas-liquid two-phase distribution cloud map, so as to obtain higher accuracy.

[0029] In the method for identifying the state of the liquid film in the internal flow field of the thin film evaporator as described above, the parameter related to i in step (4) and step (6) is the distance between the outer wall of the i-th three-dimensional vertical sub-cylindrical tube and the outer wall of the first three-dimensional vertical sub-cylindrical tube.

[0030] In the above-mentioned method for identifying the state of the liquid film in the internal flow field of the thin film evaporator, step (6) identifies the state of the liquid film in the internal flow field of the thin film evaporator according to the volume fraction mean curve, specifically as follows:

[0031] The liquid film state is divided into two states: the side close to the cylinder wall and the side close to the gas field. The side close to the cylinder wall mainly focuses on whether there are bubbles attached to the wall; while the side close to the gas field mainly focuses on the morphology of the gas-liquid interface on the surface of the liquid film.

[0032] By observing whether the mean value of the volume fraction corresponding to the position of the outer wall of the three-dimensional vertical cylinder in the volume fraction mean curve is 1, it is judged whether there are bubbles in the liquid film. If the mean value of the volume fraction is 1, it means that the liquid film on the outer wall of the three-dimensional vertical cylinder is complete, otherwise it means that there are wall bubbles;

[0033] For the liquid film surface, within the liquid film area and the interaction area between the liquid mass and the liquid film, the slope of the descending curve of the volume fraction mean curve starting from the peak value is used to judge whether the liquid film surface is flat or wavy. If it decreases linearly, it means that the liquid film surface is flat, otherwise (nonlinear decrease) it means that the liquid film surface is wavy.

[0034] That is, the state recognition of the gas-liquid interface of the liquid film needs to determine the position of the gas-liquid interface of the liquid film first, and then perform state recognition. Different types of liquid films have different areas occupied by their gas-liquid interfaces.

[0035] For a flat liquid film, its gas-liquid interface is distributed in the interaction zone, and the liquid film zone is filled with material (the bubbles on the internal wall are not considered at this time). Therefore, when identifying the gas-liquid interface state, only the interaction zone needs to be identified.

[0036] The gas-liquid interface of the wavy liquid film is distributed in the liquid film area and the interaction area. The wavy liquid film consists of two parts: one is the liquid film with wavy protrusions, and the other is the thin layer of liquid film close to the wall as a connection between the protrusions. Fig.19 As shown. The thin layer of liquid film is very thin and only plays a connecting role, and cannot reflect the key characteristic morphology of the wavy liquid film; therefore, when identifying the gas-liquid interface morphology of the wavy liquid film, only the gas-liquid interface morphology of the convex part of the liquid film is concerned. On the volume fraction mean curve, the thin layer of liquid film appears as a part with a volume fraction of 1 close to the wall. When the volume fraction starts to decrease from 1, it enters the convex liquid film area. That is, the area from the position where the volume fraction starts to decrease from 1 to the end of the interaction zone are all areas that reflect the key characteristic morphology of the gas-liquid interface of the wavy liquid film.

[0037] According to the method for identifying the state of the liquid film in the internal flow field of the thin film evaporator as described above, the viscosity of the material in the thin film evaporator is 50 to 20000 Pa·s.

[0038] Principle of the present invention:

[0039] (1) The principle of dividing the liquid film area is as follows:

[0040] The present invention restores the flow conditions inside the evaporator through CFD simulation, 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 the liquid mass and the liquid film is obtained, and then the distribution area of ​​the liquid film and the liquid mass is obtained, so as to achieve accurate division of the flow state of the flow field.

[0041] For the division of liquid mass and liquid film area, we must first find the position of its gas-liquid interface. The gas-liquid two-phase distribution cloud map can be used to pick up the gas-liquid interface and analyze the internal flow field of the thin film evaporator (three-dimensional vertical cylinder, such as Figure 3 After gridding, the volume fraction of each grid node can be determined according to the gas-liquid two-phase distribution cloud map. Since the volume fraction is an important parameter for expressing the proportion of gas and liquid phases in the grid node, the type of each grid node (gas phase, liquid phase, gas-liquid interface) can be determined. Figure 2 The volume fraction of all grid nodes in the red part is 1, indicating that these grid nodes are filled with liquid phase; the volume fraction of the royal blue part is 0, indicating that all grid nodes in this area are filled with gas phase; and the transition color part indicates that there are both gas phase and liquid phase in these grid nodes, and the volume fraction is between 0 and 1, and the color also changes with the value of the volume fraction.

[0042] In the early stage of the research, the inventors studied the gas-liquid two-phase distribution cloud diagram and the number of interface grid nodes (i.e., grid nodes corresponding to the gas-liquid interface) with radial distance (see Figure 3 ) is as follows:

[0043] Example 1: The structure of the flow field inside the thin film evaporator is simple, and the liquid film is a flat liquid film without liquid clumps; Figure 4 The left picture is Figure 3 The gas-liquid two-phase distribution cloud diagram of the axial section, the left side close to the wall is the liquid film, the volume fraction is 1, and it is displayed in red; the right side far from the wall is the gas phase, the volume fraction is 0, and it is displayed in blue; there is no gas-liquid interface in the above two parts, so the number of interface grid nodes is 0; the middle transition color part is the gas-liquid interface part, the number of interface grid nodes in this part is not 0, 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 positions in the radial direction, the following is obtained Figure 4 The right figure shows the change of the number of interface mesh nodes with radial distance. There is a peak in the spectrum, and the position corresponding to the peak is the gas-liquid interface.

[0044] Example 2: The structure of the flow field inside the thin film evaporator is relatively complex. The liquid film includes a flat liquid film part and a liquid film protrusion part, and there is no liquid mass. Figure 5 The left picture is Figure 3 The gas-liquid two-phase distribution cloud diagram of the axial section, Figure 5 The right figure shows the number of interface mesh nodes changing with radial distance. There is a peak in the spectrum, and the position corresponding to the peak is near the surface of the liquid film.

[0045] Example 3: The structure of the flow field inside the thin film evaporator is complex. The liquid film includes a flat liquid film part and a protruding liquid film part, and there are liquid clumps. Figure 6 The left picture is Figure 3 The gas-liquid two-phase distribution cloud diagram of the axial section, Figure 6 The right figure shows the number of interface mesh nodes changing with radial distance. There are two peaks in the spectrum. One peak corresponds to the position near the liquid film surface, and the other peak corresponds to the interaction area between the liquid mass and the liquid film.

[0046] The structure of the internal flow field of the thin film evaporator for high-viscosity materials is complex. The liquid film includes a flat liquid film part and a protruding part of the liquid film, and there are liquid clumps. Therefore, there are two peaks in the spectrum of the number of corresponding interface grid nodes changing with radial distance. One peak corresponds to the position near the liquid film surface, and the other peak corresponds to the interactive area between the liquid clump and the liquid film. This feature can be used to determine the interactive area between the liquid clump and the liquid film in the internal flow field of the thin film evaporator for high-viscosity materials.

[0047] However, the structure of the internal flow field of the thin film evaporator for high-viscosity materials is complex. Figure 3 The cylindrical surface corresponding to a certain radial distance is equivalent to being composed of multiple vertical bars. The number of interface mesh nodes of different vertical bars may not be the same, and may even differ significantly. Therefore, the number of interface mesh nodes of a certain vertical bar cannot represent the number of interface mesh nodes corresponding to the radial distance. The number of interface mesh nodes of the cylindrical surface corresponding to the radial distance needs to be used to represent the number of interface mesh nodes corresponding to the radial distance. In this way, the data is more real and reliable. Assuming that there are a total of 1100 mesh nodes on the cylindrical surface corresponding to the radial distance, the volume fraction data corresponding to different mesh nodes are shown in the following table:

[0048] 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

[0049] If only the mesh nodes with a volume fraction of 0.5 are considered to be gas-liquid interfaces, and the other mesh nodes are simply liquid or gas phases, then the number of interface mesh nodes on this cylindrical surface is 100; if all mesh nodes with a volume fraction between 0.4 and 0.6 are considered to be gas-liquid interfaces, then the number of interface mesh nodes on this cylindrical surface is 300;

[0050] However, if the internal flow field of the thin film evaporator for high-viscosity materials is regarded as composed of multiple cylindrical surfaces, determining the number of interface grid nodes for each cylindrical surface will result in too many cylindrical surfaces, which is not convenient for statistical calculation. For this reason, the inventors tried to divide the internal flow field of the thin film evaporator for high-viscosity materials (three-dimensional vertical cylindrical tube) into n coaxial, mutually nested, three-dimensional vertical sub-cylindrical tubes with a wall thickness of t, and then determine the number of interface grid nodes of each three-dimensional vertical sub-cylindrical tube. However, this has caused 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 grid nodes will also be too large, resulting in a large number of interface grid nodes for each three-dimensional vertical sub-cylindrical tube, which is difficult to draw the interface. The graph of the number of surface mesh nodes changing 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 a different total number of nodes in each cylinder, which in turn makes the number of interface mesh nodes in different cylinders vary greatly. This results in that simply counting the number of interface mesh nodes of each three-dimensional vertical sub-cylinder cannot accurately measure the number of interface mesh nodes in the area. For this reason, the inventor proposes to replace "the number of interface mesh nodes of a certain three-dimensional vertical sub-cylinder" with "the percentage of the number of interface mesh nodes of a certain three-dimensional vertical sub-cylinder to the total number of mesh nodes of a certain three-dimensional vertical sub-cylinder".

[0051] (2) The principle of identifying the liquid film state is as follows:

[0052] like Figures 7 to 9 As shown in the left figure, the left side of the figure is the wall position, that is, the radial distance is 0. At the same time, red is liquid, with a volume fraction of 1; blue is gas phase, with a volume fraction of 0; transition color is gas-liquid mixed phase, with a volume fraction between 0 and 1. Figures 7 to 9 As shown in the right figure, data is collected from left to right. For each radial position, the volume fractions of all grid nodes at this radial position (similar to the entire vertical line in the figure) are averaged to draw a curve of the volume fraction mean versus radial distance.

[0053] Depend on Figure 7 As can be seen from the right figure, there are three sampling positions. The left sampling position is located in the red fluid area with a volume fraction of 1; the right sampling position is located in the gas phase with a volume fraction of 0. The middle sampling position is located exactly on the gas-liquid interface. The gas-liquid interface in the figure is flat, and there is a uniform transition from the liquid phase to the gas phase. Therefore, on the curve graph, the mean volume fraction decreases linearly, transitioning linearly from 1 to 0.

[0054] like Figure 8 As shown in the left figure, at this time, many mesh nodes corresponding to radial positions contain both liquid phase, gas phase, and gas-liquid interface. At this time, we calculate the weighted average of the volume fractions by using the proportions of several different mesh nodes. Finally, we get Figure 8The volume fraction mean curve is shown in the figure on the right. It can be seen that when the gas-liquid interface presents a curved shape, the downward trend of the volume fraction mean curve is no longer linear, but in the form of a curve; whether the specific volume fraction mean curve decreases at an accelerated rate or a decelerated rate depends on the curvature of the surface of the droplet in the left figure. The curve of the wavy liquid film is similar to this curve.

[0055] Fig. 9 The following is a cloud diagram of the gas-liquid two-phase distribution in the axial section when the liquid film region is a liquid film with bubbles attached to the wall, and a schematic diagram of the mean volume fraction curve of the liquid film with bubbles attached to the wall, which is similar to the combination of the first two cases. However, since the gas-liquid phase distribution of the bubble part is exactly opposite to that of the droplet, the mean volume fraction increases from a small value at the position close to the wall, that is, the radial distance is close to 0.

[0056] It can be seen from the demonstration of the above figures that the two-dimensional curve of the mean volume fraction changing with radial distance can well describe the morphology of the liquid film.

[0057] Beneficial Effects

[0058] (1) The method of the present invention is not based on data analysis of two-dimensional images, but on three-dimensional data processing, which can be used to analyze the actual flow field inside the entire evaporator;

[0059] (2) The method of the present invention can distinguish between the liquid film region, the liquid mass region and the liquid film and liquid mass interaction region, and on this basis can identify the liquid film morphology, can reflect all the characteristics of the liquid film inside the evaporator, and can achieve quantitative description of the characteristics to a certain extent;

[0060] (3) The method of the present invention can display the state of the liquid film in the flow field in the form of a two-dimensional curve distribution. From the curve, it can be intuitively seen whether the liquid film is flat, whether there are bubbles attached to the wall, and whether there is a pure liquid phase area. It can even be obtained by further analyzing the curve to obtain the change in the surface area of ​​the liquid film and the evaporation area occupied by the bubbles attached to the wall, and then the impact on the evaporation efficiency can be estimated, providing a basis for the subsequent optimization design of the evaporator. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 It is a schematic diagram of the internal structure of a scraped film evaporator;

[0062] Figure 2 Schematic diagrams of different forms of liquid films; (a) a flat liquid film, (b) a liquid film with bubbles attached to the cylinder wall, and (c) a wavy liquid film.

[0063] Figure 3 It is a schematic diagram of radial distance; the arrow direction is the scraper movement direction and the circular wave rolling direction;

[0064] Figure 4 is a schematic diagram of Example 1; wherein the direction of the arrow is the direction of movement of the scraper;

[0065] Figure 5 is a schematic diagram of Example 2; wherein the direction of the arrow is the direction of movement of the scraper;

[0066] Figure 6 is a schematic diagram of Example 3; wherein the arrow direction is the scraper movement direction;

[0067] Figure 7 It is a gas-liquid two-phase distribution cloud diagram of the axial section when the liquid film area is a flat liquid film, and a schematic diagram of the mean value curve of the volume fraction of the flat liquid film; wherein, the direction of the arrow is the direction of the scraper movement;

[0068] Figure 8 It is a gas-liquid two-phase distribution cloud diagram of the axial section when the liquid film area is a droplet-type fractured liquid film, and a schematic diagram of the volume fraction mean curve of the droplet-type fractured liquid film; wherein, the direction of the arrow is the direction of the scraper movement;

[0069] Fig. 9 It is a gas-liquid two-phase distribution cloud diagram of the axial section when the liquid film area is a liquid film with bubbles attached to the wall, and a schematic diagram of the mean value curve of the volume fraction of the liquid film with bubbles attached to the wall; wherein, the direction of the arrow is the direction of movement of the scraper;

[0070] Fig.10 This is a schematic diagram of the structure of the in-line scraper blades inside the scraper film evaporator;

[0071] Figures 11-12 This is a schematic diagram of the theoretical distribution of flow field morphology;

[0072] Fig.13 A schematic diagram of a complex blade rotor structure used when the production capacity is higher;

[0073] Fig.14 Schematic diagram of the internal flow field of the thin film evaporator of the present invention; wherein, Figure (a) is a schematic diagram of the three-dimensional flow field structure, and Figure (b) is a cloud diagram of the local gas-liquid two-phase distribution in an axial direction;

[0074] Figures 15-16 The number of interface mesh nodes varies with radial distance under different gaps;

[0075] Fig.17 It is the mean value curve of volume fraction corresponding to the uneven liquid film in the actual thin film evaporator;

[0076] Fig.18 This is a schematic diagram of the internal flow field corresponding to the uneven liquid film in an actual thin film evaporator;

[0077] Fig.19It is the curve graph of the average value of the volume fraction corresponding to the liquid film with attached-wall bubbles in the actual thin-film evaporator;

[0078] Fig. 20 It is the schematic diagram of the internal flow field corresponding to the liquid film with attached-wall bubbles in the actual thin-film evaporator;

[0079] Among them, 1-liquid mass, 2-liquid film, 3-circular wave, 4-scraper, 5-rotating shaft, 6-cylinder wall. Specific implementation manners

[0080] The following further elaborates the present invention in combination with specific implementation manners. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0081] A method for identifying the state of the liquid film in the internal flow field of a thin-film evaporator, the specific steps are as follows:

[0082] (1) The viscosity of the material in the thin-film evaporator is 50-20000 Pa·s, and 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 three-dimensional vertical cylinder is subjected to meshing processing;

[0083] The process of meshing processing is: generate a global surface grid, and locally refine the grid at key positions such as the wall surface, the gap between the blade and the cylinder wall, and then automatically fill the volume grid. The filling grid form can adopt various structural forms such as tetrahedron, hexahedron, and the combination of hexahedron and polyhedron;

[0084] (2) The three-dimensional vertical cylinder is divided into n coaxial, mutually sleeved three-dimensional vertical sub-cylinders with a wall thickness of t each, where s < t ≤ d / 16; among them, d is the gap width of the thin-film evaporator (i.e., the distance between the blade and the inner wall of the thin-film evaporator), and s is the maximum value of the connection length between any two points in the smallest grid nodes;

[0085] (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 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-cylinder i , and calculate the ratio C of the number of interface points of the i-th three-dimensional vertical sub-cylinder 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; the grid nodes correspond to the pixels of the gas-liquid two-phase distribution cloud map; each network node 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 pixel point to the total volume of the liquid phase and the gas phase; the grid node corresponding to the gas-liquid interface is the pixel point with a corresponding volume fraction of 0.45 to 0.55 in the gas-liquid two-phase distribution cloud map;

[0086] (4) Plot the horizontal axis as i or the parameter related to i and the vertical axis as C i , find the highest peak and the second highest peak from the spectrum, the latest peak among the highest peak and the second highest peak is recorded as peak P, the area 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 area, the area between the three-dimensional vertical sub-cylinder corresponding to the end point of peak P and the inner wall of the three-dimensional vertical cylinder is the liquid mass area, the three-dimensional vertical sub-cylinder corresponding to the starting point of peak P, the three-dimensional vertical sub-cylinder corresponding to the end point of peak P and the three-dimensional vertical sub-cylinder between these two three-dimensional vertical sub-cylinders together constitute the interactive area between the liquid mass and the liquid film; wherein, 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 first three-dimensional vertical sub-cylinder;

[0087] (5) Count the volume fractions of all grid nodes in the gas-liquid two-phase distribution cloud map and perform mean processing to obtain the volume fraction mean value 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;

[0088] (6) With i or a parameter related to i as the horizontal axis, D i As the ordinate, draw a volume fraction mean curve, combine the liquid film area and the interaction area between the liquid mass and the liquid film identified in step (4), and identify the state of the liquid film in the internal flow field of the thin film evaporator according to the volume fraction mean curve; the abscissa corresponding to the spectrum in step (4) and the abscissa corresponding to the volume fraction mean curve in step (6) are the same parameter;

[0089] Among them, the state of the liquid film in the internal flow field of the thin film evaporator is identified according to the volume fraction mean curve, as follows:

[0090] By observing whether the mean value of the volume fraction corresponding to the position of the outer wall of the three-dimensional vertical cylinder in the volume fraction mean curve is 1, it is judged whether there are bubbles in the liquid film. If the mean value of the volume fraction is 1, it means that the liquid film on the outer wall of the three-dimensional vertical cylinder is complete, otherwise it means that there are wall bubbles;

[0091] For the liquid film surface, within the liquid film area and the interaction area between the liquid mass and the liquid film, the slope of the descending curve of the volume fraction mean curve starting from the peak value is used to judge whether the liquid film surface is flat or wavy. If it decreases linearly, it means that the liquid film surface is flat, otherwise (non-linear decrease), it means that the liquid film surface is wavy.

[0092] By using the method of the present invention, when the zero shear viscosity of the material is 4000 Pa·s, a graph showing the change of the interface grid node ratio with radial distance in the internal flow field of the thin film evaporator with a diameter of d=4 mm is obtained (e.g. Fig.15 As shown in the figure, there are two peaks in both figures. The second peak corresponds to the interactive area between the liquid mass and the liquid film. After the interactive area between the mass and the film is identified, the flow field area can be divided into two morphological parts. The two sides of the interactive area are the liquid film area and the liquid mass area, as shown in Fig.15 The mid-radial distance of 0 represents the wall position, then the left side of the interaction zone is the liquid film zone, and the right side of the interaction zone is the liquid mass zone.

[0093] like Fig.17 As shown in the figure, according to the mean volume fraction curve, we can see that in the liquid film area, there is a very short section with a volume fraction of 1, and then the volume fraction begins to decrease, showing a steep drop in the liquid film area and the exchange area. This morphology shows that first there is a very thin flat liquid film on the wall, and secondly, there is a wavy liquid film above this flat liquid film, resulting in a nonlinear downward slope. The flow field is screenshotted and found as follows Fig.18 As shown, its morphology is consistent with that observed in the volume fraction mean plot.

[0094] By using the method of the present invention, when the zero shear viscosity of the material is 4000 Pa·s, the present invention obtains a graph of the change of the interface grid node ratio with radial distance in the internal flow field of the thin film evaporator with d=5.5 mm (such as Fig.16 As shown in the figure, there are two peaks in both figures. The second peak corresponds to the interactive area between the liquid mass and the liquid film. After the interactive area between the mass and the film is identified, the flow field area can be divided into two morphological parts. The two sides of the interactive area are the liquid film area and the liquid mass area, as shown in Fig.16 The mid-radial distance of 0 represents the wall position, then the left side of the interaction zone is the liquid film zone, and the right side of the interaction zone is the liquid mass zone.

[0095] like Fig.19As shown in the figure, according to the mean volume fraction curve, it can be seen that in the liquid film part, the mean volume fraction first rises from about 0.4 to a peak value of 1, and then begins to decline, indicating that there are bubbles near the wall, and the area occupied by the bubbles is large; after rising to the peak value of 1, there is a short stay, and there is a certain width of pure liquid phase distribution area on the surface, that is, the liquid film area without bubble interference; then the exchange zone shows a decline, showing a linear downward trend, and the gas-liquid interface of the subsequent liquid film on the surface is relatively flat. The gas-liquid two-phase cloud map of the axial section of the flow field is extracted, as shown in the figure. Fig. 20 As shown, the liquid film state shown by the flow field is consistent with the liquid film state expressed by the curve distribution diagram.

Claims

1. A method for identifying the state of a liquid film in a thin film evaporator internal flow field, 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, and the gap width of the thin film evaporator is d, characterized in that The steps include: (1) Meshing the three-dimensional vertical cylinder; (2) dividing the three-dimensional vertical cylindrical tube into n coaxial, mutually nested three-dimensional vertical sub-cylindrical tubes with a wall thickness of t, 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 of grid nodes corresponding to the gas-liquid interface B i and the total number of mesh nodes A of the i-th three-dimensional vertical sub-cylinder i , the interface point ratio C of the i-th three-dimensional vertical sub-cylinder is 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) Plot the horizontal axis as i or the parameter related to i and the vertical axis as C i , find the highest peak and the second highest peak from the spectrum, the latest peak among the highest peak and the second highest peak is recorded as peak P, the area 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 area, the area between the three-dimensional vertical sub-cylinder corresponding to the end point of peak P and the inner wall of the three-dimensional vertical cylinder is the liquid mass area, the three-dimensional vertical sub-cylinder corresponding to the starting point of peak P, the three-dimensional vertical sub-cylinder corresponding to the end point of peak P and the three-dimensional vertical sub-cylinder between these two three-dimensional vertical sub-cylinders together constitute the interactive area between the liquid mass and the liquid film; (5) Count the volume fractions of all grid nodes in the gas-liquid two-phase distribution cloud map and perform mean processing to obtain the volume fraction mean value D of all grid nodes of the i-th three-dimensional vertical sub-cylinder i ; (6) With i or a parameter related to i as the horizontal axis, D i As the ordinate, draw a volume fraction mean curve, combine the liquid film area and the interaction area between the liquid mass and the liquid film identified in step (4), and identify the state of the liquid film in the internal flow field of the thin film evaporator according to the volume fraction mean curve; The horizontal coordinate corresponding to the spectrum in step (4) and the horizontal coordinate corresponding to the volume fraction mean curve in step (6) are the same parameter.

2. The method for identifying the state of the liquid film in the internal flow field of the thin film evaporator according to claim 1, characterized in that: t≤d / 16.

3. The method for identifying the state of the liquid film in the internal flow field of the thin film evaporator according to claim 1, characterized in that: t>s, s is the maximum length of the line connecting any two points in the minimum grid node.

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

5. The method for identifying the state of the liquid film in the internal flow field of the thin film evaporator according to claim 4, characterized in that: 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 network node to the total volume of the liquid phase and the gas phase.

6. The method for identifying the state of the liquid film in the internal flow field of the thin film evaporator according to claim 5, characterized in that: The grid nodes corresponding to the gas-liquid interface in step (3) 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 identifying the state of the liquid film in the internal flow field of the thin film evaporator according to claim 6, characterized in that: The grid nodes corresponding to the gas-liquid interface in step (3) are the pixels with volume fractions of 0.45 to 0.55 in the gas-liquid two-phase distribution cloud map.

8. The method for identifying the state of the liquid film in the internal flow field of the thin film evaporator according to claim 1, characterized in that: The parameter related to i in step (4) and step (6) is the distance between the outer wall of the i-th three-dimensional vertical sub-cylindrical tube and the outer wall of the first three-dimensional vertical sub-cylindrical tube.

9. The method for identifying the state of the liquid film in the internal flow field of the thin film evaporator according to claim 1, characterized in that: Step (6) identifies the state of the liquid film in the internal flow field of the thin film evaporator according to the volume fraction mean curve, as follows: By observing whether the mean value of the volume fraction corresponding to the position of the outer wall of the three-dimensional vertical cylinder in the volume fraction mean curve is 1, it is judged whether there are bubbles in the liquid film. If the mean value of the volume fraction is 1, it means that the liquid film on the outer wall of the three-dimensional vertical cylinder is complete, otherwise it means that there are wall bubbles; For the liquid film surface, within the liquid film area and the interaction area between the liquid mass and the liquid film, the slope of the descending curve of the volume fraction mean curve starting from the peak value is used to judge whether the liquid film surface is flat or wavy. If it decreases linearly, it means that the liquid film surface is flat, otherwise it means that the liquid film surface is wavy.

10. The method for identifying the state of a liquid film in an 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 to 20,000 Pa·s.

Images