A method for adjusting the heating process of a thin-film evaporator based on the wall film area.

Abstract

This invention belongs to the field of thin-film evaporation technology and relates to a method for adjusting the heating process of a thin-film evaporator based on the wall film area. First, the liquid phase volume fraction of the grid cells on the wall surface in the internal flow field of the thin-film evaporator is used as a criterion. A liquid phase volume fraction of 0.5 is used as a boundary to screen the liquid film region and the dry wall region. For the dry wall region, a vaporization core region and a cracked region are screened with a limit of 1 Pa. Then, the area ratio of the vaporization core region in the wall film area is calculated. By adjusting the superheat, when the area ratio of the vaporization core region in the wall film area reaches the peak value under different superheats, the optimal superheat at the maximum evaporation rate is finally applied to actual production, thereby enabling the thin-film evaporator to obtain the optimal evaporation capacity. This invention subdivides the dry wall region, determines the optimal evaporation capacity by judging the wall film area ratio under the optimal superheat, and then applies it to actual production to improve the evaporation capacity of the thin-film evaporator.

Description

Technical Field

[0001] This invention belongs to the field of thin-film evaporation technology and relates to a method for adjusting the heating process of a thin-film evaporator based on the wall film area. Background Technology

[0002] Thin-film evaporators, as important equipment for the preparation of spun fibers, have advantages such as high energy efficiency and high evaporation intensity. The internal flow field structure of a thin-film evaporator is as follows: Figure 1 As shown, under the circumferential scraping action of the serrated scraper, some material can pass through the gap between the scraper and the wall surface and form a liquid film on the inner wall of the evaporator. The remaining material forms a circular wave 1 at the leading edge of the scraper. The serrated scraper has grooves in its structure. When the circular wave 1 encounters a groove during the spiral downward transport process, it flows out of the groove and spreads circumferentially onto the liquid film. The outflowing part is called liquid stream 2. The flow rate of high-viscosity liquid materials is relatively slow. When the viscosity exceeds 0.5 Pa·s, the heat transfer coefficient of long-tube vertical or forced circulation evaporators drops rapidly, and relying solely on gravity to transport the material is too inefficient. Therefore, it is necessary to force the material to form a film using a scraper.

[0003] However, due to the scraping film formation by the scraper and the boiling heat transfer of the thin film, the film formation state on the heat transfer surface is very complex, resulting in various situations such as wall liquid film, wall-attached gas ring, and cracked liquid film. Among them, the wall-attached gas ring and cracked liquid film present a gas-solid interface where the gas phase directly contacts the cylinder wall, i.e., a dry wall phenomenon. The dry wall phenomenon reduces the actual evaporation area and lowers the evaporation efficiency of the evaporator. Currently, there is some research on the process parameters and structural optimization of thin-film evaporators in China, but research on the dry wall phenomenon is relatively limited.

[0004] The mass transfer process within a thin-film evaporator is complex, involving not only evaporation at the liquid film surface but also boiling within the liquid film itself. Existing research, to simplify calculations, largely assumes that mass transfer occurs only at the liquid film surface, treating heat transfer within the liquid film as conduction. In reality, due to the temperature difference between the heated wall and the material, bubbles are easily generated at the solid-liquid interface, leading to a dry wall phenomenon. This reduces the evaporation area, decreases evaporation efficiency, and in severe cases, can cause "drying out," damaging the equipment.

[0005] It is worth noting that when studying the bubbles generated by heating, it was found that bubbles formed near the heated wall in the evaporation field affect evaporation efficiency. Some researchers have observed in experiments that once the film ruptures, the heated surface dries quickly, and the temperature rises rapidly. Therefore, the proportion of the wall surface covered by the film (i.e., the proportion of the heated area covered by the liquid film on the wall surface of the evaporator to the total heated wall surface area) is a direct evaluation of the liquid film state in the evaporation field of a liquid film evaporator. Existing studies often simply divide the wall surface covered area into a liquid film region and a dry wall region, changing the proportion of the liquid film region by altering the structural process and external conditions (such as superheat, wall materials, etc.), increasing the surface area of ​​the liquid film on the wall, and enhancing the liquid film evaporation efficiency by increasing the surface contact area. However, due to changes in external conditions, insufficient heating capacity can lead to a decrease in evaporation performance. This indicates that a larger proportion of the liquid film region is not necessarily better; there must be an optimal proportion of film coverage for the evaporator to achieve its best evaporation capacity.

[0006] Currently, the optimal evaluation indicators for the evaporation capacity of thin-film evaporators and similar large-scale evaporators are evaporation rate, heat transfer coefficient, and heat transfer coefficient. Some studies have found that increasing the wall heat flux and increasing the liquid inlet temperature and flow rate can promote liquid evaporation, but an increase in evaporation rate does not necessarily indicate improved heat transfer performance. Since the performance changes of large-scale evaporators are directly reflected in the evaporation rate, an increase in heat transfer capacity will be directly reflected in the evaporation rate. However, the existing evaluation system suffers from the problem of relying on a single indicator, necessitating the development of new evaluation indicators for the evaporation capacity of large-scale evaporators that complement it.

[0007] The most direct way to change the heat flux of the wall is to increase the superheat of the wall. When boiling occurs at the heating point of the heated surface, its surface temperature... It must be higher than the saturation temperature of the liquid To be higher, the difference between the two This is referred to as wall superheat. Changes in superheat significantly affect the film-covered area ratio; that is, the greater the wall superheat, the more locations on the wall where dry-wall phenomena occur. However, existing research mainly focuses on the impact of superheat on the heat transfer efficiency, overall heat transfer coefficient, and evaporation rate of the evaporator, without mentioning the optimal wall film-covered area under the condition of maximum superheat at which the evaporation rate is maximized. The optimal superheat is often determined by repeatedly conducting simulation experiments to extract the maximum evaporation rate and then comparing the results to obtain the optimal superheat. Simulating each case individually is extremely time-consuming.

[0008] The literature (Study on Evaporative Wetting and Heat Transfer of Falling Film in Vertical Pipes [D]. South China University of Technology, 2020. DOI:10.27151 / d.cnki.ghnlu.2020.003455.) calculated the distribution of liquid film heat transfer coefficient under different mass flow rates, different descent heights, and different wall superheats, and summarized the variation law of liquid film heat transfer coefficient, but did not link the optimal evaporation rate with the proportion of film on the wall.

[0009] In summary, existing studies have simplistically defined the wall film area, focusing primarily on the liquid film region without considering the proportion of the wall film area under optimal superheat. To achieve a balance between high-intensity evaporation and the wall film state, it is necessary to study the wall film state of thin-film evaporators and identify the wall film state that maximizes evaporation capacity.

[0010] Therefore, it is of great significance to study a method for adjusting the heating process of a thin-film evaporator based on the wall film area in order to solve the problems existing in the prior art. Summary of the Invention

[0011] The purpose of this invention is to solve the problems existing in the prior art and provide a method for adjusting the heating process of a thin-film evaporator based on the wall film area.

[0012] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0013] The method for adjusting the heating process of a thin-film evaporator based on the wall film area includes the following steps:

[0014] (1) Based on the liquid volume fraction of the grid cells on the wall surface in the internal flow field of the thin film evaporator, the region with a liquid volume fraction of more than 0.5 is divided into the liquid film region, and the region with a liquid volume fraction of less than 0.5 is divided into the dry wall region.

[0015] (2) For the dry wall region, the region with pressure ≥ 1 Pa (pressure is the data stored in the grid cells on the wall surface in the flow field inside the thin film evaporator) is divided into the vaporization core region, and the region with pressure < 1 Pa is divided into the liquid film cracking region.

[0016] Mechanism of gasification core formation:

[0017] like Figure 3 As shown, during bubble nucleation, multiple forces, including buoyancy, cross-sectional shear force, pressure difference between water vapor 7 and liquid 8, surface tension, and gravity, play crucial roles in bubble nucleation and boiling. Figure 3As shown in (a), in large-vessel nucleation boiling (pool boiling), when buoyancy overcomes surface tension and gravity, the vaporization nuclei separate from the heated surface, and the size of the vaporization nuclei gradually increases. However, in thin-film evaporators, as... Figure 3 As shown in (b), the thickness of the liquid film will not exceed the gap between the scraper and the wall. Therefore, the vaporization nucleus will not separate from the heated surface like the nucleation in a large container. Instead, it will not be able to be wrapped by the liquid film during the growth process, causing the liquid film to crack.

[0018] like Figure 4 The diagram illustrates the evolution of vaporization nuclei. Vaporization nucleation mainly includes three stages: expansion, stretching, and cracking. In a thin-film evaporator, when transitioning from the flow field to the evaporation field, the initial liquid film is still in the initial endothermic stage, and the latent heat of the material continues to rise, but a phase change has not yet occurred. When the heat absorbed by the liquid film reaches the limit of its latent heat, bulk superheating will occur inside the thin-film evaporator, i.e., the liquid film will boil. Many vaporization nuclei begin to appear on the wall surface. These vaporization nuclei are evenly distributed on the wall surface. As the liquid film continues to absorb heat, the vaporization nuclei inside the liquid film gradually increase in size (i.e., expand), and the liquid film above the vaporization nuclei begins to stretch. At the stretching point, the liquid film contracts and narrows. At this time, the pressure inside the vaporization nuclei is much greater than the pressure of the material. As time increases, the heat absorbed by the liquid film also gradually increases, and the vaporization nuclei grow to the point where they can no longer be contained by the liquid film. The vaporization nuclei rupture the liquid film, causing it to crack. At this point, the pressure inside the liquid film gradually decreases to match the ambient pressure. This shows that the evolution from liquid film to vaporization nucleus and then to liquid film cracking is a step-by-step process.

[0019] (3) Calculate the proportion of the gasification core area in the wall film area. The proportion of the gasification core area = the area of ​​the gasification core area / (the area of ​​the liquid film area + the area of ​​the dry wall area).

[0020] like Figure 5 As shown, the percentage of 100% wall surface membrane coverage = percentage of liquid film area + percentage of dry wall area = percentage of liquid film area + percentage of vaporization core area + percentage of liquid film cracked area.

[0021] The existing technology only provides a simple division between the liquid film area and the dry wall area for the first part of the formula, which is 100% wall film area = liquid film area ratio + dry wall area ratio. However, it does not further subdivide the dry wall area ratio in the formula, i.e., 100% wall film area = liquid film area ratio + vaporization core area ratio + liquid film cracking area ratio.

[0022] (4) By adjusting the superheat, the proportion of the vaporization core area in the wall film area reaches the peak (compared to the proportion of the vaporization core area under other superheat). When the proportion of the vaporization core area reaches the peak, its evaporation rate is the largest. The superheat corresponding to this time is the optimal superheat under the maximum evaporation rate.

[0023] (5) Apply the optimal superheat at the maximum evaporation rate obtained in step (4) to actual production so that the thin film evaporator can obtain the optimal evaporation capacity.

[0024] As a preferred technical solution:

[0025] As described above, the heating process adjustment method for thin-film evaporators based on the wall film area results in a material viscosity of 0.5~6000 Pa·s at 25°C in the thin-film evaporator.

[0026] As described above, the heating process adjustment method for thin-film evaporators based on the wall film area has corresponding volume fraction data for each pixel in the gas-liquid two-phase distribution cloud map of the internal flow field of the thin-film evaporator. The liquid phase volume fraction refers to the ratio of the volume of the liquid phase at the center of each grid cell in the gas-liquid two-phase distribution cloud map of the internal flow field of the thin-film evaporator to the total volume of the liquid phase and the gas phase.

[0027] As described above, the heating process adjustment method for thin-film evaporators based on the wall film area is performed in CFD-POST post-processing software, where the division of each region in steps (1) and (2) is carried out.

[0028] As described above, the heating process adjustment method for thin film evaporators based on wall film area is as follows: Steps (1) to (3) are as follows: The internal flow field of the thin film evaporator is simulated using CFD-POST post-processing software. In the internal flow field of the thin film evaporator, near the wall, the liquid film area and the dry wall area are screened with a liquid phase volume fraction of 0.5 as the boundary. At the same time, the data of the proportion of the liquid film area and the dry wall area are directly obtained in the CFD-POST post-processing software. Next, the pressure of the dry wall area is judged, and the vaporization core area and the cracking area are screened with 1 Pa as the boundary. At the same time, the data of the proportion of the vaporization core area and the liquid film cracking area are directly obtained in the CFD-POST post-processing software. Finally, the proportion of the wall film area is calculated.

[0029] In the above-mentioned method for adjusting the heating process of a thin-film evaporator based on the wall film area, the superheat in step (3) refers to the difference between the wall temperature and the saturation temperature.

[0030] As described above, in the method for adjusting the heating process of a thin-film evaporator based on the wall film area, the peak area ratio of the vaporization core region in the wall film area in step (4) is 35-45%.

[0031] The metric proposed in this invention is the area ratio of three regions in the wall film area (liquid film region, vaporization core region, and liquid film cracking region). When the area ratio of the vaporization core region reaches its peak (the peak value is the maximum value obtained by comparing the area ratio of the vaporization core region under different superheat degrees), that is, when a large number of vaporization cores are about to transition to the liquid film cracking state but have not yet completed the transition, the evaporation capacity of the thin-film evaporator is maximized when the area ratio of the vaporization core region is between 35% and 45%.

[0032] Beneficial effects:

[0033] (1) The present invention provides a method for adjusting the heating process of a thin film evaporator based on the wall film area. The dry wall area is further divided into a vaporization core area and a cracking area. The wall superheat directly affects the proportion of the film area. There is a peak value between different superheat and evaporation rate, which is the optimal superheat. The optimal evaporation capacity is determined by judging the proportion of the wall film area under the optimal superheat. This method is then applied to actual production to improve the evaporation capacity of the thin film evaporator.

[0034] (2) The method for adjusting the heating process of a thin film evaporator based on the wall film area of ​​the present invention can directly and quickly adjust the superheat, which greatly saves verification time and computing resources. Attached Figure Description

[0035] Figure 1 Figure 1 shows the internal flow field of a thin-film evaporator. In Figure 2, (a) shows the distribution of liquid volume fraction cloud map of the material on the wall of the thin-film evaporator, with red representing material and blue representing gas; Figure 3 shows the isosurface cloud map of the liquid material; Figure 4 shows the magnified isosurface cloud map viewed from the axis; Figure 5 shows the liquid volume fraction cloud map along the axial height; and Figure 6 shows the magnified view of the red frame in Figure 7 (d).

[0036] Figure 2 This is a schematic diagram of the internal region of the thin film;

[0037] Figure 3 This is a schematic diagram of nucleo-boiling; where, Figure 3 (a) shows boiling in a large-capacity tank. Figure 3 (b) represents liquid film boiling;

[0038] Figure 4 This is a diagram illustrating the evolution of the gasification core; in which, Figure 4 In the middle (a), the initial liquid film is shown. Figure 4 (b) is the gasification core. Figure 4 In the middle (c), the gasification nucleus increases in size. Figure 4 In the middle (d), the liquid film cracks;

[0039] Figure 5A schematic diagram classifying the membrane layup states of a wall surface;

[0040] Figure 6 This is a volume fraction versus pressure contour plot at the liquid film; where, Figure 6 (a) is the liquid volume fraction diagram. Figure 6 (b) shows the pressure diagram at the liquid film.

[0041] Figure 7 Pressure contour map at the liquid film rupture point;

[0042] Figure 8 It is a liquid film state on the wall; among which, Figure 8 (a) is a cloud diagram showing the liquid volume fraction at the wall surface. Figure 8 (b) is the core gasification region. Figure 8 (c) represents the liquid film splitting region;

[0043] Figure 9 The changes in the film-coating state and evaporation rate on the wall surface with varying wall superheat; among which, Figure 9 (a) shows the liquid film region defined using existing techniques. Figure 9 (b) shows the liquid film region defined using the method of the present invention;

[0044] Figure 10 This is a schematic diagram of a small-scale thin-film evaporator, in which... Figure 10 Image (a) is a schematic diagram of the three-dimensional structure. Figure 10 (b) is a top view;

[0045] Figure 11 Verification of mesh independence for small models;

[0046] Among them, 1-circular wave, 2-liquid stream, 3-liquid film, 4-wall surface, 5-gasification core area, 6-cracked area, 7-water vapor, 8-liquid, 9-outlet, 10-scraper, 12-rotor. Detailed Implementation

[0047] 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.

[0048] The method for adjusting the heating process of a thin-film evaporator based on the wall film area is as follows:

[0049] (1) The internal flow field of the thin film evaporator was simulated using CFD-POST post-processing software. The viscosity of the material in the thin film evaporator at 25℃ was 0.5~6000 Pa·s. The liquid volume fraction of the grid cells on the wall surface in the internal flow field of the thin film evaporator was used as the criterion. The region with a liquid volume fraction above 0.5 was divided into the liquid film region, and the region with a liquid volume fraction less than 0.5 was divided into the dry wall region. At the same time, the data of the proportion of liquid film region and dry wall region were directly obtained in CFD-POST post-processing software.

[0050] Among them, each pixel of the gas-liquid two-phase distribution cloud map of the internal flow field of the thin film evaporator has corresponding volume fraction data. The liquid phase volume fraction refers to the ratio of the volume of the liquid phase at the center of each grid cell in the gas-liquid two-phase distribution cloud map of the internal flow field of the thin film evaporator to the total volume of the liquid phase and the gas phase.

[0051] (2) Pressure judgment is performed on the dry wall area. In the CFD-POST post-processing software, the area with pressure ≥1Pa is divided into the vaporization core area and the area with pressure <1Pa is divided into the liquid film cracking area. At the same time, the data of the proportion of vaporization core area and liquid film cracking area are directly obtained in the CFD-POST post-processing software.

[0052] Liquid film such as Figure 2 The red area shows a thin film spread on the wall surface 4, while the liquid film 3 in the evaporation field is not like... Figure 1 The liquid film in the flow field is uniform and continuous. Due to the heating of the wall surface 4, a phase change occurs in the material at the wall surface 4, and a portion of the material changes into water vapor 7, which adheres to the wall surface 4 and forms... Figure 2 The blue bubble region near the wall phase diagram is the vaporization core region 5 in the dry wall. As the material continues to absorb heat, the vaporization cores inside the liquid film continue to grow. When the vaporization core region 5 continues to absorb heat and grow, it will break the liquid film 3 and become cracked. These vaporization core regions 5 are ring-shaped in the circumferential direction. Unlike the cracked liquid film region 6, the water vapor 7 inside the vaporization core region 5 is not completely in contact with the outside air. A thin film adheres to the outside.

[0053] The heat transfer coefficient at the liquid film location on the same wall surface under different film-layout conditions is approximately twice that at the vaporization nucleus and about five times that at the liquid film crack location. The wall heat transfer coefficient at the liquid film crack location and the vaporization nucleus location is much smaller than that at the liquid film location.

[0054] (3) Calculate the proportion of the gasification core area in the wall film area. The proportion of the gasification core area = the area of ​​the gasification core area / (the area of ​​the liquid film area + the area of ​​the dry wall area).

[0055] (4) By adjusting the superheat, the proportion of the vaporization core area in the wall film area reaches its peak value. The corresponding superheat is the optimal superheat at the maximum evaporation rate. Superheat refers to the difference between the wall temperature and the saturation temperature. The peak proportion of the vaporization core area in the wall film area is 35-45%.

[0056] (5) Apply the optimal superheat at the maximum evaporation rate obtained in step (4) to actual production so that the thin film evaporator can obtain the optimal evaporation capacity.

[0057] The following specific embodiments illustrate a method for adjusting the heating process of a thin-film evaporator based on the wall film area, as described in detail below:

[0058] In thin-film evaporators, the viscosity range of materials is extremely wide. For low-viscosity materials, there is no need for scrapers at all, as the material will flow down the wall. This is the characteristic of falling film evaporators.

[0059] The material in the small model is an aqueous solution of CMC (carboxymethyl cellulose), with a viscosity of 4.5 Pa·s at 25°C.

[0060] The material in the large model is a lyocell (cellulose / N-methylmorpholine / water) solution with a viscosity of 4000 Pa·s at 25°C. Ultra-high viscosity materials must be simulated using a large model; using a small model will result in distorted data or even make calculations impossible.

[0061] (a) Small model approach:

[0062] (1) The area of ​​the film covering the wall is observed by simplifying the three-dimensional small model. Due to the large size of the large model, the number of meshes is large and the quality is difficult to guarantee, resulting in a long calculation cycle. Even if the calculation is done in segments, the number of meshes in a single segment is as high as 20 million, which requires huge resources. The simplified small model is small in size and its structure is scaled down proportionally to the large model, so it has a certain degree of accuracy. By simplifying the small model, we can ensure the rapid observation of evaporation characteristics, verify the large model, shorten the working cycle, and save time.

[0063] The simplified small model of this invention is reduced to approximately one-ninetieth of the large model.

[0064] (2) After simplifying the small model, the parameters of the simplified thin film evaporator small model are obtained, including the height H of the scraping section, the outer diameter R of the scraping section, the number of scraper columns in the scraping section, and the gap δ between the structure and the heating wall. According to the experimental data in the literature (Flow and Mixing Characteristics in an Agitated Thin-Film Evaporator with Vertically Aligned Blades[J]. Journal of Chemical Engineering of Japan, 1989, 22(04):346-351.), the following parameters are obtained: height H=150mm, outer diameter R=125mm, gap δ=1mm, and number of scraper columns 8.

[0065] (3) such as Figure 10 As shown, a small model is constructed based on the simplified small model parameters, and mesh independence is verified.

[0066] By refining the grid and comparing changes in key parameters, the indifference can be determined when the error between two adjacent results is less than 5%.

[0067] Increasing the number of grid cells increases computational accuracy but also incurs significant computational power and time costs. Therefore, once a certain number of grid cells is reached, key parameters remain essentially unchanged. Thus, grid independence verification is performed to prove that the number of grid cells will not cause distortion in the calculation of key parameters. Figure 11 The graph shown depicts the torque of rotor 12 at different grid numbers. Figure 11 The mesh independence verification shown applies when the number of meshes is greater than 1000 × 10⁻⁶. 4 The torque remains essentially constant.

[0068] Small models constructed, such as Figure 10 As shown, a hollow cylindrical thin-film evaporator with a diameter of 250 mm and a height of 150 mm is used, and the gap between the tip of the scraper 10 and the wall surface 4 is 1 mm.

[0069] (4) Set the internal process parameters of the small model and simulate the physical properties of the medium (CMC aqueous solution), including viscosity. and saturation temperature (90℃) is used as a quantitative parameter, while the wall temperature The parameters are input into Ansys Fluent as variables, and the curve of the wall film area ratio as a function of superheat is output. The evaporation rate is detected by the steam mass flow rate at outlet 9.

[0070] Each simulation keeps all other parameters unchanged except for the wall superheat parameter. The simulation results are then quantitatively analyzed to extract the evaporation rate for comparison. In Ansys Fluent, the mass flow rate at the outlet can be selected to monitor the gas phase (steam) over iterations. Once the mass flow rate stabilizes after each iteration, the curve showing the change in the wall film area percentage with superheat can be obtained.

[0071] Among them, the physical property parameter viscosity as follows:

[0072] Table 1 Material Parameter Table

[0073]

[0074] in, This is the consistency coefficient. Non-Newtonian exponents Zero shear viscosity It is an infinite shear viscosity;

[0075] (5) The wall membrane laying area is divided by CFD-POST post-processing, such as... Figure 5 As shown, the thin-film evaporator is divided into liquid film regions (liquid phase volume fraction V) based on the film deposition on the wall surface. f ≥0.5) and dry wall region (liquid volume fraction V) f <0.5), and the dry wall region is further divided into the vaporization core region (pressure p≥1Pa) and the liquid film cracking region (pressure p<1Pa).

[0076] The division between the liquid film region and the dry wall region is based on the liquid phase volume fraction near the wall surface. Regions with a volume fraction above 0.5% are considered liquid film regions, while regions with a volume fraction below 0.5% are considered dry wall regions. In the dry wall region, because the liquid film undergoes a phase change near the wall surface into a gas during heat exchange, the liquid phase volume fraction is extremely small. Therefore, the liquid phase volume fraction can be used to distinguish between the liquid film region and the dry wall region.

[0077] The state of the wall surface is distinguished by the pressure difference on the surface. The area with a volume fraction below 0.5 and a pressure above 1 Pa is the vaporization nucleus, and the area with a volume fraction below 0.5 and a pressure below 1 Pa is the cracking area. Figure 6 The cloud map in (a) divides the liquid film region and the dry wall region. Figure 6 The cloud map in (b) delineates the gasification core region and the dry wall region; from Figure 6It can be observed that the pressure is lower near the wall where the liquid film is higher, and higher at the vaporization core. This is because while the liquid film exchanges heat with the wall, the liquid near the wall undergoes a phase change and turns into gas. The gas expands due to heat, causing the pressure inside the vaporization core to be greater than the pressure at the liquid film. Figure 7 As can be seen from the figure, the pressure in the liquid film rupture region 6 is very low, only 0.1658 Pa. This is because a phase change continues to occur at the vaporization core, and the generated gas causes the vaporization core to grow continuously until the surface tension of the liquid film can no longer withstand the huge pressure inside the vaporization core, at which point the liquid film will rupture. The pressure at the location of the ruptured liquid film is consistent with the ambient pressure in the evaporator.

[0078] like Figure 8 As shown, taking a wall with a superheat of 4°C as an example, Figure 8 In (a), the liquid film area on the wall is 37%; Figure 8 In the middle (b) gasification core, the proportion is 32%; Figure 8 The proportion of cracked liquid film in (c) is 31%.

[0079] (6) Determine the optimal superheat at the maximum evaporation rate and the optimal wall film coverage ratio at the optimal superheat based on the obtained data;

[0080] The judgment is based on a comparison of the data extracted during the model simulation process (the data obtained in step (4) above). Figure 9 (b) When the wall superheat is 3°C, the evaporation rate and the proportion of the vaporization core region reach their peak. Therefore, the optimal superheat is determined to be 3°C, and the proportion of the vaporization core region under the optimal superheat reaches its peak, which is 40.7%.

[0081] like Figure 9 As shown in (a), the traditional method fails to distinguish between the dry wall region and the liquid film cracking region, and the evaporation rate is not correlated with the liquid film deposition rate. Figure 9 In (b), the dry wall region is further subdivided. The evaporation rate reaches its peak at a superheat of 3°C, coinciding with the peak proportion of the vaporization core region at the current superheat. Taking a superheat of 3°C as the peak value, the number of vaporization cores before the peak increases with increasing superheat. Due to the increase in energy, the liquid film region undergoes phase change vaporization. After the peak value, the number of vaporization cores decreases with increasing superheat. As energy increases, the phase change vaporization of the vaporization cores breaks through the liquid film, transforming it into liquid film cracking. Therefore, at the initial peak position, when a large number of vaporization cores are about to transform into the liquid film cracking state but have not yet transformed, the corresponding evaporation rate reaches its peak.

[0082] Therefore, under the current operating conditions, there must be an optimal wall superheat corresponding to the peak evaporation rate, so that the wall is in a state of vaporization that is about to be converted into a liquid film cracking state but has not yet been converted. At this time, the evaporation rate is optimal and the evaporation effect is the best.

[0083] (II) Large-scale model validation:

[0084] (1) Divide the scraped film section of the 50,000-ton / year large-capacity thin film evaporator into four equal sections and obtain the parameters of the scraped film evaporator, including the height H of the scraped film section, the radius of the scraped film section, and the gap δ between the scraper and the heating wall in the scraped film section. Simplify according to the axial direction and the periodic model.

[0085] (2) Obtain the temperature of the heating wall surface when the scraped film evaporator processes materials. Temperature of the liquid film Viscosity of the liquid film Rotor speed (90rpm) Input to Ansys Fluent.

[0086] Among them, liquid film viscosity for:

[0087] Table 2 Material Properties

[0088]

[0089] Where λ is the relaxation time.

[0090] (3) The effect of evaporation on the saturation temperature of the material saturation temperature was assumed for the large-capacity thin film evaporator, and the saturation temperature of the material was controlled during the simulation by using UDF (User-Defined Function).

[0091] (4) The corresponding four-stage large model, , , The data are input into Ansys Fluent, which outputs the percentage of the three types of wall film coverage for each of the four stages, as well as the total evaporation data for each stage.

[0092] (5) Among the evaporation rates of each stage under a capacity of 50,000 tons / year, the largest evaporation rate is in the second stage. The wall film area of ​​each stage is shown in Table 3. In the second stage, the wall film area ratio is similar to the optimal wall film area ratio in the small model. The optimal liquid film area ratio is in the range of 40% to 50%, the optimal gasification core ratio is in the range of 35% to 45%, and the optimal cracked liquid film area ratio is in the range of 10% to 20%.

[0093] Table 3. Wall membrane laying status at four stages

[0094]

[0095] When the evaporation rate of a thin-film evaporator reaches its maximum, the proportion of each section on the wall surface tends to a constant value, indicating the range of the optimal wall film deposition state. In actual production, to improve the evaporation capacity of a thin-film evaporator, parameters and structure are usually adjusted. During these adjustments, heat transfer deterioration is often encountered. Controlling the optimal wall film deposition state can serve as a method for monitoring this deterioration, providing theoretical support and methodological guidance for enterprises.

[0096] (6) Verification of large-scale models and actual parameters;

[0097] The simulated total evaporation rate (1297 g / s) obtained by this model has an error of 12.1% compared with the actual operating conditions. The simulated annual production capacity is 9% higher than the parameters under actual operating conditions. The error is within an acceptable range. After verification, the production capacity of the four stages is consistent with the actual operating conditions and is applicable to actual operating conditions.

[0098] This invention finds the relationship between the gasification core and the optimal evaporation rate by changing statistical methods, and uses this relationship to guide the heating process. Previously, a random superheat was used, and adjustments were made gradually, without knowing whether to increase or decrease it, requiring numerous trials and simulations, resulting in enormous time costs. However, with the statistical method for calculating the wall film area, focusing on the proportions of these three factors allows for direct and rapid adjustment of the superheat, significantly saving verification time.

Claims

1. A method for adjusting the heating process of a thin-film evaporator based on the wall film area, characterized in that... Includes the following steps: (1) Based on the liquid volume fraction of the grid cells on the wall surface in the internal flow field of the thin film evaporator, the region with a liquid volume fraction of more than 0.5 is divided into the liquid film region, and the region with a liquid volume fraction of less than 0.5 is divided into the dry wall region. (2) For the dry wall region, the region with pressure ≥ 1 Pa is divided into the vaporization core region, and the region with pressure < 1 Pa is divided into the liquid film cracking region. (3) Calculate the proportion of the gasification core area in the wall film area. The proportion of the gasification core area = the area of ​​the gasification core area / (the area of ​​the liquid film area + the area of ​​the dry wall area). (4) By adjusting the superheat, the proportion of the vaporization core area in the wall film area reaches the peak value. The superheat corresponding to this point is the optimal superheat at the maximum evaporation rate. (5) Apply the optimal superheat at the maximum evaporation rate obtained in step (4) to actual production so that the thin film evaporator can obtain the optimal evaporation capacity; In step (4), superheat refers to the difference between the wall temperature and the saturation temperature.

2. The method for adjusting the heating process of a thin-film evaporator based on the wall film area according to claim 1, characterized in that, The viscosity of the material in the thin-film evaporator at 25°C is 0.5~6000 Pa·s.

3. The method for adjusting the heating process of a thin-film evaporator based on the wall film area according to claim 1, characterized in that, Each pixel in the gas-liquid two-phase distribution cloud map of the internal flow field of the thin film evaporator has corresponding volume fraction data. The liquid phase volume fraction refers to the ratio of the volume of the liquid phase at the center of each grid cell in the gas-liquid two-phase distribution cloud map of the internal flow field of the thin film evaporator to the total volume of the liquid and gas phases.

4. The method for adjusting the heating process of a thin-film evaporator based on the wall film area according to claim 1, characterized in that, The division of each region in steps (1) and (2) is performed in the CFD-POST post-processing software.

5. The method for adjusting the heating process of a thin-film evaporator based on the wall film area according to claim 4, characterized in that, Steps (1) to (3) are as follows: The internal flow field of the thin film evaporator is simulated using CFD-POST post-processing software. In the internal flow field of the thin film evaporator, near the wall, the liquid film area and the dry wall area are screened with a liquid phase volume fraction of 0.5 as the boundary. At the same time, the data of the proportion of the liquid film area and the dry wall area are directly obtained in the CFD-POST post-processing software. Next, the pressure of the dry wall area is judged, and the vaporization core area and the cracking area are screened with 1 Pa as the boundary. At the same time, the data of the proportion of the vaporization core area and the liquid film cracking area are directly obtained in the CFD-POST post-processing software. Finally, the proportion of the film-covered area on the wall is calculated.

6. The method for adjusting the heating process of a thin-film evaporator based on the wall film area according to claim 1, characterized in that, In step (4), the peak percentage of the gasification core area in the wall film area is 35-45%.

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