Calculation Method and Application of the Theoretical Torque of the Rotor of a Scraped-Film Evaporator

Abstract

The present invention belongs to the technical field of fluid simulation and optimization of wiped film evaporators, and particularly relates to a calculation method and application of the theoretical torque of the rotor of a wiped film evaporator. The present invention proposes a theoretical torque splitting calculation method based on power characteristics, which splits the total power consumption of the wiped film evaporator into viscous torque and liquid mass load torque, reveals the contribution mechanism of different operating parameters to power, accurately locates the optimization direction, clarifies the correlation between the dynamic characteristics of the flow field and power; the theoretical torque splitting method proposed by the present invention realizes the accurate quantification of power consumption, provides a theoretical basis and a systematic solution for the energy-saving design of wiped film evaporators, and has engineering application value.

Description

Technical Field

[0001] The present invention belongs to the technical field of fluid simulation and optimization of wiped film evaporators, and particularly relates to a calculation method and application of the theoretical torque of the rotor of a wiped film evaporator. Background Art

[0002] At present, with the continuous increase in global energy demand, the problems of energy consumption and environmental pollution are becoming increasingly severe. How to improve energy utilization efficiency while reducing energy consumption and emissions has become the focus of global attention. In the industrial field, as a key heat exchange device, the energy efficiency of a wiped film evaporator directly affects the energy-saving and consumption-reduction effect of the entire system. Therefore, optimizing the design and operation mode of the wiped film evaporator and reducing power consumption have become one of the core technologies to enhance industrial energy-saving effects.

[0003] Wiped film evaporators are widely used in industries such as chemical engineering, food, and pharmaceuticals. Especially when dealing with the evaporation of high-viscosity liquids, they exhibit high thermal efficiency and thermal stability. This evaporator mainly consists of a heating jacket and a scraper. Heating steam is introduced into the inner part of the jacket, and the scraper is installed on a rotatable shaft. The gap between the scraper and the inner wall of the heating jacket is extremely small, generally ranging from 0.5 to 1.5 millimeters. The preheated feed liquid enters from the upper part of the evaporator along the tangential direction. Under the combined action of gravity and the rotating scraper, a downward-spiraling film is formed on the inner wall. During the downward process, the liquid continuously evaporates and concentrates, and finally is discharged from the bottom, while the secondary steam escapes from the top. However, due to its special structure, the energy efficiency and processing capacity of the wiped film evaporator are restricted by factors such as the flow field distribution and power consumption.

[0004] Currently, the research on the flow field of wiped film evaporators is mostly based on the structure of in-line scraper blades, and often simplifies to only retain four rows of blades. Under this assumption, a classical flow field morphology theory has been formed. As Figure 1 shown, the flow field of the in-line wiped film evaporator includes a liquid film and a liquid mass. The liquid film forms a thin layer on wall 1 and relies on scraper 2 to maintain uniformity. The liquid mass is the accumulation area of the liquid in front of scraper 2. Outside wall 1 is the heating shaft sleeve 3, and scraper 2 is driven by rotating shaft 4. The power consumption of the wiped film evaporator mainly comes from the shear force of the liquid film and the load of the liquid mass. The shear force of the liquid film is generated by the relative movement of the liquid and the wall, causing a certain viscous resistance; the load of the liquid mass comes from its own mass and the action of gravity.

[0005] The prior art has studied the energy efficiency optimization of wiped film evaporators. For example, the literature (Study on the scale-up of multi-stage agitators in laminar and transitional flow states: A computational fluid dynamics method [J]. Chemical Engineering Science, 2002, 57(21): 4617-4632.) used computational fluid dynamics (CFD) technology to study three multi-stage agitator paddles with different size ratios, obtained the corresponding power characteristic curves, and verified the reliability of the simulation results through experimental data. The literature (Numerical simulation of the flow field characteristics of non-Newtonian fluids in a wiped film evaporator [J]. Acta Physica Sinica, 2022, 71(05): 197-208.) numerically simulated the non-Newtonian fluids in a wiped film evaporator to explore the influence of the bending angle and rotational speed of the wiper on the rotor torque, and obtained the influence law of process parameters on power.

[0006] However, after obtaining the torque of the rotor of the wiped film evaporator through simulation in these prior arts, multiplying it by the angular velocity to get the power result, it is difficult to explore the specific factors affecting the power by this method, and thus it is difficult to guide the production practice. Summary of the Invention

[0007] The purpose of the present invention is to solve the problems existing in the prior art, and provide a calculation method and application of the theoretical torque of the rotor of a wiped film evaporator.

[0008] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0009] A calculation method of the theoretical torque of the rotor of a wiped film evaporator adopts the following formula:

[0010] ;

[0011] In the formula, represents the theoretical torque of the rotor of the wiped film evaporator, with the unit of N·m, which can be converted to kg·m 2 ·S -2 ;

[0012] represents the viscous torque of the rotor of the wiped film evaporator, with the unit of N·m; is one of the key factors affecting power consumption, related to the relative shear between fluid layers, and its magnitude is closely related to the viscosity of the fluid, the flow velocity gradient, and the liquid film area;

[0013] represents the liquid mass load torque of the rotor of the wiped film evaporator, that is, the load torque generated by the mass and gravity of the liquid mass itself, with the unit of N·m;

[0014] ;

[0015] In the formula, represents the moment arm, with the unit of mm;

[0016] represents the dynamic viscosity of the material, with the unit of Pa·s, which can be converted to kg·m -1 ·S -1 ;

[0017] represents the distance between the heating wall surface of the wiped-film evaporator and the central axis of the rotor, with the unit of mm;

[0018] represents the film thickness of the liquid film, with the unit of mm;

[0019] represents the circumferential contact angle, with the unit of °;

[0020] H represents the height of the wiped-film evaporator, with the unit of mm;

[0021] represents the shear rate of the liquid film flow, with the unit of s -1 ; where represents the velocity component of the fluid in the x direction, i.e., the velocity parallel to the wall surface; y represents the vertical distance between the fluid data point and the wall surface; describes the rate of change of the velocity of the fluid in the direction perpendicular to the flow direction (y direction), reflecting the relative movement between fluid layers;

[0022] ;

[0023] In the formula, represents the density of the material, with the unit of kg / m 3 ;

[0024] represents the total volume of the material in the wiped-film evaporator when the three-dimensional flow field is stable, with the unit of , and the three-dimensional flow field is stable when the following conditions are simultaneously met:

[0025] Condition 1: The residual curve (i.e., the convergence curve) starts to show a continuous downward trend, finally flattens out, reaches the order of 10 -3 magnitude, and remains at the order of 10 -3 magnitude throughout the simulation iteration;

[0026] Condition 2: The moment curve flattens out, with a fluctuation amplitude of less than 5% within 1000 iteration steps;

[0027] Condition 3: The outlet flow rate curve starts to show an upward trend and finally flattens out, that is, the difference between it and the flow rate set at the inlet remains within 5% within 1000 iteration steps;

[0028] represents the acceleration due to gravity, with a value of 9.81 m·s -2 ; The magnitude of is directly related to the volume of the liquid mass (i.e., the mass of the liquid mass). When the volume of the liquid mass increases, increases accordingly, thereby increasing the power.

[0029] As a preferred technical solution:

[0030] For the calculation method of the theoretical torque of the rotor of a scraper thin-film evaporator as described above, The process of obtaining the value of is as follows: After obtaining a stable three-dimensional flow field of the scraper thin-film evaporator through Fluent software simulation, use the CFD-Post post-processing software to extract the three-dimensional flow field diagram, then obtain the axial cross-section contour map of the three-dimensional flow field diagram, screen out the grids with a liquid volume fraction of 0.5 in the liquid part, calculate the distance between them and the heating wall surface, and then take the average value to obtain the value.

[0031] For the calculation method of the theoretical torque of the rotor of a scraper thin-film evaporator as described above, The process of obtaining the value of is as follows: First, after obtaining a stable three-dimensional flow field of the scraper thin-film evaporator through Fluent software simulation, use the CFD-Post post-processing software to extract the three-dimensional flow field diagram, then obtain the circumferential cross-section contour map of the three-dimensional flow field diagram, then remove the annular region (i.e., the liquid film part) with a width of in the outermost circle of the circumferential cross-section contour map, screen out the grids with a liquid volume fraction of 0.5 in the remaining region, use the least squares method to fit these grids around each liquid mass to obtain the circular boundary of each liquid mass, and finally calculate the distance between the center of the circular boundary of each liquid mass and the central axis of the rotor, and then take the average value to obtain the value.

[0032] For the calculation method of the theoretical torque of the rotor of a scraper thin-film evaporator as described above, The value of is obtained by using the CFD-Post post-processing software to extract the average dynamic viscosity at the junction of the liquid mass and the liquid film after obtaining a stable three-dimensional flow field of the scraper thin-film evaporator through Fluent software simulation.

[0033] For the calculation method of the theoretical torque of the rotor of a scraper thin-film evaporator as described above, The value of is obtained by using the CFD-Post post-processing software to extract the central angle corresponding to the arc surface in the contact area between the liquid mass and the liquid film after obtaining a stable three-dimensional flow field of the scraper thin-film evaporator through Fluent software simulation.

[0034] The calculation method of the theoretical torque of the rotor of a scraping thin-film evaporator as described above The value of is obtained by using the CFD-Post post-processing software to extract the average shear rate at the junction of the liquid mass and the liquid film after simulating the three-dimensional flow field of the stable scraping thin-film evaporator with the Fluent software.

[0035] The calculation method of the theoretical torque of the rotor of a scraping thin-film evaporator as described above The process of obtaining the value of is as follows: Use the Fluent software to simulate, and during this process, monitor the liquid holdup data of the scraping thin-film evaporator in real time. After obtaining the stable three-dimensional flow field of the scraping thin-film evaporator, directly extract the corresponding liquid holdup data from the data file exported by the real-time monitoring, and then obtain the value of the value of.

[0036] The present invention also provides a method for reducing the power of a scraping thin-film evaporator. After calculating the viscous torque of the rotor of the scraping thin-film evaporator and the liquid mass load torque of the rotor of the scraping thin-film evaporator respectively, compare the two. If the viscous torque of the rotor of the scraping thin-film evaporator is larger, then reduce the scraping speed and / or increase the scraping gap; if the viscous torque of the rotor of the scraping thin-film evaporator is smaller or the two are equal, then reduce the feed rate;

[0037] The calculation adopts the calculation method of the theoretical torque of the rotor of a scraping thin-film evaporator as described in any one of the above.

[0038] Beneficial effects:

[0039] 1. The present invention proposes a theoretical torque splitting calculation method based on power characteristics. This method splits the total power consumption into viscous torque and liquid mass load torque, reveals the contribution mechanism of different operating parameters to power, accurately locates the optimization direction, and clarifies the relationship between the dynamic characteristics of the flow field and power;

[0040] 2. The theoretical torque splitting method proposed by the present invention realizes the accurate quantification of power consumption, provides a theoretical basis and a systematic solution for the energy-saving design and efficient operation of scraping thin-film evaporators, and has significant engineering application value. Description of the drawings

[0041] Figure 1 is a schematic diagram of a scraping thin-film evaporator ( Figure 1 a) therein) and a schematic diagram of the flow field ( Figure 1 b) therein);

[0042] Figure 2 is a diagram of a similar slit flow field;

[0043] Figure 3 is a three-dimensional flow field diagram of a stable scraping thin-film evaporator ( Figure 3In a), the circumferential cross-sectional cloud diagram of the three-dimensional flow field of the stable scraping thin-film evaporator ( Figure 3 in b);

[0044] Figure 4 is the axial cross-sectional cloud diagram of the three-dimensional flow field of the stable scraping thin-film evaporator and the schematic diagram of the calculation method of the liquid film thickness;

[0045] Figure 5 is the fitting result of the data on the periphery of the liquid mass;

[0046] Figure 6 is the schematic diagram of the contact area between the liquid mass and the liquid film;

[0047] Figure 7 In a, the simulated torque of the rotor of the scraping thin-film evaporator at different rotational speeds, Figure 7 in b is the theoretical torque of the rotor of the scraping thin-film evaporator at different rotational speeds and its components (viscous torque, liquid mass load);

[0048] Figure 8 is that in a, the simulated torque of the rotor of the scraping thin-film evaporator at different scraping clearances, Figure 8 in b is the theoretical torque of the rotor of the scraping thin-film evaporator at different scraping clearances and its components (viscous torque, liquid mass load);

[0049] Figure 9 is that in a, the simulated torque of the rotor of the scraping thin-film evaporator at different feed rates, Figure 9 in b is the theoretical torque of the rotor of the scraping thin-film evaporator at different feed rates and its components (viscous torque, liquid mass load);

[0050] Among them, 1 - wall surface, 2 - scraper, 3 - heating shaft sleeve, 4 - rotating shaft. Specific embodiments

[0051] The present invention will be further described below in conjunction with specific embodiments. 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.

[0052] The power consumption of the scraping thin-film evaporator stems from the mechanical energy dissipation and load work in the fluid flow process, and its physical mechanism can be systematically analyzed based on the theories of fluid dynamics and rigid body mechanics; the splitting theory of power consumption is based on the principle of energy conservation, and decomposes the total power into two parts: viscous shear power and liquid mass load power, corresponding to the energy dissipation of fluid viscous effect and gravity action respectively;

[0053] Shear rate of liquid film flow Characterize the relative motion intensity between fluid layers, the magnitude of which is affected by the rotational speed of the scraper and the thickness of the liquid film; as Figure 2 shown, in the simplified model, assuming that the velocity distribution in the liquid film is linear, then:

[0054] ;

[0055] wherein, represents the shear rate of the liquid film flow, with the unit of s -1 ; represents the velocity difference between the top and bottom of the liquid film (as Figure 2 shown, = , in the formula, represents the top velocity, represents the bottom velocity), with the unit of m / s; represents the gap height, with the unit of m;

[0056] Viscous shear power is the energy dissipation generated by the fluid near the wall due to the velocity gradient, and its theoretical basis is the Navier-Stokes equation and Newton's law of internal friction; under laminar flow conditions, the shear stress is proportional to the shear rate of the liquid film flow, that is:

[0057] ;

[0058] wherein, represents the shear stress, with the unit of kg·m -1 ·s -2 ; represents the dynamic viscosity of the material, with the unit of Pa·s (which can be converted to kg·m -1 ·s -1 );

[0059] The shear stress acts on the effective area of the liquid film, and then forms a part of the viscous torque term that constitutes the rotor of the scraping thin-film evaporator:

[0060] ;

[0061] wherein, represents the viscous torque of the rotor of the scraping thin-film evaporator, with the unit of N·m; represents the moment arm (i.e., the distance from the centroid of the liquid mass to the axis of rotation), with the unit of mm; represents the contact area between the liquid mass and the liquid film, with the unit of m 2 ;

[0062] The viscous power corresponding to the viscous torque of the rotor of the scraping thin-film evaporator is:

[0063] ;

[0064] Among them, represents the viscous power corresponding to the viscous torque of the rotor of the scraper thin-film evaporator, with the unit of W; represents the angular velocity, with the unit of rad / s;

[0065] The liquid mass load power is generated by the work done by the torque of gravity on the centroid of the liquid mass, and its theoretical model is based on the rigid body statics and the law of conservation of energy; for the mechanical analysis of the gravity torque, the liquid mass is affected by gravity, so the liquid mass load torque term of the rotor of the scraper thin-film evaporator is:

[0066] ;

[0067] Among them, represents the liquid mass load torque of the rotor of the scraper thin-film evaporator, with the unit of N·m; represents the liquid mass, with the unit of kg; represents the acceleration due to gravity, and its value is 9.81 m·s -2 ;

[0068] The load power corresponding to the liquid mass load torque of the rotor of the scraper thin-film evaporator is:

[0069] ;

[0070] Among them, represents the load power corresponding to the liquid mass load torque of the rotor of the scraper thin-film evaporator, with the unit of W;

[0071] The liquid mass depends on the scraper gap and the flow field distribution; when the scraper gap decreases, the volume of the liquid mass in front of the scraper increases, resulting in an increase in the liquid mass; and the scraper rotation speed changes the dynamic accumulation form of the liquid mass through centrifugal force, indirectly affecting the instantaneous value of the liquid mass.

[0072] Based on the above physical mechanisms, the total power of the scraper thin-film evaporator can be expressed as the linear superposition of the viscous power and the load power:

[0073] ;

[0074] Among them, represents the total power of the scraper thin-film evaporator, with the unit of W;

[0075] This formula establishes the quantitative relationship between the power consumption and the flow field parameters and the operating parameters , and its theoretical rationality is based on the following assumptions and boundary conditions:

[0076] a. Quasi-steady flow assumption, that is, the dynamic change frequency of the liquid film and the liquid mass is much lower than the rotor rotation frequency, and the instantaneous power can be approximated as the steady-state value;

[0077] b. The centroid equivalent hypothesis, that is, the gravitational force of the liquid mass is equivalent to the concentrated load at the centroid, and the change of the centroid position with the dynamic flow field can be ignored;

[0078] c. The thickness of the liquid film is jointly restricted by the scraper clearance and the fluid flow rate, satisfying the mass conservation condition.

[0079] Based on the above research, the present invention proposes a calculation method for the theoretical torque of the rotor of a scraper thin-film evaporator, using the following formula:

[0080] ;

[0081] In the formula, represents the theoretical torque of the rotor of the scraper thin-film evaporator, with the unit of N·m, which can be converted to kg·m 2 ·S -2 ;

[0082] represents the viscous torque of the rotor of the scraper thin-film evaporator, with the unit of N·m; is one of the key factors affecting power consumption, related to the relative shear between fluid layers, and its magnitude is closely related to the viscosity of the fluid, the flow velocity gradient, and the liquid film area;

[0083] represents the liquid mass load torque of the rotor of the scraper thin-film evaporator, that is, the load torque generated by the mass and gravitational force of the liquid mass itself, with the unit of N·m;

[0084] ;

[0085] In the formula, represents the moment arm, with the unit of mm;

[0086] represents the dynamic viscosity of the material, with the unit of Pa·s, which can be converted to kg·m -1 ·S -1 ;

[0087] represents the distance between the heating wall surface of the scraper thin-film evaporator and the central axis of the rotor, with the unit of mm;

[0088] represents the thickness of the liquid film, with the unit of mm;

[0089] represents the circumferential contact angle, with the unit of °;

[0090] H represents the height of the scraper thin-film evaporator, with the unit of mm;

[0091] represents the shear rate of the liquid film flow, with the unit of s -1 ; where, represents the velocity component of the fluid in the x direction, that is, the velocity parallel to the wall; y represents the perpendicular distance between the fluid data point and the wall; describes the rate of change of the fluid velocity in the direction perpendicular to the flow direction (y direction), reflecting the relative motion between fluid layers;

[0092] ;

[0093] In the formula, represents the density of the material, with the unit of kg / m 3 ;

[0094] represents the total volume of the material in the wiped film evaporator when the three-dimensional flow field is stable, with the unit of , and the three-dimensional flow field is stable when the following conditions are simultaneously met:

[0095] Condition 1: The residual curve (i.e., the convergence curve) starts to show a continuous downward trend, and finally flattens out, reaching the order of 10 -3 order of magnitude, and remains at the order of 10 -3 order of magnitude along with the simulation iteration;

[0096] Condition 2: The torque curve flattens out, and the fluctuation amplitude is less than 5% within 1000 iteration steps;

[0097] Condition 3: The outlet flow rate curve starts to show an upward trend and finally flattens out, that is, the difference between it and the flow rate set at the inlet remains within 5% within 1000 iteration steps;

[0098] represents the acceleration due to gravity, and its value is 9.81m·s -2 ; is directly related to the volume of the liquid mass (i.e., the mass of the liquid mass). When the volume of the liquid mass increases, increases accordingly, thereby increasing the power.

[0099] To prove the correctness of the calculation method of the theoretical torque of the rotor of this wiped film evaporator, the verification is carried out according to the following steps:

[0100] S1. Modeling of the grid model of the wiped film evaporator (taking the in-line wiped film evaporator as an example);

[0101] S1.1. Import the geometric model of the scraper thin-film evaporator into Fluent Meshing for model mesh generation. Locally refine the mesh near the scraper edges, the wall, and the liquid film region to capture high-gradient flow features. Among them, the relevant parameters of the geometric model are as follows: the height of the scraper thin-film evaporator is 150 mm, the outer wall diameter is 250 mm, the inner wall diameter is 200 mm, the width of the annular feed inlet is 3 mm, the number of in-line scrapers is 4 rows, the scraper thickness is 3 mm, and the scraper clearance (i.e., the distance between the tip of the scraper blade and the wall) is 1 mm.

[0102] S1.2. Fill the computational domain with tetrahedral elements to ensure the mesh quality in complex geometric regions.

[0103] S1.3. By gradually refining the mesh and comparing the changes in key parameters (such as wall shear stress and liquid film thickness), ensure the independence of the mesh generation. Finally, generate a high-quality mesh file (in.msh format) for subsequent numerical simulation calculations.

[0104] S2. Simulation solution.

[0105] S2.1. Mesh model import: Import the generated high-quality mesh file into the discrete element simulation software Fluent and initialize the three-dimensional geometric model and discretized mesh data.

[0106] S2.2. Rotating reference frame setting: To accurately simulate the rotational motion inside the scraper thin-film evaporator, use the single rotating reference frame (SRF) method and set the scraper rotation speed (i.e., the rotation speed of the reference frame) to 90 rpm, thus activating the SRF motion model.

[0107] S2.3. Boundary condition definition: The material (low-viscosity non-Newtonian fluid) enters the scraper thin-film evaporator uniformly from the inlet at a certain mass flow rate of 0.025 kg / s. The wall boundary condition is set to an absolute velocity of 0, and the relative velocity of the scraper with respect to the fluid domain is also set to 0 to complete the boundary condition definition and physical model constraints.

[0108] S2.4. Solver configuration: The numerical calculation uses a pressure-based solver, which is particularly suitable for the simulation analysis of incompressible fluids and can efficiently and accurately handle the coupling problem between the pressure field and the velocity field. At the same time, use the SST k-ω turbulence model to improve the calculation accuracy.

[0109] The SST k-ω model combines the advantages of the k-ε model and the k-ω model through a blending function Use the k-ω model in the near-wall region and the k-ε model in the far field; in the flow field of a wiped thin-film evaporator, the interaction between the liquid film and the wall is the core issue. The SST k-ω turbulence model adopts the k-ω model in the near-wall region, which can accurately capture the low Reynolds number effect in the boundary layer, thus providing reliable data for viscous power calculation; in front of the wiper, the phenomena of liquid mass accumulation and flow separation are very significant. The SST k-ω model can better predict the flow characteristics in the separation zone by means of the shear stress transport term, while the traditional k-ε model has low accuracy in dealing with separated flows; the SST k-ω model combines the advantages of the k-ε model and the k-ω model, and can accurately simulate the flow in the near-wall region, separated flows, and flows with a wide range of Reynolds numbers, providing a reliable theoretical basis for power characteristic analysis and optimization design; therefore, by comparing the advantages and disadvantages of each model, the SST k-ω model is finally selected;

[0110] S2.5, Spatial Discretization Settings: In terms of spatial discretization, the momentum equation adopts the Second-Order Upwind Scheme, and the discrete accuracy is significantly improved through high-order interpolation methods, so as to more accurately capture the key feature regions in the flow field, such as the liquid film boundary layer and the liquid mass accumulation area;

[0111] S2.6, Convergence Monitoring Settings: To ensure the stability and reliability of the calculation results, set the residual convergence criterion to 1×10 -4 , and by monitoring the change trends of key parameters such as the outlet flow rate, the wiper torque, and the liquid hold-up of the evaporator in real time, until the simulation calculation is completely convergent, a stable three-dimensional flow field of the wiped thin-film evaporator is obtained (at this time, the value of the wiper torque is 0.025 N·m, and this value is the simulation torque of the rotor of the wiped thin-film evaporator); the three-dimensional flow field is stable when the following conditions are simultaneously met:

[0112] Condition 1: The residual curve (i.e., the convergence curve) begins to show a continuous downward trend and finally flattens out, reaching 10 -3 order of magnitude, and remains at 10 -3 order of magnitude along with the simulation iterations;

[0113] Condition 2: The torque curve flattens out, and the fluctuation amplitude is less than 5% within 1000 iteration steps;

[0114] Condition 3: The outlet flow rate curve begins to show an upward trend and finally flattens out, that is, within 1000 iteration steps, the difference between it and the flow rate set at the inlet is kept within 5%;

[0115] S2.7, Obtain the value of the moment arm the value of the film thickness of the liquid film the value of the dynamic viscosity of the material the value of the circumferential contact angle Value, shear rate of liquid film flow Value, total volume of material in a wiped thin-film evaporator when the three-dimensional flow field is stable Value, distance between the heating wall surface of the wiped thin-film evaporator and the central axis of the rotor Value;

[0116] The process of obtaining the value is as follows: Use CFD-Post post-processing software to extract the three-dimensional flow field diagram of a stable wiped thin-film evaporator, and obtain the circumferential cross-section cloud diagram of the three-dimensional flow field diagram (as shown in b in Figure 3 ), then remove the annular region with a width of in the outermost circle of the circumferential cross-section cloud diagram (i.e., the liquid film part), and screen the grids with a liquid volume fraction of 0.5 in the remaining region. Use the least squares method to fit these grids around each liquid mass to obtain the circular boundary of each liquid mass (as shown in Figure 5 ), and finally calculate the distance between the center of the circular boundary of each liquid mass and the central axis of the rotor, and take the average value to obtain the value (120.2 mm);

[0117] The process of obtaining the value is as follows: Use CFD-Post post-processing software to extract the three-dimensional flow field diagram of a stable wiped thin-film evaporator, and then obtain the axial cross-section cloud diagram of the three-dimensional flow field diagram (as shown in Figure 4 ), screen the grids with a liquid volume fraction of 0.5, and calculate the distance between them and the heating wall surface, and then take the average value to obtain the value (0.7762 mm); Figure 4 In , when the color on the left is orange, the value is close to 1, indicating the liquid part in the gas-liquid two-phase; when the color on the right is blue, the value is close to 0, indicating the gas phase part; the intermediate transition color region represents the transition region of liquid-gas phase change, and the default value of 0.5 is the gas-liquid separation numerical point;

[0118] The value is obtained by using CFD-Post post-processing software to extract the average dynamic viscosity at the junction of the liquid mass and the liquid film after obtaining the three-dimensional flow field of a stable wiped thin-film evaporator, The value is 0.117 Pa·s;

[0119] The value is obtained by using CFD-Post post-processing software to extract the central angle corresponding to the arc surface in the contact area between the liquid mass and the liquid film after obtaining the three-dimensional flow field of a stable wiped thin-film evaporator, =(4.7 - 0.7212)° = 3.9788°;

[0120] The value is obtained by extracting the average shear rate at the junction of the liquid mass and the liquid film using the CFD-Post post-processing software after obtaining the stable three-dimensional flow field of the wiped film evaporator. The value is 750 s -1 ;

[0121] The process of obtaining the value is as follows: After obtaining the stable three-dimensional flow field of the wiped film evaporator, the corresponding holdup data is directly extracted from the data file exported from the real-time monitoring, and then the value (3.42 m 3 ) is obtained;

[0122] S3. Calculate the theoretical torque of the rotor of the wiped film evaporator. The calculation formula is as follows:

[0123] ;

[0124] ;

[0125] ;

[0126] ;

[0127] ;

[0128] ;

[0129] In the formula, represents the theoretical torque of the rotor of the wiped film evaporator, with the unit of N·m; represents the viscous torque of the rotor of the wiped film evaporator, with the unit of N·m; represents the liquid mass load torque of the rotor of the wiped film evaporator, with the unit of N·m; represents the shear stress, with the unit of kg·m -1 ·s -2 ; represents the contact area between the liquid mass and the liquid film (as Figure 6 shown), with the unit of m 2 ; represents the distance between the heating wall surface of the wiped film evaporator and the central axis of the rotor (1.25 m); represents the height of the wiped film evaporator, with the unit of m; represents the liquid mass, with the unit of kg; represents the acceleration due to gravity, and its value is 9.81 m·s -2 ; represents the density of the material (low-viscosity non-Newtonian fluid) (998.2 kg / m 3); represents the volume of the liquid mass, with the unit of m 3 ; represents the volume of the liquid film, with the unit of m 3 .

[0130] Calculated = 0.02705 N·m; By comparing with the simulated torque of the rotor of the wiped-film evaporator, it can be seen that the relative error between the theoretical torque and the simulated torque is about 8.2% (not exceeding 20% is within the acceptable range), indicating that this calculation method has high accuracy.

[0131] In the prior art, after obtaining the torque of the rotor of the wiped-film evaporator through simulation, multiplying it by the angular velocity to get the power result. It is difficult to explore the specific factors affecting the power by this method. The present invention can overcome this defect, and the specific description is as follows:

[0132] At different rotational speeds (90 rpm, 120 rpm, 150 rpm), the simulated torques of the rotor of the wiped-film evaporator obtained by the method of the prior art are as Figure 7 shown in a of Figure 7 . It can be seen from this that as the rotational speed increases, the simulated torque of the rotor of the wiped-film evaporator gradually increases; the theoretical torques of the rotor of the wiped-film evaporator obtained by the method of the present invention are as

[0133] shown in b of Figure 8 . It can be seen from this that not only as the rotational speed increases, the theoretical torque of the rotor of the wiped-film evaporator gradually increases, but also it can be seen that when the rotational speed changes, the viscous torque changes significantly, while the load torque changes little. Therefore, the rotational speed mainly affects the change of the viscous torque term. If the viscosity of the material processed in actual production is relatively high, the viscous torque can be affected by changing the rotational speed, and thus the purpose of reducing the power can be achieved. Figure 8 shown in b of

[0134] Under different feed rates (0.15 kg / s, 0.25 kg / s, 0.35 kg / s), the simulated torque of the rotor of the scraping thin-film evaporator obtained by the method of the prior art is as Figure 9 shown in a of Figure 9 . It can be seen from this that as the feed rate increases, the simulated torque of the rotor of the scraping thin-film evaporator gradually increases; the theoretical torque of the rotor of the scraping thin-film evaporator obtained by the method of the present invention is as Figure 9 shown in b of Figure 9 . From this, it can not only be seen that as the feed rate increases, the theoretical torque of the rotor of the scraping thin-film evaporator gradually increases, but also that when the feed rate changes, the load torque changes significantly while the viscous torque changes little. Therefore, the feed rate mainly affects the change of the load torque term. If the volume of the material processed in actual production is large and affects the normal operation of the equipment, the load torque can be affected by changing the feed rate, thereby achieving the purpose of reducing power.

[0135] Based on the above research, the present invention proposes a method for reducing the power of a scraping thin-film evaporator. After separately calculating the viscous torque of the rotor of the scraping thin-film evaporator and the liquid mass load torque of the rotor of the scraping thin-film evaporator, the two are compared. If the viscous torque of the rotor of the scraping thin-film evaporator is large, the scraping speed is reduced and / or the scraping gap is increased; otherwise, the feed rate is reduced;

[0136] Calculate using the calculation method of the theoretical torque of the rotor of a scraping thin-film evaporator as described in any one of the above.

[0137] The above exploration process not only proves the accuracy and reliability of the proposed theoretical torque calculation method, but also provides strong theoretical support for the industrial design and optimization of scraping thin-film evaporators. By reasonably adjusting the scraping speed and the scraping gap to affect the viscous torque term, and reasonably controlling the feed rate to affect the load torque term, the power consumption of the scraping thin-film evaporator can be effectively reduced. Therefore, this method has broad application prospects and practical value.

Claims

1. A calculation method for the theoretical torque of the rotor of a scraping thin-film evaporator, characterized in that Use the following formula: M = M 粘性 + M 液团 ; In the formula, M represents the theoretical torque of the rotor of the scraper thin-film evaporator, with the unit of N·m; M 粘性 represents the viscous torque of the rotor of the scraping thin-film evaporator, with the unit of N·m; M 液团 represents the liquid mass load torque of the rotor of the scraper thin-film evaporator, with the unit of N·m; In the formula, r represents the torque arm, with the unit of mm; μ represents the dynamic viscosity of the material, with the unit of Pa·s; R represents the distance between the heating wall surface of the scraper thin-film evaporator and the central axis of the rotor, with the unit of mm; a represents the film thickness of the liquid film, with the unit of mm; θ represents the circumferential contact angle, with the unit of °; H represents the height of the scraper thin-film evaporator, with the unit of mm; represents the shear rate of the liquid film flow, in s -1 ; where ρ represents the density of the material, with the unit of kg / m 3 ; V 持液量 represents the total volume of the material in the scraping thin-film evaporator when the three-dimensional flow field is stable, with the unit of m 3 , and the three-dimensional flow field is stable when the following conditions are simultaneously met: Condition 1: The residual curve starts to show a continuous downward trend and finally flattens out, reaching the order of magnitude of 10, and remains at the order of magnitude of 10 throughout the simulation iterations; -3 -3 ​​ Condition 2: The torque curve tends to be flat, and the fluctuation amplitude is less than 5% within 1000 iteration steps; Condition 3: The outlet flow rate curve starts to show an upward trend and finally tends to be flat, that is, the difference between it and the flow rate set at the inlet is within 5% within 1000 iteration steps; g represents the acceleration due to gravity, whose value is 9.81 m·s -2 .

2. The calculation method of the theoretical torque of the rotor of a scraper thin-film evaporator according to claim 1, characterized in that, The process of obtaining the value of a is as follows: After obtaining a stable three-dimensional flow field of the scraper thin-film evaporator through simulation, extract the three-dimensional flow field diagram, then obtain the axial cross-section cloud diagram of the three-dimensional flow field diagram, screen out the grids with a liquid volume fraction of 0.5 in the liquid part, calculate the distance between them and the heating wall surface, and then take the average value to obtain the value of a.

3. The calculation method of the theoretical torque of the rotor of a scraper thin-film evaporator according to claim 2, characterized in that, The process of obtaining the value of r is as follows: First, after obtaining a stable three-dimensional flow field of the scraper thin-film evaporator through simulation, extract the three-dimensional flow field diagram, then obtain the circumferential cross-section cloud diagram of the three-dimensional flow field diagram, then remove the annular region with a width of a in the outermost circle of the circumferential cross-section cloud diagram, and screen out the grids with a liquid volume fraction of 0.5 in the remaining region. Use the least squares method to fit these grids around each liquid mass to obtain the circular boundary of each liquid mass. Finally, calculate the distance between the center of the circular boundary of each liquid mass and the central axis of the rotor, and then take the average value to obtain the value of r.

4. The calculation method of the theoretical torque of the rotor of a scraper thin-film evaporator according to claim 1, characterized in that, The value of μ is obtained by extracting the average value of the dynamic viscosity at the junction of the liquid mass and the liquid film after obtaining a stable three-dimensional flow field of the scraper thin-film evaporator through simulation.

5. The calculation method of the theoretical torque of the rotor of a scraper thin-film evaporator according to claim 1, characterized in that, The value of θ is obtained by extracting the central angle corresponding to the arc surface in the contact area between the liquid mass and the liquid film after obtaining a stable three-dimensional flow field of the scraper thin-film evaporator through simulation.

6. The calculation method of the theoretical torque of the rotor of a scraper thin-film evaporator according to claim 1, characterized in that, The value is obtained by extracting the average shear rate at the junction of the liquid mass and the liquid film after simulating the stable three-dimensional flow field of the scraper thin-film evaporator.

7. The calculation method of the theoretical torque of the rotor of a scraper thin-film evaporator according to claim 1, characterized in that, V 持液量 The process of obtaining the value of V is as follows: Using simulation, during which the liquid holdup data of the scraper thin-film evaporator is monitored in real time. After obtaining the stable three-dimensional flow field of the scraper thin-film evaporator, the corresponding liquid holdup data is directly extracted from the data file exported by real-time monitoring, and thus the value of V is obtained. 持液量 value.

8. Application of a calculation method for the theoretical torque of a rotor of a scraper thin-film evaporator according to any one of claims 1 to 7, characterized in that, After calculating the viscous torque of the rotor of the scraper thin-film evaporator and the liquid mass load torque of the rotor of the scraper thin-film evaporator respectively, compare the two. If the viscous torque of the rotor of the scraper thin-film evaporator is greater than the liquid mass load torque of the rotor of the scraper thin-film evaporator, then reduce the scraper speed and / or increase the scraper gap; if the viscous torque of the rotor of the scraper thin-film evaporator is less than the liquid mass load torque of the rotor of the scraper thin-film evaporator or the two are equal, then reduce the feed rate.

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