Immersion chilling process control method and system therefor

Abstract

The application relates to an immersion refrigeration process control method and system, and relates to the technical field of immersion refrigeration control, which is used for reducing the formation of large ice crystals in the refrigeration process of food, reducing the damage to the inside of the food, reducing energy consumption and reducing cost. The control method comprises the following steps: S1: dividing the refrigeration object into three stages in the refrigeration process, namely a precooling stage, a phase change stage and a supercooling stage; S2: controlling the flow rate of the refrigerant to be a low flow rate in the precooling stage and the supercooling stage, and controlling the flow rate of the refrigerant to be a high flow rate in the phase change stage. Further, the low flow rate is less than or equal to 4 m / s, and the high flow rate is greater than 4 m / s. The application is suitable for the field of immersion refrigeration control.

Description

Technical Field

[0001] The present invention relates to the technical field of immersion freezing control, and in particular to an immersion freezing process control method and a system thereof. Background Art

[0002] Frozen preservation technology is an important technology to improve the quality of my country's export products. Among the traditional freezing methods, air blast freezing and indirect contact freezing are widely used. These two methods have simple equipment and are easy to operate, but they have high energy consumption, long freezing time, and large ice crystals formed, resulting in poor quality of frozen products. Immersion freezing (ICF) is a process that uses low-temperature freezing liquid to directly contact food, that is, the frozen material is placed in a coolant for freezing, so as to achieve rapid freezing of food. Immersion freezing has the advantages of fast freezing rate, low energy consumption, low dry loss, and high freezing quality. It is widely used in food freezing processing. At present, immersion freezing has been studied in depth abroad, and in recent years, research in China has gradually increased.

[0003] Immersion freezing uses a liquid coolant, typically one with a high heat transfer coefficient, which accelerates freezing. Using a safe, non-toxic, low-temperature aqueous solution with a freezing point below 0°C as the coolant, the food is immersed in the freezing liquid, transferring heat to the liquid and freezing it. This freezing method achieves a much higher freezing rate than other methods. Food is immersed in the coolant to cool, preventing mechanical damage and minimizing the time required to pass through the zone of maximum ice crystal formation. Freezing speed is a key factor influencing frozen food quality, as it determines the size of the ice crystals. The faster the freezing rate, the smaller the ice crystals, and the better the food quality.

[0004] Based on the research on existing immersion freezing devices, it is found that the existing immersion freezing devices generally have the problems of high energy consumption and full power output, which leads to the problem of high cost. Summary of the Invention

[0005] The object of the present invention is to provide an immersion freezing process control method for ensuring that the formation of large ice crystals is reduced during the freezing process of food, thereby reducing damage to the interior of the food, while reducing energy consumption and reducing costs.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] The present invention provides an immersion freezing process control method, which includes the following steps:

[0008] S1: The freezing process of frozen materials is divided into three stages: pre-cooling stage, phase change stage and supercooling stage;

[0009] S2: The refrigerant flow rate is controlled to be low in the pre-cooling stage and the super-cooling stage, and is controlled to be high in the phase change stage.

[0010] Furthermore, in a preferred embodiment, a flow rate less than or equal to 4 m / s is considered a low flow rate, and a flow rate greater than 4 m / s is considered a high flow rate.

[0011] Furthermore, there is another preferred embodiment, the above S2 is specifically:

[0012] The freezing process of frozen materials under different flow rate conditions is simulated and analyzed to determine the influence of different flow rates on the freezing rate at different stages, so as to determine that the refrigerant flow rate is controlled to be low in the pre-cooling stage and the supercooling stage, and the refrigerant flow rate is controlled to be high in the phase change stage.

[0013] Furthermore, in another preferred embodiment, the above simulation analysis is specifically as follows:

[0014] S21: According to the freezing process of the frozen object, determine the material parameters of the simulation process: the heat released from the frozen object from room temperature to the freezing point temperature, the specific heat capacity, thermal conductivity and viscosity of the coolant;

[0015] S22: Construct the fluid mechanics calculation model of the coolant and the heat transfer equation of the freezing process;

[0016] S23: Use the fluid mechanics calculation model and the heat transfer equation of the freezing process as the calculation basis of the fluid simulation software;

[0017] S24: Construct a frozen model and put it into mesh drawing software for mesh drawing;

[0018] S25: Place the drawn grid into the fluid simulation calculation software for calculation and solution, input different flow rates of the coolant, and obtain the center temperature data;

[0019] S26: generating a curve showing the change of the core temperature over time based on the core temperature data, and generating a temperature cloud map and a heat transfer cloud map after processing;

[0020] S27: The influence of flow rate on freezing time and freezing effect is obtained based on the temperature cloud map and the heat transfer cloud map, so as to determine that the flow rate of the refrigerant is controlled to be low in the pre-cooling stage and the supercooling stage, and the flow rate of the refrigerant is controlled to be high in the phase change stage.

[0021] Furthermore, in a preferred embodiment, when drawing the above-mentioned grid, it is also necessary to set the fluid inflow and outflow directions, the fluid domain, the solid domain, the container wall and the grid size.

[0022] Furthermore, there is a preferred embodiment in which the above simulation analysis requires pre-processing, solution processing and post-processing.

[0023] Furthermore, there is a preferred embodiment, in which the above-mentioned solution processing is specifically as follows: setting the solution type to select the pressure equation solution, the velocity equation to absolute velocity, the time type to transient, applying the gravity term, and setting the gravity acceleration in the vertical direction to -9.81m / s2, the inlet to the velocity inlet, the outlet to the pressure outlet, and the center temperature point of the frozen object to the data extraction point.

[0024] The immersion freezing process control method of the present invention can be fully implemented using computer software. Therefore, correspondingly, the present invention also provides an immersion freezing process control system, the system comprising:

[0025] A storage device for dividing frozen materials into three stages during the freezing process: a pre-cooling stage, a phase change stage, and a supercooling stage;

[0026] A storage device for controlling the flow rate of the refrigerant to be low in the pre-cooling stage and the super-cooling stage, and controlling the flow rate of the refrigerant to be high in the phase change stage.

[0027] The present invention also provides a computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a processor, any one of the above-mentioned immersion freezing process control methods is executed.

[0028] The present invention also provides a computer device, which includes a memory and a processor, wherein a computer program is stored in the memory. When the processor runs the computer program stored in the memory, the processor executes any one of the above-mentioned immersion freezing process control methods.

[0029] The beneficial effects of the present invention are:

[0030] 1. The present invention provides an immersion freezing process control method. First, based on the computational fluid dynamics theory and heat transfer principles, a simulation model is constructed by numerical simulation software. The effects of different refrigerant flow rates on the freezing speed and freezing quality of frozen objects under constant and variable speed conditions are simulated, and the time-varying curve of the center temperature of the frozen object and the temperature distribution cloud map of the center section are obtained. The results show that as the refrigerant flow rate increases, the cooling and freezing time is shortened. Therefore, the present invention can achieve the effect of increasing the freezing rate and reducing energy consumption by detecting and controlling the flow rate and temperature during immersion freezing, using a low flow rate in the pre-cooling and supercooling stages of the freezing process, and a high flow rate in the phase change stage.

[0031] Furthermore, existing immersion freezing control methods typically use a fixed-speed method, where the refrigerant is frozen at a fixed speed. However, to ensure the freezing effect, full power output is used, resulting in high energy consumption. The present invention adopts a variable-speed control method for immersion freezing. That is, by controlling the refrigerant flow rate to a low rate during the pre-cooling and supercooling stages and a high rate during the phase change stage, the refrigerant flow rate is controlled to reduce freezing time while selecting different flow rates at different stages, avoiding full power output, reducing energy waste, and lowering costs.

[0032] The invention is applicable to the field of immersion freezing control. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Schematic diagram of three stages of the freezing process of the frozen product according to the present invention;

[0034] Figure 2 It is the simulation analysis flow chart of the present invention;

[0035] Figure 3 The freezing model of the present invention;

[0036] Figure 4 is a schematic diagram of grid division according to the present invention;

[0037] Figure 5 The temperature cloud map of the freezing chamber at v = 0 m / s and the temperature cloud map of the xy cross section of the freezing chamber are shown in the figure (a) and (b) respectively.

[0038] Figure 6 The temperature cloud map inside the freezing chamber and the temperature cloud map of the xy cross section when v=1m / s described in the present invention, wherein Figure (a) is the temperature cloud map inside the freezing chamber, and Figure (b) is the temperature cloud map of the xy cross section;

[0039] Figure 7 The temperature cloud map inside the freezing chamber and the temperature cloud map of the xy cross section when v=2m / s described in the present invention, wherein Figure (a) is the temperature cloud map inside the freezing chamber, and Figure (b) is the temperature cloud map of the xy cross section;

[0040] Figure 8 The temperature cloud map inside the freezing chamber and the temperature cloud map of the xy cross section when v=3m / s described in the present invention, wherein Figure (a) is the temperature cloud map inside the freezing chamber, and Figure (b) is the temperature cloud map of the xy cross section;

[0041] Figure 9 The temperature cloud map inside the freezing chamber and the temperature cloud map of the xy cross section when v=4m / s described in the present invention, wherein Figure (a) is the temperature cloud map inside the freezing chamber, and Figure (b) is the temperature cloud map of the xy cross section;

[0042] Figure 10 The temperature cloud map inside the freezing chamber and the temperature cloud map of the xy cross section when v=5m / s described in the present invention, wherein Figure (a) is the temperature cloud map inside the freezing chamber, and Figure (b) is the temperature cloud map of the xy cross section;

[0043] Figure 11 is the curve of the central temperature of the frozen product changing with time according to the present invention;

[0044] Figure 12 yes Figure 11 Magnified image of;

[0045] Figure 13 It is a bar graph of the brine flow rate and freezing time according to the present invention;

[0046] Figure 14 It is the speed setting in the fluid simulation calculation software of the present invention;

[0047] Figure 15 The temperature cloud diagram of the freezing chamber during speed change and the xy cross-section temperature cloud diagram of the present invention are shown in FIG. 1 , wherein FIG. (a) is the temperature cloud diagram of the freezing chamber, and FIG. (b) is the xy cross-section temperature cloud diagram;

[0048] Figure 16 is the variable speed freezing curve of the present invention;

[0049] Figure 17 yes Figure 16 Magnified image of;

[0050] Figure 18 It is the PID theoretical diagram of the present invention;

[0051] Figure 19 This is the cascade control principle diagram of the present invention;

[0052] Figure 20 This is a flow chart of the main program of the temperature control system of the present invention calling different subroutines. DETAILED DESCRIPTION

[0053] The specific embodiments of the present invention are further described in detail below with reference to the accompanying drawings. The following embodiments will help those skilled in the art further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that those skilled in the art may make various changes and improvements without departing from the scope of the present invention, and these are all within the scope of protection of the present invention.

[0054] Implementation method 1, see Figure 1This embodiment describes the present invention. In view of the fact that existing immersion freezing devices generally have the problem of high energy consumption and full power output, which leads to high costs, this embodiment proposes an immersion freezing process control method. By controlling the different flow rates of the refrigerant in different freezing stages, it ensures that the formation of large ice crystals in the food during the freezing process is reduced, thereby reducing damage to the interior of the food, while reducing energy consumption and costs.

[0055] The freezing process control method specifically includes the following steps:

[0056] Step S1: The freezing process of the frozen product is divided into three stages: a pre-cooling stage, a phase change stage, and a supercooling stage;

[0057] Step S2: controlling the brine flow rate to be low in the pre-cooling stage and the super-cooling stage, and controlling the brine flow rate to be high in the phase change stage.

[0058] In practical application of this embodiment, the freezing process of frozen objects is divided into three stages, such as Figure 1 As shown, it includes pre-cooling stage, phase change stage and super-cooling stage;

[0059] Among them, pre-cooling stage: This embodiment analyzes that the temperature of food or substance gradually decreases from above 0°C. During the decrease, the heat of the food or substance is gradually removed, but the water inside it still exists in liquid form. Therefore, this embodiment defines this stage as the pre-cooling stage, which is the beginning of the freezing process. The goal of this stage is to reduce the temperature of the substance to below the freezing point.

[0060] Phase Transition Stage: This embodiment analyzes that when the temperature of a food or substance drops below freezing, moisture begins to transform into solid ice crystals. The formation and growth of ice crystals directly affect the structure and taste of the food or substance. If the cooling rate is too fast, the ice crystals will form quickly and become small, which helps maintain the texture of the food or substance. However, if the cooling rate is too slow, the ice crystals will grow larger, causing structural damage to the food or substance. Therefore, this embodiment designates this stage as the phase transition stage, which is the most critical stage in the freezing process. This is because the formation and growth of ice crystals directly affect the structure and taste of the food or substance.

[0061] Supercooling Stage: This embodiment analyzes that when most of the water in a food or substance is converted into ice crystals, the substance enters the freezing stage. During this stage, the temperature of the food or substance continues to drop until it reaches the desired freezing temperature. At this point, the water in the food or substance exists primarily in the form of solid ice crystals, allowing the substance to be stored at low temperatures for long periods of time. Therefore, this embodiment defines this stage as the supercooling stage.

[0062] Understanding these three stages of the freezing process is important for optimizing freezing technology and improving the quality of frozen foods or substances. By controlling the cooling rate and temperature, the formation and growth of ice crystals can be influenced, thereby maximizing the preservation of the original structure and taste of the food or substance.

[0063] Therefore, this embodiment simulates and analyzes the freezing process of frozen materials under different flow rate conditions to determine the impact of different flow rates on the freezing rate at different stages, thereby determining that the refrigerant flow rate is controlled to be low in the pre-cooling stage and the supercooling stage (less than or equal to 4m / s is a low flow rate), and that the refrigerant flow rate is controlled to be high in the phase change stage (greater than 4m / s is a high flow rate).

[0064] Preferably, the low flow velocity is selected as 4 m / s and the high flow velocity is selected as 5 m / s.

[0065] Embodiment 2: This embodiment specifically describes step S2 in the immersion freezing process control method described in the above embodiment 1.

[0066] By simulating and analyzing the freezing process of frozen materials under different flow rate conditions, the influence of different flow rates on the freezing rate at different stages is determined, so that it is determined that the refrigerant flow rate is controlled to be low in the pre-cooling stage and the supercooling stage, and the refrigerant flow rate is controlled to be high in the phase change stage.

[0067] The simulation analysis is as follows:

[0068] Step S21: Determine the material parameters of the simulation process according to the freezing process of the frozen material: the heat released from the frozen material from room temperature to the freezing point, the specific heat capacity, thermal conductivity and viscosity of the coolant;

[0069] Step S22: constructing a fluid mechanics calculation model of the coolant and a heat transfer equation of the refrigeration process;

[0070] Step S23: using the fluid mechanics calculation model and the heat transfer equation of the freezing process as the calculation basis of the fluid simulation software;

[0071] Step S24: constructing a freezing model and placing it into mesh drawing software for mesh drawing;

[0072] Step S25: Place the drawn grid into fluid simulation software for calculation and solution, input different flow rates of the brine, and obtain the center temperature data;

[0073] Step S26: generating a curve showing the change of the core temperature over time based on the core temperature data, and generating a temperature cloud map and a heat transfer cloud map after processing;

[0074] Step S27: The influence of flow rate on freezing time and freezing effect is obtained based on the temperature cloud map and the heat transfer cloud map, so as to determine that the refrigerant flow rate is controlled to be low in the pre-cooling stage and the supercooling stage, and the refrigerant flow rate is controlled to be high in the phase change stage.

[0075] In practical application, the material parameters of the simulation process need to be determined: the freezing process of the frozen object is the heat release process of the object, and the simulation process of the design is fluid-solid heat transfer. The heat released from the frozen object from room temperature to the freezing point is:

[0076] Q1=mc1(T 初 -T 冻 )

[0077] Where m represents the mass of the frozen material and c represents the specific heat capacity of the frozen material.

[0078] The specific heat capacity, thermal conductivity and viscosity of the coolant can be obtained based on actual work.

[0079] In order to reflect the freezing process of immersion freezing, the flow process of the fluid and the heat transfer process with the frozen material, the basic theory and model calculation basis are provided for the entire freezing process, and the control equations and numerical simulation solution equations required for the fluid are explained.

[0080] The basic control equations for fluid flow include the continuity equation, the momentum conservation equation, and the energy conservation equation.

[0081] The continuity equation is:

[0082]

[0083] Where u, v, and w are the velocity components in the x, y, and z directions, respectively, in m / s. ρ is the density, in kg / m 3 .

[0084] The momentum conservation equation is:

[0085]

[0086] Where ρ is density, unit is Kg / m 3 ; P is pressure, unit is N; F bx 、F by 、F bz F is the volume force on the x, y, and z axes, in N; P xx ,P xy ,P yz is the component of P, unit N;

[0087] The energy conservation equation is:

[0088]

[0089] Among them, c p is the specific heat capacity, unit is J / kg·℃; T is the thermodynamic temperature, unit is ℃; k is the fluid heat transfer coefficient, unit is W / m 2 ℃; S T It is the part of the fluid's mechanical energy that changes into other energy under the action of viscosity, unit is J.

[0090] The above three formulas are combined into the (NS) Navier-Stokes equations, which can be solved numerically through computer calculation and iteration to calculate the computational model of fluid mechanics.

[0091] The heat transfer equation for the freezing process is:

[0092] The circulation of the refrigerant accelerates the exchange of heat between the frozen object and the refrigerant, which is called convection heat transfer. The convection heat at the contact surface between the solid and the fluid can be expressed by Newton's law of cooling:

[0093] Q=αAΔT

[0094] Where Q is the heat transfer rate, that is, the heat transfer per unit time, unit W; α is the surface heat transfer coefficient, unit W / (m 2 ·K); A is the surface area of ​​the frozen object in contact with the freezing liquid medium, unit is m 2 ΔT is the temperature difference between the food surface and the freezer, measured in degrees Kelvin. Since the surface area A of the frozen product in contact with the freezer is typically constant and the freezer temperature is determined by the refrigeration equipment, the convective heat flow is primarily related to the convective surface heat transfer coefficient, which in turn is determined by other factors, such as the type and properties of the freezer and the surface condition of the frozen product.

[0095] The governing equation for heat transfer is:

[0096]

[0097] in:

[0098]

[0099] In the formula, Vx, Vy, and Vz are the conduction rates of the medium; q' is the heat generation per unit volume; k nn is the thermal conductivity, unit is W / (mg·℃); ρ is the density; c is a constant; It is the temperature gradient in the direction of heat conduction, in K / m.

[0100] The above heat transfer control equations are set up in the fluid simulation software to set up the corresponding energy equations to help with subsequent solutions.

[0101] This embodiment requires solving the freezing process of the frozen product and generating a curve of the center temperature change over time based on the center temperature data obtained from the solution. Then, post-processing is performed to generate a temperature cloud map and a heat transfer cloud map. The overall simulation process requires pre-processing, solution processing, and post-processing. These three parts exist independently and are also necessary steps for the entire simulation. The specific simulation process is as follows Figure 2 As shown:

[0102] (1) The freezing model is simplified and constructed using modeling software.

[0103] (2) Place the constructed model into the mesh drawing software and set the basic parameters such as fluid inflow, outflow direction, fluid domain, solid domain, container wall, mesh size, etc. to draw the mesh.

[0104] (3) Place the drawn grid into the fluid simulation software for calculation and solution

[0105] (4) Generate cloud images through post-processing software.

[0106] Implementation 3: This implementation is an overall description of the immersion freezing process control method described in the above implementation;

[0107] In this embodiment, beef is used as the frozen product, and the freezing process under different flow rate conditions is simulated and analyzed. Then, the freezing rate is improved by changing the flow rate during the freezing process, and an overall analysis of the temperature curves and cloud maps that change with time are performed on the two types of simulation processes (constant speed and variable speed). It is determined that the refrigerant flow rate is controlled to be low in the pre-cooling stage and the supercooling stage (less than or equal to 4m / s is a low flow rate), and the refrigerant flow rate is controlled to be high in the phase change stage (greater than 4m / s is a high flow rate).

[0108] The freezing process of frozen materials is divided into three stages: pre-cooling stage, phase change stage and supercooling stage. Figure 1 shown.

[0109] Determine the material parameters for the simulation process:

[0110] The freezing process of frozen objects is a heat release process of the object, and the simulation process of the design is fluid-solid heat transfer. The heat released from the frozen object from room temperature to the freezing point is:

[0111] Q1=mc1(T 初 -T 冻 )

[0112] Where m represents the mass of the frozen material and c represents the specific heat capacity of the frozen material.

[0113] The specific heat capacity of beef is shown in Table 1:

[0114] Table 1

[0115] t / ℃ -5.0 -10.0 -15.0 -20.0 -25.0 C p J / (kg·℃) 5.07 4.40 3.77 3.31 3.10

[0116] The thermal conductivity of beef is shown in Table 2:

[0117] Table 2

[0118] t / ℃ -5.0 -10.0 -15.0 -20.0 -25.0 λ 1.06 1.35 1.44 1.57 1.65

[0119] The density of beef is: 1.03g / cm 2 .

[0120] The freezing medium of this embodiment is SH-7A food grade brine. Tables 3 and 4 are the attribute data of the brine at different temperatures recorded in the brine manual.

[0121] Table 3 shows the specific heat capacity of the coolant:

[0122] Table 3

[0123] t / ℃ -5.0 -10.0 -15.0 -20.0 -25.0 kj / (kg·k) 3.355 3.325 3.272 3.224 3.186

[0124] Table 4 shows the thermal conductivity of the coolant:

[0125] Table 4

[0126] t / ℃ -5.0 -10.0 -15.0 -20.0 -25.0 λ 0.372 0.360 0.354 0.350 0.345

[0127] Table 5 shows the viscosity of the coolant:

[0128] Table 5

[0129] t / ℃ -5.0 -10.0 -15.0 -20.0 -25.0 μ / mPa·s 3.97 4.11 4.37 4.53 4.81

[0130] Construct the fluid mechanics calculation model of the refrigerant and the heat transfer equation of the refrigeration process.

[0131] This embodiment requires solving the freezing process of the frozen product and generating a curve of the center temperature change over time based on the center temperature data obtained from the solution. Then, post-processing is performed to generate a temperature cloud map and a heat transfer cloud map. The overall simulation process requires pre-processing, solution processing, and post-processing. These three parts exist independently and are also necessary steps for the entire simulation. The specific simulation process is as follows Figure 2 shown.

[0132] The simulation pre-processing is as follows:

[0133] The whole model is created by 3D modeling software, and the inflow, outflow, wall, fluid domain and solid domain are set by ICEM CFD. Figure 3 As shown, the freezing chamber is a rectangular parallelepiped with a size of 200mm×200mm×100mm, and the frozen product (beef) is a small rectangular parallelepiped with a size of 40mm×30mm×20mm.

[0134] Since the model required by the present embodiment is relatively simple, the results of selecting the structural grid for division and the grid quality are as follows Figure 4 In the discretization software, the unit type of the structural three-dimensional grid division is a regular hexahedron, and there are a total of 8976 units. When the grid quality is detected to be high, the solution can be output.

[0135] The present embodiment considers that the purpose of simulation is to obtain data under a relatively ideal condition, so some factors affecting the simulation results are not considered to establish a mathematical model of a composite freezing process, so the following assumptions are made:

[0136] (1) The freezing liquid is an incompressible liquid.

[0137] (2) Exclude the decrease of water content in the frozen material during the freezing process.

[0138] (3) The freezing bin will not exchange heat with the outside world.

[0139] (4) The freezing liquid is a radiation transparent medium, and there is no thermal radiation.

[0140] (5) Since the material parameters of the frozen material and the freezing liquid will change as the temperature continuously decreases, the segmented linear function parameter input is selected.

[0141] Calculation settings need to be performed during the solution process, specifically:

[0142] Put the pre-processed model into Fluent for solution calculation. The solution type (solver) of the model is selected as pressure equation (Pressure-Based) solution, the velocity equation (Velocity Formulation) is absolute velocity (Absolute), the time type (Time) is transient (Transient), the gravity term (Gravity) is applied, and the gravity acceleration is set to -9.81 m / s2 in the vertical direction.

[0143] Set the boundary conditions. Since the frozen material is frozen at room temperature (25℃), the initial temperature of the frozen material is 25℃, and the freezing liquid needs to be cooled to -20℃ first. The temperature value is set to -20℃ during the simulation process. Since the temperature of the freezing liquid is maintained at -20~ -25℃, the related specific heat capacity, viscosity, and thermal conductivity will not change, and the constant values are set in the simulation software.

[0144] Among them, the flow inlet is a velocity inlet, and the flow outlet is a pressure outlet. The center temperature point of the frozen material is selected as the data extraction point.

[0145] The simulation results and analysis are as follows:

[0146] The simulation packets under the conditions of constant flow rate and dynamic flow rate are shown in Tables 6 and 7, and the simulation experiments are carried out according to the temperature values of the packets.

[0147] Table 6 is the constant flow rate simulation packet:

[0148] Table 6

[0149] serial number Flow rate (m / s) 1 0 2 1 3 2 4 3 5 4 6 5

[0150] Table 7 is the stage flow rate simulation packet:

[0151] Table 7

[0152]

[0153] The influence of constant flow rate on the freezing time and freezing effect of the frozen material is analyzed:

[0154] The temperature distribution of the freezing process of the immersion freezing equipment simulated by the Fluent software is shown through the overall three-dimensional structure diagram. In order to better see the temperature distribution, the freezing process and the corresponding temperature gradient cloud diagram are observed by selecting the center point x-y section, so as to better judge the freezing effect.

[0155] First, set the flow rate of the flow inlet to 0 m / s, 1 m / s, 2 m / s, 3 m / s, 4 m / s, and 5 m / s. The temperature cloud diagram generated by 6 groups of constant flow rate simulation, the center temperature detection curve of the frozen material, and the freezing data are obtained. The influence of flow rate on freezing time and freezing effect is summarized and analyzed. As shown in Figures 5 to 10 The temperature cloud diagram and the x-y plane temperature cloud diagram generated by 6 kinds of constant flow rate at the time of freezing 200s are shown. It can be seen that the distribution of temperature in the freezing bin is shown by the legend on the left side of the figure, which shows that the temperature from red to blue represents high to low. From the x-y section view, it can be seen that the internal temperature of the frozen animal is-18℃. Except for no flow rate, the freezing liquid temperature remains at-20℃. When the flow rate is 0 m / s, it can be clearly seen that heat transfer occurs between the frozen material and the freezing liquid. Single immersion freezing is direct contact between the frozen material and the freezing liquid to reduce the temperature of the frozen material to the target temperature through heat transfer. The addition of fluid field is to speed up the heat transfer by circulating the freezing liquid, so as to make the freezing rate larger and the freezing time shorter.

[0156] The center point data of the frozen material of 6 groups of simulation are extracted to obtain the temperature curve changing with time as shown in Figure 11 and Figure 12It can be seen that when the freezing begins, the center temperature drops rapidly, but there will be an inflection point at a certain time, and the curve slope of the center temperature drop gradually tends to be flat. With the increase of the coolant flow rate, the inflection point will move to the left along the coordinate axis. This is because when the food is frozen, the free water molecules in the cells will condense into small ice crystals, and the small ice crystals will slowly aggregate to form ice crystal bands. The freezing time before the inflection point is the time through the formation of the maximum ice crystal band. The shorter this period of time, the higher the freezing quality of the food. Therefore, it can be known from the simulation data that with the increase of the coolant flow rate, the freezing quality of the frozen material is improved. With the increase of the coolant flow rate, the freezing time gradually decreases.

[0157] According to Figures 11 to 13 It can be seen that increasing the coolant flow rate will affect the freezing speed of beef, and with the increase of the coolant flow rate, the freezing time of the frozen material will gradually decrease. But the improvement of freezing efficiency shows a decreasing trend, which means that with the increase of the coolant flow rate, the degree of improvement of freezing efficiency gradually decreases, and finally tends to zero.

[0158] Among them, the coolant flow rate and the freezing time efficiency are shown in Table 8.

[0159] Table 8

[0160] Flow rate (m / s) Freezing time (s) Year-on-year growth (%) 0 1780 \ 1 897 49.6 2 804 10.3 3 713 12.3 4 684 4.06 5 666 2.63

[0161] It can be seen that the coolant flow rate is saturated at 4m / s due to the influence of the size of the frozen material block, and the simulation results show that the freezing rate improves slowly with the continuous increase of the flow rate.

[0162] Analysis of the influence of changing flow rate on the freezing time and freezing effect of the frozen material:

[0163] According to the above analysis, the higher the flow rate, the shorter the time required in the simulation process, but the power required is also higher, and the energy consumption is also more. Therefore, this embodiment proposes to change the flow rate at different temperature stages during the freezing process to simulate. To maintain high freezing rate and reduce energy consumption.

[0164] In the freezing process, 4℃ to -4℃ is the temperature interval when water changes from ice-water mixture to ice crystal, the maximum ice crystal formation band and the maximum water density. Therefore, the flow rate is increased at 4℃ to -4℃ to ensure the formation of small ice crystals and reduce the damage to the cells of the frozen material, which reflects the advantages of immersion freezing. The time required to reduce the temperature of the frozen material from 4℃ to -4℃ at a constant flow rate of 6 groups of flow rate is shown in Table 9.

[0165] Table 9

[0166] Flow rate (m / s) Time(s) 0 233 1 150 2 133 3 122 4 114 5 109

[0167] An expression that changes with temperature and thus changes the flow rate is set by using the expression function in the Fluent software. The expression is: IF (T<=269.15[K], X[m / s], IF (T>=277.15[K], X[m / s], Y[m / s])). Where Y is the flow rate at 4°C to -4°C, and X is the flow rate at other temperatures. For example Figure 14 A cross-sectional view of the expression set in the software and a flow rate curve changing with temperature.

[0168] According to the research purpose and the analysis of the constant flow rate, the freezing time required in different stages of the freezing process at different flow rates is obtained, and 10 groups of simulations are developed to summarize and analyze, and an accurate flow rate combination is obtained to provide a high freezing rate and energy-saving flow rate combination for subsequent research and development of the control board and the experimental stage. The overall change of the temperature cloud map during the freezing process is not large, so this embodiment only shows the temperature cloud map and three-dimensional temperature image of one group of flow rate combination (3-4-4 m / s) as shown in Figure 15

[0169] According to the simulated data, the temperature curve is summarized as shown in Figure 16 Figure 16 is the overall temperature curve of the 10 groups of images, Figure 17 is an enlarged image of the inflection point starting from 4°C.

[0170] The time required for variable-speed freezing and the time difference value are obtained as shown in Table 10.

[0171] Table 10

[0172]

[0173] According to the above analysis of the influence of constant flow rate on freezing time and freezing effect of frozen objects and the influence of variable flow rate on freezing time and freezing effect of frozen objects, the corresponding energy consumption is calculated.

[0174] The freezing time of 16 groups of constant speed and variable speed is solved by simulation, and the energy calculation of the simulation results of the two speeds is performed.

[0175] Constant speed energy consumption calculation:

[0176] Referring to the existing immersion freezing experimental equipment, the liquid pump power (maximum) is 48w·h; the freezer unit is 165w·h; the controllable flow rate of the pump is 1m / s, 2m / s, 3m / s, 4m / s, 5m / s. Finally, the time solved by Table 2-9 is multiplied to solve the formula:

[0177] W=p·t

[0178] The results are sorted out to obtain the constant speed energy consumption as shown in Table 11. ​​

[0179] Table 11

[0180] Flow rate (m / s) Time (t) Energy consumption (w·h) 1 897 55.096 2 804 51.456 3 713 49.6036 4 684 48.3275 5 666 48.4256

[0181] Variable speed energy consumption calculation:

[0182] This embodiment is to accelerate the flow rate at 4°C to -4°C, so the time to increase the flow rate in 10 groups of variable speed freezing simulation is shown in Table 12. The power consumption calculation formula is obtained by multiplying the time required at 4°C to -4°C with the power required to increase the flow rate and adding the power of other flow rates multiplied by the remaining time:

[0183] W = p 快 · t 快 + p 慢 · t 慢

[0184] Table 12 is the time required for freezing at 4°C to -4°C:

[0185] Table 12

[0186]

[0187] The variable speed energy consumption is shown in Table 13:

[0188] Table 13

[0189]

[0190] Result analysis:

[0191] According to the above Table 11 and Table 13, it can be seen that as the flow rate is continuously accelerated, the energy consumption gradually decreases. When the full power is running, the energy consumption generated is 48.4256 W. In the calculation of variable speed energy consumption, the energy consumption required by the 4-5-4 flow rate group is 48.3193 W. It can be seen that the energy consumption required under variable speed condition is reduced. The existing equipment has low power, and when a large-scale immersion freezing equipment is designed later, the amount of energy consumption reduction will be greater, and when the freezing equipment is running at full power, it will also cause damage to the service life of the liquid pump. Therefore, high flow rate is used when ice crystals are formed (phase change stage) and low flow rate is used at other temperatures, which can not only improve the freezing rate, but also protect the experimental equipment. Therefore, this embodiment adopts 4 m / s as the slow flow rate of variable speed freezing. Thus, ideas are provided for the subsequent control system, and control board accuracy and experimental correctness are judged by providing control data for subsequent experimental data.

[0192] To summarize, this embodiment solves the time required for the maximum ice crystal band and the time required for 16 groups of flow rates through 16 groups of simulation analysis of constant speed and variable speed, and finally calculates the energy consumption to obtain the control panel program of the flow rate group required for temperature control in subsequent experiments, and makes idealized data for the correctness of the experimental data of subsequent experiments, determines the correctness of the experimental data, and performs the overall design of the control system based on the simulation results.

[0193] Embodiment 4: This embodiment provides a temperature control system for an immersion freezing device, which is used to implement the immersion freezing process control method described in the above embodiment.

[0194] The control system includes a temperature acquisition module, a liquid pump drive module, a host computer, a data transmission module, a control module and a refrigeration equipment control module;

[0195] The host computer is used to exchange data with the control module through the data transmission module;

[0196] The temperature acquisition module is used to collect the temperature of the external environment of the frozen object in real time, and convert the collected temperature signal into an electrical signal and send it to the control module;

[0197] The control module is used to control the power of the liquid pump drive module through a cascade control strategy and a PID control algorithm according to the temperature signal of the frozen object and the influence of the external environment on the freezing liquid;

[0198] The control module is also used to control the start and stop of the refrigeration equipment control module according to the real-time temperature of the frozen object.

[0199] In practical applications, this embodiment involves designing a temperature acquisition module, a liquid pump driver module, a host computer, a data transmission module, a control module, and a refrigeration equipment control module to ensure that food reaches the desired freezing temperature in the shortest possible time while minimizing energy consumption. The control module receives temperature signals from the temperature acquisition module and adjusts the power of the liquid pump driver module based on these signals and a pre-defined control strategy (cascade control and PID control algorithm). This control increases the freezing rate by controlling the flow rate of the refrigerant, reduces energy consumption, minimizes the formation of large ice crystals, and preserves the nutritional content of the food. Furthermore, the refrigeration equipment control module is activated and deactivated based on the real-time temperature of the frozen food, minimizing energy consumption.

[0200] The data transmission module realizes the communication between the upper computer and the control module through USB-to-serial communication. The "DB9 interface" between the upper computer and the control module is connected through a serial signal line, and the "RS-232 standard" is used to transmit data signals in the serial signal line. Since the RS-232 level standard signal cannot be directly recognized by the controller, the embodiment adopts a level conversion chip to convert it into a "TTL standard" level signal that can be recognized by the controller. The serial port baud rate is configured as 11500, the data bits are 8, there is no parity check bit, the stop bit is 1, and the serial port selection is COM3 or COM5.

[0201] The STM32F103C8T6 single-chip microcomputer is used as the main control chip of the control module to complete data acquisition, transmission, display and other functions. The STM32F103C8T6 single-chip microcomputer is a 32-bit microcontroller based on the ARM Cortex-M kernel STM32 series, with a program memory capacity of 64KB, a required voltage of 2V-3.6V, and a working temperature of -40℃-85℃.

[0202] The DS18B20 temperature sensor is used as the temperature acquisition module. Two DS18B20 temperature sensors are connected to measure the temperature of the frozen object and the external environment, and the collected temperature signals are converted into electrical signals and stored in the single-chip microcomputer.

[0203] The power supply voltage of the liquid pump driving module and the refrigeration equipment control module is 220V AC, but the power supply voltage of the circuit board is 24V. In order to protect the entire circuit board, two relays for converting 3.3V DC to 220V AC are connected. The relay is an Ouyan DC-to-AC SSR-25DA with an actual current of 12A.

[0204] The 24V motor, 3.3V single-chip microcomputer and liquid crystal display screen also need to be powered. The external 220V AC power supply is adjusted to 24V by a power adapter and connected to the control board, and then converted to 12V, 5V and 3.3V, so the power module circuit is designed.

[0205] The temperature control system proposed in this embodiment needs to accurately control the frozen environment to be stable at a temperature of-20℃-25℃, so the DS18B20 temperature sensor is selected as the temperature data acquisition element of the control system. The temperature control system should be able to maintain stable performance during long-term operation and will not be affected by small changes in the external environment or the aging of internal components. Therefore, this paper selects the STM32 single-chip microcomputer as the control core to design the control system. The hardware part is made into a modularized form for easy maintenance and optimization.

[0206] Implementation 5: This implementation is to specifically explain the cascade control strategy and PID control algorithm embedded in the control module described in Implementation 1 above.

[0207] The interaction process between the cascade control strategy and the PID control algorithm is:

[0208] Set up two control closed loops, the inner loop and the outer loop, with the outer loop as the main loop and the inner loop as the secondary loop, and the two closed loops work together;

[0209] The main loop is used to convert the difference between the rated temperature and the actual temperature of the frozen object into the set value of the secondary loop through the PID control algorithm;

[0210] The secondary loop is used to control the power of the liquid pump drive module according to the set value and the difference between the collected temperature of the refrigerant and the actual temperature.

[0211] In practical applications, this embodiment adopts a combination of a cascade control strategy and a PID control algorithm to achieve power control of the liquid pump drive module.

[0212] Among them, Figure 18 As shown, the PID control algorithm uses the deviation between the set value and the actual measured value to perform linear algebraic operations through proportional (P), integral (I), and differential (D) information to gradually reduce the system deviation to zero, thereby achieving the purpose of precise control. The purpose of this embodiment is to reduce energy consumption and increase the freezing rate by intelligently controlling the difference between the real-time temperature of the frozen object and the rated temperature, thereby improving the immersion freezing effect. Therefore, the digital position PID control algorithm is selected. The specific PID calculation formula is:

[0213]

[0214] Among them, T i 、T d are all time constants. K d =K p T d .

[0215] like Figure 19 As shown in the figure, two control loops are designed, with the inner loop being the secondary loop and the outer loop being the primary loop. The primary loop is a fixed-value control system, while the secondary loop is a follow-up control system. The two closed loops work together to accurately control the temperature of the frozen product. The input of the main regulator is the rated temperature of the frozen product, and the output of the main regulator is the set value of the secondary regulator. The output of the secondary regulator controls the power of the liquid pump, which affects the flow rate and thus cools the refrigerant.

[0216] In the main loop, e1(t) is the difference between the setpoint temperature of the frozen material and the actual temperature of the frozen material y1(t). e2(t) is the difference between the liquid pump controller input and the actual temperature of the frozen liquid y2(t).

[0217] The main regulator output u1(t) is:

[0218]

[0219] The output u2(t) of the sub-regulator is:

[0220]

[0221] u2(t) is the driving voltage acting on the liquid pump.

[0222] Among them, T i 、T d are all time constants.

[0223] The temperature control system described in this embodiment is a thermal inertia system, and the heat transfer process can be affected by adjusting the water flow rate through a water pump. The temperature control system is a second-order system.

[0224] In a temperature control system, the water pump speed directly affects the water flow rate, which in turn affects the heat transfer rate and temperature response.

[0225] The heat transfer process of the temperature control system can be expressed as the following differential equation:

[0226]

[0227] Where, T(t) represents the temperature response equation; Q(t) represents the pump speed; K represents the gain of the system; τ is the time constant; ζ represents the damping ratio;

[0228] Perform Laplace transform to convert the equation into frequency domain, and the differential equation can be expressed as:

[0229] τ 2 s 2 T(s)+2ζτsT(s)+T(s)=KQ(t)

[0230] By simplifying the above equation and extracting T(s), we can obtain the transfer function G(s):

[0231]

[0232] Where Q(s) is the pump speed and T(s) is the temperature.

[0233] When the pump speed increases from 0 m / s to 5 m / s, the temperature decreases from 25°C to -18°C. Calculate the system gain.

[0234]

[0235] Wherein, time constant τ = 30s; damping ratio ζ = 0.7; ζ > 1 is overdamping; ζ = 1 is critical damping; 0 < ζ < 1 is underdamping;

[0236] Substitute the parameters and calculate:

[0237]

[0238] Implementation 6: This implementation is to specifically explain the control module described in the above implementation;

[0239] The control module also embeds the main program, temperature sensor subroutine, digital PID algorithm subroutine, PWM subroutine, display subroutine, host computer reading subroutine, and relay control subroutine;

[0240] The main program is used to control the temperature of the immersion freezing equipment by calling different subroutines at regular intervals.

[0241] In actual application of this embodiment, the program of the temperature control system is compiled in a long polling structure. All programs are integrated into a main program and the main program is called in a self-loop to work.

[0242] The whole procedure is as follows Figure 20 As shown, the program of each function is initialized first and then enters the system loop to first read the PID value set by the host computer and the detected temperature to calculate the PWM value, and at the same time all the data are displayed on the screen.

[0243] Among them, the digital PID algorithm subroutine is specifically as follows:

[0244] Write a microcontroller program based on the positional PID formula. Err(k) represents the current temperature error minus the previous temperature error. Since the temperature control system has three temperature control stages (20°C to 4°C, 4°C to -4°C, and -4°C to -18°C), Δu(k) in the formula is set to the PWM_DUTU value in the program, and Derror represents the accumulated error. This represents the accumulated error and the PWM value generated by the PID iterative calculation.

[0245] The PID formula is:

[0246] Δu(k)=K p err(k)+K i ∑err(k)+K d (err(k)-err(k-1))

[0247] Where: k is the sampling number; err(k) is the kth error; u(k) is the output; K p constant.

[0248] The relay on-off subroutine is as follows: According to the design of the control system, the condenser and the compression pump need to be shut down at -18 degrees Celsius, so the PA12 and PA15 pins of the microcontroller are used to control the relay to stop being powered on at -18 degrees Celsius.

[0249] The specific subroutine of calling the upper computer numerical value is as follows: The overall PID program needs to be measured through experiments to determine the specific K P , K I If you modify the program to control K P , K I The value is too cumbersome and inconvenient, so the value can be changed at any time by calling the host computer. The host computer sends the changed value to the STM32 microcontroller through the serial port, and the microcontroller stores the data sent by the host computer in an array named USART_RX_BUF[*].

[0250] The OLED display subroutine is as follows: Set the STM32103 IO pins connected to the OLED module. Set the PB11-PB15 IO pins connected to the OLED module as outputs. Write the subroutine according to the timing diagram to meet the requirements.

[0251] The foregoing description is merely an embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of the claims.

Claims

1. An immersion freezing process control method, characterized in that: The immersion freezing process control method is implemented based on an immersion freezing device temperature control system, which includes a temperature acquisition module, a liquid pump drive module, a host computer, a data transmission module, a control module and a refrigeration equipment control module; The upper computer is used to exchange data with the control module through the data transmission module; the temperature acquisition module is used to acquire the temperature of the external environment of the frozen object in real time, and convert the acquired temperature signal into an electrical signal and send it to the control module; the control module is used to control the power of the liquid pump drive module through the cascade control strategy and PID control algorithm according to the temperature signal of the frozen object and the influence of the external environment on the refrigerant; the control module is also used to control the start and stop of the refrigeration equipment control module according to the real-time temperature of the frozen object; the interaction process of the cascade control strategy and the PID control algorithm is: set two control closed loops, namely the inner loop and the outer loop, and the outer loop is the main loop and the inner loop is the sub-loop, and the two closed loops work with each other; the main loop is used to convert the difference between the rated temperature and the actual temperature of the frozen object into the set value of the sub-loop through the PID control algorithm; the sub-loop is used to control the power of the liquid pump drive module according to the set value and the difference between the collected temperature of the refrigerant and the actual temperature; The control method is: S1: The freezing process of frozen materials is divided into three stages: pre-cooling stage, phase change stage and supercooling stage; S2: The refrigerant flow rate is controlled to be low in the pre-cooling stage and the super-cooling stage, and is controlled to be high in the phase change stage.

2. The immersion freezing process control method according to claim 1, characterized in that: Less than or equal to 4m / s is a low flow rate, and greater than 4m / s is a high flow rate.

3. The immersion freezing process control method according to claim 2, characterized in that: S2 is specifically: The freezing process of frozen materials under different flow rate conditions is simulated and analyzed to determine the influence of different flow rates on the freezing rate at different stages, so as to determine that the refrigerant flow rate is controlled to be low in the pre-cooling stage and the supercooling stage, and the refrigerant flow rate is controlled to be high in the phase change stage.

4. The immersion freezing process control method according to claim 3, characterized in that: The simulation analysis is as follows: S21: According to the freezing process of the frozen object, determine the material parameters of the simulation process: the heat released from the frozen object from room temperature to the freezing point temperature, the specific heat capacity, thermal conductivity and viscosity of the coolant; S22: Construct the fluid mechanics calculation model of the coolant and the heat transfer equation of the freezing process; S23: Use the fluid mechanics calculation model and the heat transfer equation of the freezing process as the calculation basis of the fluid simulation software; S24: Construct a frozen model and put it into mesh drawing software for mesh drawing; S25: Place the drawn grid into the fluid simulation calculation software for calculation and solution, input different flow rates of the coolant, and obtain the center temperature data; S26: generating a curve showing the change of the core temperature over time based on the core temperature data, and generating a temperature cloud map and a heat transfer cloud map after processing; S27: The influence of flow rate on freezing time and freezing effect is obtained based on the temperature cloud map and the heat transfer cloud map, so as to determine that the flow rate of the refrigerant is controlled to be low in the pre-cooling stage and the supercooling stage, and the flow rate of the refrigerant is controlled to be high in the phase change stage.

5. The immersion freezing process control method according to claim 4, characterized in that: When drawing the mesh, you also need to set the fluid inflow and outflow direction, fluid domain, solid domain, container wall and mesh size.

6. The immersion freezing process control method according to claim 4, characterized in that: During simulation analysis, pre-processing, solution processing and post-processing are required.

7. The immersion freezing process control method according to claim 6, characterized in that: The specific solution process is as follows: set the solution type to pressure equation, velocity equation to absolute velocity, time type to transient, apply gravity term, and set the gravity acceleration in the vertical direction to -9.81m / s 2 The inlet is the velocity inlet, the outlet is the pressure outlet, and the center temperature point of the frozen object is the data extraction point.

8. Immersion freezing process control system, characterized in that, The system is implemented based on the immersion freezing process control method described in claim 1.

9. A computer-readable storage medium, characterized in that The computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the immersion freezing process control method according to any one of claims 1 to 7 is executed.

10. A computer device, characterized in that: The device includes a memory and a processor, wherein a computer program is stored in the memory. When the processor runs the computer program stored in the memory, the processor executes the immersion freezing process control method according to any one of claims 1 to 7.

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