Immersion type freezing process control method and system

Abstract

The invention discloses a dipping type freezing process control method and a dipping type freezing process control system, relates to the technical field of dipping type freezing control, and is used for reducing the formation of large ice crystals in a freezing process of food so as to reduce the damage to the interior of the food and reduce energy consumption and cost at the same time. The control method comprises the following steps that S1, the freezing process of a frozen object is divided into three stages including the pre-cooling stage, the phase change stage and the supercooling stage; and S2, the flow speed of the secondary refrigerant is controlled to be low in the precooling stage and the supercooling stage, and the flow speed of the secondary refrigerant is controlled to be high in the phase change stage. Furthermore, the flow velocity smaller than or equal to 4 m / s is low, and the flow velocity larger than 4 m / s is high. The method is suitable for the field of dipping type freezing control.

Description

Technical Field

[0001] The 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] Freezing 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, 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, usually a liquid with a high heat transfer coefficient, which is conducive to accelerating the freezing speed. Using a safe and non-toxic low-temperature aqueous solution with a freezing point below 0°C as a coolant, the food is immersed in the low-temperature freezing liquid, so that the heat of the food is transferred to the liquid and frozen. The freezing rate of this freezing method is much higher than other freezing methods. The food is immersed in the coolant to cool down, there is no mechanical damage, and it can pass through the maximum ice crystal formation zone in the shortest time. Freezing speed is one of the main factors affecting the quality of freezing. The speed of freezing determines the size of the ice crystal. The faster the freezing speed, the smaller the ice crystal, and the better the quality of the food.

[0004] Based on the research on the 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 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, the control method comprising the following steps:

[0008] S1: The freezing process of frozen objects is divided into three stages: precooling stage, phase change stage and supercooling stage;

[0009] S2: The flow rate of the refrigerant is controlled to be low in the precooling stage and the supercooling stage, and the flow rate of the refrigerant is controlled to be high in the phase change stage.

[0010] Furthermore, there is a preferred embodiment in which a flow rate less than or equal to 4 m / s is a low flow rate, and a flow rate greater than 4 m / s is 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, there is a 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 refrigeration 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 freezing model and put it into mesh drawing software for mesh drawing;

[0018] S25: Put 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 of the change of the center temperature over time according to the center 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, there is a preferred embodiment, when drawing the above-mentioned grid, it is also necessary to set the fluid inflow and outflow directions, fluid domain, solid domain, container wall and grid size.

[0022] Furthermore, there is a preferred embodiment that 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 to solve, the velocity equation to be absolute velocity, the time type to be transient, applying the gravity term, and setting the gravity acceleration in the vertical direction to -9.81m / s2, the inlet to be the velocity inlet, the outlet to be the pressure outlet, and the center temperature point of the frozen object to be the data extraction point.

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

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

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

[0027] The present invention also provides a computer-readable storage medium, on which a computer program is stored. 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 the memory stores a computer program, and 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, a simulation model is constructed by numerical simulation software based on computational fluid dynamics theory and heat transfer principles. The influence of different refrigerant flow rates on the freezing speed and freezing quality of frozen objects under constant speed and variable speed conditions is 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, the existing immersion freezing control method usually adopts a fixed speed mode, that is, the coolant is frozen in a fixed speed mode, but in order to ensure the freezing effect, full power output will be used, resulting in high energy consumption. The present invention adopts a variable speed mode to control the immersion freezing control, that is, by controlling the coolant flow rate to a low flow rate in the precooling stage and the supercooling stage, and controlling the coolant flow rate to a high flow rate in the phase change stage, while reducing the freezing time, different flow rates are selected in different stages, avoiding full power output, reducing energy waste, and reducing costs.

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

[0033] Figure 1 It is a schematic diagram of three stages of the freezing process of the frozen object of 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 It is the temperature cloud diagram inside the freezing chamber and the temperature cloud diagram of the xy cross section at v=0m / s of the present invention, wherein Figure (a) is the temperature cloud diagram inside the freezing chamber, and Figure (b) is the temperature cloud diagram of the xy cross section;

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

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

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

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

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

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

[0044] Fig.12 yes Fig.11 A magnified image of

[0045] Fig.13 It is a bar graph of the flow rate of the coolant and the freezing time of the present invention;

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

[0047] Fig.15 It is the temperature cloud diagram of the freezing chamber during speed change and the temperature cloud diagram of the xy cross section of the freezing chamber according to the present invention, wherein Figure (a) is the temperature cloud diagram of the freezing chamber, and Figure (b) is the temperature cloud diagram of the xy cross section;

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

[0049] Fig.17 yes Fig.16 A magnified image of

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

[0051] Fig.19 It is the cascade control principle diagram of the present invention;

[0052] Fig. 20 It is a flow chart of the main program of the temperature control system described in the present invention calling different subprograms. DETAILED DESCRIPTION

[0053] The specific embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

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

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

[0056] Step S1: dividing the frozen product into three stages during the freezing process: a pre-cooling stage, a phase change stage, and a super-cooling stage;

[0057] Step S2: Control the coolant flow rate to be low in the precooling stage and the supercooling stage, and control the coolant flow rate to be high in the phase change stage.

[0058] In actual application of this embodiment, the freezing process of frozen objects is divided into three stages, such as Figure 1 As shown, it includes a precooling stage, a phase change stage and a supercooling stage;

[0059] Among them, pre-cooling stage: This embodiment analyzes the temperature of food or substance gradually decreasing from above 0°C. During the decreasing process, 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 change stage: This embodiment analyzes that when the temperature of food or substance drops below the freezing point, the water begins to transform into solid ice crystals. The formation and growth of ice crystals will directly affect the structure and taste of food or substance. If the cooling rate is too fast, ice crystals will form quickly and become very small, which helps to maintain the texture of food or substance; but if the cooling rate is too slow, ice crystals will grow larger, resulting in structural damage to food or substance. Therefore, this embodiment defines this stage as the phase change stage, which is the most critical stage in the freezing process. Because the formation and growth of ice crystals will directly affect the structure and taste of food or substance.

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

[0062] Understanding these three stages of the freezing process is of great significance 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 affected, thereby maximizing the preservation of the original structure and taste of food or substances.

[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 is a specific description of 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 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.

[0067] The simulation analysis is as follows:

[0068] Step 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;

[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 a mesh drawing software for mesh drawing;

[0072] Step S25: putting the drawn grid into the fluid simulation calculation software for calculation and solution, inputting different flow rates of the coolant, and obtaining the center temperature data;

[0073] Step S26: generating a curve of the core temperature changing with time according to 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 according to 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 precooling stage and the supercooling stage, and that the flow rate of the refrigerant is controlled to be high in the phase change stage.

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

[0076] Q1=mc1(T 初 -T 冻 )

[0077] Among them, 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 object, 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 controlling fluids include the continuity equation, the momentum conservation equation, and the energy conservation equation.

[0081] The continuity equation is:

[0082]

[0083] Among them, u, v, 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 Kg / m 3 ; P is pressure, unit N; F bx 、F by 、F bz is the volume force of F on the x, y, and z axes, in N; P xx ,P xy ,P yz is the component of P, in N;

[0087] The energy conservation equation is:

[0088]

[0089] Among them, c p is the specific heat capacity, in J / kg·℃; T is the thermodynamic temperature, in ℃; k is the fluid heat transfer coefficient, in 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, and then the computational model of fluid mechanics can be calculated.

[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 heat convection heat transfer. The convection heat at the contact surface between the solid and the fluid can be expressed by Newton's cooling law:

[0093] Q=αAΔT

[0094] Where Q is the heat transfer rate, that is, the heat transfer per unit time, in W; α is the surface heat transfer coefficient, in W / (m 2 ·K); A is the surface area of ​​the frozen object in contact with the freezing liquid medium, unit: m 2 ; ΔT is the temperature difference between the food surface and the freezing liquid, in K. Since the frozen surface area A in contact with the freezing liquid is usually a constant, and the freezing liquid temperature is determined by the freezing equipment, it can be seen that the convective heat transfer heat flow is mainly related to the convective surface heat transfer coefficient, which is determined by other factors such as the type and physical properties of the freezing liquid 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, 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 heat conduction direction, 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 assist in the subsequent solution.

[0101] This implementation method needs to solve the freezing process of the frozen object and generate a curve of the central temperature change over time based on the central temperature data obtained by the solution, and then perform post-processing 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 maps through post-processing software.

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

[0107] In this embodiment, beef is used as the frozen product, and a simulation analysis is performed on the freezing process under different flow rate conditions. Then, a simulation analysis is performed by changing the flow rate during the freezing process to increase the freezing rate, and an overall analysis of the temperature curves and cloud diagrams that change with time in the two types of simulation processes (constant speed and variable speed) is performed. Thus, 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 objects is divided into three stages: precooling 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 for objects, and the simulation process for the design is fluid-solid heat transfer. The heat released from the room temperature to the freezing point temperature is:

[0111] Q1=mc1(T 初 -T 冻 )

[0112] Among them, 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 <![CDATA[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 refrigerant. Tables 3 and 4 are the property data of the refrigerant at different temperatures recorded in the refrigerant 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 coolant and the heat transfer equation of the refrigeration process.

[0131] This implementation method needs to solve the freezing process of the frozen object and generate a curve of the central temperature change over time based on the central temperature data obtained by the solution, and then perform post-processing 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 overall model is created in 3D modeling software, and the inflow, outflow, wall, fluid domain, and solid domain are set through ICEM CFD. Figure 3 As shown, the freezing chamber is a rectangular parallelepiped with dimensions of 200 mm × 200 mm × 100 mm, and the frozen product (beef) is a small rectangular parallelepiped with dimensions of 40 mm × 30 mm × 20 mm.

[0134] Since the model required by this implementation is relatively simple, the results and mesh quality of the structural grid are shown in Figure 2. Figure 4 In the discretization software, the structural three-dimensional grid division unit type is regular hexahedron, with a total of 8976 units. When the detection grid quality is high, the solution can be output.

[0135] This embodiment takes into account that the purpose of simulation is to obtain data under relatively ideal conditions, so some factors that affect the simulation results are not considered to establish a mathematical model of the composite freezing process, so the following assumptions are made:

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

[0137] (2) Eliminate the reduction of water content in frozen products during the freezing process.

[0138] (3) The freezer does not exchange heat with the outside world.

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

[0140] (5) Since the material parameters of the frozen material and the refrigerant will change as the temperature continues to decrease, the piecewise linear function parameter input is selected.

[0141] Calculation settings are required during solution processing, specifically:

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

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

[0144] The inlet is the velocity inlet, and the outlet is the pressure outlet. The central temperature point of the frozen object is selected as the data extraction point.

[0145] The simulation results and analysis are:

[0146] The simulation groups 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 groups.

[0147] Table 6 shows the constant flow rate simulation grouping:

[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 shows the grouping of stage flow rate simulation:

[0151] Table 7

[0152]

[0153] Analyze the influence of constant flow rate on freezing time and freezing effect of frozen products:

[0154] The temperature distribution of the freezing process of the immersion freezing equipment simulated by Fluent software is presented 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 xy section to better judge the freezing effect.

[0155] First, the flow rate at the inlet is set to 0m / s, 1m / s, 2m / s, 3m / s, 4m / s, and 5m / s. The temperature cloud map generated by the 6 sets of constant flow rate simulations and the frozen product center temperature detection curves are used to obtain the freezing data. The influence of flow rate on freezing time and freezing effect is summarized and analyzed. Figures 5 to 10 As shown in the figure, the temperature cloud diagrams and the temperature cloud diagrams on the xy plane generated by freezing for 200s with 6 constant flow rates can be seen. It can be seen that the temperature distribution in the freezing chamber is from high to low from red to blue. From the xy cross-sectional diagram, it can be seen that the internal temperature of the frozen animal is -18°C. Except when there is no flow rate, the temperature of the freezing liquid is kept at -20°C. When the flow rate is 0m / s, it can be clearly seen that heat is transferred between the frozen object and the freezing liquid. Single immersion freezing is the direct contact between the frozen object and the freezing liquid, and the temperature of the frozen object is reduced to the target temperature through heat transfer. Adding a fluid field accelerates the transfer of heat through the circulation of the freezing liquid, making the freezing rate greater and the freezing time shorter.

[0156] Extract the data of the center point of the 6 simulated frozen objects to obtain the curve of temperature change over time. Fig.11 and Fig.12As shown. It can be seen that when freezing begins, the center temperature drops rapidly, but after a certain period of time, an inflection point will appear, and the slope of the curve of the center temperature drop gradually tends to be gentle. As the flow rate of the coolant increases, 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 gather to form an ice crystal belt. The freezing time before the inflection point is the time to form the largest ice crystal belt. The shorter this period of time, the higher the freezing quality of the food. Therefore, it can be seen from the simulation data that as the flow rate of the coolant increases, the freezing quality of the frozen food improves. As the flow rate of the coolant increases, the freezing time gradually decreases.

[0157] according to Figures 11 to 13 It can be seen that increasing the flow rate of the refrigerant will affect the freezing speed of beef. As the flow rate of the refrigerant increases, the freezing time of the frozen food will gradually decrease. However, the improvement of the freezing efficiency shows a decreasing trend, which means that as the flow rate of the refrigerant increases, the improvement of the freezing efficiency gradually decreases and eventually tends to zero.

[0158] Among them, the refrigerant flow rate and 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 due to the influence of the size of the frozen object, the refrigerant flow rate reaches a saturated state at 4m / s. The simulation results show that the freezing rate increases very slowly if the flow rate is continuously increased.

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

[0163] According to the above analysis, the higher the flow rate, the shorter the time required in the simulation process, but the higher the power required and the more energy consumed. Therefore, this embodiment proposes to adopt a method of changing the flow rate at different temperature stages during the freezing process to perform simulation. This can achieve both maintaining an efficient freezing rate and reducing energy consumption.

[0164] During the freezing process, 4℃~-4℃ is the temperature range when water goes from ice-water mixture to ice crystals, the maximum ice crystal formation zone and the water density is the highest. Therefore, the flow rate is increased at 4℃~-4℃ to ensure the formation of small ice crystals, reduce the damage to the frozen cells, and reflect the advantages of immersion freezing. When simulating at a constant flow rate, the time required for the six flow rates to reduce the final temperature of the frozen product from 4℃ to -4℃ 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] By using the expression function in Fluent software, we can set an expression that changes the flow rate as the temperature changes. 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℃~-4℃, and X is the flow rate at other temperatures. Fig.14 The cross-sectional diagram and the flow velocity versus temperature curve for the expressions set in the software.

[0168] According to the research purpose and the analysis of the constant flow rate, the freezing time required for different stages of the freezing process at different flow rates is obtained, and 10 groups of simulations are formulated as shown in Table 7 for summary analysis, and an accurate flow rate combination is obtained to provide a flow rate combination with high freezing rate and energy saving for the subsequent 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 a group of flow rate groups (3-4-4m / s) The temperature cloud map and three-dimensional temperature image are shown in Figure 7. Fig.15 shown.

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

[0170] The time required for variable speed freezing and the time difference 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 the freezing time and freezing effect of frozen materials and the influence of variable flow rate on the freezing time and freezing effect of frozen materials, corresponding energy consumption calculations are carried out respectively.

[0174] The freezing time of 16 groups of fixed speed and variable speed is solved through simulation, and the energy calculation of the simulation results of the two speeds is carried out below.

[0175] Calculation of energy consumption at constant speed:

[0176] Refer to the existing immersion freezing experimental equipment, where the liquid pump power (maximum): 48w·h; refrigeration unit: 165w·h; pump controllable flow rate is 1m / s, 2m / s, 3m / s, 4m / s, 5m / s. Finally, multiply it with the time term solved in Table 2-9 to obtain the solution formula:

[0177] W=p·t

[0178] The results of the constant speed energy consumption are 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] Calculation of variable speed energy consumption:

[0182] This embodiment is to speed up the flow rate at 4℃~-4℃, so the time of increasing the flow rate for 10 groups of variable speed freezing simulations is summarized as shown in Table 12. The power consumption calculation formula is obtained by multiplying the time required for 4℃~-4℃ by the power required for the increased 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 shows the time required for freezing at 4℃~-4℃:

[0185] Table 12

[0186]

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

[0188] Table 13

[0189]

[0190] Result analysis:

[0191] According to the above Tables 11 and 13, it can be seen that as the flow rate continues to increase, the energy consumption gradually decreases. When the full-power operation is performed, the energy consumption generated is 48.4256W. In the calculation of the variable speed energy consumption, the energy consumption required by the 4-5-4 flow rate group is 48.3193W. It can be seen that the energy consumption required under the variable speed condition is reduced. The existing equipment has a low power. When a large-scale immersion freezing equipment is subsequently designed, the reduction in energy consumption will be greater, and when the freezing equipment is fully powered, it will also damage the service life of the liquid pump. Therefore, when ice crystals are formed (phase change stage), using a high flow rate and using a low flow rate at other temperatures can not only increase the freezing rate, but also protect the experimental equipment. Therefore, this embodiment adopts 4m / s as the slow flow rate for variable speed freezing. Thereby providing ideas for the construction of the subsequent control system and providing control data for the subsequent experimental data to judge the accuracy of the control board and the correctness of the experiment.

[0192] In summary, 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 analyses of constant speed and variable speed, and finally calculates the energy consumption to obtain the control board program for 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 an overall design of the control system based on the simulation results.

[0193] Embodiment 4: This embodiment provides an immersion freezing device temperature control system, and the system 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 value 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 application, in order to ensure that the food can reach the required freezing temperature in the shortest time and the required energy consumption is lower, the temperature acquisition module, liquid pump drive module, host computer, data transmission module, control module and refrigeration equipment control module are designed respectively; the control module is responsible for receiving the temperature signal from the temperature acquisition module, and adjusting the power of the liquid pump drive module according to these temperature signals and the preset control strategy (cascade control and PID control algorithm), thereby controlling the flow rate of the refrigerant, increasing the freezing rate, reducing energy consumption, reducing the formation of large ice crystals, and maintaining the nutritional content of the food. At the same time, the start and stop of the refrigeration equipment control module is controlled according to the real-time temperature of the frozen object to minimize energy consumption.

[0200] Among them, the data transmission module realizes the communication between the host computer and the control module through USB to serial communication, and the connection between the host computer and the "DB9 interface" of the control module is established through the serial signal line, and the serial signal line uses the "RS-232 standard" to transmit data signals. Since the RS-232 level standard signal cannot be directly recognized by the controller, this embodiment uses a level conversion chip to convert it into a "TTL standard" level signal that can be recognized by the controller, and configures the serial port baud rate to 11500, the data bit is 8, there is no parity bit, the stop bit is 1, and the serial port selection is COM3 or COM5.

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

[0202] The DS18B20 temperature sensor is used as the temperature acquisition module. By connecting two DS18B20 temperature sensors to measure the temperature of the frozen object and the external environment, 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 drive module and the refrigeration equipment control module is 220 AC, but the power supply voltage of the circuit board is 24V. In order to protect the entire circuit board, two relays that convert DC 3.3V to AC 220V are connected. The relay is Aoyan DC-AC SSR-25DA, and the actual current is 12A.

[0204] It is also necessary to power the 24v motor, 3.3v microcontroller and LCD display. The external power supply 220v AC is adjusted to 24v through the power adapter and connected to the control board, and then 24v is converted to 12v, 12v to 5v, and 5v to 3.3v, so the power module circuit is designed.

[0205] The temperature control system proposed in this implementation needs to be able to accurately control the freezing environment to stabilize at -20℃~25℃, so the DS18B20 temperature sensor is selected as the component for temperature data acquisition of the control system. The temperature control system should be able to maintain stable performance during long-term operation, and will not cause performance degradation due to slight changes in the external environment or aging of internal components. Therefore, this paper chooses to design the control system with the STM32 microcontroller as the control core. And the hardware part is made modular 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 the above implementation 1;

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

[0208] Two control closed loops are set, namely 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 application, this embodiment adopts a combination of cascade control strategy and PID control algorithm to realize power control of the liquid pump drive module.

[0212] Among them, Fig.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, so that the system deviation is gradually reduced to zero, thereby achieving the purpose of precise control. The purpose of this embodiment is to reduce energy consumption and enhance the freezing rate by intelligently controlling the difference between the real-time temperature of the frozen object and the rated temperature, thereby improving the effect of immersion freezing. Therefore, the numbers in the position PID control algorithm are selected, and the PID calculation formula is specifically as follows:

[0213]

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

[0215] like Fig.19 As shown, two control closed loops are designed, in which the inner loop is the secondary loop and the outer loop is the main loop. The main loop is a fixed value control system, and the secondary loop is a follow-up control system. The two closed loops work together to accurately control the temperature of the frozen object. The input of the main regulator is the rated temperature of the frozen object, and the output of the main regulator is the set value of the secondary regulator. The power of the liquid pump is controlled by the output value of the secondary regulator. The power affects the flow rate, thereby cooling the frozen liquid.

[0216] In the main loop, e1(t) is the difference between the rated value of the frozen material temperature and the actual frozen material temperature y1(t). e2(t) is the difference between the liquid pump controller input and the actual refrigerant temperature 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 get the transfer function G(s):

[0231]

[0232] Among them, Q(s) is the pump speed and T(s) is the temperature.

[0233] When the pump speed increases from 0m / s to 5m / s, the temperature will decrease from 25℃ to -18℃. 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 has embedded main program, temperature sensor subprogram, digital PID algorithm subprogram, PWM subprogram, display subprogram, host computer reading subprogram, and relay control subprogram;

[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 with a long polling structure, and all programs are integrated into a main program, which is called in a self-loop manner to work.

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

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

[0244] Write a microcontroller program based on the positional PID formula. Among them, err(k) is temperature1_temp-SET_Temp in the program, which represents the temperature error of one time minus the temperature error of the previous time. Since the temperature control of the temperature control system is divided into three stages (20℃~4℃, 4℃~-4℃, -4℃~-18℃), Δu(k) in the formula is set to the PWM_DUTU value in the program, and Derror represents the accumulated error value. Among them, it represents the accumulated value of the 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 for calling the upper computer numerical value is as follows: The overall PID program needs to be tested 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 specific subroutine for OLED display is as follows: Set the IO of STM32103 connected to the OLED module. Set the PB11-PB15 IO ports connected to the OLED module as outputs. Write the subroutine according to the timing diagram to meet the requirements.

[0251] The above description is only the implementation mode of the present invention and is not limited to the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the scope of the claims of the present invention.

Claims

1. An immersion freezing process control method, characterized in that: The method is: S1: The freezing process of frozen objects is divided into three stages: precooling stage, phase change stage and supercooling stage; S2: The flow rate of the refrigerant is controlled to be low in the precooling stage and the supercooling stage, and the flow rate of the refrigerant 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 refrigeration 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 freezing model and put it into mesh drawing software for mesh drawing; S25: Put 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 of the change of the center temperature over time according to the center 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 select the pressure equation solution, the velocity equation to absolute velocity, the time type to transient, apply the 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 central temperature point of the frozen object is the data extraction point.

8. Immersion freezing process control system, characterized in that: The system includes: A storage device for dividing the frozen material into three stages during the freezing process: a pre-cooling stage, a phase change stage, and a super-cooling stage; A storage device for controlling the flow rate of the refrigerant to a low flow rate in the pre-cooling stage and the super-cooling stage, and controlling the flow rate of the refrigerant to a high flow rate in the phase change stage.

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 comprises a memory and a processor, wherein a computer program is stored in the memory, and 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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