Spherical-like fruit pressure difference precooling control method and system

Abstract

A spherical-like fruit differential pressure precooling control method comprises the steps that a three-dimensional random stacking geometric model of spherical-like fruits is established through computer simulation software, a three-dimensional heat and mass transfer model is established for the spherical-like fruits and a wind tunnel, and fluid flowing in the wind tunnel and heat and mass transfer behaviors of fluid and the spherical-like fruits are predicted; and the cooling curve of the sphere-like fruit under different working condition parameters and the weight loss ratio after pre-cooling are calculated, so that the working condition of the differential pressure pre-cooling equipment is optimized. The invention further provides a spherical-like fruit differential pressure precooling system which can be used for verifying the accuracy of the model and precooling spherical fruits, is composed of a refrigerating system, a water curtain, a low-temperature constant-temperature circulating pump, an anemograph and a thermocouple and can collect experimental data. Through combined application of the control method and equipment, precise regulation and control of pre-cooling parameters can be achieved, so that the pre-cooling process of the sphere-like fruits is optimized, the pre-cooling efficiency is improved, and the weight loss rate of the fruits is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of food preservation, and particularly to a control method and system for the differential pressure precooling of spherical fruits. Background Art

[0002] After fruits and vegetables are picked, they still have a large amount of field heat. Therefore, freshly picked fruits usually have a high temperature and strong respiration. If not cooled in time, it will accelerate water evaporation, affect the quality, and even cause diseases. Therefore, quickly eliminating the field heat of fruits is a key measure to extend their shelf life. Precooking technology can quickly reduce the temperature of fruits, inhibit respiration, and reduce water loss, thereby extending the shelf life and maintaining nutritional value. Currently, the main post-harvest precooling technologies include differential pressure precooling, ice water precooling, and vacuum precooling, etc. Among them, differential pressure precooling has broad development prospects in the field of fruit preservation due to its advantages of large processing capacity and fast precooling speed.

[0003] Taking litchi as an example, due to its thin wax layer, many stomata, rough and cracked peel surface, water is easily evaporated. At the same time, the degree of suberization of the litchi peel is relatively low, and it is difficult to form an effective barrier to reduce water loss. In addition, there is no vascular bundle or other structure connecting the litchi peel and the pulp, and water supply cannot be obtained from the pulp. Therefore, during conventional differential pressure precooling, if the precooling working conditions are inappropriate, the water loss of litchi may be aggravated, resulting in a decrease in commercial value. Therefore, optimizing the control method of differential pressure precooling is particularly important for the preservation of litchi.

[0004] Different fruits have different requirements for the working conditions of differential pressure precooling. Traditional experimental testing methods usually require a large amount of manpower and material resources, which restricts the further development of differential pressure precooling technology. In recent years, computer simulation software has been widely used in the numerical simulation of food production and processing processes, which can effectively reduce the consumption of manpower and material resources caused by experiments. However, the existing numerical simulation methods mainly focus on the change of fruit temperature over time, and do not fully consider the weight loss rate of fruits during differential pressure precooling, which to a certain extent affects the comprehensive evaluation and optimization of the differential pressure precooling effect. Summary of the Invention

[0005] The main object of the present invention is to propose a control method and system for the differential pressure precooling of spherical fruits, which can obtain the working conditions of differential pressure precooling most suitable for spherical fruits according to the results of simulation, and regulate the working conditions of differential pressure precooling equipment, so that the temperature of spherical fruits can be quickly reduced to the precooling temperature and the weight loss rate during differential pressure precooling can be reduced.

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

[0007] A control method for the differential pressure precooling of spherical fruits, which uses computer simulation software to establish a three-dimensional random packing geometric model of spherical fruits, and establishes a three-dimensional heat and mass transfer model for spherical fruits and a wind tunnel to predict the fluid flow in the wind tunnel and the heat and mass transfer behavior between the fluid and spherical fruits, and calculates the cooling curve of spherical fruits under different operating parameters and the weight loss rate after precooling;

[0008] According to the obtained cooling curve of spherical fruits and the weight loss rate after precooling, optimize the operating conditions of the differential pressure precooling equipment to precool spherical fruits.

[0009] As an optimization, a differential pressure precooling equipment is used to verify the accuracy of the heat and mass transfer model.

[0010] As an optimization, the method for establishing a three-dimensional random packing geometric model of spherical fruits is as follows: use the computer simulation software Altair EDEM based on the discrete element method, simplify and simulate spherical fruits as spheres, set multiple simulated spheres as packing materials, set a box as a container, add a gravity effect to the spheres, and let them fall randomly from the top of the box into the box to generate a three-dimensional random packing geometric model.

[0011] As an optimization, the steps for establishing a three-dimensional heat and mass transfer model include:

[0012] Import the center coordinates and sphere diameter data of the three-dimensional random packing geometric model into the simulation software COMSOL Multiphysics, and set a cube as the wind tunnel to generate a three-dimensional geometric model;

[0013] Set the geometric model and physical field parameters, including:

[0014] A: Set the boundary conditions and initial conditions according to the required operating parameters of spherical fruits;

[0015] B: Set the physical property parameters according to the material properties of spherical fruits;

[0016] C: Divide the geometric model using free tetrahedral meshes and refine the meshes of spherical fruits;

[0017] D: Set the steady-state solver for the fluid flow field to perform the solution calculation, and set the transient solver for the heat and mass transfer physical field to perform the solution calculation;

[0018] E: Set the physical field interfaces as turbulence, SST interface, moist air heat transfer interface and moisture transport interface in the air to calculate the velocity of the fluid flow field, the temperature of the air domain, the temperature of spherical fruits and the weight loss rate of spherical fruits after precooling during the precooling process of the physical model.

[0019] As an optimization,

[0020] The governing equations of the three-dimensional heat and mass transfer model are:

[0021] Flow behavior of fluid flow field:

[0022]

[0023] Among them, ρ a is the density of air, u is the flow field velocity, P is the air pressure, μ is the dynamic viscosity of air, μ T is the turbulent viscosity;

[0024] Heat transfer behavior:

[0025]

[0026] Q r =0.003f(1.8T l +32) g (5)

[0027] Q e =L i G (6)

[0028] Among them, ρ l is the density of spherical fruit, c p,l is the specific heat capacity of spherical fruit, T l is the temperature of spherical fruit, t is the pre-cooling time, λ l is the thermal conductivity of spherical fruit, Q r is the respiratory heat of spherical fruits, Q e is the transpiration heat of spherical fruits, c p,a is the specific heat capacity of air, T a is the air temperature, λ a is the thermal conductivity of air, f and g are the respiration coefficients of fruits, L v is the latent heat of evaporation, G is the mass transfer source term;

[0029] Mass transfer behavior:

[0030]

[0031] n·g v,evap =k skin M v (a w c sat -c v ) if (a ω c sat >c v ) (9)

[0032] Among them, M v is the molar mass of water, cv is the water vapor concentration, D v,a is the water vapor diffusion coefficient of air, is the relative humidity, c sat is the saturated vapor pressure concentration, g v,evap is the moisture loss flux on the fruit surface, k skin is the fruit evaporation rate factor, a w is the water activity of the fruit;

[0033] The density, thermal conductivity, and specific heat capacity of the spheroidal fruit are set as specific fixed values and input into the model, and then the control equations of the three-dimensional heat and mass transfer model are solved using the computer simulation software COMSOL Multiphysics to obtain the relationship between the temperature of each point of the spheroidal fruit and time and the weight loss rate.

[0034] As an optimization, the process of verifying the accuracy of the heat and mass transfer model is as follows: Place the spheroidal fruit in a pressure difference precooling device, measure the temperature change during the precooling process of the spheroidal fruit and the mass before and after precooling, obtain the cooling curve and weight loss rate of the spheroidal fruit, then use the origin analysis software to perform polynomial fitting on the cooling curve, calculate its 95% confidence interval and 95% prediction interval, and compare with the simulated cooling curve to verify the accuracy of the model.

[0035] As an optimization, the pressure difference precooling device includes:

[0036] A wind tunnel device for generating and controlling air flow;

[0037] A refrigeration system for controlling the temperature of the cavity of the pressure difference precooling device;

[0038] A water curtain for increasing the humidity of the cavity environment of the pressure difference precooling device and reducing the air temperature;

[0039] A low-temperature constant temperature circulating pump for supplying low-temperature water source to the water curtain;

[0040] An anemometer for measuring the air inlet velocity of the wind tunnel device;

[0041] A thermocouple for measuring the temperature of the cavity of the pressure difference precooling device and the temperature of the spheroidal fruit.

[0042] As an optimization, the method for precooling the spheroidal fruit is as follows: Start the pressure difference precooling device, set the temperature of the refrigeration system and the low-temperature constant temperature circulating pump, and the air inlet velocity of the wind tunnel device. When the temperature and humidity of the cavity of the pressure difference precooling device reach the experimental target values, place the spheroidal fruit in a basket in a randomly stacked manner, and then place the basket containing the spheroidal fruit in the wind tunnel of the pressure difference precooling device, and take it out when the temperature of the spheroidal fruit drops to the target precooling temperature;

[0043] Randomly select 5 spherical fruits, measure their masses before and after precooling to obtain the weight loss rate.

[0044] A spherical fruit differential pressure precooling system for the above control method, comprising:

[0045] A prediction module, used to establish a three-dimensional heat and mass transfer model, predict the temperature and weight loss rate data of spherical fruits, and calculate the most suitable differential pressure precooling working conditions for spherical fruits;

[0046] A differential pressure precooling equipment module, used to precool spherical fruits according to the required working conditions.

[0047] As a preference, the differential pressure precooling equipment module includes a wind tunnel device, a refrigeration system, a water curtain, a low-temperature constant-temperature circulation pump, an anemometer, and thermocouples;

[0048] The wind tunnel device is arranged inside the differential pressure precooling equipment cavity for generating and controlling air flow;

[0049] The refrigeration system includes an axial flow fan, an evaporator, and a condenser. The evaporator and the axial flow fan are located inside the differential pressure precooling equipment cavity;

[0050] The water curtain is arranged inside the differential pressure precooling equipment cavity;

[0051] The low-temperature constant-temperature circulation pump is connected to the water curtain and supplies low-temperature water source to the water curtain;

[0052] The anemometer is arranged at the air inlet of the wind tunnel device;

[0053] The number of thermocouples is multiple, and they are arranged on the inner wall surface of the differential pressure precooling equipment cavity and inside the spherical fruits.

[0054] Generally speaking, the present invention has the following advantages:

[0055] 1. The present invention establishes a three-dimensional random packing geometric model through the computer simulation software Altair EDEM, simulates the packing method of spherical fruits after harvesting, and improves the accuracy of the simulation results.

[0056] 2. The present invention establishes a three-dimensional heat and mass transfer model through the computer simulation software COMSOL Multiphysics, predicts the temperature change during the differential pressure precooling process of spherical fruits, has universality. For spherical fruits of different types, different sizes and different working conditions, only need to modify the model parameters and run the program again to obtain the prediction results, can conveniently obtain the prediction results under different working conditions, is convenient for quickly selecting better working condition parameters for precooling, and saves the experimental cost and time.

[0057] 3. The present invention constructs a differential pressure precooling device, verifies the accuracy of the heat and mass transfer model, and provides reference and guidance for regulating the operating conditions of the differential pressure precooling device. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 is a flowchart of the control method of the present invention;

[0059] Figure 2 is a schematic structural diagram of the three-dimensional geometric model in the embodiment;

[0060] Figure 3 is a comparison diagram of the simulated value and the experimental value of the temperature reduction curve of the litchi sample in the embodiment;

[0061] Figure 4 is a comparison diagram of the simulated value and the experimental value of the weight loss rate of the litchi sample in the embodiment;

[0062] Figure 5 is a schematic structural diagram of the differential pressure precooling device in the embodiment.

[0063] Figure 6 is the weight loss rate after differential pressure precooling under different relative humidities.

[0064] Figure 7 is a diagram of the average temperature change during differential pressure precooling under different air supply temperatures.

[0065] Figure 8 is a diagram of the change of the temperature coefficient of variation under different air supply temperatures.

[0066] Among them, 1 is an axial flow fan, 2 is an evaporator, 3 is a wind tunnel device, 4 is a water curtain, and 5 is a basket. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0067] Embodiment 1

[0068] The present invention will be further described in detail below by taking litchi as an example, but the embodiments of the present invention are not limited thereto.

[0069] As Figure 1 shown, this embodiment provides a control method for differential pressure precooling of spherical fruits, uses the computer simulation software Altair EDEM to establish a three-dimensional random packing geometric model, and simulates the packing method of litchi after harvest in the precooling box. Specifically:

[0070] The litchi is simplified and simulated as a sphere, and simulation spheres with different diameters (28mm, 30mm, 32mm) are set as packing materials. Then, 10*10*10mm 3The box is used as a container, and a gravitational force is applied to the sphere so that it falls from a random position at the top of the box into the box to generate a three-dimensional random packing geometric model. Then, the center coordinates and sphere diameter data of each sphere are exported and used for the following simulation.

[0071] Use the computer simulation software COMSOL Multiphysics to establish a three-dimensional heat and mass transfer model for litchi and the wind tunnel, predict the fluid flow in the wind tunnel and the heat and mass transfer process between the fluid and litchi, and obtain the simulated values of the litchi cooling curve and weight loss rate. In this embodiment, taking the litchi temperature dropping to the target pre-cooling temperature of 6°C as an example, specifically:

[0072] (1) First, import the center coordinates and sphere diameter data of the spheres generated by Altair EDEM into COMSOL Multiphysics, and establish a cuboid with dimensions of 250*100*100mm 3 to establish a three-dimensional geometric model, as Figure 2 shown;

[0073] (2) Set the physical field interfaces as turbulence, SST interface, humid air heat transfer interface, and moisture transport interface in air;

[0074] (3) Set the boundary conditions and initial conditions according to the required operating condition parameters. Specifically:

[0075] The initial temperature of the sample is 25°C, and the initial temperature of the environment is 5°C. Define the inlet as fully developed flow conditions, and calculate with 5 different wind speeds (1.2 m / s, 1.5 m / s, 1.8 m / s, 2.1 m / s, 2.4 m / s) respectively. Apply the boundary condition of suppressing backflow at the outlet. Set all solid surfaces as no-slip boundary conditions, including the wind tunnel wall surface and all surfaces of the litchi;

[0076] (4) Set the physical property parameters according to the material properties of litchi, as shown in Table 1.

[0077] Table 1 Physical property parameters of litchi material

[0078]

[0079] (5) Use the built-in mesh generation tool in COMSOL Multiphysics to generate a mesh. Use free tetrahedral meshes to mesh the geometric model, and refine the mesh of the geometric model of the spherical fruit-like object;

[0080] (6) Set the steady-state solver for the fluid flow field to perform the solution calculation, and set the transient solver for the heat and mass transfer physical field to perform the solution calculation. The transient solution of the fluid flow field is required for the heat and mass transfer physical field and is input through the dependent variable setting;

[0081] (7) Set the transient solver to solve until 25 min with a time step of 10 s;

[0082] (8) Predict the cooling curve and weight loss rate of the litchi samples and compare with the subsequent verification results.

[0083] The governing equations of the three-dimensional heat transfer model are:

[0084] Flow behavior of the fluid flow field:

[0085]

[0086] where ρ a is the density of air, u is the flow field velocity, P is the air pressure, μ is the dynamic viscosity of air, and μ T is the turbulent viscosity;

[0087] Heat transfer behavior:

[0088]

[0089] Q r =0.003f(1.8T l +32) g

[0090] Q e =L v G

[0091] where ρ l is the density of the spherical fruit, c p,l is the specific heat capacity of the spherical fruit, T l is the temperature of the spherical fruit, t is the precooling time, λ l is the thermal conductivity of the spherical fruit, Q r is the respiratory heat of the spherical fruit, Q e is the transpiration heat of the spherical fruit, c p,a is the specific heat capacity of air, T a is the temperature of air, λ a is the thermal conductivity of air, f and g are the respiratory coefficients of the fruit, L v is the latent heat of transpiration, and G is the mass transfer source term;

[0092] Mass transfer behavior:

[0093]

[0094] n·g v,evap =k skin M v (a w c sat -c v ) if (aω c sat >c v )

[0095] Among them, M v is the molar mass of water, c v is the water vapor concentration, D v,a is the water vapor diffusion coefficient of air, is the relative humidity, c sat is the saturated vapor pressure concentration, g v,evap is the moisture loss flux on the fruit surface, k skin is the fruit evaporation rate factor, a w is the water activity of the fruit.

[0096] The density, thermal conductivity, and specific heat capacity of the spherical fruit are set as specific fixed values and input into the model. Then, the control equations of the three-dimensional heat and mass transfer model are solved using the computer simulation software COMSOL Multiphysics to obtain the relationship between the temperature of each point of the spherical fruit and time and the weight loss rate.

[0097] Use a pressure difference precooling device to conduct a pressure difference precooling experiment on litchi to verify the accuracy of the simulation results. The pressure difference precooling device includes: a wind tunnel device for generating and controlling airflows; a refrigeration system for controlling the temperature of the pressure difference precooling device cavity; a water curtain for increasing the humidity of the pressure difference precooling device cavity environment and reducing the air temperature; a low-temperature constant-temperature circulation pump for supplying low-temperature water source to the water curtain; an anemometer for measuring the air inlet velocity of the wind tunnel device; and a thermocouple for measuring the temperature of the pressure difference precooling device cavity and the temperature of the spherical fruit.

[0098] In this embodiment, temperature data is obtained by inserting a thermocouple into the litchi. The specific steps of conducting a pressure difference precooling experiment using the pressure difference precooling device are as follows:

[0099] (1) Randomly select 5 litchis for each experiment, mark them on the peel as samples for measuring the weight loss rate, and weigh their masses;

[0100] (2) Start the pressure difference precooling device, set the temperatures of the refrigeration system and the low-temperature constant-temperature circulation pump, and adjust the wind tunnel device so that its air inlet velocity reaches the specified velocity for each experiment (1.2 m / s, 1.5 m / s, 1.8 m / s, 2.1 m / s, 2.4 m / s). When the temperature at the air inlet of the wind tunnel in the pressure difference precooling device cavity drops to 5°C and the humidity is 90%, place the litchis in a random stacking manner in a basket of 10*10*10 mm 3 , and then place the basket containing the litchis in the wind tunnel of the pressure difference precooling device. At the same time, turn on the recording software of the temperature sensor to collect sample temperature data;

[0101] (3) When the temperature of the quasi-spherical fruit drops to 6 °C, take it out and weigh the mass of 5 marked litchis.

[0102] (4) To avoid the contingency of the results, 3 parallel experiments were carried out.

[0103] (5) Compare and analyze the collected data with the calculation results of the model.

[0104] Such as Figure 3 and Figure 4 shown, the cooling curve of litchi, 95% confidence interval and 95% prediction interval were plotted using the average value of the measured values, and the measured values were compared with the predicted values. In addition, to further determine the effectiveness of the model, the measured values and predicted values were compared based on the root mean square error (RMSE). It was found that the RMSE value of the litchi temperature at the inlet was 0.69 °C, the litchi temperature at the outlet was 0.74 °C, and the RMSE value of the weight loss rate was 0.01%, which proved the effectiveness of the heat and mass transfer model of the present invention.

[0105] When verifying the accuracy of the heat and mass transfer model using the measured values of the differential pressure precooling equipment, if the accuracy does not reach the expected target, the model parameters can be adjusted and the simulation and verification can be continued.

[0106] When the above control method is actually used, the shape and size of the simulation sphere can be adjusted according to the fruits to be precooled as required, and multiple working condition parameters can be selected for simulation according to the actual situation to select a more appropriate precooling working condition.

[0107] In this embodiment, the weight loss rate of litchi after differential pressure precooling at different air supply humidities was also calculated by the above heat and mass transfer model. Such as Figure 6 shown, it can be seen from the figure that the increase of the environmental relative humidity can reduce the weight loss rate of litchi. Therefore, a higher humidity of 90% was selected as the most suitable humidity condition.

[0108] Select the air supply speed of 2.4 m / s and the air relative humidity of 90%. The change trend of the average temperature with time during the differential pressure precooling process of litchi at different air supply temperatures (i.e., the temperature at the air inlet of the wind tunnel in the differential pressure precooling equipment cavity) of 2 °C, 5 °C, and 8 °C was calculated by the above heat and mass transfer model. Such as Figure 7 shown, the results show that as the air supply temperature decreases, the absolute value of the slope of the cooling curve increases, that is, the cooling rate accelerates, and the average temperature of the fruit drops more rapidly. Figure 8 Shows the change trend of the temperature coefficient of variation (COV T ) of litchi during the differential pressure precooling process under different air supply temperature conditions. It can be seen that the COV [[ID=3)4]] T both show a trend of first increasing and then decreasing, but their values are significantly different at different air supply temperatures. The lower the air supply temperature, the COVT The higher the overall value is, the worse the pre-cooling uniformity is during the entire pre-cooling process. A supply air temperature of 2°C will cause uneven pre-cooling of lychees, easily resulting in local over-cooling and local insufficient pre-cooling; while 8°C has better pre-cooling uniformity, but its cooling rate is slow and it cannot reach the target pre-cooling temperature of 5°C. Therefore, the most suitable supply air temperature can be selected as 5°C. In this embodiment, the preferred operating conditions can be selected as a wind speed of 2.4 m / s, a supply air humidity of 90%, and a supply air temperature of 5°C.

[0109] Embodiment 2

[0110] A spherical fruit differential pressure pre-cooling system for implementing the control method of Embodiment 1, the system includes:

[0111] A prediction module, used to establish a three-dimensional heat and mass transfer model, predict the temperature and weight loss rate data of spherical fruits, and calculate the most suitable differential pressure pre-cooling operating conditions for spherical fruits;

[0112] A differential pressure pre-cooling equipment module, used to pre-cool spherical fruits according to the required operating conditions.

[0113] As Figure 5 shown, the differential pressure pre-cooling equipment module includes a wind tunnel device, a refrigeration system, a water curtain, a low-temperature constant-temperature circulation pump, an anemometer, and thermocouples; the wind tunnel device, the refrigeration system, the water curtain, the low-temperature constant-temperature circulation pump, the anemometer, and the thermocouples can all use existing products.

[0114] The wind tunnel device is arranged inside the differential pressure pre-cooling equipment cavity, used to generate and control air flow. Spherical fruits can be stacked in the wind tunnel, and the wet and cold air flow passes through the gaps between fruits and vegetables to achieve forced convective heat and mass transfer, achieving the effect of cooling fruits and vegetables; the refrigeration system includes an axial flow fan, an evaporator, a condenser, etc., and the evaporator and the axial flow fan are located inside the differential pressure pre-cooling equipment cavity; the water curtain is arranged inside the differential pressure pre-cooling equipment cavity; the low-temperature constant-temperature circulation pump is connected to the water curtain and supplies low-temperature water source to the water curtain; the anemometer is arranged at the air inlet of the wind tunnel device; the number of thermocouples is multiple, and they are arranged on the inner wall surface of the differential pressure pre-cooling equipment cavity and inside the spherical fruits.

[0115] The above embodiments are the preferred embodiments of the invention, but the embodiments of the invention are not limited by the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the invention shall be equivalent replacement methods and are all included in the protection scope of the invention.

Claims

1. A control method for the differential pressure precooling of spherical fruits, characterized in that, A three-dimensional random packing geometric model of spherical-like fruits was established using computer simulation software, and a three-dimensional heat and mass transfer model of spherical-like fruits and a wind tunnel was established to predict the fluid flow in the wind tunnel and the heat and mass transfer behavior between the fluid and the spherical-like fruits, and the cooling curve of the spherical-like fruits under different operating parameters and the weight loss rate after precooling were calculated; According to the obtained cooling curve of the spherical fruits and the weight loss rate after precooling, the operating conditions of the pressure difference precooling equipment were optimized, and the spherical-like fruits were precooled.

2. A control method for the differential pressure precooling of spherical fruits according to claim 1, characterized in that: A pressure difference precooling equipment was used to verify the accuracy of the heat and mass transfer model.

3. A control method for pressure difference precooling of quasi-spherical fruits according to claim 1, characterized in that, The method for establishing the three-dimensional random packing geometric model of spherical-like fruits was as follows: Using the computer simulation software AltairEDEM based on the discrete element method, the spherical-like fruits were simplified and simulated as spheres, multiple simulated spheres were set as packing materials, a box was set as a container, and a gravity effect was added to the spheres to make them fall randomly from the top of the box into the box to generate a three-dimensional random packing geometric model.

4. A control method for pressure difference precooling of spherical fruits according to claim 1, characterized in that The steps for establishing the three-dimensional heat and mass transfer model included: The center coordinates and sphere diameter data of the three-dimensional random packing geometric model were imported into the simulation software COMSOL Multiphysics, and a cube was set as the wind tunnel to generate a three-dimensional geometric model; The geometric model and physical field parameters were set, including: A: The boundary conditions and initial conditions were set according to the required operating parameters of the spherical-like fruits; B: The physical property parameters were set according to the material properties of the spherical-like fruits; C: The geometric model was divided using free tetrahedral meshes, and the meshes of the spherical-like fruits were refined; D: The steady-state solver was used to solve the fluid flow field, and the transient solver was used to solve the heat and mass transfer physical field; E: The physical field interfaces were set as turbulence, SST interface, heat transfer interface of humid air, and moisture transport interface in air to calculate the velocity of the fluid flow field, the temperature of the air domain, the temperature of the spherical-like fruits, and the weight loss rate of the spherical-like fruits after precooling during the precooling process of the physical model.

5. A control method for pressure difference precooling of spherical fruits according to claim 1, characterized in that The control equations of the three-dimensional heat and mass transfer model were: The flow behavior of the fluid flow field: where ρ a is the density of air, u is the flow field velocity, P is the air pressure, μ is the dynamic viscosity of air, and μ T is the turbulent viscosity; The heat transfer behavior: Q r = 0.003f(1.8T l + 32) g (5) Q e = L v G(6) Among them, ρ l is the density of the spherical fruit, c p,l is the specific heat capacity of the spherical fruit, T l is the temperature of the spherical fruit, t is the precooling time, λ l is the thermal conductivity of the spherical fruit, Q r is the respiratory heat of the spherical fruit, Q e is the transpiration heat of the spherical fruit, c p,a is the specific heat capacity of air, T a is the temperature of air, λ a is the thermal conductivity of air, f and g are the respiratory coefficients of the fruit, L v is the latent heat of transpiration, G is the mass transfer source term; The mass transfer behavior: n·g v,evap = k skin M v (a w c sat - c v ) if (a ω C sat > C v ) (9) Among them, M v is the molar mass of water, c v is the water vapor concentration, D v,a is the water vapor diffusion coefficient of air, is the relative humidity, c sat is the saturated vapor pressure concentration, g v,evap is the moisture loss flux on the fruit surface, k skin is the fruit evaporation rate factor, a w is the water activity of the fruit; The density, thermal conductivity, and specific heat capacity of the spherical-like fruits were set as specific fixed values and input into the model, and then the computer simulation software COMSOL Multiphysics was used to solve the control equations of the three-dimensional heat and mass transfer model to obtain the relationship between the temperature of each point of the spherical-like fruits and time and the weight loss rate.

6. The control method for differential pressure precooling of a spherical fruit according to claim 2, characterized in that, The process for verifying the accuracy of the heat and mass transfer model was as follows: The spherical-like fruits were placed in the pressure difference precooling equipment, the temperature changes during the precooling process of the spherical-like fruits and the mass before and after precooling were measured to obtain the cooling curve and weight loss rate of the spherical-like fruits, and then the origin analysis software was used to perform polynomial fitting on the cooling curve, calculate its 95% confidence interval and 95% prediction interval, and compare them with the simulated cooling curve to verify the accuracy of the model.

7. A control method for the differential pressure precooling of spherical fruits according to claim 1, characterized in that, The pressure difference precooling equipment included: A wind tunnel device for generating and controlling airflows; A refrigeration system for controlling the temperature of the cavity of the pressure difference precooling equipment; A water curtain for increasing the humidity of the cavity environment of the pressure difference precooling equipment and reducing the air temperature; A low-temperature constant-temperature circulating pump is used to supply low-temperature water source to the water curtain; An anemometer is used to measure the air inlet velocity of the wind tunnel device; Thermocouples are used to measure the temperature of the pressure difference pre-cooling equipment cavity and the temperature of spherical-like fruits.

8. A control method for differential pressure precooling of quasi-spherical fruits according to claim 7, characterized in that, The method for pre-cooling spherical-like fruits is as follows: Start the pressure difference pre-cooling equipment, set the temperatures of the refrigeration system and the low-temperature constant-temperature circulating pump, and the air inlet velocity of the wind tunnel device. When the temperature and humidity of the pressure difference pre-cooling equipment cavity reach the experimental target values, place the spherical-like fruits randomly stacked in a basket, and then place the basket containing the spherical-like fruits in the wind tunnel of the pressure difference pre-cooling equipment. Take out the spherical-like fruits when their temperature drops to the target pre-cooling temperature; Randomly select 5 spherical-like fruits and measure their masses before and after pre-cooling to obtain the weight loss rate.

9. A quasi-spherical fruit differential-pressure precooling system for implementing the control method according to any one of claims 1 to 8, characterized in that, It includes: A prediction module is used to establish a three-dimensional heat and mass transfer model, predict the temperature and weight loss rate data of spherical-like fruits, and calculate the most suitable pressure difference pre-cooling working conditions for spherical-like fruits; A pressure difference pre-cooling equipment module is used to pre-cool spherical-like fruits according to the required working conditions.

10. A quasi-spherical fruit pressure difference precooling system according to claim 9, characterized in that, The pressure difference pre-cooling equipment module includes a wind tunnel device, a refrigeration system, a water curtain, a low-temperature constant-temperature circulating pump, an anemometer and thermocouples; The wind tunnel device is arranged inside the pressure difference pre-cooling equipment cavity and is used to generate and control air flow; The refrigeration system includes an axial flow fan, an evaporator and a condenser. The evaporator and the axial flow fan are located inside the pressure difference pre-cooling equipment cavity; The water curtain is arranged inside the pressure difference pre-cooling equipment cavity; The low-temperature constant-temperature circulating pump is connected to the water curtain and supplies low-temperature water source to the water curtain; The anemometer is arranged at the air inlet of the wind tunnel device; The number of thermocouples is multiple, and they are arranged on the inner wall surface of the pressure difference pre-cooling equipment cavity and inside the spherical-like fruits.

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