Hot air drying method for joint optimization of microwaves and mathematical models of Chinese yams

Abstract

According to the Chinese yam microwave and mathematical model combined optimization hot air drying method, Chinese yam slices are dried in a microwave pretreatment and hot air drying mode, browning of the Chinese yam slices can be inhibited to a certain extent through microwave pretreatment, nutritional ingredients are reserved to the maximum extent, the drying time is shortened, and the drying efficiency is improved. And the drying rate is improved. Besides, the method utilizes a COMSOL mathematical model to realize the simulation of hot air drying of different Chinese yam slices by changing different parameters, so that the change of temperature and moisture content in the drying process of the Chinese yam slices can be obtained, the optimal drying process is simulated, a large amount of test time and cost are saved, and the research efficiency is improved. And the method has certain theoretical guiding significance on optimization of a hot air drying process.

Description

Technical Field

[0001] The invention belongs to the technical field of fruit and vegetable drying processing, and relates to a hot air drying method for yam jointly optimized by microwave and mathematical model. Background Art

[0002] Fresh yam has a high moisture content. If appropriate control measures are not taken, microbial spoilage can occur, affecting shelf life and storage stability, resulting in significant losses. Furthermore, yam is seasonal, and consumer demand for it is no longer limited to the harvest season but exists year-round, anytime and anywhere. To meet this demand and facilitate market circulation and export trade, extending the effective storage life of yam is particularly important. Therefore, dehydration and drying of yam has enormous development potential and broad application prospects.

[0003] At present, common methods for drying yam include natural drying, hot air drying, infrared drying, microwave drying, freeze drying, etc. Hot air drying is currently the most commonly used drying method, but discoloration and quality deterioration are very likely to occur during the hot air drying process. Microwave drying has the disadvantages of high equipment investment cost, large one-time investment, and small penetration depth. It usually needs to be used in combination with hot air drying, and can solve the problems of discoloration and quality deterioration during hot air drying. The drying rate when microwave and hot air drying are used together is closely related to the drying conditions. Therefore, it is necessary to further optimize the pretreatment and drying process of the material to maximize the drying rate and reduce the drying energy consumption.

[0004] During the hot air drying process of yam, the process parameters of hot air drying are closely related to air temperature, flow rate, material structure, moisture content, shape and quality requirements. This process involves multiple variables, so how to quickly and accurately obtain the optimal working parameters is a technical problem that technicians in this field urgently need to solve. Summary of the Invention

[0005] The purpose of the present invention is to provide a hot air drying method for yam by combined optimization of microwave and mathematical model, so as to solve the problem that working parameters of yam hot air drying are difficult to determine.

[0006] To achieve the above object, the present invention adopts the following technical solutions: The present application provides a hot air drying method for yam jointly optimized by microwave and mathematical model, the method comprising: S01: The yam is washed, peeled, and cut into uniform yam slices at a 45° angle.

[0007] Fresh, mold-free, mechanically undamaged and uniform in thickness and size iron stick yam is selected as raw material and stored in a 4±1℃ environment for no more than 7 days. The initial moisture content of the iron stick yam is 76±1.75% by mass percentage. The selected yam is cleaned and peeled with a ceramic knife. The yam is cut into uniform slices at a 45° angle.

[0008] S02: The yam slices are uniformly placed on the material tray and subjected to microwave treatment for 80-90s at 500-600W. Within 30s after the treatment, the yam slices are transferred to a hot air drying oven for constant temperature drying. The moisture content of the yam slices is calculated to obtain the actual test results.

[0009] The yam slices are uniformly placed on the material tray and subjected to microwave treatment for 80-90s at 500-600W. After the microwave treatment, the yam slices are quickly transferred to a hot air drying oven within 30s, uniformly and unfolded on the drying net, and the drying net is placed in the middle position of the drying oven. The drying oven is preheated for at least 1h. The yam slices are subjected to constant temperature drying at a temperature of 70℃, a relative humidity of 6%, and a hot air speed of 1.0m / s. The weight of the yam slices is measured every 30 minutes with an electronic balance, and the moisture content of the yam slices is calculated. After each weighing, the yam slices are immediately returned to the hot air drying oven. When the moisture content of the yam slices is less than 10%, the drying is completed, and the actual test results are obtained.

[0010] In this application, the calculation formula of the moisture content of the yam slices is: (1) Wherein, m t is the mass of the yam slices at time t; and m d is the absolute dry mass of the yam slices.

[0011] S03: Based on the COMSOL mathematical model, a hot air drying physical model of the yam slices is established according to the size of the yam slices and the drying oven. The hot air drying physical model includes a fluid heat transfer module, a dilute substance transfer module, and a solid mechanics module.

[0012] Considering the complexity of the model, the accuracy of the calculation, and the length of the calculation time, the following assumptions are made before establishing the hot air drying physical model of the yam slices: the yam slices are uniform and isotropic continuous media; the moisture diffusion inside the yam slices conforms to Fick's law; the phase change of the solid, liquid and gas phases inside the yam slices is ignored; the air in the drying process is an ideal gas, and the flow rate and temperature distribution in the oven are constant; and the moisture diffuses from the inside to the surface of the yam in liquid form and vaporizes on the surface.

[0013] Run COMSOL Multiphysics software, select a three-dimensional model in COMSOL simulation software, select forced convection heat transfer coefficient according to Reynolds coefficient, select fluid heat transfer, dilute substance transfer, solid mechanics three modules for transient research. In this application, the actual size of the drying box is taken as the basis, and each structure of the drying box is restored as much as possible to improve the similarity and reduce the relative error rate between the simulation value and the true value. Therefore, the length, width and height of the drying box, the size of the drying tray, the position and size of the air inlet and outlet are measured to obtain the physical size of the drying box. Then, according to the physical size of the drying box and the size of the shanyao slice, a shanyao slice hot air drying physical model is constructed.

[0014] S04: defining material properties for the hot air drying physical model.

[0015] The material properties are defined for the fluid heat transfer module, the dilute substance transfer module and the solid mechanics module respectively. Specifically: The fluid heat transfer module defines the initial value of the fluid, and calculates the effective moisture diffusion coefficient of the shanyao slice, the heat transfer control equation of the shanyao slice in the hot air drying process and the heat transfer control equation at the boundary of the model according to the initial value. The initial value of the fluid in this application includes hot air drying medium temperature, dynamic viscosity, thermal conductivity, constant pressure heat capacity and boundary heat source. Specifically, the initial temperature of air and shanyao slice is set to room temperature; the convection heat transfer boundary condition is set, the contact surface of the material and air is determined as the boundary heat source, and the moisture content of the shanyao slice at the drying end point is 10%.

[0016] The calculation formula of the effective moisture diffusion coefficient of the shanyao slice is: (2) Wherein, M R is the moisture ratio of the shanyao slice; D eff is the effective moisture diffusion coefficient; L is the thickness of the shanyao slice; t is the hot air drying time.

[0017] The heat transfer control equation of the shanyao slice in the hot air drying process is: (3) (4) In formula (3), (4), ρ is the density of the shanyao slice; c p is the specific constant pressure heat capacity of the shanyao; q is the volume heat source; T is the internal temperature of the shanyao slice; λ is the thermal conductivity of the shanyao slice; hfg is the latent heat of vaporization of water; k c is the mass transfer coefficient of yam slices; c b is the moisture concentration of the air; c Slice the yam at any time t moisture concentration.

[0018] The governing equation for heat transfer at the model boundary is: (5) in, n is the unit normal vector; h s is the convective heat transfer coefficient of yam slices; T air is the hot air temperature; λ is the thermal conductivity of yam slices; T The internal temperature of the yam slice; D eff is the effective water diffusion coefficient; c Slice the yam at any time t moisture concentration.

[0019] The Diluted Species Transport module defines transfer properties, including specific heat capacity, thermal conductivity, density, Young's modulus, Poisson's ratio, mass transfer coefficient, and moisture loss boundary. The mass transfer coefficient is the bulk, the flux type is external convection, and the moisture loss boundary is the surface of the yam slice in contact with the air.

[0020] The mass transfer control equation of yam slices during air-heat drying is: (6) in, c Slice the yam at any time t Moisture concentration; D eff is the effective water diffusion coefficient.

[0021] The mass transfer governing equation at the model boundary is: (7) in, n is the unit normal vector; D eff is the effective water diffusion coefficient; c Slice the yam at any time t Moisture concentration; h s is the convective heat transfer coefficient of yam slices; c b is the moisture concentration of the air; c0 is the initial moisture concentration of yam slices.

[0022] The Solid Mechanics Module defines a linear elastic material with isotropic material symmetry and determines the hygroscopic expansion coefficient.

[0023] S05: Dividing the hot air drying physical model into a grid; dividing the grid into an air domain and a yam slice domain; when the temperature change rate of the sample center point solved with different grid sizes is less than 0.1%, determining the optimal grid size for solution.

[0024] To ensure computational accuracy while optimizing computational efficiency and resource utilization, the hot air drying model is divided into different grids. In this application, the hot air drying model is divided into two grids: one for the air domain, where fluid dynamics is selected; and the other for the yam slice domain, where general physics is selected. When the temperature change rate at the sample center point is less than 0.1% for different grid sizes, the optimal grid size for the yam slice solution is that grid size.

[0025] S06: Solve the hot air drying physical model to obtain a hot air drying simulation result of the yam slices; the hot air drying simulation result includes the simulated moisture content and temperature.

[0026] The hot air drying physical model is solved to obtain the simulation results of hot air drying of yam slices. The choice of solver needs to find a balance between accuracy and calculation speed according to specific needs, and then reasonably select and configure the solver by simplifying the grid size to ensure the simulation accuracy while optimizing the calculation efficiency and resource utilization. In this application, a transient solver is used in conjunction with a fully coupled solution mode to find the balance between accuracy and calculation speed by adjusting the grid size, and then the hot air drying physical model is solved. The solution result is the simulation result of hot air drying of yam slices, including the moisture content, temperature, etc. after simulation.

[0027] S07: Fitting the actual test results and the hot air drying simulation results to determine the relative error rates of moisture content and temperature; when the relative error rates are less than 5%, the physical model for hot air drying of yam slices is applicable.

[0028] The actual test results and the hot air drying simulation results were fitted to calculate the relative error rates of moisture content and temperature. When the relative error rate is less than 5%, the physical model for hot air drying of yam slices is applicable.

[0029] The calculation formula for the relative error rate is: (8).

[0030] The present invention has the following beneficial effects: (1) In the present application, yam slices are dried by microwave pretreatment and hot air drying. Microwave pretreatment can inhibit the browning of yam slices to a certain extent, the nutritional components are retained to the maximum extent, and the drying time is shortened and the drying rate is increased.

[0031] (2) In this application, the COMSOL mathematical model is used to simulate the hot air drying of different yam slices by changing different parameters. The changes in temperature and moisture content during the drying process of yam slices can be obtained, and the optimal drying process can be simulated, which saves a lot of experimental time and cost, improves research efficiency, and has certain theoretical guiding significance for the optimization of hot air drying process. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic diagram of the placement of yam slices in a hot air drying oven; Figure 2 It is a physical model of hot air drying of yam slices; Figure 3 This is the mesh division diagram of the physical model of hot air drying of yam slices; Figure 4 These are the model verification comparison pictures of yam slices, where Figure a is the image of the yam slices before drying, and Figure b is the image of the yam slices after drying. DETAILED DESCRIPTION

[0033] The technical solution of the present invention is further explained and illustrated below by taking yam slices with a thickness of 3±0.5 mm dried by hot air at 70°C as an example.

[0034] The present invention provides a hot air drying method for yam by combined optimization of microwave and mathematical model, which includes: S01: Select fresh, mold-free, mechanically undamaged, and uniformly thick iron stick yams as the raw material. Refrigerate at 4±1°C for no more than 7 days. The initial moisture content of the iron stick yams is 76±1.75% by mass. Clean and select the yams. Peel the iron stick yams with a ceramic knife. Take the uniformly sized, central portion of the yam and cut it at a 45° angle into uniform slices with a diameter of 25±2mm and a thickness of 3±0.5mm.

[0035] S02: Place the yam slices evenly on the material tray and microwave them at 560W for 90s. After the microwave treatment, quickly transfer them to the hot air drying oven within 30s, spread them evenly and without folding on the drying net, and place the drying net in the middle of the drying oven, as shown in the attached figure. Figure 1The drying oven was preheated for at least 1 hour and dried at 70°C, 6% relative humidity, and a hot air velocity of 1.0 m / s. The weight of the yam slices was measured every 30 minutes using an electronic balance to calculate their moisture content. After each weighing, the slices were immediately returned to the hot air drying oven. Drying was completed when the moisture content fell below 10%, yielding the actual test results.

[0036] In the embodiment of the present application, the calculation formula for the moisture content of yam slices is: (1) Among them, m t is the mass of yam slices at time t; m d It is the absolute dry material mass of yam slices.

[0037] S03: Establishing a physical model for hot air drying of yam slices Taking into account the complexity of the model, the accuracy of the calculation and the length of the calculation time, the assumptions made before establishing the physical model of hot air drying of yam slices are as follows: yam slices are uniform, isotropic continuous media; the moisture diffusion inside the yam slices conforms to Fick's theorem; the solid, liquid and gas phase changes inside the yam slices are ignored; the air in the drying process is an ideal gas, and the flow rate and temperature distribution in the oven are constant; moisture diffuses from the inside to the yam surface in liquid form and vaporizes on the surface.

[0038] In the embodiment of the present application, in order to restore the various structures of the drying box as much as possible, improve the similarity, and reduce the relative error rate between the simulated values ​​and the real values, the length, width, and height of the drying box, the size of the drying tray, the position and size of the air inlet and outlet, and the size of the yam slices are measured, and the multi-physical field related to the hot air drying of yam slices is selected. According to the physical size of the drying box and the size of the yam slices, a physical model of the hot air drying of yam slices is constructed. Specifically, as shown in the attached figure, Figure 2 As shown, the length, width and height of the box are 46cm, 34cm and 45cm respectively; the length, width and height of the drying tray are 39.5cm, 32cm and 4cm respectively, located in the middle of the drying box, with a hole diameter of 0.5cm; the yam slices are 25mm in diameter and 3mm in thickness, with a total of 16 slices, and each yam slice is 4cm apart left and right and 3cm apart up and down.

[0039] During the specific operation, run COMSOL Multiphysics software, select the three-dimensional model in the COMSOL simulation software, select the forced convection heat transfer coefficient according to the Reynolds coefficient, and select the three modules of fluid heat transfer, dilute material transfer, and solid mechanics for transient research.

[0040] S04: Define material properties for the Fluid Heat Transfer Module, Diluted Species Transport Module, and Solid Mechanics Module respectively. Specifically: (1) Fluid heat transfer module defines the initial value of the fluid Initial fluid values ​​are defined for the fluid heat transfer module. Based on these initial values, the effective moisture diffusion coefficient of the yam slices, the governing heat transfer equations for the yam slices during air-heat drying, and the governing heat transfer equations at the model boundaries are calculated. The initial fluid values ​​in this embodiment include parameters such as the hot air drying medium temperature, dynamic viscosity, thermal conductivity, constant-pressure heat capacity, and boundary heat sources.

[0041] Specifically, the initial temperatures of the air and yam slices were set to room temperature, 25°C; the convection heat transfer boundary conditions were set, and the contact surface between the material and the air was determined to be the boundary heat source. The moisture content of the yam slices at the drying end point was 10%.

[0042] The calculation formula for the effective water diffusion coefficient of yam slices is: (2) in, M R is the moisture ratio of yam slices; D eff is the effective water diffusion coefficient; L is the thickness of yam slices; t This is the hot air drying time.

[0043] The heat transfer control equation of yam slices during air-heat drying is: (3) (4) In formula (3) and (4), ρ is the density of yam slices; c p is the specific heat capacity at constant pressure of yam; q is the volume heat source; T The internal temperature of the yam slice; λ is the thermal conductivity of yam slices; h fg is the latent heat of vaporization of water; k c is the mass transfer coefficient of yam slices; c b is the moisture concentration of the air; c Slice the yam at any time t moisture concentration.

[0044] The governing equation for heat transfer at the model boundary is: (5) in, n is the unit normal vector; h sis the convective heat transfer coefficient of yam slices; T air is the hot air temperature; λ is the thermal conductivity of yam slices; T The internal temperature of the yam slice; D eff is the effective water diffusion coefficient; c Slice the yam at any time t moisture concentration.

[0045] (2) Diluted Species Transport Module defines transport properties The dilute species transfer module defines transfer properties, namely, the water diffusion coefficient, to facilitate the calculation of the mass transfer governing equations for yam slices during the air-heat drying process and at the model boundaries. The transfer properties in this embodiment include specific heat capacity, thermal conductivity, density, Young's modulus, Poisson's ratio, mass transfer coefficient, and moisture loss boundary. The mass transfer coefficient is the bulk, the flux type is external convection, and the moisture loss boundary is the surface of the yam slice in contact with the air.

[0046] The mass transfer control equation of yam slices during air-heat drying is: (6) in, c Slice the yam at any time t moisture concentration; D eff is the effective water diffusion coefficient.

[0047] The mass transfer governing equation at the model boundary is: (7) in, n is the unit normal vector; D eff is the effective water diffusion coefficient; c Slice the yam at any time t moisture concentration; h s is the convective heat transfer coefficient of yam slices; c b is the moisture concentration of the air; c 0 is the initial moisture concentration of yam slices.

[0048] (3) The solid mechanics module defines a linear elastic material, where the material symmetry is isotropic and the hygroscopic expansion coefficient is determined.

[0049] S05: To ensure the calculation accuracy and optimize the calculation efficiency and resource utilization, the hot air drying physical model is divided into different grids. Figure 3As shown, in the embodiment of the present application, the hot air drying physical model is divided into two grids, one is the air domain, and fluid dynamics is selected; the other is the yam slice domain, and general physics is selected. The number of edge units is 79260; the number of boundary units is 472025; the number of units is 8340420; and the minimum unit mass is 0.03465. The maximum unit size of the box is 28.1; the minimum unit size is 8.38; the maximum unit growth rate is 1.15; the curvature factor is 0.6; and the resolution in narrow areas is 0.7. The maximum unit size of the tray and sample is 36.8; the minimum unit size is 4.6; the maximum unit growth rate is 1.45; the curvature factor is 0.5; and the resolution in narrow areas is 0.6. When the temperature change rate of the sample center point solved by different grid sizes is less than 0.1%, the optimal grid size for solving yam slices is this grid size.

[0050] S06: Solve the hot air drying physical model to obtain the hot air drying simulation results of yam slices. The choice of solver needs to find a balance between accuracy and calculation speed according to specific needs, and then reasonably select and configure the solver by simplifying the grid size to ensure the simulation accuracy while optimizing the calculation efficiency and resource utilization. In the embodiment of the present application, a transient solver is used in conjunction with a fully coupled solution mode to find the balance between accuracy and calculation speed by adjusting the grid size, and then solve the hot air drying physical model. The solution result is the hot air drying simulation result of yam slices, including the moisture content, temperature, etc. after simulation.

[0051] S07: Fit the actual test results and the hot air drying simulation results to calculate the relative error rate of moisture content and temperature. If the relative error rate is less than 5%, the physical model for hot air drying of yam slices is applicable.

[0052] The calculation formula for the relative error rate is: (8).

[0053] As attached Figure 4 As shown in the figure, the errors between the actual and predicted moisture content and temperature at the drying endpoint were 4.50% and 3.03%, respectively, both less than 5%, demonstrating the feasibility of this model. Therefore, this model can be used to optimize the subsequent hot air drying process of yam slices.

[0054] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A hot air drying method for Chinese yam by combined optimization of microwave and mathematical model, characterized in that: include: The yam is washed, peeled, and cut into even slices at a 45° angle; The yam slices were evenly placed on a material tray, microwaved at 500-600W for 80-90s, and then placed in a hot air drying oven for constant temperature drying within 30s after the treatment. The moisture content of the yam slices was calculated to obtain the actual test results; Based on the COMSOL mathematical model, a hot air drying physical model for yam slices was established according to the size of yam slices and the drying oven. The hot air drying physical model includes a fluid heat transfer module, a dilute species transfer module, and a solid mechanics module. defining material properties for the hot air drying physical model; The hot air drying physical model is divided into a grid; the grid is divided into an air domain and a yam slice domain; when the temperature change rate of the sample center point solved with different grid sizes is less than 0.1%, the optimal grid size is determined; Solving the hot air drying physical model to obtain a hot air drying simulation result of the yam slices; the hot air drying simulation result includes the simulated moisture content and temperature; The actual test results and the hot air drying simulation results are fitted to determine the relative error rates of moisture content and temperature; when the relative error rates are less than 5%, the physical model for hot air drying of yam slices is applicable.

2. The hot air drying method for yam by microwave and mathematical model combined optimization according to claim 1, characterized in that: The constant temperature drying conditions are: temperature 70°C, relative humidity 6%, and hot air speed 1.0 m / s.

3. The hot air drying method for yam by microwave and mathematical model combined optimization according to claim 1, characterized in that: In the actual test results, the calculation formula for the moisture content of the yam slices is: Among them, m t is the mass of yam slices at time t; m d It is the absolute dry material mass of yam slices.

4. The hot air drying method for yam by microwave and mathematical model combined optimization according to claim 1, characterized in that: The assumptions made before establishing the physical model of hot air drying of yam slices are as follows: yam slices are a uniform, isotropic continuous medium; the moisture diffusion inside the yam slices conforms to Fick's theorem; the phase transitions of solid, liquid, and gas inside the yam slices are ignored; the air during the drying process is an ideal gas, and the flow rate and temperature distribution in the oven are constant; moisture diffuses from the inside to the yam surface in liquid form and vaporizes on the surface.

5. The hot air drying method for Chinese yam by microwave and mathematical model combined optimization according to claim 1, characterized in that: The physical model of hot air drying of yam slices is established based on the size of yam slices and drying oven, including: Measure the physical dimensions of the drying oven, including the length, width, and height of the oven, the dimensions of the drying tray, and the location and dimensions of the air inlets and outlets; A physical model for hot air drying of yam slices is constructed according to the physical size of the drying oven and the size of the yam slices.

6. The hot air drying method for Chinese yam by microwave and mathematical model combined optimization according to claim 1, characterized in that: Defining material properties for the hot air drying physical model includes: The fluid heat transfer module defines the initial value of the fluid, which includes the temperature of the hot air drying medium, dynamic viscosity, thermal conductivity, constant pressure heat capacity, and boundary heat source; The diluted species transport module defines transport properties, including specific heat capacity, thermal conductivity, density, Young's modulus, Poisson's ratio, mass transfer coefficient, and water loss boundary; The Solid Mechanics Module defines linear elastic materials.

7. The hot air drying method for Chinese yam by microwave and mathematical model combined optimization according to claim 1, characterized in that: The hot air drying physical model is solved using a transient solver in combination with a fully coupled solution mode.

Images