CN110020465B - Simulation analysis method for liquid canned food heat sterilization process based on CFD - Google Patents

Simulation analysis method for liquid canned food heat sterilization process based on CFD Download PDF

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CN110020465B
CN110020465B CN201910200727.7A CN201910200727A CN110020465B CN 110020465 B CN110020465 B CN 110020465B CN 201910200727 A CN201910200727 A CN 201910200727A CN 110020465 B CN110020465 B CN 110020465B
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刘东红
陈舒航
刘懿
周建伟
叶兴乾
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Zhejiang University ZJU
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    • A—HUMAN NECESSITIES
    • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
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    • A23L3/00—Preservation of foods or foodstuffs, in general, e.g. pasteurising, sterilising, specially adapted for foods or foodstuffs
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Abstract

The invention discloses a simulation analysis method of a CFD-based liquid canned food heat sterilization process, which comprises the following steps: collecting data, simplifying the data, and establishing a can heat transfer model; determining a control equation, an initial condition and a boundary condition based on the model, dividing the grid, importing the grid into COMSOL Multiphysics software, and carrying out CFD solution; and carrying out corresponding post-processing based on the result of the CFD solution, and analyzing the heat sterilization process. By the method, the change conditions of the fluid flow speed, the temperature, the pressure, the humidity and other physical quantities at any time and any position of the liquid canned food in the heat sterilization process to be researched can be given, and the physical field change and the heat sterilization effect in the heat sterilization process can be more accurately analyzed. The invention is suitable for liquid canned food with different can shapes and sizes and different contents, is beneficial to accurately searching the position of a cold spot before a heat penetration test is carried out, determines the shortest sterilization time and finally realizes the optimization of the sterilization process.

Description

Simulation analysis method for liquid canned food heat sterilization process based on CFD
Technical Field
The invention relates to the field of food processing, in particular to a simulation analysis method for a liquid canned food heat sterilization process based on CFD (computational fluid dynamics)
Background
Canned foods are widely used in the food industry because of their good sealability and sterilization measures. Thermal sterilization is the most common sterilization means in food processing due to its effectiveness and convenience. Computational fluid dynamics (cfd) is a numerical simulation of fluid flow using control equations, and the distribution of physical quantities at various locations in a flow field and the change of these physical quantities with time are obtained by solving a system of equations.
The heat sterilization of liquid canned foods generally involves fluid heat transfer, and it is difficult to obtain the temperature distribution at every position in the can at any time by the conventional method, and the accuracy of judging the cold spot and the minimum sterilization intensity in the can is insufficient. And the simulation of the heat sterilization process is carried out based on the CFD method, so that the changes of the temperature, the speed, the pressure, the slowest heating area and the sterilization intensity of different types of food in the process can be effectively predicted. The simulation result is applied to parameters such as optimization of geometric dimension, heat treatment temperature, treatment time and the like, the sterilization intensity is ensured, the sterilization time is reduced, the flavor and the nutrition level of the product are improved, and the energy consumption is saved. The accurate simulation model can improve the accuracy and the effectiveness of the test, and provides a basis for the research and development of the equipment and the determination of the optimal control scheme.
The can top gap refers to a gap left between the surface or liquid level of the canned food and the upper cover of the can container, and is used for maintaining a certain vacuum degree and balancing the pressure difference between the inside and the outside of the can so as to avoid appearance deformation such as convex angle, flat can and the like in the heat sterilization process.
At present, the CFD method is applied to a few examples for analyzing the heat sterilization process of the canned food, and the influence of the headspace on heat transfer is mostly ignored, or the evaporation and condensation phenomena of liquid existing at the headspace are not considered, so that the pressure change in the sterilization process cannot be accurately considered. Ignoring the headspace also produces a bias in the prediction of temperature and results in less accurate calculation of the minimum sterilization value.
Disclosure of Invention
The invention provides a simulation analysis method for analyzing the change conditions of temperature, pressure and thermal death rate in cans of liquid canned food under different thermal sterilization processes based on a CFD (computational fluid dynamics) technology so as to optimize the thermal sterilization process of the corresponding canned food.
In order to achieve the purpose, the technical scheme adopted by the invention is a simulation analysis method for analyzing the heat sterilization process of liquid canned food based on a CFD technology, which comprises the following steps:
a. acquiring data based on the liquid canned food to be analyzed and the heat sterilization process to be analyzed, and establishing a can heat transfer model after simplifying the data;
b. determining a control equation, an initial condition and a boundary condition based on the model, dividing a grid, introducing the grid into COMSOLULTIPhysics software, and carrying out CFD (computational fluid dynamics) solution;
c. and carrying out corresponding post-processing based on the result of the CFD solution, and analyzing the heat sterilization process.
Further, the step a specifically comprises:
the data collected includes: the density, dynamic viscosity, constant pressure heat capacity, heat conductivity coefficient, geometric dimension of the can, liquid level height in the can, size of top gap, and sterilizing temperature time-varying curve in the heat sterilization process of the liquid food in the can; the simplified processing is as follows: the liquid and the gas in the tank body are approximately uniform and symmetrical; the thickness of the tank wall is approximately uniform; the liquid has no slippage on the inner wall of the tank; the tank body is approximately not deformed in the heating process; the temperature at the outer wall of the can is approximately the sterilization temperature and the interface between the solution in the can and the top gas is the wet surface where the water evaporates.
The heat transfer model of the can is as follows: and a two-dimensional axisymmetric model is established based on the geometric dimension, the liquid level height and the top clearance of the can.
Further, the step b specifically comprises:
the control equation is: and determining a CFD control equation by taking laminar flow, heat transfer in wet air and moisture transfer in air as physical fields to be analyzed and combining three multi-physical field coupling relations of heat and humidity, moisture transfer and non-isothermal flow. The initial conditions were: the speed of each direction is zero, the initial temperature is room temperature, the initial humidity is the relative humidity of the environment, and the initial pressure is the vacuum degree of the can. The boundary conditions include: temperature boundary conditions at the outer wall of the can, evaporation rate function on the wet surface and boundary heat sources, no slip boundary of the can wall. The mesh generation method comprises the following steps: and (4) carrying out mesh generation according to the requirement of the physical field on the precision. And then setting the solving time length according to the process requirements, and carrying out CFD calculation in COMSOL Multiphysics to obtain the change condition of the corresponding physical quantity of each point along with the time in the heat sterilization process.
Further, the step c specifically comprises:
and (3) analyzing the physical field change in the heat sterilization process by applying CFD post-treatment: analyzing the change of the flow speed of the solute and the top gas in the can through a flow chart, analyzing the temperature change of different stages in the heat sterilization process through a temperature distribution cloud chart, and analyzing the pressure change in the can through a pressure contour line. And (3) calculating the sterilization intensity F value and the food nutrition retention C value according to the temperature by combining a thermal sterilization intensity formula and a food quality reduction formula so as to analyze the killing degree of the food in the can to the microorganisms and the nutrition retention condition in the thermal sterilization process.
The invention has the beneficial effects that: the change conditions of the fluid flow speed, the temperature, the pressure, the humidity and other physical quantities at any time and any position of the liquid canned food in the heat sterilization process to be researched can be given, and the physical field change and the heat sterilization effect in the heat sterilization process can be more accurately analyzed. The invention is suitable for liquid canned food with different can shapes and sizes and different contents, is beneficial to accurately searching the position of a cold spot before a heat penetration test is carried out, determines the shortest sterilization time and finally realizes the optimization of the sterilization process. The invention can analyze the pressure change condition in cans of different types under different heat sterilization processes, so as to control the pressure change condition of the sterilization kettle and the maximum pressure resistance value of the cans of manufacturers and avoid the quality problems of can expansion or tube collapse and the like.
Drawings
Fig. 1 is a schematic view of a simplified model of a liquid food can according to the present invention;
FIG. 2 is a fluid velocity field flow diagram;
FIG. 3 is a cloud of temperature profiles for different sterilization stages;
FIG. 4 is a field distribution of microbial lethality at the end of sterilization;
fig. 5 is a graph showing the change in the internal pressure of the can.
Detailed Description
The invention adopts the technical scheme that a simulation analysis method for analyzing the heat sterilization process of liquid canned food based on a CFD technology comprises the following specific steps:
a. acquiring data based on the liquid canned food to be analyzed and the heat sterilization process to be analyzed, and establishing a can heat transfer model after simplifying the data; the method specifically comprises the following steps:
the canned food to be analyzed was a 1.0% CMC solution that can be used to simulate the heat sterilization process of liquid canned food, and was filled in can No. 7113, and the functions of density, dynamic viscosity, constant pressure heat capacity, and thermal conductivity varying with temperature were obtained by measurement and parameter fitting, as shown in table 1.
TABLE 11.0% CMC solution thermophysical parameter characterization
Figure BDA0001997349190000031
The geometric dimension of the can is as follows: the height of the can is 106.3mm, the radius is 38.1mm, and the liquid level in the can is 95 mm. Thus, a two-position axisymmetric geometric model is constructed and the following simplification processing is carried out: the liquid and the gas in the tank body are approximately uniform and symmetrical; the thickness of the tank wall is approximately uniform; the liquid has no slippage on the inner wall of the tank; the tank body is approximately not deformed in the heating process; the temperature at the outer wall of the can is approximately the sterilization temperature. The interface between the solution in the tank and the top gas is the wet surface where the water evaporates.
A two-dimensional axisymmetric model is established based on the geometry, level height and headspace size (distance from the level to the interior of the can top) of the can, as shown in fig. 1.
The heat sterilization process in the case is a heat sterilization process with the temperature rise time of 20min, the cooling time of 15min and the heat preservation time of 30min at the working temperature of 121 ℃.
b. Determining a control equation, an initial condition and a boundary condition based on the model, dividing a grid, introducing the grid into COMSOLULTIPhysics software, and carrying out CFD (computational fluid dynamics) solution; the method specifically comprises the following steps:
the control equation is: the method comprises the following steps of determining a CFD control equation by taking laminar flow, heat transfer in wet air and moisture transfer in air as physical fields to be analyzed and combining three multi-physical field coupling relations of heat and humidity, moisture transfer and non-isothermal flow, wherein the CFD control equation comprises a Navie-Stokes equation, a continuity equation and an energy conservation equation, and the three control equations are as follows:
the Navier-Stokes equation:
Figure BDA0001997349190000041
continuity equation:
Figure BDA0001997349190000042
wherein the content of the first and second substances,
Figure BDA0001997349190000043
is the fluid velocity, p is the fluid pressure, ρ is the fluid density, μ is the hydrodynamic viscosity, F is the external force experienced by the fluid,
Figure BDA0001997349190000047
representing the gradient operator, I is a matrix of all 1's.
Energy conservation equation:
Figure BDA0001997349190000044
Figure BDA0001997349190000045
wherein the content of the first and second substances,
Figure BDA0001997349190000046
is the fluid velocity, p is the fluid pressure, p is the fluid density, CpIs constant pressure heat capacity, T is temperature, q is heat, k is thermal conductivity, Q, Qp、QvdRespectively the heat generated by the external heat source and the pressure working and the viscous heating.
The initial conditions were: the speed of each direction is zero, the initial temperature is room temperature, and the initial humidity is the relative humidity of the environment; the boundary conditions are as follows: generating a mathematical expression by the change of the sterilization temperature in different sterilization processes along with time, wherein the mathematical expression is used as a temperature boundary condition of the outer wall of the can, and the evaporation rate is given to be 10m/s on the wet surface of the interface water evaporation of the solution and the top gas in the can; and refining mesh division is carried out according to the requirement of fluid dynamics on precision, and the solution domain is divided into 5971 units.
Setting the solving time length according to the process requirements, taking 30s as the step length, and carrying out CFD calculation in COMSOL Multiphysics to obtain the change condition of the corresponding physical quantity of each point along with the time in the heat sterilization process, such as temperature, pressure, speed, relative humidity, vortex intensity and the like.
c. And carrying out corresponding post-processing based on the result of the CFD solution, and analyzing the heat sterilization process. The method specifically comprises the following steps:
the CFD post-treatment is used for analyzing the change of a physical field in the heat sterilization process, the velocity change of the solute and the top gas in the can is analyzed through a flow chart (figure 2), and the temperature change in different stages in the heat sterilization process is analyzed through a temperature distribution cloud chart (figure 3). And (3) calculating the sterilization intensity F value and the food nutrition retention C value according to the temperature by combining a thermal sterilization intensity formula and a food quality reduction formula so as to analyze the killing degree of the food in the can to the microorganisms and the nutrition retention condition in the thermal sterilization process. Fig. 5 is a field distribution of microbial lethality at the end of sterilization whereby the minimum lethality in the can is found to determine the can cold spot.
The accuracy of the model is verified through the actual temperature-time curve of the specific position by using the temperature sensor, and the error between the simulated value and the measured value is 1.92-4.89%, which shows that the method can better perform simulation analysis on the heat sterilization process.
Through CFD post-processing analysis, the physical field distribution and the change condition in the heat sterilization process can be intuitively reflected, so that a practitioner can better understand the heat and mass transfer process involved in the process, the search of cold spots in the can is facilitated, and the shortest sterilization time required for reaching a certain sterilization degree is more accurately determined.
The method can obtain pressure change condition (figure 5) in the can during heat sterilization, and compared with the traditional method of calculating pressure by temperature, the accuracy is greatly improved. The manufacturer can set the pressure of each stage in the sterilization process under different head space heights in an auxiliary mode through simulation, so that the problem of can appearance deformation such as can expansion and flat can is avoided.

Claims (2)

1. A simulation analysis method of a CFD-based liquid canned food heat sterilization process is characterized by comprising the following steps:
a. acquiring data based on the liquid canned food to be analyzed and the heat sterilization process to be analyzed, and establishing a can heat transfer model after simplifying the data;
b. determining a control equation, an initial condition and a boundary condition based on the model, dividing a grid, introducing the grid into COMSOLULTIPhysics software, and carrying out CFD (computational fluid dynamics) solution;
c. performing corresponding post-processing based on the result of the CFD solution, and analyzing the heat sterilization process;
the step a is specifically as follows:
the data collected includes: the density, dynamic viscosity, constant pressure heat capacity, heat conductivity coefficient, geometric dimension of the can, liquid level height in the can, size of top gap, and sterilizing temperature time-varying curve in the heat sterilization process of the liquid food in the can; the simplified processing is as follows: the liquid and the gas in the tank body are approximately uniform and symmetrical; the thickness of the tank wall is approximately uniform; the liquid has no slippage on the inner wall of the tank; the tank body is approximately not deformed in the heating process; the temperature at the outer wall of the can is approximately the sterilization temperature, and the interface of the solution in the can and the gas at the top is a wet surface for water evaporation;
the heat transfer model of the can is as follows: a two-dimensional axisymmetric model is established based on the geometric dimension, the liquid level height and the size of a top gap of the can;
the step b is specifically as follows:
the control equation is: determining a CFD control equation by taking laminar flow, heat transfer in wet air and moisture transfer in air as physical fields to be analyzed and combining three physical field coupling relations of heat and humidity, moisture transfer and non-isothermal flow; the initial conditions were: the speed of each direction is zero, the initial temperature is room temperature, the initial humidity is the relative humidity of the environment, and the initial pressure is the vacuum degree of the can; the boundary conditions include: temperature boundary conditions at the outer wall of the can, evaporation rate function and boundary heat source on the wet surface, no slip boundary of the can wall; the mesh generation method comprises the following steps: mesh generation is carried out according to the requirement of the physical field on the precision; and then setting the solving time length according to the process requirements, and carrying out CFD calculation in COMSOL Multiphysics to obtain the change condition of the corresponding physical quantity of each point along with the time in the heat sterilization process.
2. The method according to claim 1, wherein step c specifically is:
and (3) analyzing the physical field change in the heat sterilization process by applying CFD post-treatment: analyzing the change of flow speed of solute and top gas in the can through a flow chart, analyzing the temperature change of different stages in the heat sterilization process through a temperature distribution cloud chart, and analyzing the pressure change in the can through a pressure contour line; and calculating the sterilization intensity F value and the food nutrition retention C value by combining a thermal sterilization intensity formula and a food quality reduction formula according to the temperature.
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JP2003181597A (en) * 2001-12-18 2003-07-02 Mitsubishi Motors Corp Resin model and method for forming the same
CN101356990A (en) * 2008-09-04 2009-02-04 南京财经大学 Food hyperpressure low-temperature sterilization method
CN106777765A (en) * 2016-12-31 2017-05-31 浙江大学 Easily thermal sterilization runout compensation method

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* Cited by examiner, † Cited by third party
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JP2003181597A (en) * 2001-12-18 2003-07-02 Mitsubishi Motors Corp Resin model and method for forming the same
CN101356990A (en) * 2008-09-04 2009-02-04 南京财经大学 Food hyperpressure low-temperature sterilization method
CN106777765A (en) * 2016-12-31 2017-05-31 浙江大学 Easily thermal sterilization runout compensation method

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