Optimization Design of Insulation Materials and Thickness Based on Box Insulation Performance

To ensure that the air inside the box does not fall below -20 °C after cooling for four hours, With the help of CFD numerical simulation technology, the composition materials and thickness have been optimized and designed in this paper. In this paper, there are two kinds of materials for box wall panel, one is insulation material 1 with smaller specific heat capacity and steel, the other is insulation material 2 with larger specific heat capacity. The simulation analysis results of two types of box wall panels show that the air temperature at edges and corners is difficult to meet the requirements. By using local thickening, this situation can be greatly improved.


Introduction
Computational fluid dynamics is used to analyze the physical phenomena such as fluid motion and heat conduction through computer numerical calculation and image display.Its basic idea is to replace the fields of physical quantities, such as velocity field and pressure field, which are continuous in the time domain and space domain, with a set of variable values on a limited number of discrete points, establish algebraic equation about the relationship between field variables at these discrete points through certain principles and methods, and then solve the algebraic equation to obtain the approximate values of field variables.By relying on CFD numerical simulation, quantitative calculation results of flow field parameters can be provided within a wide flow range, which is convenient for analyzing the impact of various flow parameters and flow field collection structures on flow patterns, and can quickly evaluate and optimize design schemes.It can also guide subsequent experimental verification to avoid blind and repetitive design processes, thereby improve design quality, shorten the development cycle and reduce development costs.
To meet insulation performance requirements of box, the insulation layer material and thickness of box wall were optimized with CFD numerical simulation technology in this paper, which can provide design basis and theoretical support for the box structure design.

Numerical simulation
2.1 Mathematical model [1][2][3] 1) Mass conservation equation: the increase in mass in a fluid microelement per unit time is equivalent to the net mass flowing into the microelement at the same time interval.The equation is as follows: 2) Momentum conservation equation: the rate of change of momentum of a microelement over time is equal to the sum of various external forces acting on the microelement.The equation is as follows: 3) Energy conservation equation: the increase rate of energy in a microelement is equal to the net heat flux entering the microelement plus the work done by physical and surface forces on the microelement.This law is actually the first law of thermodynamics.The equation is as follows:

Structural model
The calculation domain is composed of the box wall and its air domain, and the structure of the calculation domain is shown in Figure 1.

Grid Model
Compared with unstructured grids, structured grids have fewer quantities, lower computational costs, smaller truncation errors, and higher computational accuracy [4] .In order to divide more structural grids, the computational domain is divided into blocks.
After partitioning, the computational domain in this paper can be fully divided into hexahedral structured grids, as shown in Figure 2.

Analysis Settings
Material parameters.The thermal performance parameters of insulation materials used in this simulation analysis are shown in Table 1.2.5.1Thebox wall is composed of steel plate and insulation layer.The steel plate is 8mm thick, and the insulation layer is made of insulation material 1.
The surface temperature of the air domain, i.e. the temperature at the interface between the air domain and the box wall, is the lowest temperature in the air domain.Figure 3 shows the surface temperature distribution of the air domain when the insulation layer is 80mm thick.
From the figure, it can be seen that the highest temperature is approximately -21.5 ℃, located on the center of the air domain surface, which does not meet the design requirements.From Figure 4, it can be seen that the minimum temperature is about -23.9 ℃, located at the vertex of the air domain; and the maximum temperature is about -18.3 ℃, located on the center of the air domain surface; From Figure 5, it can be seen that the areas with temperatures below -20 ℃ are mainly located at edges and corners.According to statistics, the volume of areas with temperatures above -20 ℃ accounts for approximately 93.5%.The structural model was modified by local thickening, as shown in Figure 7.The thickened area only surrounds the edges and corners of the box, with a width of 100mm and a thickness of 8mm.From Figure 6, it can be seen that the area with a temperature higher than -20 ℃ accounts for approximately 97.6%.
2.5.2The box wall is composed of an insulation layer.Insulation layer is made of insulation material 2. Figures 10 to 12 respectively shows the surface temperature distribution of the air domain, the area where temperature is lower than -20 ℃ and the volume proportion of different temperature ranges on the surface of the air domain when the insulation layer is 65mm thick.From Figure 10, it can be seen that the minimum temperature is about -22.4 ℃, and the maximum temperature is about -16.9 ℃; From Figure 11, it can be seen that on the surface of the air domain, areas with temperatures below -20 ℃ are mainly located at the edges and corners.According to statistics, the volume of areas with temperatures above -20 ℃ accounts for approximately 97.4%.The structural model was modified as Figure 7 shows.Figures 13 to 14 respectively shows the surface temperature distribution of the air domain and the volume proportion of different temperature ranges on the surface of the air domain when the thick of the insulation layer is 65mm and the insulation layer was local thickened with a frame structure which width is 100mm and thickness is 8mm.According to statistics, areas with temperatures above -20 ℃ account for approximately 99.9%.

Conclusions
To ensure that the air inside the box does not fall below -20 ℃ after cooling for four hours, the composition materials and thickness have been optimized and designed in this paper with CFD numerical simulation technology.
The numerical simulation analysis of insulation layer thickness optimization based on the above working conditions shows that: 1) When the box wall is composed of 8mm thick steel plate and 82mm thick insulation layer made of insulation material 1.The volume of the area with a temperature above -20 ℃ accounts for about 93.5%, and mainly located at the edges and corners.When using insulation material 1 to locally thicken the edges and corners by 8mm, the volume proportion of the area with a temperature above -20 ℃ increases to about 97.6%.
2) When the box wall is composed of 65mm thick insulation layer made of insulation material 2. The volume of the area with a temperature above -20 ℃ accounts for about 97.4%, and mainly located at the edges and corners.When using insulation material 2 to locally thicken the edges and corners by 8mm, the volume proportion of the area with a temperature above -20 ℃ increases to about 99.9%.

Figure 3 .
Figure 3. Surface temperature distribution of the air domain

Figure 9 .Figure 11 .Figure 12 .
Figure 9. Surface temperature distribution of the air domain when insulation layer is 62mm thick

Figure 13 .
Figure 13.Surface temperature distribution of the air domain when local thickened

Table 1 .
Insulation Material Parameters Working conditions.The initial temperature of the box wall and its internal air domain is 25 ℃, and the surrounding temperature outside the box is -60 ℃ 32.5 Analysis and Discussion of Calculation Results