CN110619149B

CN110619149B - Chameleon-like super shell layer aiming at heat conduction and heat convection

Info

Publication number: CN110619149B
Application number: CN201910751408.5A
Authority: CN
Inventors: 黄吉平; 须留钧
Original assignee: Fudan University
Current assignee: Fudan University
Priority date: 2019-08-15
Filing date: 2019-08-15
Publication date: 2022-11-18
Anticipated expiration: 2039-08-15
Also published as: CN110619149A

Abstract

This patent belongs to thermodynamics technical field, specifically is a chameleon class super shell to heat-conduction and thermal convection. According to the chameleon-like super shell layer, the heat conduction process is described by a Fourier law, and the heat convection process is described by a Darcy law; the equivalent properties (equivalent thermal conductivity and equivalent permeability) of the shell layer change with changes in the properties of the surrounding environment. The shell layer is made of anisotropic material, and the radial parameter of the shell layer is far larger than the tangential parameter. The theoretical feasibility is verified by finite element simulation, and the intelligent shell layer is beneficial to realizing that the functional device is suitable for different working environments, thereby having wider practicability.

Description

Chameleon-like super shell for heat conduction and heat convection

Technical Field

The invention belongs to the technical field of thermodynamics, and particularly relates to a chameleon-like super shell aiming at heat conduction and heat convection.

Background

Generally, objects have their own inherent properties (thermal conductivity, permeability, \8230;) that are difficult to vary with environmental changes. This inherent property can lead to a problem: once some devices are designed, their properties are determined. This situation is not conducive to the development of intelligent, multifunctional devices, and can greatly limit their application range. In the design of the thermal metamaterial, this problem also exists, namely: the corresponding device must be designed according to the parameters of the environment.

Inspired by the chameleon phenomenon in the biology world, the invention aims to design a shell layer, so that equivalent parameters (equivalent thermal conductivity and equivalent magnetic conductivity) of the shell layer can also change along with the change of environmental parameters, and the shell layer is called as a chameleon-like super shell layer as the chameleon can change the self color according to the environmental color.

Disclosure of Invention

The invention aims to provide a chameleon-like super shell layer aiming at heat conduction and heat convection, which has a simple structure and excellent performance.

The allochroic dragon super shell aiming at heat conduction and heat convection comprises a shell, and the equivalent parameters (equivalent heat conductivity and equivalent permeability) of the shell can be changed along with the change of the environmental temperature parameter. This phenomenon is similar to the phenomenon in biology in which chameleon changes color according to the environment, and is called chameleon-like nanosheet. The physical process of the shell device comprises a heat conduction process described by the Fourier law and a heat convection process described by the Darcy law, so that the device is suitable for the heat conduction and heat convection processes.

In the invention, the thermal conductivity and permeability of the shell layer need to be designed. Thermal conductivity can regulate heat conduction, and permeability can regulate heat convection. The chameleon-like phenomenon is realized by calculating equivalent parameters of the shell layer and finding out certain special conditions.

In the invention, the equivalent thermal conductivity and the equivalent permeability of the shell layer are respectively described by Fourier law and Darcy law, and the specific description is as follows.

As shown in fig. 1, consider a microfluidic system where the flow of water through a porous medium transfers heat. Assume thermal conductivities of core and background are both κ ₁ ＝(1-f)κ _s +fκ _f ，κ _s Is the thermal conductivity of porous medium, kappa _f Is the water thermal conductivity, f is the porosity; all permeabilities are sigma ₁ The shell layer has anisotropic thermal conductivity

κ _rr For radial thermal conductivity, κ _θθ As tangential thermal conductivity and anisotropy of permeability

σ _rr Is radial permeability, σ _θθ Is the tangential permeability. Namely, the shell material is anisotropic (including thermal conductivity and permeability). Hereinafter, unless otherwise specified, anisotropy is expressed in cylindrical coordinates. Let the internal diameter of the shell be r ₁ Outer diameter of r ₂ . According to the requirement of the chameleon-like super shell, the equivalent parameters of the shell are required to be constantly equal to the parameters of the surrounding environment. Because the parameters of the shell layer and the environment are always equal, the equivalent parameters of the core-shell structure are also always equal to the parameters of the core, and the conversion into the mathematical language is that:

κ _e ＝κ ₁ ,σ _e ＝σ ₁ ， (1)

wherein, κ _e Is the equivalent thermal conductivity, σ, of the core-shell structure _e Is the equivalent permeability of the core-shell structure.

The problem then translates into calculating the equivalent thermal conductivity and equivalent permeability of the core-shell structure and finding certain specific conditions so that the requirements of equation (1) are met. The equivalent thermal conductivity and equivalent permeability with respect to the core-shell structure can be calculated by the following methods:

wherein the content of the first and second substances,

is the area fraction of the nucleus that is,

as to the degree of anisotropy of the thermal conductivity,

the degree of permeability anisotropy. According to the formula (2), some specific conditions are required to be found to meet the requirement of the chameleon-like super shell,

this condition is:

κ _θθ ＜＜κ ₁ ＜＜κ _rr ,σ _θθ ＜＜σ ₁ ＜＜σ _rr (3)

when the formula (3) is satisfied, the phenomenon of the chameleon-like super shell can be realized.

The invention has the advantages that:

(1) The invention can realize the intellectualization of the device;

(2) The invention has simple structure and parameters, and is suitable for steady-state and unsteady-state processes;

(3) The invention is applicable to both thermal conduction and thermal convection processes.

Drawings

FIG. 1 is a schematic view of a microfluidic system.

Fig. 2 is a theoretical calculation result. When the thermal conductivity or permeability of the core is changed in a large range, the equivalent parameters/core parameters of the core-shell are all about 1, and the error range is less than 5%, which indicates that the device meets the requirements of the chameleon-like super-shell layer.

Fig. 3 is a simulation result of placing the system in a horizontal thermal field and a horizontal pressure field. Case 1 corresponds to a thermal conductivity of 6Wm for regions I and III ^-1 K ^-1 Permeability of 5X 10 ^-12 m ² (ii) a Case 2 corresponds to regions I and III having thermal conductivities of

Wm ^-1 K ^-1 A permeability of

m ² The two parameters are expressed in a rectangular coordinate system. Throughout this patent, the thermal conductivity of the hyper shell is

Wm ^-1 K ^-1 Permeability of

m ² Thermal conductivity of common shell layer is 30Wm ^-1 K ^-1 Permeability of 10 ^-12 m ² The material of the comparative shell and the material parameters of the regions I and III are the same, and the moldDimension of the mimetic system being d ₀ ＝10 ^-4 And m is selected. The temperature difference applied horizontally was 40K and the pressure difference applied horizontally was 200Pa.

Fig. 4 is a simulation result of placing the system in other temperature fields or pressure fields. Case 3 corresponds to a thermal conductivity of 60Wm for regions I and III ^-1 K ^-1 Permeability of 5X 10 ^-13 m ² . For the two left columns, the temperature difference applied horizontally was 40K and the pressure difference applied vertically was 200Pa. For the two columns on the right, a heat source is applied to the upper left corner, and the lower boundary and the right boundary are at low temperature; the lower right corner is applied with high pressure, the left and upper boundaries are applied with low pressure, the temperature difference is still 40K, and the pressure difference is still 200Pa.

FIG. 5 is the simulation results of changing the shape of the chameleon-like nanoshell. For squares, the inner side length is 3.2X 10 ^-5 m, outer side length of 5.12X 10 ^-5 And m is selected. For complex shapes, the equation can be given by the parametric equation x =2 × 10 ^-6 [10+cosθ-cos(2θ)+2sin(5θ)]cosθ,y＝(10/7)×10 ^-6 [10+cosθ-cos(2θ)+2sin(5θ)]sin θ, θ ∈ [0,2 π ]), the outer boundary being 1.5 times the inner boundary. The remaining parameters are the same as in fig. 2.

FIG. 6 is a simulation result of using isotropic materials to achieve a chameleon-like hyper-shell. Thermal conductivity of Material A3000 Wm ^-1 K ^-1 Permeability of 10 ^-14 m ² (ii) a Thermal conductivity of Material B0.3 Wm ^-1 K ^-1 Permeability of 10 ^-10 m ² 。

Fig. 7 is a simulation result of transient simulation of 0.001s and 0.002 s.

Detailed Description

The present invention will be described in detail below with reference to specific examples and drawings, but the present invention is not limited thereto.

FIG. 1 shows a schematic of a chameleon-like nanosheet. By designing the parameters of the shell layer according to the formula (3), the equivalent thermal conductivity and the equivalent permeability of the shell layer can be changed along with the change of the parameters of the surrounding environment, as shown in fig. 2.

Fig. 3 shows the simulation results in the horizontal temperature field and the horizontal pressure field (the first and three columns show the temperature distribution and the second and four columns show the pressure distribution). The simulation used the commercial software COMSOL multiple thesics, which combines a heat conduction-convection module and a darcy's law module. As can be seen from the simulation: when the shell satisfies the formula (3), the equivalent parameters of the shell can be changed no matter what the environment around the shell is, so that the change of the external contour of the shell is not caused, as can be seen by comparing the external contours of the shells in the first row and the third row of fig. 3. However, a common shell cannot change with the change of the environmental parameters, which results in distortion of the isotherm outside the shell, as can be seen from the distorted isotherm outside the shell in the second row of fig. 3.

Furthermore, the shell can also accommodate more complex temperature and pressure fields. The first and second columns in fig. 4 show the simulation results of the horizontal temperature field and the vertical pressure. The two columns on the right in fig. 4 show the non-uniform temperature field and the non-uniform pressure field. Comparing the temperature distribution and the pressure distribution outside the first and the third row of shells in fig. 4, it is demonstrated that the allochroic torron-like super shell is also suitable for the non-uniform field. And the different temperature and pressure distributions outside the shell layer in the second row of the figure 4 show that the common shell layer has no adaptability to the environment, thereby highlighting the advantages of the design of the invention.

In addition, the circular chameleon-like super shell can be popularized to any shape, and the device can be designed to any shape by comparing the temperature distribution and the pressure distribution outside the first row and the third row of shells in fig. 5. Similarly, the common shell in the second row of FIG. 5 still does not allow for environmental compatibility.

The allochroic dragon super shell layers are all prepared from anisotropic materials. However, in practice, isotropic materials have wide applicability. We have therefore realized the chameleon-like nanoshells in the first row of fig. 1, 2, 3, 4 (see fig. 6) as well using the layered structure of the two materials, and simulation results have validated our design. Wherein the actual material corresponding to the material A can be carbon nanotubes, and the actual material corresponding to the material B can be nano aerogel.

Finally, we verified the transient results of the device, namely: the evolution of the temperature distribution over time. The results show that the temperature evolution outside the shells of the first and third rows in fig. 7 is completely consistent, illustrating the transient feasibility of the device.

Claims

1. A chameleon-like super shell for heat conduction and heat convection is characterized in that equivalent parameters of the shell are as follows: the equivalent thermal conductivity and the equivalent permeability change with the change of the ambient temperature; the physical process of the shell comprises a heat conduction process described by Fourier law and a heat convection process described by Darcy law;

the shell material is anisotropic and is determined by the following design method:

water in the micro-flow system flows in the porous medium for heat transfer; thermal conductivity of both core and background is κ ₁ ＝(1-f)κ _s +fκ _f ，κ _s Is the thermal conductivity of porous medium, kappa _f Is the water thermal conductivity, f is the porosity; permeability is sigma ₁ The shell layer has anisotropic thermal conductivity

κ _rr For radial thermal conductivity, κ _θθ For tangential thermal conductivity, the permeability is anisotropic

σ _rr Is radial permeability, σ _θθ Is the tangential permeability; namely, the shell layer material is anisotropic and comprises heat conductivity and permeability; let the internal diameter of the shell be r ₁ Outer diameter of r ₂ (ii) a According to the requirement of the chameleon-like super shell layer, the equivalent parameters of the core-shell structure are equal to the parameters of the core, and the mathematical expression is as follows:

κ _e ＝κ ₁ ,σ _e ＝σ ₁ ， (1)

wherein, κ _e Is the equivalent thermal conductivity, σ, of the core-shell structure _e Equivalent permeability for core-shell structures;

calculating the equivalent thermal conductivity and the equivalent permeability of the core-shell structure, and finding out specific conditions to meet the requirements of the formula (1); the equivalent thermal conductivity and equivalent permeability for the core-shell structure were calculated by the following method:

wherein the content of the first and second substances,

is the area fraction of the nucleus that is,

as to the degree of anisotropy of the thermal conductivity,

the degree of permeability anisotropy; according to the formula (2), the material characteristics required for realizing the chameleon-like super shell are obtained as follows:

κ _θθ ＜＜κ ₁ ＜＜κ _rr ,σ _θθ ＜＜σ ₁ ＜＜σ _rr ， (3)。