阅读说明：本技术 一种针对热辐射的热透明装置 (Thermal transparent device for thermal radiation ) 是由 黄吉平 须留钧 于 2019-08-15 设计创作，主要内容包括：本发明属于热力学技术领域,具体为一种针对热辐射的热透明装置。本发明热透明装置为核壳结构,该核壳结构的存在不影响壳层外部的温度分布,好像核壳结构根本不存在一样,具有热透明的效果。本发明涉及的热输运过程包含热传导和热辐射两个部分,分别采用傅里叶定律和Rosseland扩散近似所描述。本发明通过设计材料的热导率和Rosseland平均消光系数,可实现室温和高温的热透明现象。本发明经过理论分析和有限元模拟验证热透明装置的可行性。本发明为实现高温条件辐射主导的热透明装置提供了有效的方案,具有广阔应用前景,比如,可用于欺骗红外探测、热辐射防护等。(The invention belongs to the technical field of thermodynamics, and particularly relates to a thermal transparent device aiming at thermal radiation. The thermal transparent device is of a core-shell structure, the temperature distribution outside the shell layer is not influenced by the core-shell structure, and the thermal transparent device has the thermal transparent effect as if the core-shell structure does not exist at all. The heat transport process involved in the present invention comprises two parts, heat conduction and heat radiation, described by fourier law and Rosseland diffusion approximation respectively. According to the invention, the thermal transparency phenomenon at room temperature and high temperature can be realized by designing the thermal conductivity and the Rosseland average extinction coefficient of the material. The feasibility of the thermal transparency device is verified through theoretical analysis and finite element simulation. The invention provides an effective scheme for realizing the heat transparent device with radiation dominance under high temperature condition, and has wide application prospect, for example, the invention can be used for deception infrared detection, heat radiation protection and the like.)

1. A heat transparent device aiming at heat radiation is characterized in that the device is of a core-shell structure, the temperature distribution outside a shell layer is not influenced by the core-shell structure, and the device has the heat transparent effect as if the core-shell structure does not exist at all.

2. The thermal transparent device according to claim 1, wherein the equivalent parameters of the core-shell structure are the same as the background by designing two parameters of the thermal conductivity and the Rosseland average extinction coefficient of the core-shell structure, thereby eliminating the influence of the core-shell structure on the background and realizing the thermal transparent effect; the concrete description is as follows:

for a core-shell structure, let the thermal conductivity of the core be κ_cRelative refractive index of n_cRosseland mean extinction coefficient β_c(ii) a The shell layer has a thermal conductivity of κ_sRelative refractive index of n_sRosseland mean extinction coefficient β_s(ii) a Considering the situation of geometric anisotropy, let the length of three semi-axes of the kernel be lambda respectively_c1、λ_c2、λ_c3(ii) a The three semi-axial lengths of the shell layers are respectively lambda_s1、λ_s2、λ_s3(ii) a The equivalent thermal conductivity of the core-shell structure satisfies the following relational expression (1):

and the equivalent radiation parameters of the core-shell structure satisfy the following relational expression (2):

wherein, γ_s＝n_s ²/β_s，γ_c＝n_c ²/β_c；f＝λ_c1λ_c2λ_c3/λ_s1λ_s2λ_s3Is the volume fraction of the nucleus, d_ciAnd d_siRespectively being a core and a shellThe shape factor in the i-1, 2,3 direction is calculated from the following formula (3):

wherein λ is₁、λ₂、λ₃Are the semiaxial lengths of the ellipsoid in three directions, respectively.

3. The thermally transparent device of claim 2, wherein the third axial length of the ellipsoid is assumed to tend to infinity, namely: lambda [ alpha ]₃When ∞, degenerate to the two-dimensional case; then the form factor described by equation (3) is reduced to:

4. the thermal transparency device according to claim 1,2 or 3, characterized in that for thermal transparency in a non-steady state, the thermal diffusivity of the material is further considered, and the thermal diffusivity of each region including the core, the shell and the background is the same, and the optimization is performed to determine the optimized value; the thermal diffusivity is numerically equal to the thermal conductivity (κ) divided by the heat capacity density volume (ρ c).

Technical Field

The invention belongs to the technical field of thermodynamics, and particularly relates to a thermal transparent device for thermal radiation.

Background

In existing device designs, most thermal metamaterials are designed to operate at room temperature, and heat conduction is the primary mode of heat transport. However, as the temperature increases, the thermal radiation becomes non-negligible and thus these thermal metamaterials no longer function properly. Existing devices fail to address the problem of thermal radiation, which greatly limits the practical application of many devices at high temperatures, such as thermal protection.

To solve this problem, the present invention proposes an effective medium theory in which the treatment of thermal radiation is based on the Rosseland diffusion approximation. This theory helps to design a thermally transparent radiation metamaterial, even taking into account anisotropic geometry. Theoretical analysis was further confirmed by finite element analysis. Simulation results show that the thermal transparent device performs well in both steady and non-steady states. The invention not only provides an effective medium theory for controlling heat radiation, but also designs a heat transparent device. The results have application prospect in the aspect of thermal radiation regulation.

Disclosure of Invention

The invention aims to provide a heat transparent device with simple structure and excellent heat transparent effect

The thermal transparent device provided by the invention is directed at thermal radiation, and particularly has a core-shell structure, the existence of the core-shell structure does not influence the temperature distribution outside a shell layer, and the thermal transparent device has the thermal transparent effect as if the core-shell structure does not exist at all no matter under the conditions of room temperature or high temperature.

The heat transparent device for heat radiation provided by the invention relates to a heat transport process comprising two parts of heat conduction and heat radiation, which are respectively described by Fourier law and Rosseland diffusion approximation. Under room temperature conditions, heat conduction is the main heat transport process; but at high temperatures in the thousands of kelvin, thermal radiation becomes the dominant process. The present invention focuses on the realization of a radiation dominated, thermally transparent device at high temperature.

According to the invention, two key parameters, namely the thermal conductivity and the Rosseland average extinction coefficient of the core-shell structure, are designed, so that the equivalent parameters of the core-shell structure are the same as the background, the influence of the core-shell structure on the background is eliminated, and the purpose of thermal transparency can be achieved.

The invention is applicable to both two and three dimensions.

The invention provides a heat transparent device aiming at heat radiation, which relates to the calculation of equivalent properties of a core-shell structure, wherein the basic rule is approximately described by Fourier law and Rosseland diffusion, and the specific description is as follows:

for a core-shell structure, let the thermal conductivity of the core be κ_cRelative refractive index of n_cRosseland mean extinction coefficient β_c(ii) a The shell layer has a thermal conductivity of κ_sRelative refractive index of n_sRosseland mean extinction coefficient β_s. Considering the situation of geometric anisotropy, let the length of three semi-axes of the kernel be lambda_c1、λ_c2、λ_c3(ii) a The three semi-axial lengths of the shell layers are respectively lambda_s1、λ_s2、λ_s3. The equivalent thermal conductivity for such a core-shell structure satisfies the following relation (1):

and the equivalent radiation parameters of the core-shell structure satisfy the following relational expression (2):

wherein γ is n²The,/beta, can be viewed as the radiation parameter, (i.e., gamma_s＝n_s ²/β_s，γ_c＝n_c ²/β_c)；f＝λ_c1λ_c2λ_c3/λ_s1λ_s2λ_s3Is the volume fraction of the nucleus, d_ciAnd d_siThe shape factors of the core-shell in the i-1, 2,3 directions, respectively, and d is taken for simplicity_ciAnd d_siIs uniformly expressed as d_iIt can be calculated from the following formula:

wherein λ is₁、λ₂、λ₃Are the semiaxial lengths of the ellipsoid in three directions respectively; when lambda is₁、λ₂、λ₃Three semi-axial lengths λ of the cores respectively_c1、λ_c2、λ_c3When d is greater than_iIs d_ci(ii) a When lambda is₁、λ₂、λ₃Three half-axis lengths λ of the shells respectively_s1、λ_s2、λ_s3When d is greater than_iIs d_si(ii) a d is the sign of the differential, and a is the integration parameter (from 0 to ∞ integration). This is an equivalent parameter calculation method when the core-shell structure is three-dimensional, and certainly, this method can also be directly degenerated to a two-dimensional situation, and only the third axial length of the ellipsoid needs to be assumed to be infinite, that is: lambda [ alpha ]₃When ∞, degenerate to the two-dimensional case; then the form factor described by equation (3) can be simplified as:

through the formulas (1) and (2), the equivalent thermal conductivity and the equivalent radiation parameter of the core-shell structure can be calculated, and the two parameters are matched with the background parameter, so that the thermal transparency effect can be obtained.

In the case of thermal transparency mainly under a steady state, for thermal transparency under an unsteady state, the thermal diffusion coefficient of the material is further considered, that is, the thermal diffusion coefficients of each region (including the core, the shell and the background) are the same, and an optimized value is determined through optimization processing; the thermal diffusivity is numerically equal to the thermal conductivity (κ) divided by the heat capacity density volume (ρ c).

The invention has the advantages that:

(1) the invention can solve the problem of heat radiation under Roseeland diffusion approximation;

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

(3) the invention is applicable to geometric anisotropy.

The invention has wide application prospect, for example, can be used for deception infrared detection, thermal radiation protection and the like.

Drawings

FIG. 1 is a schematic of a core-shell structure.

Fig. 2 is the results of a thermal transparency steady state simulation. The simulated size is 10 x 10cm²Relative folding of all areasRefractive index of 1, background thermal conductivity of 1Wm^-1K^-1Rosseland average extinction coefficient of 100m^-1. The simulation used three temperature profiles: 273-313K, the heat conduction in the temperature range is far greater than the heat radiation effect; 273-673K, the heat conduction in the temperature interval is far equivalent to the heat radiation effect; 273-4273K, the heat conduction in the temperature range is far less than the heat radiation effect. Wherein the radius of the core in (a) - (c) is 2 cm; the thermal conductivity is 2Wm^-1K^-1Rosseland mean extinction coefficient of 50m^-1(ii) a The radius of the shell layer is 3 cm; the thermal conductivity is 0.62Wm^-1K^-1Rosseland average extinction coefficient 161.1m^-1. (d) The half axis size of the core in the step (a) and the step (f) is 2.5 cm and 1.25 cm; the thermal conductivity is 0.5Wm^-1K^-1Rosseland mean extinction coefficient of 200m^-1(ii) a The half shaft size of the shell layer is 3cm and 2.08 cm; the thermal conductivity is 1.61Wm^-1K^-1Rosseland mean extinction coefficient of 62m^-1. (d) And (f) shows a reference temperature distribution, namely, the original core-shell structure area is filled with the background material. Where the dashed circles and ellipses show the position of the core-shell structure for convenience of comparison with simulations in the first and second columns

Fig. 3 is a result of thermal transparency transient simulation. The parameters are exactly the same as in fig. two (e), but the density and heat capacity of the material need to be additionally considered. Wherein the background has a heat capacity density volume of 10⁶Jm^-3K^-1The heat capacity density of the core is 5X 10⁵Jm^-3K^-1The shell has a heat capacity density of 1.61X 10⁶Jm^-3K^-1。

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.

Figure 1 shows a schematic of the core-shell structure. The core-shell parameters are designed through the formulas (1) and (2) and are matched with the background parameters, so that the thermal transparency function can be realized. This theory can address the ellipsoid/ellipse problem of geometric anisotropy, but requires that the core-shell structure be confocal (concentric for a circular core-shell structure).

A thermally transparent steady state simulation diagram is shown in fig. 2. The simulation used the commercial software COMSOL MULTIPHYSICS. To reflect the role of thermal radiation in the heat transport process, we have adopted three temperature distributions. According to the Stephen-Boltzmann law, the following can be known: the radiation power is proportional to the fourth power of the temperature. The effect of the heat radiation is increased sharply as the temperature is gradually increased. The temperature profile taken is therefore: 273-313K, wherein heat conduction is the main transportation mode; 273-673K, wherein the heat radiation has a contribution equivalent to the heat conduction; 273-4273K, wherein the heat radiation is already a dominant heat transport mode. Observing the temperature distribution outside the core-shell structure in the first and second columns of fig. 2, they are exactly the same as the distribution outside the corresponding dashed lines in the last column. This shows that the core-shell structure has no influence on the temperature distribution of the background as if the core-shell structure does not exist, thus achieving the effect of thermal transparency. From the simulation results, it can be seen that the thermally transparent device performs very well from a room temperature region, which is entirely dominated by thermal conduction, to a high temperature region, which is almost dominated by thermal radiation.

The above is the steady state simulation results of thermal transparency, but the present invention is not limited thereto, and the thermal transparency device is also applicable to non-steady state. For this purpose, two additional parameters, namely the heat capacity and the density of the material, need to be considered. For convenience of discussion, a new physical quantity is defined, namely: the thermal diffusivity, numerically equal to the thermal conductivity (κ) divided by the heat capacity density volume (ρ c). In the steady state design, the thermal conductivity and radiation parameters of the material are designed according to equations (1) and (2). In the non-stationary state, in addition to the original requirements of equations (1) and (2), another condition needs to be satisfied, i.e. to ensure that the thermal diffusivity of each region (core, shell, background) is the same. For this reason, the heat capacity density volume needs to be designed intentionally. After ensuring that the thermal diffusivity is the same for each region, finite element simulations were performed. With the existing parameters, the system takes approximately 60 minutes to reach final steady state. Fig. 3 shows the temperature distribution of the system at 10, 20 and 60 minutes respectively, and the temperature distribution of the background can be observed to find that: the isotherms are substantially straight. This illustrates that the thermally transparent device can also operate in an unstable state.