Hao-Chun Zhang

Transient Coupled Heat Transfer Inside a Scattering mMedium with Graded Refractive Index

Transient heat transfer of coupled radiation and conduction within a semitransparent parallel plane slab of absorbing-emitting-scattering gray medium with graded index is investigated. The refractive index of the medium varies continuously along the slab thickness, and both boundary surfaces are specular and semitransparent. The medium is discretized into a number of sublayers with a constant refractive index in each, in which the radiative transfer is numerically simulated using a multilayer radiative transfer model. The multilayer model is developed using a ray-tracing method based on a zonal method. Comparison of present results with previous research shows excellent agreement, which helps to verify the validity of the multilayer radiative transfer model of a medium with a graded refractive index. Effects of optical thickness, scattering albedo, and gradient of refractive index on transient thermal behavior are examined for a medium with nonlinear refractive index distributions. It is found that temperature peaks also appear in the inner regions of a graded refractive index medium with two semitransparent surfaces subjected to external radiation even though there is no convection cooling on the surfaces and the absorbing coefficient (or extinction coefficient) is spatially uniform in the medium.

Ray-Tracing/Nodal-Analyzing Model for Transient Thermal Behavior of a Scattering Medium with a Variable Refractive Index

ABSTRACT Transient combined radiation-conduction inside a semitransparent scattering gray medium with a graded index is examined. By nodal analysis, the radiative source term of the energy equation is deduced through radiative transfer coefficients (RTCs), which are deduced through a multilayer model of radiative transfer, established by employing a ray-tracing method based on the zonal method. The basic idea is that a medium with a continually variable refractive index is discretized into certain number of isothermal sublayers with constant refractive index in each sublayer and the radiative transfer in a curve inside the graded index medium is regarded as that in a straight line within each sublayer. The present results accord very well with previous results. The results show that the multilayer model for radiative transfer to simulate the radiative transfer in a graded refractive index medium is reliable and correct. In addition, effects of convection boundary conditions, scattering albedo, opaque surface emission, and refractive index distribution on transient thermal behavior are also investigated for the graded index medium.

Coupled radiation-conduction heat transfer in an anisotropically scattering slab with mixed boundaries

Abstract Ray tracing method combined with Hottels zonal method is used to establish radiation transfer model in anisotropic scattering media. The transmission progress of radiation intensity in the absorbing, emitting and anisotropically scattering medium is divided into two sub-progresses: emitting–attenuating–reflecting progress and absorbing–scattering progress. For the medium with both surfaces being semitransparent and mirror-like, radiative transfer coefficients are developed. Energy equation of transient coupled radiation and conduction heat transfer is solved by the fully implicit control-volume method. In this paper, it needs only to disperse spatial location while spatial solid angle is integrated directly. On this basis, effects of conduction–radiation parameter, spectral refractive index and scattering phase functions etc., on the temperature field and radiative heat flux field within an anisotropic scattering medium are examined. The results show that: the linear part in linear or nonlinear scattering phase function can produce great difference of the temperature field and radiative heat flux field between anisotropic scattering medium and isotropic scattering medium.

Control and design heat flux bending in thermal devices with transformation optics

We propose a fundamental latent function of control heat transfer and heat flux density vectors at random positions on thermal materials by applying transformation optics. The expressions for heat flux bending are obtained, and the factors influencing them are investigated in both 2D and 3D cloaking schemes. Under certain conditions, more than one degree of freedom of heat flux bending exists corresponding to the temperature gradients of the 3D domain. The heat flux path can be controlled in random space based on the geometrical azimuths, radial positions, and thermal conductivity ratios of the selected materials.

Reliability of stray light calculation code using the Monte Carlo method

This paper mainly discusses the validity and the reliability of the stray light calculation code using the Monte Carlo method in an optical system. As a new method, the symmetrical test of radiation in two-dimensional media (STRTDM) is presented to verify the characteristics of pseudorandom numbers. The method can provide great flexibility and simplicity under various calculation conditions through iteratively comparing the performance of the pseudorandom number generators (PNGs) until a sufficiently good PNG is obtained. After a series of tests have been performed—including tests for the characteristics of a PNG, for the reliability of probability models, and for the summation reciprocity relationships of the radiative exchange factor—the results show the reliability of the stray light calculation code in this paper. Moreover, the Monte Carlo computer code is verified by comparing its predictions with previously published results for optical geometries similar to ours.

Investigating the Thermodynamic Performances of TO-Based Metamaterial Tunable Cells with an Entropy Generation Approach

Active control of heat flux can be realized with transformation optics (TO) thermal metamaterials. Recently, a new class of metamaterial tunable cells has been proposed, aiming to significantly reduce the difficulty of fabrication and to flexibly switch functions by employing several cells assembled on related positions following the TO design. However, owing to the integration and rotation of materials in tunable cells, they might lead to extra thermal losses as compared with the previous continuum design. This paper focuses on investigating the thermodynamic properties of tunable cells under related design parameters. The universal expression for the local entropy generation rate in such metamaterial systems is obtained considering the influence of rotation. A series of contrast schemes are established to describe the thermodynamic process and thermal energy distributions from the viewpoint of entropy analysis. Moreover, effects of design parameters on thermal dissipations and system irreversibility are investigated. In conclusion, more thermal dissipations and stronger thermodynamic processes occur in a system with larger conductivity ratios and rotation angles. This paper presents a detailed description of the thermodynamic properties of metamaterial tunable cells and provides reference for selecting appropriate design parameters on related positions to fabricate more efficient and energy-economical switchable TO devices.

Response Surface Methodology Control Rod Position Optimization of a Pressurized Water Reactor Core Considering Both High Safety and Low Energy Dissipation

Response Surface Methodology (RSM) is introduced to optimize the control rod positions in a pressurized water reactor (PWR) core. The widely used 3D-IAEA benchmark problem is selected as the typical PWR core and the neutron flux field is solved. Besides, some additional thermal parameters are assumed to obtain the temperature distribution. Then the total and local entropy production is calculated to evaluate the energy dissipation. Using RSM, three directions of optimization are taken, which aim to determine the minimum of power peak factor Pmax, peak temperature Tmax and total entropy production Stot. These parameters reflect the safety and energy dissipation in the core. Finally, an optimization scheme was obtained, which reduced Pmax, Tmax and Stot by 23%, 8.7% and 16%, respectively. The optimization results are satisfactory.

Directed Thermal Diffusions through Metamaterial Source Illusion with Homogeneous Natural Media

Owing to the utilization of transformation optics, many significant research and development achievements have expanded the applications of illusion devices into thermal fields. However, most of the current studies on relevant thermal illusions used to reshape the thermal fields are dependent of certain pre-designed geometric profiles with complicated conductivity configurations. In this paper, we propose a methodology for designing a new class of thermal source illusion devices for achieving directed thermal diffusions with natural homogeneous media. The employments of the space rotations in the linear transformation processes allow the directed thermal diffusions to be independent of the geometric profiles, and the utilization of natural homogeneous media improve the feasibility. Four schemes, with fewer types of homogeneous media filling the functional regions, are demonstrated in transient states. The expected performances are observed in each scheme. The related performance are analyzed by comparing the thermal distribution characteristics and the illusion effectiveness on the measured lines. The findings obtained in this paper see applications in the development of directed diffusions with minimal thermal loss, used in novel “multi-beam” thermal generation, thermal lenses, solar receivers, and waveguide.

Optimum Structural Design of Thermal Protection Using Photonic Crystal Material Considering Thermophysical Properties in Micro/Nanoscale

With the rapid development of the supersonic aircraft technology, tremendously, the aircraft Mach numbers get higher and higher, but on the other hand, the working condition become worse and worse. The photonic crystal material which is formed by the periodic micro/nanoscale structures can generate the photonic band gaps, and the photonic band gaps could reflect the energy of the electromagnetic wave effectively. Consequently, the photonic crystal material turns into the newly-developing hotspot on the field of thermal protection for the supersonic aircraft. In this paper, the aircraft states of Mach 6 are set as the target operating condition, and 5 optimum proposals are presented for the structures of typical photonic crystal material. The energy which gets into the body material is calculated; Based on the theory of the electromagnetic field, using the method of transmission matrix and Plane Wave Expansion (PWE), the characteristics of the photonic band gaps for one-and-three dimensional photonic crystals are calculated. Finally, the characteristics of the photonic band gaps are discussed, and optimal design for the performance of the photonic crystal material thermal protection are proposed.Copyright