Statistical Multiscale Analysis of Transient Conduction and Radiation Heat Transfer Problem in Random Inhomogeneous Porous Materials

This paper is devoted to the homogenization and statistical multiscale analysis of a transient heat conduction problem in random porous materials with a nonlinear radiation boundary condition.A novel statistical multiscale analysis method based on the two-scale asymptotic expansion is proposed.In the statistical multiscale formulations,a unified linear homogenization procedure is established and the second-order correctors are introduced for modeling the nonlinear radiative heat transfer in random perforations,which are our main contributions.Besides,a numerical algorithm based on the statistical multiscale method is given in details.Numerical results prove the accuracy and efficiency of our method for multiscale simulation of transient nonlinear conduction and radiation heat transfer problem in random porous materials.

The Influence of <i>γ</i>-Irradiation on Thermoemf and Heat Conduction of Ln_0.01Sn_0.99Se (Ln - Pr, Tb, Er) Monocrystals

Monocrystals are synthesized and grown according to Bridgman-Stockbarger method, and the influence of rare earth elements (REE) as well as of γ-irradiation on electrophysical properties of (SnSe)1-x - (SnSe)x (Ln - Pr, Tb, Er) alloy system is investigated. During transition from SnSe to (SnSe)1-x - (SnSe)x solid solutions, a partial compensation of charge carriers occurs and additional scattering centres appear. At low temperatures T after irradiation with γ-quanta, thermoemf in samples of p-type conduction becomes higher and decreases in those of n-type conduction. In addition, under the influence of γ-rays radiation, defects come into being causing a decrease in lattice heat conduction and a rise in electron heat conduction. REE impurities are supposed to be positioned among the points of crystal lattice during irradiation with γ-quanta and, moreover, Frenkel defects are formed.

Combining the complex variable reproducing kernel particle method and the finite element method for solving transient heat conduction problems

In this paper, the complex variable reproducing kernel particle （CVRKP） method and the finite element （FE） method are combined as the CVRKP-FE method to solve transient heat conduction problems. The CVRKP-FE method not only conveniently imposes the essential boundary conditions, but also exploits the advantages of the individual methods while avoiding their disadvantages, then the computational efficiency is higher. A hybrid approximation function is applied to combine the CVRKP method with the FE method, and the traditional difference method for two-point boundary value problems is selected as the time discretization scheme. The corresponding formulations of the CVRKP-FE method are presented in detail. Several selected numerical examples of the transient heat conduction problems are presented to illustrate the performance of the CVRKP-FE method.

A Concise Model and Analysis for Heat-Induced Withdrawal Reflex Caused by Millimeter Wave Radiation

In this study, we consider the heat-induced withdrawal reflex caused by exposure to an electromagnetic beam. We propose a concise dose-response relation for predicting the occurrence of withdrawal reflex from a given spatial temperature profile. Our model is distilled from sub-step components in the ADT CHEETEH-E model developed at the Institute for Defense Analyses. Our model has only two parameters: the activation temperature of nociceptors and the critical threshold on the activated volume. When the spatial temperature profile is measurable, the two parameters can be determined from test data. We connect this dose-response relation to a temperature evolution model for electromagnetic heating. The resulting composite model governs the process from the electromagnetic beam deposited on the skin to the binary outcome of subject’s reflex response. We carry out non-dimensionalization in the time evolution model. The temperature solution of the non-dimensional system is the product of the applied power density and a parameter-free function. The effects of physical parameters are contained in non-dimensional time and depth. Scaling the physical temperature distribution into a parameter-free function greatly simplifies the analytical solution, and helps to pinpoint the effects of beam spot area and applied power density. With this formulation, we study the theoretical behaviors of the system, including the time of reflex, effect of heat conduction, biological latency in observed reflex, energy consumption by the time of reflex, and the strategy of selecting test conditions in experiments for the purpose of inferring model parameters from test data.

Conducting Rubber Force Sensor: Transient Characteristics and Radiation Heating Effect

Compression force sensors are indispensable to tactile sensors in humanoid robots. We are investigating the application of low-cost electrically conducting rubber sheets to force sensors, of which the biggest problem is its poor reproducibility. We have found that the deposition of aluminum by a vacuum evaporation method shows such an excellent characteristic that the sensor can be used in a wide range under 10.33 N/cm2. In this article, we investigated time response of the sensors and also studied how the radiation heating during the vacuum evaporation process for Al deposition affected their sensing property. We found that the radiation heating induces deterioration from the point of view of standard deviation of the output voltage of the sensors at a transient region. We convince that a low-temperature Al deposition method should be developed to form electrodes on the electrical conducting rubber sensors.

Flow of Burgers' fluid over an inclined stretching sheet with heat and mass transfer

Effects of heat and mass transfer in the flow of Burgers fluid over an inclined sheet are discussed. Problems formulation and relevant analysis are given in the presence of thermal radiation and non-uniform heat source/sink. Thermal conductivity is taken temperature dependent. The nonlinear partial differential equations are simplified using boundary layer approximations. The resultant nonlinear ordinary differential equations are solved for the series solutions. The convergence of series solutions is obtained by plotting theη-curves for the velocity, temperature and concentration fields. Results of this work describe the role of different physical parameters involved in the problem. The Deborah numbers corresponding to relaxation time(β1 and β2) and angle of inclination(α) decrease the fluid velocity and concentration field. Concentration field decays as Deborah numbers corresponding to retardation time(β3) and mixed convection parameter(G) increase. Large values of heat generation/absorption parameters A/B, and the temperature distribution across the boundary layer increase. Numerical values of local Nusselt number,-θ′(0), and local Sherwood number,-f′(0), are computed and analyzed. It is found that θ′(0) increases with an increase in β3.

Numerical manifold method for steady-state nonlinear heat conduction using Kirchhoff transformation

The numerical manifold method(NMM)introduces the mathematical and physical cover to solve both continuum and discontinuum problems in a unified manner.In this study,the NMM for solving steady-state nonlinear heat conduction problems is presented,and heat conduction problems consider both convection and radiation boundary conditions.First,the nonlinear governing equation of thermal conductivity,which is dependent on temperature,is transformed into the Laplace equation by introducing the Kirchhoff transformation.The transformation reserves linearity of both the Dirichlet and the Neumann boundary conditions,but the Robin and radiation boundary conditions remain nonlinear.Second,the NMM is employed to solve the Laplace equation using a simple iteration procedure because the nonlinearity focuses on parts of the problem domain boundaries.Finally,the temperature field is retrieved through the inverse Kirchhoff transformation.Typical examples are analyzed,demonstrating the advantages of the Kirchhoff transformation over the direct solution of nonlinear equations using the NewtonRaphson method.This study provides a new method for calculating nonlinear heat conduction.

铁路车辆车窗隔热系数计算及隔热性能分析

根据铁路车辆车窗结构特点,将其分为安装结构和中空玻璃两部分,分别通过一维稳态导热公式和中空玻璃稳定状态下U值计算方法计算隔热系数,再通过面积贡献率叠加计算整窗隔热系数,并验证了方法的可行性。利用隔热系数计算方法研究了安装结构材料导热系数和厚度、中空层气体厚度与种类、玻璃表面辐射率、中空玻璃材质与厚度对隔热系数的影响。结果表明,车窗安装结构隔热性能主要取决于是否有效隔断热桥,非金属隔断层的厚度越大,热阻越高,隔热性能越好。中空玻璃获得良好隔热性能的前提是具有较优的中空层厚度,在此基础上采用镀Low-E膜玻璃可以有效降低隔热系数,填充惰性气体、使用PC板代替玻璃或增加玻璃厚度均可一定程度降低车窗隔热系数,但效果不明显。

A possible mechanism for magnetar soft X-ray/γ-ray emission

Once the energies of electrons near the Fermi surface obviously exceed the threshold energy of the inverse β decay,electron capture（EC） dominates inside the magnetar.Since the maximal binding energy of the 3 P 2 neutron Cooper pair is only about 0.048 MeV,the outgoing high-energy neutrons（E k（n） 60 MeV） created by the EC can easily destroy the 3 P 2 neutron Cooper pairs through the interaction of nuclear force.In the anisotropic neutron superfluid,each 3 P 2 neutron Cooper pair has magnetic energy 2μ n B in the applied magnetic field B,where μ n = 0.966 × 10 23 erg.G 1 is the absolute value of the neutron abnormal magnetic moment.While being destroyed by the high-energy EC neutrons,the magnetic moments of the 3 P 2 Cooper pairs are no longer arranged in the paramagnetic direction,and the magnetic energy is released.This released energy can be transformed into thermal energy.Only a small fraction of the generated thermal energy is transported from the interior to the surface by conduction,and then it is radiated in the form of thermal photons from the surface.After highly efficient modulation within the star＇s magnetosphere,the thermal surface emission is shaped into a spectrum of soft X-rays/γ-rays with the observed characteristics of magnetars.By introducing related parameters,we calculate the theoretical luminosities of magnetars.The calculation results agree well with the observed parameters of magnetars.

Optimization of Compressed Ceramics Fiber Insulating Layers

Compressed thin layers of ceramic fiber insulation are used as high temperature insulating layers as well as mechanical support for catalyst coated ceramic monoliths in automotive emission control devices. Minimization of energy losses, choice of material and thickness of com- pressed insulating layer are based on knowledge of their thermal physical properties. Currently, consistent meas- urements of materials in a compressed state, as they would be in emission control applications, are absent due to the absence of suitable methods for s,wh tests. A test method was developed for measurement of the thermal conductivity of compressed thin fiber layers. This paper summarizes the results of thermal conductivity and diffu- sivity measurements of 27 compressed fiber alumina -sili- ca -vermiculite materials in the range of 200 -950℃. Thermal physical properties as a function of temperature, density/mechanical pressure, thickness and composition of insulating layers are presented. The whole set of exper- imental data is generalized on 3D surface plots and de- scribed by polynomial functions. The possible heat trans- fer mechanisms governing apparent thermal conductivity of pressed insulation layers are discussed.

Numerical simulation of laser-induced plasma in background gas considering multiple interaction processes

Laser-induced plasma is often produced in the presence of background gas,which causes some new physical processes.In this work,a two-dimensional axisymmetric radiation fluid dynamics model is used to numerically simulate the expansion process of plasma under different pressures and gases,in which the multiple interaction processes of diffusion,viscosity and heat conduction between the laser ablated target vapor and the background gas are further considered,and the spatio-temporal evolutions of plasma parameters(species number density,expansion velocity,size and electron temperature)as well as the emission spectra are obtained.The consistency between the actual and simulated spectra of aluminum plasma in 1 atm argon verifies the correctness of the model and the numerical simulation,thus providing a refinement analysis method for the basic research of plasma expansion in gases and the application of laser-induced breakdown spectroscopy.

Investigating the thermal conductivity of materials by analyzing the temperature distribution in diamond anvils cell under high pressure

Investigating the thermal transport properties of materials is of great importance in the field of earth science and for the development of materials under extremely high temperatures and pressures.However,it is an enormous challenge to characterize the thermal and physical properties of materials using the diamond anvil cell(DAC)platform.In the present study,a steady-state method is used with a DAC and a combination of thermocouple temperature measurement and numerical analysis is performed to calculate the thermal conductivity of the material.To this end,temperature distributions in the DAC under high pressure are analyzed.We propose a three-dimensional radiative-conductive coupled heat transfer model to simulate the temperature field in the main components of the DAC and calculate in situ thermal conductivity under high-temperature and high-pressure conditions.The proposed model is based on the finite volume method.The obtained results show that heat radiation has a great impact on the temperature field of the DAC,so that ignoring the radiation effect leads to large errors in calculating the heat transport properties of materials.Furthermore,the feasibility of studying the thermal conductivity of different materials is discussed through a numerical model combined with locally measured temperature in the DAC.This article is expected to become a reference for accurate measurement of in situ thermal conductivity in DACs at high-temperature and high-pressure conditions.

Convection of Radiating Gas in a Vertical Channel through Porous Media

The free convection flow of radiating gas between two vertical thermally conducting walls through porous medium in the presence of a uniform gravitational field has been studied. Closed form solutions for the velocity and temperature have been obtained in the optically thin limit case when the wall temperatures are varying linearly with the vertical distance. It is observed that the fluid velocity increases and the temperature difference between the walls and the fluid decreases with an increase in the radiation parameter. It is also observed that both the fluid velocity and temperature in the flow field increase with an increase in the porosity parameter. It is found that the fluid velocity decreases while the temperature increases with an increase in the thermal conductance of the walls. Further, it is found that radiation causes to decrease the rate of heat transfer to the fluid, thereby reducing the effect of natural convection.

超高导热金属基复合涂层材料制备

随着电子技术的迅猛发展、集成电路的密集化以及电子器件的小型化,对系统热管理及散热技术提出了更高要求。制备一种超高导热涂层,应用在汽车散热器上,通过特性测定,该涂层材料相比以往,有效提高了其散热系数,大大缓解了由于发动机温度过高,使发动机燃烧不正常、机油变质、点火系统工作不正常等影响汽车正常行驶的情况得以解决。

热湿舒适性智能织物的研究进展

针对当前空调制冷与供暖中造成的能源消耗激增与实现“双碳”的全球目标之间出现巨大矛盾,实现由智能织物对人体自身热湿舒适性调节从而降低能耗成为亟需解决的问题。根据当前热湿舒适性智能织物的研究,概述了人体热湿舒适性调节原理;介绍了由高性能材料(如高红外线反射、高导热、高红外线透过材料)制备的热湿舒适性智能织物以及通过纤维或织物结构控制实现的智能织物(如保暖、吸湿快干、智能热湿调节织物)。分析了不同调节方式的智能织物制备方法及现阶段面临的困难和挑战;提出可制备新型热湿刺激响应纤维,通过纤维的低成本、大规模生产达到智能热湿调节织物的生产及广泛应用;展望了热湿舒适性智能织物在“双碳”背景下,推动智能服装发展的应用前景。

复杂背景下三维弯曲表面红外测温修正

航空发动机涡轮叶片的断裂脱落与叶片温度密切相关,准确测量涡轮叶片温度对航空发动机的安全运行具有重要意义。针对涡轮叶片材料发射率不均匀、周围高温物体反射严重以及探测角度变化等因素导致传统红外辐射测温方法精度难以保证的问题,基于辐射传输理论与红外热成像测温原理,分析了对红外测温结果影响的主要因素,提出了复杂背景下三维弯曲表面红外辐射测温的修正模型。通过设计实验测量获得弯曲表面的发射率、双向反射分布函数、角系数等重要参数,根据提出的温度修正模型,得到弯曲表面红外修正温度。通过与典型位置上热电偶的测温结果对比,测温误差由修正前的4%左右下降到1%以内,证明了所提的修正模型具有较高的精度和适应性,可为涡轮叶片气动传热试验技术的提升和航空发动机等重大装备的研制提供支撑。

准稳态下常功率平面热源法测不良热导体热导率及COMSOL模拟

准稳态法是测量不良热导体热导率的一种常用方法,样品材质、保温层、加热功率等都会对测量结果造成影响。本文采用分离变量法,以有机玻璃为例,求解了厚度均匀的无限大不良热导体两侧同时施加均匀的指向中心面的热流密度时的热传导方程,得到样品各处的温度随时间变化的严格解。在初始加热时间不到十分钟的状态下采用截断近似,并在准稳态下用常功率平面热源加热法测量了不良热导体的热导率k及比热c。在此基础上,采用四块有机玻璃板和两个加热薄膜外包泡沫和聚丙烯(PP)阻热材料的模型,作了COMSOL多物理场耦合仿真,得到的理论、实验及仿真结果三者符合得很好。

太阳辐射热利用优先的户内空间适配组合模式研究

针对户型空间布局如何更直接高效地利用太阳能资源的问题,以太阳能资源富集的拉萨地区为例,通过对当地住宅户内太阳辐射热空间分布规律分析和设计要素整合,量化户内太阳辐射热利用分区,提出空间适应性布局概念及适配应用模式,探索住宅建筑直接利用太阳辐射热实现节能减排的绿色建筑设计新方式。

锅炉炉渣在冷渣管内的传热特性

基于离散元法,建立并验证了颗粒-颗粒热传导、颗粒-流体-颗粒热传导及颗粒-颗粒热辐射模型,采用温度概率密度函数作为传热效应的评价指标,研究了填充率对冷渣管内高温炉渣颗粒传热的影响。结果表明:填充率降低,料床内颗粒的传热更均匀,高温核缩小的速度更快。传热后期,填充率越小,概率密度函数峰值所对应的量纲一温度越大。不同填充率下,各传热途径所传递的热流量总体上波动不大,颗粒料床内辐射传热传递的热流量占总传热流量的59%,是影响料床内颗粒温度降低的主要因素,其次为颗粒-流体-颗粒热传导(占24%),最后为颗粒间的热传导(占16%)。

Theoretical and experimental advances on heat transfer and flow characteristics of metal foams

Open cell metal foam can be applied to greatly improve thermal performance of heat sink and heat exchanger,so that it has been widely used in the fields of thermal(or heat)control system of aerospace vehicle and energy utilization system and become a very important topic for research in the aerospace thermophysics field,and more and more attentions have been attracted.The optimal design of metal foam heat transfer devices is based on the understanding the flow and heat transfer characteristics in metal foam.This article reviews some recent progresses of theoretical and experimental researches on heat transfer enhancement and flow characteristics of metal foam.We found that the pore cell simplification models of metal foams generally fall into four categories,among which the most commonly used cell model is Kelivin model.Some exploratory works performed by the current authors are also introduced,such as the effect of boundary conditions on the heat transfer enhancement;the theoretical modelling of interfacial convective heat transfer taking into account heat conduction between foam ligaments;and the flow characteristics under relatively high velocity.The analytical results show that the flow characteristics of metal foam at relatively high speed are completely different from those at low speed,a further thorough study of the heat transfer and flow characteristics of metal foam is necessarily required.In this paper,two types of partial filling techniques are discussed.The heat transfer performance of partially filled tubes was evaluated by both the performance evaluation criteria and the performance evaluation plot of enhanced heat transfer techniques oriented for energy-saving.The results show that the filling type of metal foam have a significant impact on its heat transfer enhancement performance.Therefore,the filling method of metal foam should be further studied,in order to optimize the thermophysical properties of heat transfer devices.