Measuring the thermal conducitivity of uranin

A convenient experimental method for measuring the thermal conductivity of uranin (fluorecein sodium, C₂₀H₁₀O₅Na₂) is described. Two similar blocks of uranin, produced from a strong uranin/water solution, were exposed to one-dimensional steady-state conduction. It was found that, for a mean bulk temperature ranging from ambient up to 55°C, the uranin has a constant thermal conductivity of 0.43 W/mK. Above these temperatures, the material begins to soften and the thermal conductivity is seen to decrease     

Calculation of thermal conductivity of polar-nonpolar gas mixtures

Summary An approximate method, an empirical one, a semi theoretical one, and the procedure due to Lindsay and Bromley are examined for prediction of thermal conductivity of polar-nonpolar gas mixtures. With a modification of the approximate method we find, by analyzing experimental and calculated results of twelve different systems, that prediction of thermal conductivity is possible with an uncertainty of about 2%. This is important because experimental data on such gas systems are rare. The methods also permit computation of thermal conductivity at moderately high temperatures and for multicomponent mixtures.     

Numerical prediction of the thermal conductivity of fibers

The need to determine the thermal conductivity of fibers for design purposes of new composite materials and the inherent difficulties in the direct measurement of the thermal conductivity of fibers motivated the present work due to its importance for energy conservation purposes. In this work, a correlation formula is developed to predict the thermal conductivities of fiber as function of the effective thermal conductivity of a fiber-reinforced composite laminates and their constituents which are easy to measure. The parallel and series thermal models of composite walls have been utilized in developing this correlation equation. The coefficients of this formula can be given as functions of the voids volume fraction for each fiber to resin volume ratio considered. The validity of the models is verified through finite element analysis. This model also shows excellent agreement with the available experimental values.     

The effective thermal conductivity of insulation materials reinforced with aluminium foil at low temperatures

The effective thermal conductivity (ETC) of multilayer thermal insulation materials was experimentally investigated as a function of temperature (0–25?°C). The materials consisted of binary/ternary glass wools or ternary expanded polystyrene foams reinforced with aluminium foil. The experimental measurements were performed using a guarded hot plate with temperature differences of 5, 10 and 15?°C. The results indicated that significant correlations exist between ETC and the characteristics of the materials with decreasing temperature. The ETC decreases with reinforcement with aluminium foil at the same temperature or with temperature differences of 5 and 15?°C. In addition, it was clearly observed that the ETC decreases sharply with decreased temperature. Consequently, reflective materials may reduce the ETC at low temperatures.     

A prediction model for the effective thermal conductivity of nanofluids considering agglomeration and the radial distribution function of nanoparticles

Conventional heat transfer fluids usually have low thermal conductivity, limiting their efficiency in many applications. Many experiments have shown that adding nanosize solid particles to conventional fluids can greatly enhance their thermal conductivity. To explain this anomalous phenomenon, many theoretical investigations have been conducted in recent years. Some of this research has indicated that the particle agglomeration effect that commonly occurs in nanofluids should play an important role in such enhancement of the thermal conductivity, while some have shown that the enhancement of the effective thermal conductivity might be accounted for by the structure of nanofluids, which can be described using the radial distribution function of particles. However, theoretical predictions from these studies are not in very good agreement with experimental results. This paper proposes a prediction model for the effective thermal conductivity of nanofluids, considering both the agglomeration effect and the radial distribution function of nanoparticles. The resulting theoretical predictions for several sets of nanofluids are highly consistent with experimental data.     

Measurement of the effective thermal conductivity of a Mg–MgH2 packed bed with oscillating heating

The magnesium–magnesium hydride–hydrogen-system (Mg–MgH₂–H₂) offers, because of its combined hydrogen and heat storage capacity, the possibility to design hydride heat pumps and heat stores. For such industrial application systems based on cylindrically formed reactors filled with an active magnesium powder, the effective thermal conductivity limits the time in which the metal hydride alloy is charged and discharged with hydrogen. Determination of this transport coefficient is of fundamental importance for the optimum design of magnesium hydride reactors. The complex interrelation of the different transport mechanisms in a metal hydride packed bed and the hitherto undefined rule that the solid effective thermal conductivity behaves as a function of the hydrogen concentration, requires a reliable and simple-to-realize measuring method so as to determine the effective thermal conductivity of a magnesium hydride bed. In the present study, a report is given for the first time on the initiation of a measuring technique with oscillating change of temperature in a non-permeated packed bed of fine-grained material. The measurement of the effective thermal conductivity can ensue by tailoring the problem-specific mathematical result to the experimentally recorded temperature-time function. The effective thermal conductivity of the magnesium hydride bed varies between 2 and 8 W/(m K) in a temperature range of 523–653 K.     

考虑瞬态效应的承载隔热多功能结构拓扑优化

同时满足承载和隔热要求的多功能结构在高超飞行器热防护结构设计中倍受关注.实际隔热材料通常承载能力弱,而高承载材料隔热性能差,如何在有限空间内协同结构的承载与隔热成为关键问题.高超飞行器气动加热时间有限,存在加热时间短、热荷载变化大的特点.因此结构设计需要考虑时间因素和瞬态效应,而现有稳态传热与承载的多功能协同优化设计模...     

Concurrent topology optimization for minimization of total mass considering load-carrying capabilities and thermal insulation simultaneously

The present work introduces a novel concurrent optimization formulation to meet the requirements of lightweight design and various constraints simultaneously. Nodal displacement of macrostructure and effective thermal conductivity of microstructure are regarded as the constraint functions, which means taking into account both the load-carrying capabilities and the thermal insulation properties. The effective properties of porous material derived from numerical homogenization are used for macrostructural analysis. Meanwhile, displacement vectors of macrostructures from original and adjoint load cases are used for sensitivity analysis of the microstructure. Design variables in the form of reciprocal functions of relative densities are introduced and used for linearization of the constraint function. The objective function of total mass is approximately expressed by the second order Taylor series expansion. Then, the proposed concurrent optimization problem is solved using a sequential quadratic programming algorithm, by splitting into a series of sub-problems in the form of the quadratic program. Finally, several numerical examples are presented to validate the effectiveness of the proposed optimization method. The various effects including initial designs, prescribed limits of nodal displacement, and effective thermal conductivity on optimized designs are also investigated. An amount of optimized macrostructures and their corresponding microstructures are achieved.     

The direct effect of interfacial nanolayers on thermal conductivity of nanofluids

One of the most important features of nanofluids is their thermal conductivity. In this article, a new model for thermal conductivity is proposed based on the combination of a statistical model and thermal convection caused by Brownian motion of nanoparticles with considering the effect of interfacial nanolayers among nanoparticles and base fluids. This model is compared with Al₂O₃ in deionized water and CuO in deionized water (based nanofluids of spherical particles) using a number of theoretical and experimental thermal conductivity models, after that the experimental results have been made available in the open literature. In this model, an interfacial nanolayer is influenced directly on both parts of static and dynamic effective thermal conductivity. The present model shows good agreement with the experimental result of nanofluids and gives better predictions compared to models used for nanofluids in this article. This model is purely theoretical and in order to achieve it, experimental results have no effect.     

材料热传导系数随温度变化函数的反演方法

材料热传导系数的反演是一类典型的热传导逆问题。针对材料热传导系数随温度变化的情况,本文将材料的热传导系数值按温度区间分段离散,建立了通过材料边界点的温度测量来反演各温度区间热传导系数值的遗传算法和伴随方程法。通过典型算例分析和考虑测量噪声、系统噪声的反演计算结果分析表明：所建立的两种反演算法都是可行有效的,受测量随机噪...     

Carbonate-salt-based composite materials for medium- and high-temperature thermal energy storage

This paper discusses composite materials based on inorganic salts for medium- and high-temperature thermal energy storage application. The composites consist of a phase change material （PCM）, a ceramic material, and a high thermal conductivity material. The ceramic material forms a microstructural skeleton for encapsulation of the PCM and structural stability of the composites; the high thermal conductivity material enhances the overall thermal conductivity of the composites. Using a eutectic salt of lithium and sodium carbonates as the PCM, magnesium oxide as the ceramic skeleton, and either graphite flakes or carbon nanotubes as the thermal conductivity enhancer, we produced composites with good physical and chemical stability and high thermal conductivity. We found that the wettability of the molten salt on the ceramic and carbon materials significantly affects the microstructure of the composites.     

Effects of thermal radiation on MHD viscous fluid flow and heat transfer over nonlinear shrinking porous sheet

This paper investigates the effects of thermal radiation on the magnetohy-drodynamic (MHD) flow and heat transfer over a nonlinear shrinking porous sheet. The surface velocity of the shrinking sheet and the transverse magnetic field are assumed to vary as a power function of the distance from the origin. The temperature dependent viscosity and the thermal conductivity are also assumed to vary as an inverse function and a linear function of the temperature, respectively. A generalized similarity transformarion is used to reduce the governing partial differential equations to their nonlinear coupled ordinary differential equations, and is solved numerically by using a finite difference scheme. The numerical results concern with the velocity and temperature profiles as well as the local skin-friction coefficient and the rate of the heat transfer at the porous sheet for different values of several physical parameters of interest.     

Effective thermal conductivity of wire-woven bulk Kagome sandwich panels

Thermal transport in a highly porous metallic wire-woven bulk Kagome (WBK) is numerically and analytically modeled. Based on topology similarity and upon introducing an elongation parameter in thermal tortuosity, an idealized Kagome with non-twisted struts is employed. Special focus is placed upon quantifying the effect of topological anisotropy of WBK upon its effective conductivity. It is demonstrated that the effective conductivity reduces linearly as the porosity increases, and the extent of the reduction is significantly dependent on the orientation of WBK. The governing physical mechanism of anisotropic thermal transport in WBK is found to be the anisotropic thermal tortuosity caused by the intrinsic anisotropic topology of WBK.     

Methods of calculating thermal conductivity of binary mixtures involving polyatomic gases

Summary The experimental binary thermal conductivity data of nineteen different gas pairs have been discussed and the competence of the rigorous, approximate and empirical procedures to represent them is investigated and discussed. In addition a new semi-theoretical method is suggested and tested. The suggested semi-theoretical procedure works very satisfactorily and is of good accuracy. It also compares favourably with the other methods. We also suggest a procedure for estimating thermal conductivity values at high temperature. This is an interesting and useful suggestion in view of the great practical need and their meagre availability.     

Thermal analysis of annular fins with temperature-dependent thermal properties

The thermal analysis of the annular rectangular profile fins with variable thermal properties is investigated by using the homotopy analysis method （HAM）. The thermal conductivity and heat transfer coefficient are assumed to vary with a linear and power-law function of temperature, respectively. The effects of the thermal-geometric fin parameter and the thermal conductivity parameter variations on the temperature distribution and fin efficiency are investigated for different heat transfer modes. Results from the HAM are compared with numerical results of the finite difference method （FDM）. It can be seen that the variation of dimensionless parameters has a significant effect on the temperature distribution and fin efficiency.     

A Highly Conductive Porous Medium for Solid–Gas Reactions: Effect of the Dispersed Phase on the Thermal Tortuosity

The heat transfer in a highly conductive material constituted by a graphite matrix in which a granular phase is dispersed is studied. The effective thermal conductivity of this anisotropic porous composite medium used in solid–gas reactors can vary largely with the component fractions. The effect of the dispersed grains on the deformable structure of the matrix is considered. A model developed on the basis of thermal tortuosity by analogy with mass transfer is adequately correlated with experimental results.     

Effect of temperature on the effective thermal conductivity of n-tetradecane-based nanofluids containing copper nanoparticles

Nanofluids were prepared by dispersing Cu nanoparticles(~20nm) in n-tetradecane by a two-step method.The effective thermal conductivity was measured for various nanoparticle volume fractions(0.0001-0.02) and temperatures(306.22-452.66 K).The experimental data compares well with the Jang and Choi model.The thermal conductivity enhancement was lower above 391.06 K than for that between306.22 and 360.77 K.The interfacial thermal resistance increased with increasing temperature.The effective thermal conductivity enhancement was greater than that obtained with a more viscous fluid as the base media at 452.66 K because of nanoconvection induced by nanoparticle Brownian motion at high temperature.     

A micromechanical model to describe thermal fatigue and bowing of marble

Marble slabs are frequently used as façade panels to externally cover buildings. In some cases a bowing of such façade panels after a certain time of environmental exposure is experienced. The bowing is generally accompanied by a reduction of strength which increases with increasing degree of bowing. In the present paper, a theoretical model to calculate the progressive bowing and the thermal fatigue of marble slabs submitted to temperature cycles is presented. The model, developed within the framework of fracture mechanics, takes into account the mechanical microstructural characteristics of the marble as well as the actual cyclic temperature field in the material. The slabs are subjected to a thermal gradient along their thickness (due to different values of temperature between the outer and inner sides of the slab) as well as to thermal fluctuation on the two sides of the slab due to daily and seasonal temperature excursions. This thermal action causes a stress field which can locally determine microcracks due to decohesion of calcite grains. Stress intensification near the cracks occurs and leads to crack propagation in the slab. Such crack propagation under thermal actions is evaluated and the corresponding deflection (bowing) is calculated. Some examples are presented which show the strong influence of material microstructure on the degree of bowing.     

Thomson and rotation effects during photothermal excitation process in magnetic semiconductor medium using variable thermal conductivity

This study investigates a strong magnetic field acting over an elastic rotator semiconductor medium. The Thomson effect due to the magnetic field during the photothermal transport process is studied, and the thermoelectricity theory is used to explain the behavior of waves in the homogenous and isotropic medium under the effect of variable thermal conductivity. The variable thermal conductivity is considered as a linear function of the temperature. The two-dimensional deformation equations are used to describe the overlaps among plasma, electrical, thermal, and magneto-elastic waves.The charge density of inertia-particles is considered as a function of time for studying the induced electric current. The normal mode analysis is used to obtain the exact solutions of the physical field distributions as part of this phenomenon. To obtain the complete solutions of the physical field quantities, the certain mechanical loads, electromagnetic effects, thermal effects, and plasma recombination process are applied herein. The results of the physical distributions are graphically depicted and discussed in consideration of the internal heat source, rotation, and Peltier coefficient.