Periodisches Hitzdrahtverfahren zur Messung von Wärme- und Temperaturleitfähigkeit von geringen Stoffmengen

A measuring procedure for the simultaneous determination of the thermal conductivity and thermal diffusivity of small quantities is described. The procedure is suited for high-viscous fluids and for powdery material. The measuring principle is based on the transient hot-wire method. A sinusoidal alternating current flows through a thin platinum wire and heats up the wire periodically. This results in thermal waves, which penetrate into the surrounding sample. The amplitude and the phase shift of the thermal waves depend on the thermal diffusivity “a” and the thermal conductivity “λ” of the sample. The temperature oscillation in the sample is measured by means of the platinum wire, which is simultaneously applied as a resistance thermometer. The values measured for water and glycerine correspond well to those given in literature. Results of the effective thermal conductivity and the effective thermal diffusivity of zeolite powder under pressurized hydrogen are also discussed. The advantage of this measuring procedure is that only a sample of 13 ml is needed for the test.     

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.     

Thermoelastic Stress Analysis of Structures Under Natural Vibrations

The paper focuses on stress analyses of structures subjected to excitation forces operating at resonant frequencies. The structures are analysed experimentally using the Thermoelastic Stress Analysis (TSA) technique. Experiments are carried out for fixed-free beams of different dimensions and materials, and also for a steel rectangular plate with clamped edges. These structures are excited by a shaker via a stinger. For materials with low thermal conductivity, the agreement between the theory, numerical results and experimental results is excellent. As the thermal conductivity of the material is increased, the correspondence is not as close. This is because of non-adiabatic behaviour. The implications of these results are discussed in detail in the paper and a means of deriving the severity of heat transfer is provided. Other factors that influence the TSA results from structures under natural loading are also discussed.     

Effective Thermal Conductivity of Liquid-Saturated Coatings and Their Liquid Vaporisation Behaviour

The effective thermal conductivity is an important element in understanding the thermal response to heating of a paper coating, e.g. during drying in heatset web-offset, and thus it not only affects the drying efficiency but also affects print quality detriments like web fluting. This study examines both the effective thermal conductivity of liquid-saturated ground calcium carbonate coating structures as well as the vaporisation behaviour from these structures. The liquids used for saturation were mineral oil and water in order to resemble ink and fountain solution, respectively, both of which are present in the traditional heatset web-offset process. The effective thermal conductivities of liquid-saturated coating structures are discussed in regard to the corresponding unsaturated systems by using a Lumped Parameter Model. It is shown that the liquid saturation has a dominant effect in determining the effective thermal conductivity. Since this effect is not fully captured by the model, other mechanisms like an apparent pigment–pigment connectivity increase by liquid bridging and the role of liquid in changing the contact resistance during the measurement of thermal conductivity are discussed. In addition, the transformation of three-dimensional structures to an equivalent two-dimensional modelling is evaluated. The vaporisation behaviour of mineral oil and water is studied by a thermogravimetric analysis. By following the changes in maximum evaporation temperatures and evaporation rates, the addition of binder is seen to lead to a reduction in the vaporisation rate of both liquids. Since there is little to no interaction between the liquids and the binder, the confinement caused by the geometry change induced by binder addition is identified as the mechanism resulting in elevated vapour pressure within the structure.     

The influence of bound molecules on the thermal conductivity in the critical region

Summary An attempt has been made to interpret the thermal conductivity data of CO₂ in the critical region by considering the gas to be a mixture of clusters. Additional heat transfer due to the formation and the breaking of clusters has also been considered.In contradiction to the recent experimental results, the calculated thermal conductivity values do not show a sharp maximum in the critical region. The reasons for this disagreement have been discussed.     

Anisotropic thermal conductivity in sheared polypropylene

We discuss the anisotropy of the thermal conductivity tensor in polymer flow in this paper. Isotactic polypropylene (iPP) specimens were deformed by injection moulding at high shear rates and by steady shear at low shear rates, and were then quenched. The thermal conductivities parallel and perpendicular to the shear direction were measured using modulated differential scanning calorimetry (MDSC) in accordance with the ASTM E1952-01. The measured results showed that the thermal conductivity of the sheared polymer was anisotropic with an increase in the shear direction. The thermal conductivity can be regarded as varying either with the strain or the stress, as suggested by Van den Brule (1989). In addition to the Van den Brule mechanism, crystallization during flow also changes the thermal conductivity and this effect may often be dominant. Suggestions for procedures in processing computations, based on both effects, are given.     

Flow of variable thermal conductivity fluid due to inclined stretching cylinder with viscous dissipation and thermal radiation

The aim of the present study is to investigate the flow of the Casson fluid by an inclined stretching cylinder. A heat transfer analysis is carried out in the presence of thermal radiation and viscous dissipation effects. The temperature dependent thermal conductivity of the Casson fluid is considered. The relevant equations are first simplified under usual boundary layer assumptions, and then transformed into ordinary differential equations by suitable transformations. The transformed ordinary differential equations are computed for the series solutions of velocity and temperature. A convergence analysis is shown explicitly. Velocity and temperature fields are discussed for different physical parameters by graphs and numerical values. It is found that the velocity decreases with the increase in the angle of inclination while increases with the increase in the mixed convection parameter. The enhancement in the thermal conductivity and radiation effects corresponds to a higher fluid temperature. It is also found that heat transfer is more pronounced in a cylinder when it is compared with a flat plate. The thermal boundary layer thickness increases with the increase in the Eckert number. The radiation and variable thermal conductivity decreases the heat transfer rate at the surface.     

A Semi-analytical Model for Pressure-Dependent Permeability of Tight Sandstone Reservoirs

In tight gas reservoirs, permeability is pressure dependent owing to pore pressure reduction during the life of the reservoir. Empirical models are commonly used to describe pressure-dependent permeability. In this paper, it was discussed a number of issues which centered around tight sandstone pressure-dependent permeability experiment, first to apply core aging on permeability test and then to develop a new semi-analytical model to predict permeability. In tight sandstone permeability test experiment, the microinterstice between core and sleeves resulted in over estimation of dependency of permeability on pressure. Then, a new semi-analytical model was developed to identify the relation between permeability and fluid pressure in tight sandstone, which indicates there is a linear relation between pore pressure changes and the inverse of permeability to a constant power. Pressure-dependent permeability of 8 tight sandstone core samples from Ordos Basin, China, was obtained using the modified procedure, and results were perfectly matched with the proposed model. Meanwhile, the semi-analytical model was also verified by pressure-dependent permeability of 16 cores in the literature and experiment results of these 24 cores were matched by empirical models and the semi-analytical model. Compared with regression result of commonly used empirical models, the semi-analytical model outperforms the current empirical models on 8 cores from our experiment and 16 cores from the literature. The model verification also indicates that the semi-theoretical model can match the pressure-dependent permeability of different rock types. In addition, the permeability performance under reservoir condition is discussed, which is divided into two stages. In most tight gas reservoirs, the permeability performance during production is located in stage II. The evaluation result with proposed experiment procedure and the stress condition in stage II will reduce permeability sensitivity to stress.     

Steady and Unsteady Heat Transfer in a Channel Partially Filled with Porous Media Under Thermal Non-Equilibrium Condition

Steady and pulsatile flow and heat transfer in a channel lined with two porous layers subject to constant wall heat flux under local thermal non-equilibrium (LTNE) condition is numerically investigated. To do this, a physical boundary condition in the interface of porous media and clear region of the channel is derived. The objective of this work is, first, to assess the effects of local solid-to-fluid heat transfer (a criterion indicating on departure from local thermal equilibrium (LTE) condition), solid-to-fluid thermal conductivity ratio and porous layer thickness on convective heat transfer in steady condition inside a channel partially filled with porous media; second, to examine the impact of pulsatile flow on heat transfer in the same channel. The effects of LTNE condition and thermal conductivity ratio in pulsatile flow are also briefly discussed. It is observed that Nusselt number inside the channel increases when the problem is tending to LTE condition. Therefore, careless consideration of LTE may lead to overestimation of heat transfer. Solid-to-fluid thermal conductivity ratio is also shown to enhance heat transfer in constant porous media thickness. It is also revealed that an increase in the amplitude of pulsation may result in enhancement of Nusselt number, while Nusselt number has a minimum in a certain frequency for each value of amplitude.     

Heat flow in functionally graded pipes with isothermal boundary conditions

The steady state two-dimensional temperature field within a functionally graded pipe having isothermal inner and outer surfaces is calculated. The analysis is based upon the theory of complex variables. An expression, which depends upon the spatial variation of the conductivity, is suggested for the equivalent homogeneous thermal conductivity.     

Determination of specific heat and true thermal conductivity of glass from dynamic temperature data

A method to determine the true specific heat and true thermal conductivity for glass and other semitransparent materials from dynamic temperature data is presented. A unique fabrication technique to obtain high quality dynamic temperature data from glass test plates employing thermocouples fused to the glass is described. The true thermal conductivity and specific heat of float glass has been measured using these techniques, and the results are compared with the scant data available in the literature. Sensitivity of the measured specific heat and thermal conductivity to sources of uncertainty is identified and these are discussed.     

Effective thermal conductivity of dispersed materials

Measurements have been made by the comparison method for the effective thermal conductivity of dispersed materials which consist of substances with different thermal conductivities. The applicability of existing predicting formulae is discussed in detail as comparing their predicted values with the present data. A new predicting formula is proposed through analyzing the experimental data, the numerical results, and also the data obtained with the electrolytic-bath. It is found that the proposed formula has a wider range of applicability than that of previously reported ones.     

Prediction of thermal conductivity of air voided-fiber-reinforced composite laminates part II: 3D simulation

Applying the fundamental definition of thermal conductivity to a unit cell of unidirectional fiber reinforced composite with air voids, one can deduce simple empirical formula to predict the thermal conductivity of the composite material with estimated air void volume percent. The inherent 3-D problem is modeled using finite element analysis. The model is tested at different fiber to resin volume ratios and various fibers to resin thermal conductivity ratios for three different air void volume percent. The air voids are modeled as cylindrical shapes with different lengths aligned with fiber direction. Two prediction schemes have been developed through the present work. One is to predict the longitudinal thermal conductivity and the other is to predict the transverse thermal conductivity of the fibers. Also, the model can be used to estimate the voids volume percent if the fiber thermal conductivity has been provided. Such expression can, also, serve as useful guides for quality and perfect bonding for material development.     

The macroscopic coefficients of thermal conductivity and diffusion in microinhomogeneous solids

A method is given for calculating the macroscopic coefficients of thermal conductivity and diffusion for microinhomogeneous solids whose local coefficients of thermal conductivity (or diffusion) form an ergodic homogeneous stray field. In the case of marked isotropy of the field of the local coefficients, the calculations are taken to a conclusion. The final formulas for the structure are not much more complicated than the corresponding first-approximation formulas. The results of calculations for certain other cases are also given. The effect of anisotropy of the crystallites in polycrystalline material on the coefficients of thermal conductivity and diffusion is discussed.One of the main problems in the mechanics of microinhomogeneous bodies is the determination of the macroscopic constants from the corresponding microscopic characteristics. The assumption regarding the small inhomogeneity used by a number of authors [1, 2] is not applicable in the case of isotropic polycrystalline aggregates consisting of substantially anisotropic crystallites, stochastic reinforced media media, etc. The so-called self-consistent field method [3] opens up some interesting prospects, but this method is an approximate one and its errors have not yet been assessed. Nevertheless, by making certain fairly general assumptions about the correlation properties of the inhomogeneity, it is possible to obtain final accurate formulas for such macroscopic properties of the solids as the coefficients of thermal conductivity, diffusion, elasticity, and thermal expansion. Below we consider some of the simplest problems involved in determining the macroscopic constants which form a second-order tensor and which characterize the distribution of a certain scalar quantity in a microinhomogeneous body.     

多相材料传热微结构的多目标优化设计

提出基于散热弱度的材料微结构热传导性能的预测方法,分别从理论和数值上验证该方法与均匀化方法的等效性;推导出微结构等效热传导系数的灵敏度计算格式,建立传热微结构拓扑优化的数学模型.以二维、三维多相材料等效热传导系数的加权组合为目标,采用凸规划对偶优化算法和二次型周长约束进行材料微结构的设计和材料分布的棋盘格控制.数值算例表明基于散热弱度的传热材料微结构设计是可行、有效的,可以为实际的材料设计提供依据.     

Non-Darcy effects on convective boundary layer flow past a semi-infinite vertical plate in saturated porous media

The composite effects of viscosity, porosity, buoyancy parameter, thermal conductivity ratio and non-Darcy effects of Brinkman friction and Forscheimmer quadratic drag on the mixed convection boundary layer flow past a semi-infinite plate in a fully-saturated porous regime are theoretically and numerically investigated using Keller’s implicit finite-difference technique and a double-shooting Runge-Kutta method. The Brinkman Forcheimer-extended Darcy model is implemented in the hydrodynamic boundary layer equation. The effects of the various non-dimensional thermofluid parameters, viz Grashof number, Darcy number, and Forchheimer number, and also porosity, thermal conductivity and viscosity parameters on the velocity and temperature fields are discussed. Computations for both numerical schemes are made where possible and found to be in excellent agreement.     

空心材料导热性能预测研究

本文建立了一种预测空心材料导热性的方法。研究了空心材料的导热性。用柱形空心材料分析了体分比和孔洞的排列方式对整体材料导热性的影响，用圆柱形、方柱形空心材料和含裂纹材料，分析了空心形状对材料导热性的影响。同他人的实验结果和某些现有的理论模型比较表明，本文方法是有效的。本文的结果能够很好地解释实验结果。     

Boundary layer flow and heat transfer past a stretching plate with variable thermal conductivity

In the present paper, the boundary layer flow of viscous incompressible fluid over a stretching plate has been considered to solve heat flow problem with variable thermal conductivity. First, using similarity transformation, the components of velocity have been obtained. Then, the heat flow problem has been considered in two ways: (i) prescribed surface temperature (PST), and (ii) prescribed stretching plate heat flux (PHF) in case of variable thermal conductivity. Due to variable thermal conductivity, temperature profile has its two part—one mean temperature and other temperature profile induced due to variable thermal conductivity. The related results have been discussed with the help of graphs.     

Unsteady hydromagnetic Couette flow of dusty fluid with temperature dependent viscosity and thermal conductivity

In the present paper the unsteady Couette flow and heat transfer of a dusty conducting fluid between two parallel plates with temperature dependent viscosity and thermal conductivity are studied. A constant pressure gradient and an external uniform magnetic field are applied. The governing coupled momentum and energy equations are solved numerically using finite differences. The effect of the variable viscosity and thermal conductivity of the fluid and the uniform magnetic field on the velocity and temperature fields for both the fluid and dust particles is discussed.     

Locally similar solutions for hydromagnetic and thermal slip flow boundary layers over a flat plate with variable fluid properties and convective surface boundary condition

This paper presents heat transfer process in a two-dimensional steady hydromagnetic convective flow of an electrically conducting fluid over a flat plate with partial slip at the surface of the boundary subjected to the convective surface heat flux at the boundary. The analysis accounts for both temperature-dependent viscosity and temperature dependent thermal conductivity. The local similarity equations are derived and solved numerically using the Nachtsheim-Swigert iteration procedure. Results for the dimensionless velocity, temperature and ambient Prandtl number within the boundary layer are displayed graphically delineating the effect of various parameters characterizing the flow. The results show that momentum boundary layer thickness significantly depends on the surface convection parameter, Hartmann number and on the sign of the variable viscosity parameter. The results also show that plate surface temperature is higher when there is no slip at the plate compared to its presence. For both slip and no-slip cases surface temperature of the plate can be controlled by controlling the strength of the applied magnetic field. In modelling the thermal boundary layer flow with variable viscosity and variable thermal conductivity, the Prandtl number must be treated as a variable irrespective of flow conditions whether there is slip or no-slip at the boundary to obtain realistic results.