Measuring and modeling vapor boundary layer growth during transient diffusion heat and moisture transfer in cellulose insulation

In this paper, an experimental test facility that permits continuous measurements of transient heat and moisture transfer in porous media is applied to study the vapor boundary layer in cellulose insulation. The experiment measures the relative humidity, temperature and moisture accumulation within the cellulose specimen with a fully developed flow of air at a controlled temperature and humidity provided above the surface. These experimental results are used to verify a mathematical model, which is used to develop an expression for moisture diffusivity (α_m) that is analogous to thermal diffusivity, and takes into consideration moisture storage. The moisture diffusivity is used to calculate the vapor density in the boundary layer and the size of vapor boundary layer in cellulose insulation. It is found that the moisture storage effect has a very significant effect on the vapor boundary layer and cannot be ignored. For cellulose insulation, the size of the vapor boundary layer may be over predicted by a factor of ten when moisture storage is neglected.     

Determination of thermal parameters of poor conductors by transient techniques

The authors compare the transient hot wire method and the parallel wire method of determining thermal conductivity, using the second to find the thermal diffusivity of two materials. The wires are sandwiched between two samples of the material to be investigated. The influence of pressure is also studied in order to identify the measurement conditions that can be easily achieved. A method is chosen to evaluate thermal parameters and to determine the field of data to be used in relation to sample dimensions.     

Graphical determination of drying process and moisture transfer parameters for solids drying

In this paper a simple graphical method is proposed to determine the drying moisture transfer parameters such as moisture diffusivity and moisture transfer coefficient for solid products. Once the lag factor and the drying coefficient are obtained from the experimental moisture content data, the proposed graphical method can be used to estimate the drying moisture transfer parameters in a quick and efficient manner. Drying time can also be easily determined for a solid whose drying process parameters are known. Two illustrative examples are given to highlight the importance of the topic and validate the use of the present methodology for practical drying applications.     

Measurement of apparent solid-side mass diffusivity of a water vapor-silica gel system

A measurement of the apparent solid-side mass diffusivity of water vapor adsorbed in a regular density silica gel is performed by using a constant-pressure thermal gravimetrical apparatus. The diameter of the silica gel particles is 2 mm. Six adsorption isotherms, individually correspond to 5.1, 22.2, 34.3, 49.5, 64.4 and 79.6 °C, are measured. The covered range of moisture content is from 0% to 40%. Using a previously developed model, which considers both surface (film) heat and mass transfer resistances, the measured uptake curves yield the apparent solid-side mass diffusivities. The apparent solid-side mass diffusivity is expressed as a function of temperature and moisture content. The thermal effect and importance of surface mass transfer resistance are individually discussed.     

Measurement of thermal conductivity, thermal diffusivity and heat capacity of highly porous building materials using transient plane source technique

This paper presents both experimental and theoretical works concerning evaluation of the thermal conductivity, thermal diffusivity and heat capacity of wood composites. Moreover, the aim of this study is to show that the transient plane source technique originally used for measuring thermal properties of isotropic materials can be spread worthy of heat capacity, thermal conductivity and thermal diffusivity measurements of highly porous materials. Measurements of the thermal conductivity, thermal diffusivity and heat capacity have been performed at room temperature (20 ± 0.5°C) and normal pressure. An attempt has been made to predict the thermal diffusivity of wood composites from the predicted values of thermal conductivity using a Verma et al's model based on Ohms law and the calculated values of heat capacity using the enthalpy concept. The predicted values by the proposed model are compared with the values of the thermal diffusivity measured using the TPS method. A comparison shows a good agreement.     

Simultaneous measurements of thermal conductivity and thermal diffusivity of insulators,fluids and conductors using the transient plane source (TPS) technique

Simultaneous measurements of thermal conductivity and thermal diffusivity of composite red-sand bricks, glycerine and mercury have been made at room temperature by the recently developed transient plane source (TPS) technique. This paper describes, in brief, the theory and the experimental conditions for the simultaneous measurements of thermal conductivity and thermal diffusivity of insulators, fluids and metals. The source of heat is a hot disc made out of bifilar spirals. The disc also serves as a sensor of temperature increase in the sample. The measured values of the thermal conductivity and thermal diffusivity of these samples are in agreement with the values reported earlier using other methods. The advantage of the TPS technique is the simplicity of the equipment, simultaneous information on thermal conductivity and thermal diffusivity, and also the applicability of the technique to insulators, fluids and metals.     

An exact modelling for thermal diffusivities of solid objects exposed to heating

A new simple model for determining the thermal diffusivities of solid geometrical objects (i.e., infinite slab, infinite cylinder, and sphere) being heated in a heating medium is proposed. In the unsteady-state heat transfer modelling, the lag factor and heating coefficient for heating applications were well-defined as being in a cooling process. In addition, new characteristic equations for the case of 0 < Bi < 100 in the transient heat transfer were developed instead of the existing complicated equations, and these were employed. In order to test the present model, the literature heat transfer data were used as an example and the use of the model was described in detail. The results of this study indicate that the heating coefficient has a direct influence on the thermal diffusivity and that the present model permits the determination of the thermal diffusivity values of solid geometrical objects in a simple and accurate manner.     

石墨烯粉末导热性能的研究

采用瞬态电热技术测量了5~6层纯石墨烯粉末中石墨烯的热扩散率,其值为1.15×10-5 m2/s,相应的导热系数为18.000 W/(m·K)。借助导热仪研究了不同密度下石墨烯粉末导热系数的变化情况,发现其导热系数与密度成正比。密度由0.02增加到0.22g/cm3时,导热系数总体提升了8.09%。另通过实验得到了含水率对石墨烯粉末导热性能的影响。实验结果显示,石墨烯粉末含水率由0.0%增加到99.8%的过程中,导热系数先是上升随后下降最终直线降至最低点(约为0.765~1.030 W/(m·K))。其中当含水率达到96.7%时,混合物(石墨烯与水)的导热系数提高了62.80%。该研究为石墨烯热应用及热管理提供了理论支撑。     

Modeling of rates of moisture ingress into photovoltaic modules

Encapsulant materials are used in photovoltaic devices for mechanical support, electrical isolation, and protection against corrosion. During long-term exposure of photovoltaic modules to environmental stress, the ingress of water into the module is correlated with decreased performance. By using diffusivity measurements for water through ethylene vinyl acetate (EVA), we have modeled moisture ingress using finite-element analysis with atmospheric data from various locations such as Miami, Florida. This analysis shows that because of the high diffusivity of EVA, even an impermeable glass back-sheet is incapable of preventing significant moisture ingress from the edges for a 20–30-year lifetime. Once moisture penetrates a module, it can condense and increase corrosion rates. Significantly reducing moisture ingress requires a true hermetic seal, the use of an encapsulant loaded with a desiccant, or the use of an encapsulant with a very low diffusivity.     

The Lock-in Heating-Cooling Method for the Measurement of the Thermal Diffusivity of Solid Materials

A new test method is presented for the on-field nondestructive measurement of the thermal diffusivity of solid materials. A periodic thermal disturbance is supplied to the inspected material by a thermoelectric source based on the Peltier effect. This can alternate heating and cooling stages and provide, if properly controlled, a harmonic disturbance with null net heat flux. A steady-periodic temperature field can thus be induced within the specimen. The diffusivity of the material is then estimated by monitoring the propagation of the temperature cycles along the optically accessible surface of the specimen, adjacent to the thermal input surface area. A camera for infrared thermography is used for nonintrusive surface temperature measurement. At the current stage of development, the focus is on the accurate reproduction of the theoretical model on which the method is based. Ease of operation and portability of the test equipment are also pursued. However, tests on thin specimens of materials with known properties give measurements in encouraging agreement with the nominal values.     

Estimation des propriétés thermiques à l'échelle millimétrique par méthodes périodiques: Résolution du problème direct et du problème inverse

Estimation of thermal properties by periodic methods: direct problem and inverse problem solving. This article presents two processes of thermal diffusivity measurement for homogeneous material. The experimental bench uses a periodic method dedicated to millimeter scale study. Simulation of this experimental method is studied in detail. The case of a homogeneous material of unknown diffusivity to be measured is studied. Sensitivity coefficient and calculus of Cramer–Rao bounds (BCR) prove the good conditioning of diffusivity estimation and illustrate the action of thermal losses. Simulations of Monte Carlo compare performances of least squares estimator to optimal performances defined by the BCR. Two experimental processes are validated by a study of the iron ARMCO chosen as material of reference.     

Thermal diffusivity estimation using numerical inverse solution for 1D heat conduction

This work presents an improved apparatus and a numerical approach to obtain the estimate of thermal diffusivity of complex materials. Transient thermal response at the axis of cylindrical sample is measured when boundary temperature is suddenly changed. Instead of assuming an ideal step temperature excitement, a measured temperature of a material boundary was employed. An iterative procedure, based on minimizing a sum of squares function with the Levenberg–Marquardt method, is used to solve the inverse problem. A graphical user interface is built to enable easy use of the inverse thermal diffusivity estimation method. The reference materials used to evaluate the method are Agar water gel, glycerol and Ottawa quartz sand.     

Effective thermal parameters of layered films: An application to pulsed photothermal techniques

Pulsed photothermal techniques provide useful methods based on linear relations between measurable quantities to obtain the thermal diffusivity and thermal conductivity of homogeneous materials. In this work, the effective thermal parameters of two-layered films are defined starting from an homogeneous layer which at the surfaces, produces the same temperature fluctuations and the same photothermal signal that the composite heated by a fast pulse-laser. Our theoretical model predicts that the effective thermal parameters of the layered system can only be calculated in the limit when the laser pulse duration is smaller tan the characteristic time of each layer, respectively. The temperature distribution is calculated in each layer by using the Fourier integral and the time-dependent one-dimensional heat diffusion equation with appropriate boundary conditions according to the experimental conditions. Within this approximation, we found an analytical expression for both, the effective thermal diffusivity and thermal conductivity which depend significantly on the thickness and the thermal parameters of each film.     

A design method to determine the optimal distribution and amount of insulation for in-ground heat storage tanks

The seasonal sensible heat storage model developed by Hooper and Attwater [1] is modified to describe the thermal behaviour of the soil regime surrounding cylindrical, in-ground, heat storage tanks with optimally distributed insulation. The model assumes steady-state heat transfer, and the surrounding soil is considered to be homogeneous and isotropic. Changes in soil thermal properties due to moisture migration, whether driven by thermal or hydrostatic gradients, are assumed negligible [2]. The optimal distribution is determined using the method of Lagrange multipliers. It is shown that the marginal cost per unit of energy lost and per unit of tank surface area must be the same at all points on the surface of the tank as the condition for minimum total heat loss with a given total investment in insulation. This condition appears to apply for all axi-symmetric in-ground tank geometries. For a given volume of insulation, the incremental increase in storage efficiency with an optimal redistribution of the insulation is a function of tank geometry. The problem of determining the optimal total investment in insulation for a given marginal cost of fuel is described and a method of solution is outlined.     

Investigation Methods for Thermal Shock Crack Problems of Functionally Graded Materials–Part II: Combined Analytical-Numerical Method

Fractures phenomena can be often found in functionally graded materials (FGMs) subjected to thermal shock loadings. This paper aims to develop a set of analytical-numerical methods for analyzing the mixed-mode thermal shock crack problems of a functionally graded plate (FGP). First, a domain-independent interaction energy integral method is developed for obtaining the mixed-mode transient thermal stress intensity factors (TSIFs). A perturbation method is adopted to obtain the transient temperature field. Then an analytical-numerical method combining the interaction energy integral method, a perturbation method, and the finite element method is developed to solve the present crack problem. Particularly, the influences of the materials parameters, crack length, and crack angle on the TSIFs and the crack growth angle are investigated. The results show that the present analytical-numerical method can be used to solve the thermal shock crack problem with high efficiency. The present work will be significant for the fracture mechanics analysis and design of FGM structures.     

LOCAL THERMAL CHARACTERIZATION OF INNER GUN BARREL REFRACTORY METALLIC COATINGS

Large-caliber gun barrels are coated, on their internal surface, by hard and refractory metallic layers. The thermophysical properties of such coatings are generally very different of bulk materials ones, and liable to evolve following the thermal cyclings. Nevertheless, the knowledge of these properties is necessary for modeling the gun barrels behavior. In the present work, a photothermal microanalysis method has been used to measure the local thermal diffusivity within such a coating. The periodic excitation is localized on a micrometer-scale spot and the temperature evolution in the thermally excited zone is monitored by photoreflection on a spot with similar size. Local thermal properties are identified from thermal transfer function analysis, i.e., phase-lag evolution versus frequency. Results obtained on an aged chromium coating, on which several local diffusivity measurements have been undertaken along the depth, are presented. The diffusivity is higher near the surface than toward the interior of the coating, which has undergone less important heating during the gun lifetime. This behavior is compared to the microhardness evolution along the coating depth.     

A modelling study for moisture diffusivities and moisture transfer coefficients in drying of solid objects

This article presents an effective analytical model for determining the moisture diffusivities and moisture transfer coefficients for solid objects (namely, infinite slab, infinite cylinder, sphere; and also for irregularly shaped objects, by using a shape factor) subject to drying applications in a medium. The unsteady-state moisture diffusion analysis is used on the basis of two important criteria: 0·1 <Bi < 100 and Bi > 100. The drying coefficients and lag factors were employed. The analytical models are then verified using available experimental data taken from the literature. The results show that the method presented here can be used to determine the moisture diffusion coefficients and moisture transfer coefficients for such solid objects in a simple and accurate manner for a variety of drying applications.     

Measurement of the through-plane thermal conductivity of carbon paper diffusion media for the temperature range from −50 to +120 °C

Carbon paper, a fibrous material, is often used as the gas diffusion layer in polymer electrolyte membrane (PEM) fuel cells, which are being vigorously developed as a zero-emission power source for transportation applications. The temperature field and heat transfer in this material is determined by its thermal conductivity and diffusivity, which are directly dependent on the operating temperature. In this work, we use a quasi-steady method known as the thermal capacitance (slug) method to experimentally measure the through-plane thermal conductivity of TORAY carbon paper for a temperature range from −50 to +120 °C. The effects of compression and PTFE loading on the overall thermal conductivity are also investigated. Compression leads to a decrease in thermal resistance between the carbon fibers; hence, an increase in the overall thermal conductivity. However, it is also found that this thermal resistance is highly dependent on the temperature and the PTFE loading. In contrast with our in-plane thermal conductivity measurements from a previous study, the through-plane thermal conductivity is found to increase with an increase in temperature in this study. This finding suggests that the thermal expansion of the carbon fibers is a direction dependent quantity.