Thermal Conductivity Of Fibrous Materials Research Articles

РАСЧЕТНО-ЭКСПЕРИМЕНТАЛЬНОЕ ИССЛЕДОВАНИЕ ЭФФЕКТИВНОЙ ТЕПЛОПРОВОДНОСТИ ВОЛОКНИСТЫХ МАТЕРИАЛОВ

The article is devoted to the evaluation of the effect of porosity on the effective thermal conductivity of thermal insulation materials. The main factors influencing the thermal conductivity of the material, such as density, the type of porous structure of the material and humidity, are considered. The method of measuring the thermal conductivity by the stationary heat flow method and the hot zone method is described. A method for calculating the effective thermal conductivity of fibrous materials is presented. A computational and experimental study of the effective thermal conductivity is carried out and the results are analyzed.

Thermal conductivity of two-dimensional disordered fibrous materials defined by interfiber thermal contact conductance and intrinsic conductivity of fibers

A general framework for the theoretical analysis and numerical calculations of the effective thermal conductivity of two-dimensional homogeneous and isotropic disordered fibrous materials is developed in this work based on the model of random contacts between soft-core spherocylinders. The analysis accounts for the interfiber contact conductance and intrinsic conductivity of fibers and is performed in a wide space of governing parameters that includes the fiber aspect ratio, Biot number calculated for a single thermal contact between fibers, and material density ranging from values corresponding to the percolation threshold up to those characteristic of dense fiber networks. For dense networks, exact theoretical equations for the thermal conductivity of materials composed of spherocylinders with an arbitrary aspect ratio and Biot number are derived. The effect of the intrinsic conductivity of fibers on the thermal transport in fibrous materials is found to depend on the density of contacts and can be significant in sufficiently dense fiber networks even if the Biot number for a single thermal contact is small. Semiempirical corrections to the theoretical equations are derived for small and moderate fiber densities. The power law exponent describing the approximate dependence of the conductivity on the density parameter is found to vary from values close to 1 up to values exceeding 2 when evaluated within different finite ranges of the density parameter. This finding explains the variability of scaling laws for thermal conductivity of fibrous materials suggested in the literature based on numerical simulations performed in different regions of the space of material parameters.

Analytical Fractal Model for Calculating Effective Thermal Conductivity of the Fibrous Porous Materials.

Accurately predicting the effective thermal conductivity of the fibrous materials is highly desirable but remains to be a challenging work. In this paper, the microstructure of the porous fiber materials is analyzed, approximated and modeled on basis of the statistical self-similarity of fractal theory. A fractal model is presented to accurately calculate the effective thermal conductivity of fibrous porous materials. Taking the two-phase heat transfer effect into account, the existing statistical microscopic geometrical characteristics are analyzed and the Hertzian Contact solution is introduced to calculate the thermal resistance of contact points. Using the fractal method, the impacts of various factors, including the porosity, fiber orientation, fractal diameter and dimension, rarified air pressure, bulk thermal conductivity coefficient, thickness and environment condition, on the effective thermal conductivity, are analyzed. The calculation results show that the fiber orientation angle caused the material effective thermal conductivity to be anisotropic, and normal distribution is introduced into the mathematic function. The effective thermal conductivity of fibrous material increases with the fiber fractal diameter, fractal dimension and rarefied air pressure within the materials, but decreases with the increase of vacancy porosity.

Evaluation of different measurements for effective thermal conductivity of fibrous materials

Effective thermal conductivity is generally recognized as the intrinsic factor to reveal the thermal responses of fibrous materials. Here, two typical measurements, the step-wise transient method and the guarded hot plate method, were utilized to identify their feasibility for the effective thermal conductivity of fibrous materials (non-woven fabric and twill fabric) with different stacking layers.

Numerical determination of the effective thermal conductivity of fibrous materials. Application to proton exchange membrane fuel cell gas diffusion layers

The knowledge of physical properties, such as the thermal conductivity, plays an important role in the management of the heat transfer in fibrous materials like PEFC gas diffusion layers. Measurement of thermal conductivity by experimental means is not easy (due to the anisotropy and the high porosity), therefore the availability of experimental data is rather limited. In this paper, the numerical determination of the effective thermal conductivity of fibrous materials is investigated using a three-dimensional approach. Two different fiber geometries were studied with randomly generated fiber structures with overlapping and non-overlapping fibers. The corresponding anisotropic thermal conductivities are computed through the solution of the energy transport equation. The results were validated through a comparison with existing experimental data and the influence of different parameters such as fiber orientation, fiber diameter and binding material were investigated.

Lattice Boltzmann modeling of the effective thermal conductivity for fibrous materials

This paper presents a full set of numerical methods for predicting the effective thermal conductivity of natural fibrous materials accurately, which includes a random generation-growth method for generating micro morphology of natural fibrous materials based on existing statistical macroscopic geometrical characteristics and a highly efficient lattice Boltzmann algorithm for solving the energy transport equations through the fibrous material with the multiphase conjugate heat transfer effect considered. Using the present method, the effective thermal conductivity of random fibrous materials is analyzed for different parameters. The simulation results indicate that the fiber orientation angle limit will cause the material effective thermal conductivity to be anisotropic and a smaller orientation angle leads to a stronger anisotropy. The effective thermal conductivity of fibrous material increases with the fiber length and approach a stable value when the fiber tends to be infinite long. The effective thermal conductivity increases with the porosity of material at a super-linear rate and differs for different fiber location distribution functions.

Conductive thermal conductivity of fibrous materials under transient gas flow

The results are reported from computational studies of the conductive thermal conductivity of fibrous materials working at pressures correspnding to transient gas flow in pores.

Heat transfer through textile fabrics: mathematical model

Recently a wide range of materials have become available in the form of fibres or fabrics for use in electrochemical reactors and other applications. The problem of heat transfer through fabric materials is theoretically solved—a model representing the plain weave fabrics is given, which facilitates the application of the heat transfer equations. As a result the effective thermal conductivity of fibrous materials in a fabric form, yarn form, or even in fibre form could be predicted.

Heat transfer through textile fabrics: mathematical model

Recently a wide range of materials is available in the form of fibres or fabrics for use in electrochemical reactors and other applications. The problem of heat transfer through fabric materials is theoretically solved—a model representing the plain weave fabrics is given, which facilitates the application of the heat transfer equations. As a result the effective thermal conductivity of fibrous materials in a fabric form, yarn form, or even in fibre form could be predicted.