A Model of Moist Polymer Foam and a Scheme for the Calculation of Its Thermal Conductivity

Abstract

This paper proposes a method for determining an effective value of the thermal conductivity for moist, highly porous rigid polymer foams. The model of moist foam based on an ordered structure with interpenetrating components was developed in accordance with the moisture distribution in the pore space. With small moisture content, isolated water inclusions are formed, and the pore space is considered as a binary system (vapor-gas mixture and water) with isolated inclusions. With an increase in moisture content, isolated water inclusions merge, forming a continuous layer, and pore space is considered as a binary system of interpenetrating components. The thermal conductivity of the vapor-gas mixture is represented as the sum of the thermal conductivity of the dry gas and the thermal conductivity of the vapor caused by the diffusion transfer of vapor in the pore space, taking into account the coefficient of vapor diffusion resistance. Using the proposed scheme of calculation, a computational experiment was performed to establish the influence of the vapor diffusion, moisture content, and average temperature of the foam on its thermal conductivity.

Effect of Moisture on the Thermal Conductivity of Cellulose and Aramid Paper Impregnated with Various Dielectric Liquids

“…These studies supplement the knowledge regarding heat transport in the transformer and can be used to calculate the temperature rise in the transformer, both at the design and operation stages. The influence of moisture on the thermal conductivity of materials is a very important issue, not only in terms of the operation of insulation systems of power devices but also in many other cases, such as the energy efficiency of buildings [44,45].…”

Dependencies for Determining the Thermal Conductivity of Moist Capillary-Porous Materials

“…The structure of the moist material in the form of a unit cell with a drop of underhygroscopic liquid concentrated at its node is presented in Figure 1a. Depending on the value of the contact angle θ, the surface of this drop of liquid can be both concave and convex [11]. As presented in [5] and confirmed by calculations made for the unit cell under consideration, the form (shape) of inclusion has a negligible effect on the effective thermal conductivity.…”

“…Energies 2020, 13, x FOR PEER REVIEW 5 of 13 An illustration of the dependency ψpʹʹ = f (θ, c), determined using Equations ( 6)-( 8), is shown in Figure 3. This figure also shows the dependency ψpʹ = f (θ, c) determined in [11]. These dependencies render it possible to assess the structure of the moist open-porous material as a three-component system, and then, using appropriate formulas, determine its effective thermal conductivity.…”

“…The boundary values of the pore moisture ψ p and ψ p " depend on the contact angle θ and the properties of the capillary-porous material structure. For the adopted hard skeleton model of the porous material (Figure 1a), the values of ψ p can be determined using the dependency ψ p = f (θ, c) presented in [11]. When deriving these dependencies, it is assumed that in the process of sorption moisture there is no capillary condensation and the maximum hygroscopic moisture of the material is very low (ψ p ≈ 0.002), and therefore it is not considered.…”

“…Therefore, this paper is another attempt to develop a simple calculation method enabling the determination of the thermal conductivity of capillary-porous materials, which can be used to solve various engineering problems (design of building partitions, assessment of existing buildings, etc.). The first attempt was to develop a calculation scheme presented in [11], proposed for determining the effective thermal conductivity of polyurethane foams. The dependencies presented in the current publication are the result of further work in the chosen direction, aimed at simplifying the calculation procedures and extending the possibility of their application to other materials with capillary-porous structure.…”