A 3D model to predict the influence of nanoscale pores or reduced gas pressures on the effective thermal conductivity of cellular porous building materials

Abstract

Cellular porous materials are frequently applied in the construction industry, both for structural and insulation purposes. The progressively stringent energy regulations mandate the development of better performing insulation materials. Recently, novel porous materials with nanopores or reduced gas pressures have been shown to possess even lower thermal conductivities because of the Knudsen effect inside their pores. Further understanding of the relation between the pore structure and the effective thermal conductivity is needed to quantify the potential improvement and design new optimized materials. This article presents the extension of a 3D numerical framework simulating the heat transfer at the pore scale. A novel methodology to model the reduced gas-phase conductivity in nanopores or at low gas pressures is presented, accounting for the 3D pore geometry while remaining computationally efficient. Validation with experimental and numerical results from the literature indicates the accuracy of the methodology over the full range of pore sizes and gas pressures. Combined with an analytical model to account for thermal radiation, the framework is applied to predict the thermal conductivity of a nanocellular poly(methyl methacrylate) foam experimentally characterized in the literature. The simulation results show excellent agreement with less than 5% difference with the experimental results, validating the model’s performance. Furthermore, results also indicate the potential improvements when decreasing the pore size from the micrometre to the nanometre range, mounting up to 40% reduction for such high-porosity low-matrix-conductivity materials. Future application of the model could assist the design of advanced materials, properly accounting for the effect of reduced pore sizes and gas pressures.

Biobased foams for thermal insulation: material selection, processing, modelling, and performance

“…In contrast, van de Walle et al developed a model which employs micro-CT scanning and/or stochastic generation algorithms to get a more accurate representation of a true foam structure. 116,117 An overview of the simulation process is given in Fig. 9.…”

The impact of pore structure parameters on the thermal conductivity of porous building blocks

“…The 3D image-based thermal simulation model [32][33][34][35] used in this study is briefly outlined here. This model performs heat transfer simulations on a cubic material sample, via three main steps, of which a graphic overview is depicted in Figure 1.…”

“…Firstly, the influence of nanometre-sized pores on the conduction via the solid matrix remains an open question. However, since materials with low thermal conductivity are inherently envisioned when producing insulating porous building blocks, potential pore size effects on the solid matrix conductivity are deemed negligible as discussed in [34,35]. For conduction via the pore gas, the presence of nanoscale pores invalidates the use of Fourier's first law, due to the Knudsen effect.…”

“…This study therefore examines the interacting impacts of different pore structure parameters on the ETC of porous building blocks. A pore-scale heat transfer simulation framework [32][33][34][35], see Section 2, is used to predict the ETC of a wide range of generic pore structures with varying properties. Therefore, this study is effected in two steps.…”

Three-dimensional geometry, pore parameter, and fractal characteristic analyses of medium-density fiberboard by X-ray tomography, coupling with scanning electron microscopy and mercury intrusion porosimetry

“…Building materials such as wood-based panels have been identified as a major source of indoor air pollutants (Lee et al, 2018; Tham, 2016). Material structure parameters, such as porosity ( ε ), pore connectivity (Mu et al, 2015), and fractal characteristics, have a close relationship to volatile organic compound (VOC) diffusion and mass transfer (Liu et al, 2015; Van de Walle and Janssen, 2020). Therefore, measuring the pore parameters and establishing the structure of the pores to obtain the characteristic parameters of building materials are crucial to controlling IAQ and have drawn widespread attention (Hicklin et al, 2018).…”