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To design foams with specific properties, it is important to be able to predict their effective properties. The objective of this study is to compare and quantify the effect of the geometrical parameters on the thermal properties. Then the study uses both analytical and numerical homogenization methods to connect micro-scale parameters to the macro-scale thermal behavior. Several statistical studies of the morphology have been used to model the different categories of PU foams. The foam is modeled by a periodic structure of open, closed, or mixed cells. For each type of cell, different idealized shapes of a single cell are used, while keeping a constant porosity for each category. In the case of mixed cells, the effect of the relative positioning of cells is investigated. The results show that the effective thermal conductivity is considerably impacted by the cell shape. Moreover, the geometrical modeling of the struts, in the heuristic modeling, has also an important effect. Furthermore, the relative position of gas cavities, in the spherical representation, influences the effective thermal conductivity. However, the distribution of open and closed cells, in the case of mixed cell foam, has a limited effect. KEYWORDS numerical homogenization, polyurethane foam, shape of cells, thermal conductivity, 1 | INTRODUCTION Polyurethane (PU) foam materials are presented in various industrial applications, [1] such as the thermal insulation of buildings. [2] PU foams of different densities are suited for [...]

Many theoretical and empirical models exist to predict the effective thermal conductivity of polymer foams. However, most of the models only consider the effect of porosity, while the pore size effect is ignored. The objective of this study is to understand the effect of pore size on the thermal conductivity of polymer foams, especially when it reduces to the micro and nanometer scales. A wide range of pore sizes from 1 nm to 1 mm were studied in conjunction with the porosity effect using finite element analysis and molecular dynamics simulation methods. Experimental data was used to validate the modeling result. It is shown that the pore size has significant effect on thermal conductivity, even for microcellular and conventional foams. The contribution of heat conduction through air is negligible when pore size is reduced to the micrometer scale. The extremely low thermal conductivity of nanofoams is attributed to extensive diffusive scattering of heat carriers in the solid phase of polymer matrix, instead of air. This study provides quantitative understanding of the pore size effect on thermal conductivity of polymer foams. It is also shown that polyetherimide (PEI) nanofoams could have a thermal conductivity as low as 0.015 W/m-K. POLYM, ENG. SC., 53:1901-1909, 2013. © 2013 Society of Plastics Engineers, INTRODUCTION The thermal conductivity of polymer foams is of significant interest in both academia and industry, especially when the pore size is reduced to the micro and nanometer scales (1), [...]

The self-consistency of a proposed three-dimensional geometric model of open-celled Duocell foam is evaluated. The foam samples have displayed a higher dependence on relative density and less dependence on interstitial fluid and the proposed metal foam geometric model is found to be self-consistent with respect to both its geometric and thermal properties.

In this paper, we present our recent experimental results on buoyancy- induced convection in aluminum metal foams of different pore densities [corresponding to 5, 10, 20, and 40 pores per in. (PPI)] and porosities (0.89-0.96). The results show that compared to a heated surface, the heat transfer coefficients in these heat sinks are five to six times higher. However, when compared to commercially available heat sinks of similar dimensions, the enhancement is found to be marginal. The experimental results also show that for a given pore size, the heat transfer rate increases with porosity, suggesting the dominant role played by conduction in enhancing heat transfer. On the other hand, if the porosity is held constant, the heat transfer rate is found to be lower at higher pore densities. This can be attributed to the higher permeability with the larger pores, which allows higher entrainment of air through the porous medium. New empirical correlations are proposed for the estimation of Nusselt number in terres of Rayleigh and Darcy numbers. We also report our results on novel finned metal foam heat sinks in natural convection. Experiments were conducted on aluminum foams of 90% porosity with 5 and 20 PPI with one, two, and four aluminum fins inserted in the foam. All of these heat sinks were fabricated in-house. The results show that the finned rueral foam heat sinks are superior in thermal performance compared to the normal metal foam and conventional finned heat sinks. The heat transfer increases with an increase in the number of fins. However, the relative enhancement is found to decrease with each additional fin. The indication is that there exists an optimum number of fins beyond which the enhancement in heat transfer, due to increased surface area, is offset by the retarding effect of over-lapping thermal boundary layers. Similar to normal metal foams, the 5 PPI samples are found to give higher values of h compared to the 20 PPI samples due to higher permeability of the porous medium. Future work is planned to arrive at the optimal heat sink configuration for even larger enhancement in heat transfer. [DOI: 10.1115/1.2229225]