No abstract

Several studies in the literature have been devoted to the permeability prediction of metal foams and stacked fibres, including the representative unit cell (RUC) model. This study is the first to involve a comparative analysis for the permeability and specific surface area prediction of different versions of the RUC model for fibrous media, i.e. the 2D RUC models for in-plane and through plane flow, the 3D RUC model, the two-strut RUC models for in-plane and through plane flow and the three-strut RUC model. These models are chosen due to their simple rectangular geometry, as well as its satisfying performance in comparison with other models and experimental data as proven elsewhere in the literature for fibrous media. The Darcy and Forchheimer permeability coefficients of these models are presented of which some are novel contributions. A geometric approach requiring measured geometric parameters and a combined geometric-kinetic approach requiring measured permeability coefficients are included for determining the specific surface area for all the models of which the majority of expressions are new. Also novel to this study is the comparative analysis of the 2D and 3D RUC modelling approaches to that of the two-strut and three-strut approaches. An evaluation of which models would suit fibre-like media and which would suit foam-like media is also included with regard to permeability and specific surface area predictions, based on geometric features and model predictive ability. The models are evaluated through comparison with other models from the literature as well as available experimental and numerical data.

Knowledge of the geometric and kinematic parameters of porous foams are of great importance since it is used in a wide variety of industrial multiphase flow applications that require optimal functionality, e.g. gas filters, heat exchangers and catalyst supports. The large external surface area and high porosity of metal foams provide good chemical resistance, enhanced heat and mass transfer properties and low pressure drops. Four generic geometric models will be considered to characterize the metal foam geometry, namely the cubic unit cell, tetrakaidecahedron, dodecahedron and rectangular representative unit cell (RUC) models, as well as three kinetic approaches from the literature in order to predict the specific surface area (SSA). Two sets of experimental data from the literature will then be compared to the SSA model predictions of the geometric approach and to the SSA values obtained from the kinetic approach. A comparative analysis reveals that the most geometrically complex tetrakaidecahedron model indeed provides the best correspondence with the experimental data for the SSAs, followed by the geometrically simplest RUC model. The latter model, in addition, provides accurate results for the kinetic approach. The advantage of the RUC model is that it is the only geometric model that provides both a geometric and kinetic approach, and, as a result of its relatively simple geometry it is geometrically adaptable towards anisotropy. The Klinkenberg effect will also be considered to determine the influence on the predictions of the SSAs dependency on the permeability coefficients for different fluid phases.