Convective heat and mass transfer modelling at air-porous material interfaces: Overview of existing methods and relevance

Accurate predictions of convective heat and mass transfer at air-porous material interfaces are essential in many engineering applications, one example being optimisation of industrial drying processes with respect to energy consumption and product quality. For porous-material modelling purposes, simplified convective transfer coefficients (CTCs) are often used to avoid explicit air-flow modelling. Alternatively, conjugate models have been introduced recently and are being more widely used. Conjugate modelling has the advantage that it does not require the use of CTCs or of the heat and mass transfer analogy. Instead, these CTCs can be identified a-posteriori. In this study, an overview of the existing methods to predict convective heat and mass transfer at air-porous material interfaces is given, with a specific focus on conjugate modelling. The improved accuracy of this approach is indicated based on two case studies, namely hygroscopic loading and convective drying. A large spatial and temporal variability of the CTCs is found by means of conjugate modelling. This approach provides increased accuracy, which is especially relevant for complex flow problems, such as in industrial drier systems. However, the sensitivity to the convective boundary conditions can be limited in some cases, e.g. for hygroscopic loading. Instead of improving accuracy significantly here, conjugate modelling will rather impose an additional modelling effort, which often requires conjugate model code development as these models are not readily available. Before embarking on a conjugate modelling study, it is advised to perform a sensitivity analysis with respect to the convective boundary conditions: in some cases, sufficient accuracy can be obtained using empirical CTCs from literature.