|Bundle of capillaries drying kinetics continuous model relative permeability capillary pressure control volume method|
Being one of the most complex processes encountered in engineering, the drying of porous media has a vital role in many different industrial fields. In principle, the transport phenomena in the drying of porous media can be modeled using a continuous or discrete approach. The continuous approach is based on a description of the system as a fictitious continuum by using effective coefficients of heat and mass transfer. In the discrete approach the drying of porous media is represented by a network of pores and transport phenomena are directly described at the pore level. In developing a continuous drying model, the volume averaging technique can be used to derive a system of macroscopic transport equations from a set of basic transport laws at microscopic level for gas, liquid and solid phases. The derived system represents the conservation equations of mass, energy and momentum, in which the average state variables (moisture content, temperature and gas pressure) and a set of effective parameters are employed. These effective parameters have strong effects on the material drying characteristics and must be determined experimentally or must be modeled with a great care about the material microscopic structure. In general, the problem of determining the model effective parameters is yet to be solved and deserves careful attention. These parameters are capillary pressure curve, liquid and gas permeabilities, effective diffusivity, and effective thermal conductivity. As a first step in gaining a basic knowledge about how the material microstructure affects its drying kinetics, in this work, the porous medium are represented by a bundle of capillaries with a radius distribution to compute the mentioned effective parameters. In this model, the material pore size distribution is considered as the key to build a link between the material microstructure and its macro drying behaviour. By varying the mean pore radius and the broadness of the distribution as well as the number of modes (mono-modal and bi-modal distributions), the influence of pore size distribution on effective parameters and on drying behaviour is analysed. This analysis is realized with the help of the control volume method and the numerical results are presented as temporal evolution of local moisture content, temperature and gas pressure as well as overall drying curves. The continuous model for the bundle of capillaries geometry is compared with two discrete models, a one-dimensional capillary model and a pore network model using an equivalent geometry. A good agreement between the continuous and the discrete approaches is found. In addition to the study of the influence of pore size distribution on drying behaviour, the continuous model is also used to investigate the influence of sample size on drying time, where a reference material (light concrete) is considered. The results are compared with a simple diffusion model and a receding front model. Besides the numerical modelling of drying, several experimental techniques were used to characterize the pore structure, the pore size distribution, the sorption equilibrium and the drying kinetics of gamma alumina particles of diameter 4.8 mm.