Drying theory and application of porous media
The drying process of wet materials, particularly thermal drying, is fundamentally a coupled heat and mass transfer phenomenon. During this process, the material absorbs heat from the surrounding drying medium, which causes the heat to move from the exterior to the interior of the material. At the same time, moisture within the material migrates from the inside to the outside until the desired moisture level is achieved. As a result, the heat and mass transfer occurring in the boundary layer between the fluid and the solid material plays a critical role in understanding and optimizing the drying process.
According to Luikov’s classification, dried porous solids can be categorized into three main types: typical colloids, typical capillary porous media, and colloidal-capillary porous media. These porous structures are widely present in various industrial sectors. Examples include rocks, minerals, ceramics, building materials, insulation materials, catalysts, plant roots and leaves, as well as agricultural and food products. In fact, porous media form the core focus of industrial drying technologies.
It is important to note that the study of moisture migration during drying, along with the development of theoretical models for moisture transport in solids, has become a foundational system for advancing drying technologies. Traditional models, such as the continuous medium hypothesis, have been widely used. However, to better capture the characteristics of mesoscale structures, the volume average theory was introduced. Over the past decade, research trends have increasingly focused on integrating knowledge from neighboring disciplines to analyze and compare different continuum-based models. This has highlighted the limitations of traditional theories and led to the development of alternative approaches, such as pore network models, multi-scale methods, and fractal theory.
These emerging methodologies offer new insights into the complex behavior of moisture movement in porous media. Among them, the pore network model represents a significant shift in drying theory, focusing on the mesoscale structure of porous materials and aiming to link microstructural properties with macroscopic transport phenomena. As research progresses, the pore network approach is expected to play an even more vital role.
Multi-scale methods have already found applications in many fields, especially when dealing with complex systems where traditional methods fall short. Based on the analysis of the structural and transport characteristics of porous media, it is reasonable to consider applying multi-scale techniques to the drying process. A successful multi-scale approach should involve a clear classification of scales, representative modeling at each level, and ultimately achieve information fusion across scales. Pore network and fractal theories can serve as effective tools in this scale integration.
Fractal geometry, in particular, has shown great potential in various scientific fields and holds promising applications in the study of moisture transport in both dry and wet porous media. Its ability to describe irregular and complex structures makes it a powerful tool for improving our understanding of drying processes. Overall, these advanced theories and methods are essential for the future development of efficient and sustainable drying technologies.
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