A multi-scale model for impingement drying of porous slab

Abstract

Air impingement drying of a planar sheets of porous materials offers advantages of high and controllable heat and mass transfer rates. The governing heat and mass transfer phenomena involve multi-scale physical mechanisms ranging from mesoscopic pore scale to macroscopic jet scale. This work presents a pore-scale heat and mass transfer model of an unsaturated porous material based on statistically self-similar fractal scaling laws of pore structures. Furthermore, the effective transport coefficients are derived and applied to develop a cross-scale mathematical model for hot air impingement drying of a slab made of an unsaturated porous material. The flow and thermal fields of jet impingement and drying characteristics of unsaturated porous material are modeled numerically. The quantitative correlations between the mesoscopic pore structure and macroscopic heat and mass transfer properties are explored. The proposed fractal pore-scale model of porous material and cross-scale mathematical model of jet impingement show good agreement with experimental data. The effects of jet velocity, porosity as well as pore and tortuosity fractal dimensions on drying rate are discussed in detail. The computational results show that the mesoscopic pore structures indicate significant effect on the drying rate during later stage of drying. Results of this study provide useful guidance for efficient design of impingement of porous materials. • Meso-scale heat and mass transfer model of unsaturated porous material is proposed. • Multi-scale model for laminar impingement drying of porous slab is developed. • Impingement drying performance of porous slab is predicted. • Effect of mesoscopic structure on drying kinetics of porous material is explored.