Study Of Heat And Mass Transfer In Deformable Porous Media And Strengthening Technology

Porous media exist in both natural and artificial systems,playing a critical role in sustaining almost everything in the world through the transfer of energy,mass,and momentum.In nature and artificial processes,the solid skeleton structures of porous media are not fixed and undergo deformation during heat and mass transfer,resulting in significant changes to their shape and properties.Deformable porous media have a complex structure,which in turn makes internal energy,mass,and momentum transfer problems difficult to solve.These challenges include thermo-hydro-mechanical bi-directional coupling,multi-scale feature evolution,deformation regime,and strengthening mechanism.This paper presents a comprehensive investigation of the heat and mass transfer process in deformable porous media and ultrasonic-assisted technology,using a systematic approach that includes experimental,numerical,and theoretical analysis.We have built a visualized hot air drying and regeneration test platform to analyze the drying kinetics and multi-scale deformation characteristics of three representative deformable porous materials.On the representative element volume scale,pore scale and cell scale,a bidirectional coupled thermos-hydromechanical mathematical model of the deformable porous media was established,and the spatial and temporal distribution of moisture fraction,temperature,solid skeleton stress and deformation displacement were analyzed to explore the deformation mechanism during heat and mass transfer in deformable porous media.We developed an experimental platform for ultrasonic-assisted hot air drying and regeneration,and proposed an acoustic-coupled thermohydro-mechanical model,sheds light on the mechanism of ultrasonic enhancement to heat and mass transfer processes.The detailed research contents and conclusions of this work are as follows:(1)A visualized test platform was used to conduct hot air drying and regeneration experiments for activated carbon foam,municipal sludge,and apple slices.The drying kinetics and multiscale deformations of different materials were investigated.The whole regeneration process of activated carbon foam can be divided into constant rate stage and falling rate stage.The regeneration rate of the constant rate stage is increased with the hot air temperature,while the hot air temperature has minimal impact on the regeneration rate at falling rate stage.There are three main deformation regimes in the regeneration process of activated carbon foam: skeleton shrinkage,closed pore crack,and skeleton union.By adjusting the two pore fractal dimensions,closed pore ratio,and average pore throat diameter,it is possible to control the deformation of activated carbon foam.The drying rate and volume shrinkage of municipal sludge increase as the sludge thickness decreases.Reducing the sludge thickness can not only shorten the drying time and increase the volume shrinkage of the sludge,but also can decrease energy consumption.The cell walls of apple tissue contracted and deformed,leading to an increase in cell interspaces.The browning index and total chromaticity value of apple slices increased significantly with increasing drying temperature.(2)We developed bidirectional coupled thermo-hydro-mechanical mathematical models for both saturated and unsaturated deformable porous media at REV scale.The numerical results were compared to experimental data to confirm the validity and accuracy of the model.We investigated the drying kinetics and diffusion coefficient evolution of liquid and gas phases,compared various stresses(e.g.,thermal and hygroscopic)inside deformable porous media,and studied total deformation displacement and macroscopic shrinkage.During the constant rate drying stage,porous media experience a sharp decrease in porosity and an increase in shrinkage rate.As the drying process progresses to the falling rate stage,porosity and shrinkage tend to stabilize over time,while a fluid interface forms between the dry and wet zones.The capillary diffusion of liquid water decreases over time,whereas the convective diffusion increases and stabilizes.During the constant rate drying stage,positive stress increased with time before stabilizing in the falling rate drying stage.Shear stress has a persistent impact on the deformation of deformable porous media.Thermal stress exerts less influence on porous media deformation than hygroscopic stress.The impact of non-uniform shrinkage can be partially mitigated by regulating the shape of the outer surface of the deformable porous media.(3)We established a 3D thermo-hydro-mechanical model of deformable porous media at the pore scale and confirmed its validity through experimental results.The closed pores within deformable porous media impede the heat and mass transfer process and often lead to stress concentration,with stress values around these areas reaching up to three times higher than elsewhere in the material.Due to an increase in effective stress,cracks and deformations tend to occur preferentially on the skeleton within the closed-pore region.Fractal theory is used to quantify the deformation of the porous skeleton.The pore area fractal dimension increases over time,indicating an increase in effective porosity.Additionally,the pore tortuosity fractal dimension gradually decreases,suggesting that pore channels in the deformable porous media become straighter.The pore scale model was expanded to simulate fruit tissue drying,revealing distinct water content distribution patterns within cells and cell walls.During the drying process,the local deformation of cell walls is influenced by the contraction difference between adjacent cells.The total deformation displacement value of the cell walls is approximately 7.62% higher than that of the cell interspaces.(4)An experimental platform was constructed to investigate the effectiveness of ultrasonic-assisted hot air drying under varying conditions and power levels.Results showed that ultrasonic treatment can effectively enhance the effective diffusivity of moisture and lower the activation energy required for desorption.In a specific range of ultrasonic power,the presence of ultrasound can reduce unit energy consumption,and there is a critical threshold of ultrasonic power that minimizes system unit energy consumption.The effect of ultrasonic power on the apparent structure of deformable porous materials was analyzed indepth using three-dimensional image processing.The results showed that after ultrasonic treatment,the specific surface area of activated alumina increased and its adsorption capacity became stronger.The ultrasonic action induced delamination within the sludge,resulting in an increase in porosity with increasing thickness.By employing ultrasonic-assisted hot air drying,apple slices can maintain their microstructure,minimize surface shrinkage deformation,and retain their color.(5)An acoustic-coupled thermo-hydro-mechanical(THM)model was developed to investigate the heat and mass transfer mechanisms inside deformable porous media during ultrasonic-assisted hot air drying.Ultrasound can improve the orderliness of the diffusion velocity field of moisture within deformable porous media,with a corresponding increase in velocity magnitude as power increases.The presence of ultrasound increases the temperature gradient,reducing resistance to heat transfer in materials.The average synergy was directly proportional to the ultrasonic power,indicating that ultrasound enhances the heat and mass transfer process in the porous material.Ultrasonic treatment altered the material’s deformation properties,with higher power levels contributing to a reduction in sample volume shrinkage.Specifically,the sample showed a 15.4% decrease in volume shrinkage when exposed to 180 W ultrasound compared to non-ultrasound treated samples.Furthermore,the energy flow during ultrasonic-assisted hot air drying of deformable porous materials was analyzed.The unique effects and synergistic mechanisms of ultrasound were summarized,and a comprehensive evaluation system for ultrasonic strengthening technology was proposed.This work investigated the thermo-hydro-mechanical properties of deformable porous media at different scales.It not only developed new ideas and research interests for the simulations of heat and mass transfer in deformable porous media,but also explored ultrasonic-enhanced heat and mass transfer technology via theoretical analysis,experiments and simulations.Moreover,it offered both theoretical and technical support to enhance the potential applications of deformable porous materials in various fields.