Multi-scale Simulation On Impinging Jet Drying Of Unsaturated Porous Materials

The porous photoelectric materials indicate a few advantages including relative low density,high specific surface area,easy to be functional,high specific strength,good permeability and heat insulation ability,etc.Thus,it plays an increasingly important role in optoelectronic devices,semiconductor,energy conversion,energy storage,fluorescent sensors,which has brought new opportunities to the photoelectric field.However,the process of preparing porous optoelectronic materials is very complicated,and the drying technology is particularly important.As one of new drying method,air impinging jet drying technology has been widely used in the field of drying porous materials due to its advantages of high heat transfer efficiency,fast drying speed,compact structure and good uniformity after drying.The heat and mass transfer process of impingement jet drying porous materials spans from microscopic pore scale to macroscopic jet scale,involving multiple physical mechanisms such as heat transfer,fluid flow,gas diffusion and phase transformation,and its dynamic behavior and transport mechanism are very complex.Most of the current studies focused on the effect of jet structure and size parameters such as nozzle,the nozzle arrangement,impact height,jet velocity and temperature on the drying characteristics.It is lack of understanding on the heat and mass transfer properties and mechanisms of the unsaturated porous materials,the quantitative relationship between drying characteristics and thermal physical properties of porous material has not yet been established.Therefore,the fractal geometry theory is adopted to characterize the complex pore structures of unsaturated porous materials,and develop the pore-scale heat and mass transfer model in order to deduce the analytical expressions of thermal conductivity,permeability,and diffusion coefficient.And a multi-scale mathematical model for impinging jet drying unsaturated porous material is proposed based on the finite element method.The influence of microstructure characteristics of porous materials on macroscopic drying characteristics is studied.The mainly contributions of this thesis include the following aspects:(1)Thermophysical properties of unsaturated porous materials.Based on fractal geometry theory,a pore-scale heat and mass transfer model of unsaturated porous materials is presented,and analytical expressions of permeability,thermal conductivity and gas diffusivity are derived accordingly.The proposed fractal model is validated by comparison with experimental data.The results show that the predicted values by the fractal model are in good agreement with the experimental results.The effective thermal conductivity decreases with the increase of porosity and the decrease of liquid saturation.The effective permeability and gas diffusivity both increase with the enhancement of porosity.In addition,the increase of pore and tortuosity fractal dimensions reduces the effective thermal conductivity and gas diffusion coefficient.They show significant effect on the relative permeability of nonwetting phase,but indicate marginal influence on the relative permeability of wetting phase.(2)Heat and mass transfer enhancement of gas impingent jet.Mathematical and physical models of two-dimensional gas impingent jet are developed based on finite element method,and the thermal and flow fields are explored in detail.The effect of jet velocity and impinging height on local heat transfer rate are discussed.The results show that w the local heat transfer rate in the target surface increases continuously with the increase of jet velocity,however,the increasing amplitude decreases continuously.With the increase of impact height,the convective heat transfer rate in the stagnation zone decreases first and then increases.(3)Gas impingent jet drying of unsaturated porous materials.Mathematical and physical models of two-dimensional gas impingent jet drying of unsaturated porous materials are presented by using the analytical expressions of thermophysical properties of porous materials and finite element method.The variation laws of jet flow field,temperature field and concentration field are studied.The results show that the increment amplification of drying rate decreases with the increase of air velocity.The pore structure has significant effect on the transport properties of porous material.Thus,it shows evident influence on the drying rate at the late drying stage.The drying rate first increases and then decreases with the increase of porosity.In the case of constant porosity,the increase of pore fractal dimension and tortuosity fractal dimension can decrease the drying rate.By employing the effective transport parameters of unsaturated porous materials,a multi-scale model of gas impinging jet drying is developed to understand the impingement drying mechanism.It also provides an effective method to predict and optimize impinging jet drying porous materials system.The present numerical methods and results provide the theoretical basis and technical support for the preparation of porous photoelectric material,which further promote the development of the gas jet drying technology.