Study On Migration Of VOC In Fractal Porous Building Materials And Its Behavior Effected By Electrostatic Field

Air quality of indoor environment has a significant impact on physical and psychological health,and living quality of human.Volatile organic compounds(VOC)emitted from building materials is one important reason for worsening the indoor air quality,and causing the various sick building syndromes.Therefore,as a typical pollution source of VOC,the migration behavior characteristics and enhancing methods of mass transfer of VOC in building materials should be developed to control the emission of VOC from building materials radically,which has incurred major international research interest.Nowadays,the migration behavior mechanism of VOC in porous building materials is not fully understood at the pore scale.In addition,almost no studies have been concentrated on the mass transfer methods of VOC in building materials using the nonthermal field.In this paper,the fractal reconstruction method is introduced to generate the porous structure of building materials.The migration behaviors of VOC in porous building materials.including desorption,diffusion and emission are investigated at multiscale level,and the impact mechanism of external electrostatic field on the migration of VOC is explored.The results and conclusions are summarized as follows:(1)The static chamber experiment system is designed and assembled so as to study the emission characteristics of formaldehyde from particle board.In the experiment,the transient concentration curves of formaldehyde in the static chamber are obtained,and the effects of temperature and electric field intensity on the key emission parameters are analyzed using C-history method.The experimental results indicate that the temperature field has a significant impact on the key emission parameters of formaldehyde in building materials.The mass diffusion coefficient increases with the temperature increases obviously;by contrast,the partition coefficient of formaldehyde decreases with the temperature rises monotonously.In addition,the external electrostatic field has a certain influence on the key emission parameters,especially on the partition coefficient.The exponential function relationship is presented between the electric field intensity and the partition coefficient of formaldehyde.(2)Based on the fractal theory,the fractal characteristics of porous building materials are described,and the porous structures of building materials are reconstructed.Through the microscopic observation of microstructure,the anisotropic porous structure of building materials is presented,and the fractal geometry is appropriate for describing the topological characteristics of porous building materials.According to the obtained two dimensional porous structure of building materials,the random midpoint displacement algorithm,which is derived from the fractional Brownian motion theory,is introduced to reconstruct two dimensional and three dimensional porous materials.In addition,using a structure filter transformation,the anisotropic fractional Brownian structure is obtained to generate anisotropic porous media which can be applied to reconstruct anisotropic porous structure of building materials.(3)Lattice Boltzmann model derived from the kinetic theory of mixture gas is developed to simulate VOC transfer in porous structure of materials.The results show that the fractal characteristics have an obvious impact on the effective diffusion coefficient of porous materials.With the decreasing Hurst exponent,the tortuosity raises obviously which causes the reduction of effective diffusion coefficient.For anisotropic porous materials,the difference of effective diffusion coefficient at different diffusion direction is observed.The effective diffusion coefficients in the privileged direction vary approximately linearly with the porosity,meanwhile,the nonlinear variation is appeared in the perpendicular to privileged direction.With regard to the multicomponent mixtures diffusion in porous materials,the effect of Knudsen diffusion resistance is more significant for the VOC species with larger molecular weight in contrast to the smaller molecular weight species such as nitrogen.(4)At the molecular scale,the impact mechanism of external electrostatic field on the adsorption between formaldehyde and the cellulose monomer molecules is explained based on density functional theory.The research results indicate that the adsorption potential is reduced with the applied electric field intensity increases,and the exponential relationship between the electric field intensity and the adsorption potential is observed,and which is agree with the prediction of experimental results qualitatively.(5)The adsorption/desorption process is coupled into LBM model to study the migration of VOC in porous building materials and the influence of external electrostatic field.The numerical results show that the concentration of formaldehyde in porous structure increases due to the enhanced VOC desorption under the action of external electrostatic field.Therefore,the emission rate of formaldehyde is improved owing to the increasing pressure and concentration gradient.With the duration of migration process,the difference of decline rate of formaldehyde is reduced applied by the different electric field intensity.In summary,the above investigations gain a further insight into the migration mechanism of VOC in porous building materials,which provides a theoretical support for the predication of migration behavior of VOC in porous building materials.The effect of external electrostatic field on the migration of VOC is explored to develop the enhancing methods of mass transfer of VOC in building materials.