The Research On Anomalous Diffusion Of VOCs And Formaldehyde In Porous Indoor Building Materials

Volatile organic compounds(VOCs)emitted by porous wood-based panels with fractal structure seriously pollute indoor environment.In order to improve indoor air quality,it is of great significance to study the anomalous diffusion characteristics and quality transfer law of VOCs and formaldehyde released from wood-based panels.In this paper,the mass transfer of VOC and formaldehyde in wood-based panels and the impacts of different environmental factors on their emission characteristics are discussed,respectively.The main contents are as follows:1)According to the results of mercury intrusion porosimetry(MIP)tests of wood-based panels,VOC emissions from porous wood-based panels belongs to Knudsen diffusion is firstly determined by comparing the pore diameter of internal channel with VOC molecular free path.Different from the studies which VOC diffusion type in building materials is attributed to Fick diffusion,a time-fractional mass transfer model related to fractal dimension is proposed to analyze Knudsen diffusion characteristics,and the effect of areal porosity on surface emission is analyzed,and the analytical solution of the model was obtained for the first time.It is further proved that the finite difference scheme is solvable,unconditionally stable and convergent,and numerical simulation result and experimental data match well.Compared with the model of Little et al.,Deng and Kim,it can reveal the heavy-tailed phenomenon more accurately.In addition,the influences of the fractal dimension d_f,areal porosityεand delay time parameterλon VOCs emission are analyzed.2)Based on the results of MIP tests,it is firstly confirmed that the formaldehyde release from wood-based panels belongs to the transition zone diffusion,and a general time fractional mass transfer model is established to describe the non-fickian emission of pollutants(including formaldehyde,VOCs and SVOCs),the areal porosity of wood-based panel,the chemical reaction of formaldehyde and adsorption characteristics of SVOCs are taken into account,the analytical solutions in three different cases are obtained with Laplace transform method.It is also proved that the finite difference scheme of mass transfer equation is solvable,conditionally stable and unconditionally convergent.The numerical results show that the model is effective according to the maximum simulation relative error of formaldehyde release independent experiments of 5 wood-based panels and numerical simulation.In addition,the correlations between the fractal dimension and the time fractional derivative parameter,delay time parameter,areal porosity are proposed firstly.The effects of chemical reaction coefficient and delay time parameters on formaldehyde emission,and the changing trend of physical diffusivity and chemical reaction formation rate of formaldehyde within wood-based panel are further revealed.3)The effects of environmental factors on VOCs and formaldehyde emission in wood-based panels are studied by using the general time-fractional mass transfer model.Based on the fractional order mass transfer model,three groups of independent experimental data of formaldehyde release under different environmental factors are numerically simulated,the correlations between environmental factors(temperature T,relative humidity RH,air exchange rate N)and key release parameters are preliminatively illustrated according to the simulation results.In this paper,the developed time fractional mass transfer models and the influence of environmental factors on pollutant emission characteristics can provide important theoretical basis and guidance for further prediction and control of pollutant diffusion in indoor building materials,and lay a foundation for providing new methods and ideas of subsequent research.