Multi-Scale Mass Transfer Mechanisms And Emission Characteristics Of VOC In Porous Building Materials

As the main source of indoor air pollution,volatile organic compounds(VOCs)are widespread in indoor decoration materials.In recent years,the rapid advancement of building energy conservation made buildings tend to be highly enclosed and airtight.VOCs accumulate in the indoor air and have a great impact on the health and work efficiency of the indoor crowd.Depending on the data released by the world health organization(WHO),about 4 million people died prematurely due to non-communicable diseases caused by indoor air pollution each year.The existing researches on VOC emission characteristics of building materials are mostly focused on the phenomenological level.However,exploring the complex pore characteristics of porous structures and the differential mass transfer mechanisms is the basis for determining VOC emission characteristics of building materials,which is also the premise of guiding VOC control strategies.In view of this,by being combined of theoretical analysis,experimental test and numerical calculation,this thesis investigated the multi-scale mass transfer mechanisms and emission characteristics of VOC in porous building materials.The specific research contents and academic achievements are as follows:Breaking the dimensional barriers of the traditional continuum model,the fractal geometry theory was applied in the prediction of VOC effective diffusion coefficient of porous building materials.Considering the diameter variability of the mass transfer channels in building materials,a multistage series-connection fractal capillary-bundle(MSFC)model was developed based on the fractal capillary-bundle model.Through the analysis of pore structures of porous building materials,the distribution characteristics of macro-mesopores and the connection modes of mass transfer networks were explored.Thus,an innovative mass transfer mode was proposed,which included the fractal distribution of mass transfer channels composed by macropores and the auxiliary mass transfer characteristics of mesopores.In addition,the accuracy of the MSFC model was verified by the mercury intrusion porosimetry(MIP)tests and the environmental chamber experiments.Compared with the classic diffusivity models,the VOC concentrations predicted by the MSFC model were more consistent with the experimental data.Based on the adsorption potential theory,the influences of heterogeneous active sites formed by porous structure on the adsorption characteristics of VOC molecules were interpreted.A dual-scale calculation model of the partition coefficient was deduced based on the Dubinin-Radushkevich and Freundlich adsorption isotherm,respectively,for micropores and macro-mesopores.Compared with the partition coefficient model which was derived based on Langmuir monolayer adsorption theory,the dual-scale calculation model took the influence of pore structure difference into consideration.To verify the accuracy of the theoretical prediction model,a new experimental method was developed to measure the partition coefficient and initial emittable concentration in step temperature condition.Through fitting with the experimental data,the prediction results can be expanded to the determination of the partition coefficients of the homologues.Furthermore,by using the partition coefficient of macro-mesopores as a passing parameter,the effective diffusion coefficient can be converted to the apparent diffusion coefficient,which is suitable for the single-phase model.The relationship between the apparent diffusion coefficient and temperature was established.The multi-scale desorption mechanism of VOC molecules was systematically analyzed,and then,the mathematical characterization of the distribution law of adsorption potential in porous building materials was proposed,instead of using the mean or empirical formula to calculate the adsorption potential.An analytical formula for the distribution of multiple initial emittable concentration was derived based on the nonuniform desorption criterion.This criterion described the relationship between the probability density distribution of molecular kinetic energy and the adsorption potential distribution.Meanwhile,in view of similar chemical properties between homologues,the correlation of the adsorption potential of homologous was deduced,by means of which the initial emittable concentration of a variety of homologues can be simultaneously predicted with few experiments.Through the sensitivity analysis of VOC emission of building materials,the influences of the key emission parameters and their main control factors were discussed.In addition,for the three VOC source control methods,namely,uniform blending,surface obstructing and substrate fixation of adsorbent,the prediction system of the key parameters of VOC emission in building materials was established,which can be utilized to predict and evaluate the effects of the three VOC control methods.The results showed that the inhibition effect of uniform blending of adsorbents on VOC emission was the best,the effect of surface obstructing was significant in short-term emission process,while substrate fixation was suitable for long-term emission.The prediction of the source control effect of VOC can provide a theoretical approach for selecting the VOC control strategies in engineering application.