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Abstract

Volatile organic compounds (VOC) are major indoor air pollutants. Physical models have been developed to predict VOC source (emission) and sink behavior (sorption) of building materials. They frequently adopt the conventional convection approach using a third-kind boundary condition. This conventional convection approach in conjunction with the commonly used Sherwood number correlation is based on the assumptions of constant wall concentration at the material-air interface and quasi-steady convective mass transfer in the fluid (air). In this study, the validity of these assumptions is theoretically investigated. An analytical model using the conventional convection approach and a numerical conjugate mass transfer model are developed. The conjugate mass transfer models consider unsteady two-dimensional laminar forced convection over a flat plate coupled with unsteady one-dimensional diffusion and sorption within the porous solid through the concentration and the flux continuities at the material-air interface. The simulation results indicate that the assumptions can lead to a significant overestimation of the wall concentration especially in the early transfer phase. When the effect on the VOC source/sink behavior is quantified by the total transfer time, which is the time required to emit/absorb 99% of the maximum transferable VOC mass, the analytical model results in less than 5% error in the predicted value when VOC transfer is controlled by internal diffusion, i.e., Biot number larger than 9 for (ε + K) 100.

Keywords

modeling indoor air quality mass transfer building materials conjugate mass transfer numerical mass transfer pollutants emission

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