Influence of chemical reactions and turbulent diffusion on the formation of local pollutant concentration distributions

Abstract

The mechanism of structure formation of non-uniform pollutant concentration distributions in indoor environments have been investigated over the past several decades to determine effective ventilation designs. In this study, numerical analyses of local pollutant concentration distributions in indoor environments are performed based on computational fluid dynamics techniques. In particular, the influence of gas phase chemical reactions and turbulent diffusion on the formation of the local pollutant concentration is parametrically analyzed, and the structures are quantitatively investigated using the index for ventilation efficiency, namely the net escape velocity (NEV) concept. The NEV concept represents the substantive velocity scale, combining advection and diffusion velocity of pollutant at a point, and functions as an index to determine the pollutant concentration at that point. Sensitivity analyses as functions of the first Damköhler number (Da) and the turbulent Schmidt number (Sct) were performed, and the influence of Sct on pollutant concentration distributions was more significant compared with that of Da. When Sct was changed from 0.5 to 1.0, the significant NEV increase, especially that in the stagnant flow region, could be attributed to the enhanced pollutant discharge efficiency due to turbulent diffusion in addition to convective flow. Thus, NEV could be used to quantitatively assess the changes in pollutant concentrations accompanying the change in Da or Sct.