RANS simulation of near-field dispersion of reactive air pollutants

In conventional modeling of air pollution dispersion, pollutants are treated as passive scalars or inert species even though most of them are chemically reactive [1]. Chemical reactions contribute to pollutant dispersion via the generation and depletion of pollutants, in addition to other two mechanisms: advection and turbulent diffusion. This study investigated how chemical reactions affect near-field pollution dispersion by integrating the simple NOx-O3 chemistry into RANS-based computational fluid dynamics (CFD) simulation. CFD simulation was used to model a mixed emission of NO and NO2 from a short stack attached to a building into ambient O3, prompting chemical reactions between the NO, NO2, and O3. Various degrees of chemical reactivity were modeled by varying the Damkh¨oler number (Da) between 0.073 and 4.363. The results showed significant chemical reactivity for cases where Da [NO] > 1, while cases with Da [NO] < 1 had pollutant dispersion patterns similar to inert species. Noticeable modifications in concentrations were detected at ground level, where the NO concentration was depleted and NO2 concentration increased significantly. A budget analysis revealed major contributions of chemistry and turbulent diffusion to plume dispersion in the surroundings, while advection mainly carried the pollutants downstream from the source.
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Distribution of K [NO2] in the vertical center plane (y/H = 0) for (a) Case 1 – inert NO2 (b) Case 2 – Da [NO]:Da [O3] = 0.073:0.727, and (c) Case 4 – Da [NO]:Da [O3] = 2.181:7.271.
Budget analysis for (a) Case 1 – NO inert, (b) Case 2 – Da [NO]:Da [O3] = 0.073:0.727, (c) Case 4 – Da [NO]:Da [O3] = 2.181:7.271, and (d) Case 5 – Da [NO]:Da [O3] = 4.363:7.271 in the vertical center plane (y/H = 0)