The effects of thermal stratification on airborne transport within the urban roughness sublayer

• Published in: International journal of heat and mass transfer 2022-03, Vol.184, p.122289, Article 122289
• Main Authors: Cai, Junjie, Chen, Jingtan, Cheng, Haimei, Zi, Shuangfei, Xiao, Jinchao, Xia, Fan, Zhao, Jiyun
• Format: Article
• Language: English
• Subjects: Atmospheric diffusion; Atmospheric dispersion; Building codes; Computational fluid dynamics (CFD); Dispersion; Flow stability; Hazardous materials; Heat; Heat exchange; Heat transfer coefficients; High temperature; Indicators; Mass transfer; Pollutants; Pollution transport; Positive feedback; Richardson number; Roughness; Scalar fluxes; Stability tests; Street canyons; Temperature gradients; Thermal stratification; Transfer coefficients; Urban environments; Urban roughness sublayer (RSL)
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2021.122289

•The proposed morphology captures and reproduces the 3-D transport characteristics better.•The lateral momentum exchange induced by roughness elements suppresses the spanwise transport of pollutants towards outside.•The gap between heat and pollutant transfer coefficients (HTC and PTC) is one order of magnitude.•The bottleneck of the HTC/PTC is the source-to-canyon process.•The stability threshold is proved to be Rb ≈ 0.7. The release of airborne hazardous substances in the urban roughness sublayer (RSL) is a risk to human health. The atmospheric dispersion of these materials should be contained to mitigate their adverse consequences. This study investigates the effects of atmospheric stability on the air, heat, and pollutant transport within RSL by a validated 3-D RANS code. A building morphology with height variance is proposed for better representing the real urban environment and characterizing the complex interactions between the roughness elements and the flow regimes. The thermal stratification stability test subject to the ground-inflow temperature difference is extended to span from strongly unstable to moderately stable. The quantitative indicators, including air exchange rate, heat removal rate, pollutant removal rate, heat transfer coefficient, and pollutant transfer coefficient, are introduced and analyzed. The correlations between indicators and thermal stabilities are established, which provides explicit expressions to describe the influence of the changing bulk Richardson number (Rb). Results show that the design of height-variance elements enables a stronger lateral momentum towards the target street canyon, which suppresses the spanwise dispersion of pollutants. The discussions upon heat and pollutant transport based on stability categories show a high Rb-dependence of heat/mass transfer efficiency. The stability threshold is demonstrated to be Rb ≈ 0.7, where the flow speed near the ground approaches zero. As a result, the high temperature gradient is formed and acts as positive feedback to facilitate a more stratified condition. The accurate and rational correlations obtained in this study will save the computational cost considerably for further research. Also, the available results will be a reference for environmentally friendly designs.