The influence of aspect ratios and wall heating conditions on flow and passive pollutant exposure in 2D typical street canyons

Deep street canyons and unfavourable meteorological conditions usually induce high pollutant exposure. Validated by experimental data, this paper employs computational fluid dynamic simulations with RNG k-ε model to investigate the flow, and passive pollutant dispersion within scale-model two-dimensional street canyons(H = 3 m). As a novelty, this paper quantifies the impacts of various wall heating scenarios(bottom, leeward/windward wall and all-wall heating), ambient velocity(Uref = 0.5–2 m s−1, Froude numbers Fr = 0.25–4.08, Reynolds numbers Re = 95602–382409) and aspect ratios(building height/street width, AR = 0.5, 0.67, 1, 2, 3) on personal intake fraction for entire streets( ). The governing equations are implicitly discretized by a finite volume method (FVM) and the second-order upwind scheme with Boussinesq model for quantifying buoyancy effects. The SIMPLE scheme is adopted for the pressure and velocity coupling. In most isothermal cases, one-main-vortex structure exists as AR = 0.5–3( = 0.43–3.96 ppm and 1.66–27.51 ppm with Uref = 2 and 0.5 m s−1). For non-isothermal cases with Fr = 4.08(Uref = 2 m s−1), wind-driven force dominates urban airflow as AR = 0.5–1 and four heating conditions attain similar (0.39–0.43 ppm, 0.57–0.60 ppm, 0.91–0.98 ppm). As AR = 2, windward and all-wall heating get two-vortex structures with greater (3.18–3.33 ppm) than others( = 2.13–2.21 ppm). As AR = 3, leeward-wall heating slightly reduces (~3.72–3.96 ppm), but the other three produce two-vortex structures with greater (6.13–10.32 ppm). As Fr = 0.25(Uref = 0.5 m s−1), leeward-wall heating always attains smaller (1.20–7.10 ppm) than isothermal cases(1.66–27.51 ppm) as AR = 0.5–3, however the influence of the other three is complicated which sometimes raises or reduces . Overall, smaller background wind speed (Fr = 0.25) with two-vortex structures attains much larger . Special attention is required at night(all-wall heating), noon(bottom-heating) and cloudy period(no-wall heating) as AR = 2–3, while it is during windward-wall heating and cloudy period for AR = 0.5–1. •As Fr = 4.08, wind-driven force dominates the urban airflow as AR = 0.5–1.•As Fr = 0.25, most heating conditions would lead to a lower .•Formation of single main vortex is the most efficient way to decrease the .•Leeward heating condition always decreases the as Fr = 0.25 and 4.08.